Stepper motorFrom Wikipedia, the free encyclopedia

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Frame 1: The top electromagnet (1) is turned on, attracting the nearest tooth of a gear-shaped iron rotor.

With the teeth aligned to electromagnet (1), they will be slightly offset from electromagnet (2).

Frame 2: The top electromagnet (1) is turned off, and the right electromagnet (2) is energized, pulling the nearest

teeth slightly to the right. This results in a rotation of 3.6° in this example.

Frame 3: The bottom electromagnet (3) is energized; another 3.6° rotation occurs.

Frame 4: The left electromagnet (4) is enabled, rotating again by 3.6°. When the top electromagnet (1) is again

enabled, the teeth in the sprocket will have rotated by one tooth position; since there are 25 teeth, it will take 100

steps to make a full rotation in this example.

Because of power requirements, induction of the windings, and temperature management, motors cannot be

powered directly by most digital controllers. Some circuitry that can handle more power — a motor controller such as

an H-bridge — must be inserted between digital controller and motor's windings. The above image shows the basic

circuit of a motor controller that can also sense motor current. The circuitry to control one winding of a motor is

shown; a stepper motor would use a circuit that could control four windings, and a normal DC motor would need

circuitry to control two windings. All of this circuitry is typically incorporated in an integrated H-bridge chip.

A stepper motor (or step motor) is a brushless, synchronous electric motor that can divide a full

rotation into a large number of steps. The motor's position can be controlled precisely without any

feedback mechanism (see Open-loop controller), as long as the motor is carefully sized to the

application. Stepper motors are similar to switched reluctance motors (which are very large stepping

motors with a reduced pole count, and generally are closed-loop commutated.)

Contents

 [hide]

1 Fundamentals of operation

2 Stepper motor characteristics

3 Open-loop versus closed-loop commutation

4 Types

5 Two-phase stepper motors

o 5.1 Unipolar motors

o 5.2 Bipolar motor

6 Higher-phase count stepper motors

7 Stepper motor drive circuits

o 7.1 L/R drive circuits

o 7.2 Chopper drive circuits

8 Phase current waveforms

o 8.1 Full step drive (two phases on)

o 8.2 Wave drive

o 8.3 Half stepping

o 8.4 Microstepping

9 Theory

o 9.1 Pull-in torque

o 9.2 Pull-out torque

o 9.3 Detent torque

10 Stepper motor ratings and specifications

11 Applications

12 See also

13 References

14 External links

[edit]Fundamentals of operation

Stepper motors operate differently from DC brush motors, which rotate when voltage is applied to

their terminals. Stepper motors, on the other hand, effectively have multiple "toothed"

electromagnets arranged around a central gear-shaped piece of iron. The electromagnets are

energized by an external control circuit, such as a microcontroller. To make the motor shaft turn, first

one electromagnet is given power, which makes the gear's teeth magnetically attracted to the

electromagnet's teeth. When the gear's teeth are thus aligned to the first electromagnet, they are

slightly offset from the next electromagnet. So when the next electromagnet is turned on and the first

is turned off, the gear rotates slightly to align with the next one, and from there the process is

repeated. Each of those slight rotations is called a "step", with an integer number of steps making a

full rotation. In that way, the motor can be turned by a precise angle.

[edit]Stepper motor characteristics

1. Stepper motors are constant power devices.

2. As motor speed increases, torque decreases. (most motors exhibit maximum torque when

stationary, however the torque of a motor when stationary 'holding torque' defines the ability

of the motor to maintain a desired position while under external load).

3. The torque curve may be extended by using current limiting drivers and increasing the

driving voltage (sometimes referred to as a 'chopper' circuit; there are several off the shelf

driver chips capable of doing this in a simple manner).

4. Steppers exhibit more vibration than other motor types, as the discrete step tends to snap

the rotor from one position to another (called a detent). The vibration makes stepper motors

noisier than DC motors.

5. This vibration can become very bad at some speeds and can cause the motor to lose torque

or lose direction. This is because the rotor is being held in a magnetic field which behaves

like a spring. On each step the rotor overshoots and bounces back and forth, "ringing" at its

resonant frequency. If the stepping frequency matches the resonant frequency then the

ringing increases and the motor comes out of synchronism, resulting in positional error or a

change in direction. At worst there is a total loss of control and holding torque so the motor

is easily overcome by the load and spins almost freely.

6. The effect can be mitigated by accelerating quickly through the problem speeds range,

physically damping (frictional damping) the system, or using a micro-stepping driver.

7. Motors with a greater number of phases also exhibit smoother operation than those with

fewer phases (this can also be achieved through the use of a micro stepping drive)

[edit]Open-loop versus closed-loop commutation

Steppers are generally commutated open loop, i.e. the driver has no feedback on where the rotor

actually is. Stepper motor systems must thus generally be over engineered, especially if the load

inertia is high, or there is widely varying load, so that there is no possibility that the motor will lose

steps. This has often caused the system designer to consider the trade-offs between a closely sized

but expensive servomechanism system and an oversized but relatively cheap stepper.

A new development in stepper control is to incorporate a rotor position feedback (e.g.

an encoder or resolver), so that the commutation can be made optimal for torque generation

according to actual rotor position. This turns the stepper motor into a high pole count brushless

servo motor, with exceptional low speed torque and position resolution. An advance on this

technique is to normally run the motor in open loop mode, and only enter closed loop mode if the

rotor position error becomes too large — this will allow the system to avoid hunting or oscillating, a

common servo problem.

[edit]Types

There are three main types of stepper motors:[1]

1. Permanent Magnet Stepper (can be subdivided in to 'tin-can' and 'hybrid', tin-can being a

cheaper product, and hybrid with higher quality bearings, smaller step angle, higher power

density)

2. Hybrid Synchronous Stepper

3. Variable Reluctance Stepper

4. Lavet type stepping motor

Permanent magnet motors use a permanent magnet (PM) in the rotor and operate on the attraction

or repulsion between the rotor PM and the stator electromagnets. Variable reluctance (VR) motors

have a plain iron rotor and operate based on the principle that minimum reluctance occurs with

minimum gap, hence the rotor points are attracted toward the stator magnet poles. Hybrid stepper

motors are named because they use a combination of PM and VR techniques to achieve maximum

power in a small package size.

[edit]Two-phase stepper motors

There are two basic winding arrangements for the electromagnetic coils in a two phase stepper

motor: bipolar and unipolar.

[edit]Unipolar motors

A unipolar stepper motor has two windings per phase, one for each direction of magnetic field. Since

in this arrangement a magnetic pole can be reversed without switching the direction of current, the

commutation circuit can be made very simple (eg. a single transistor) for each winding. Typically,

given a phase, one end of each winding is made common: giving three leads per phase and six

leads for a typical two phase motor. Often, these two phase commons are internally joined, so the

motor has only five leads.

A microcontroller or stepper motor controller can be used to activate the drive transistors in the right

order, and this ease of operation makes unipolar motors popular with hobbyists; they are probably

the cheapest way to get precise angular movements.

Unipolar stepper motor coils

(For the experimenter, one way to distinguish common wire from a coil-end wire is by measuring the

resistance. Resistance between common wire and coil-end wire is always half of what it is between

coil-end and coil-end wires. This is because there is twice the length of coil between the ends and

only half from center (common wire) to the end.) A quick way to determine if the stepper motor is

working is to short circuit every two pairs and try turning the shaft, whenever a higher than normal

resistance is felt, it indicates that the circuit to the particular winding is closed and that the phase is

working.

[edit]Bipolar motor

Bipolar motors have a single winding per phase. The current in a winding needs to be reversed in

order to reverse a magnetic pole, so the driving circuit must be more complicated, typically with

an H-bridge arrangement (however there are several off the shelf driver chips available to make this

a simple affair). There are two leads per phase, none are common.

Static friction effects using an H-bridge have been observed with certain drive topologies[citation needed].

Because windings are better utilized, they are more powerful than a unipolar motor of the same

weight. This is due to the physical space occupied by the windings. A unipolar motor has twice the

amount of wire in the same space, but only half used at any point in time, hence is 50% efficient (or

approximately 70% of the torque output available). Though bipolar is more complicated to drive, the

abundance of driver chip means this is much less difficult to achieve.

An 8-lead stepper is wound like a unipolar stepper, but the leads are not joined to common internally

to the motor. This kind of motor can be wired in several configurations:

Unipolar.

Bipolar with series windings. This gives higher inductance but lower current per winding.

Bipolar with parallel windings. This requires higher current but can perform better as the winding

inductance is reduced.

Bipolar with a single winding per phase. This method will run the motor on only half the available

windings, which will reduce the available low speed torque but require less current.

[edit]Higher-phase count stepper motors

Multi-phase stepper motors with many phases tend to have much lower levels of vibration, although

the cost of manufacture is higher. These motors tend to be called 'hybrid' and have more expensive

machined parts, but also higher quality bearings. Though they are more expensive, they do have a

higher power density and with the appropriate drive electronics are actually better suited to the

application[citation needed], however price is always an important factor. Computer printers may use hybrid

designs.

[edit]Stepper motor drive circuits

Stepper motor performance is strongly dependent on the drive circuit. Torque curves may be

extended to greater speeds if the stator poles can be reversed more quickly, the limiting factor being

the winding inductance. To overcome the inductance and switch the windings quickly, one must

increase the drive voltage. This leads further to the necessity of limiting the current that these high

voltages may otherwise induce.

[edit]L/R drive circuits

L/R drive circuits are also referred to as constant voltage drives because a constant positive or

negative voltage is applied to each winding to set the step positions. However, it is winding current,

not voltage that applies torque to the stepper motor shaft. The current I in each winding is related to

the applied voltage V by the winding inductance L and the winding resistance R. The resistance R

determines the maximum current according to Ohm's law I=V/R. The inductance L determines the

maximum rate of change of the current in the winding according to the formula for an Inductor dI/dt =

V/L. Thus when controlled by an L/R drive, the maximum speed of a stepper motor is limited by its

inductance since at some speed, the voltage U will be changing faster than the current I can keep

up. In simple terms the rate of change of current is L X R (e.g. a 10mH inductance with 2 ohms

resistance will take 20 ms to reach approx 2/3rds of maximum torque or around 0.1 sec to reach

99% of max torque). To obtain high torque at high speeds requires a large drive voltage with a low

resistance and low inductance. With an L/R drive it is possible to control a low voltage resistive

motor with a higher voltage drive simply by adding an external resistor in series with each winding.

This will waste power in the resistors, and generate heat. It is therefore considered a low performing

option, albeit simple and cheap.

[edit]Chopper drive circuits

Chopper drive circuits are also referred to as constant current drives because they generate a

somewhat constant current in each winding rather than applying a constant voltage. On each new

step, a very high voltage is applied to the winding initially. This causes the current in the winding to

rise quickly since dI/dt = V/L where V is very large. The current in each winding is monitored by the

controller, usually by measuring the voltage across a small sense resistor in series with each

winding. When the current exceeds a specified current limit, the voltage is turned off or "chopped",

typically using power transistors. When the winding current drops below the specified limit, the

voltage is turned on again. In this way, the current is held relatively constant for a particular step

position. This requires additional electronics to sense winding currents, and control the switching, but

it allows stepper motors to be driven with higher torque at higher speeds than L/R drives. Integrated

electronics for this purpose are widely available.

[edit]Phase current waveforms

A stepper motor is a polyphase AC synchronous motor (see Theory below), and it is ideally driven by

sinusoidal current. A full step waveform is a gross approximation of a sinusoid, and is the reason

why the motor exhibits so much vibration. Various drive techniques have been developed to better

approximate a sinusoidal drive waveform: these are half stepping and microstepping.

Different drive modes showing coil current on a 4-phase unipolar stepper motor

[edit]Full step drive (two phases on)

This is the usual method for full step driving the motor. Both phases are always on. The motor will

have full rated torque.

[edit]Wave drive

In this drive method only a single phase is activated at a time. It has the same number of steps as

the full step drive, but the motor will have significantly less than rated torque. It is rarely used.

[edit]Half stepping

When half stepping, the drive alternates between two phases on and a single phase on. This

increases the angular resolution, but the motor also has less torque (approx 70%) at the half step

position (where only a single phase is on). This may be mitigated by increasing the current in the

active winding to compensate. The advantage of half stepping is that the drive electronics need not

change to support it.

[edit]Microstepping

What is commonly referred to as microstepping is actually "sine cosine microstepping" in which the

winding current approximates a sinusoidal AC waveform. Sine cosine microstepping is the most

common form, but other waveforms are used [1]. Regardless of the waveform used, as the

microsteps become smaller, motor operation becomes more smooth, thereby greatly reducing

resonance in any parts the motor may be connected to, as well as the motor itself. Resolution will be

limited by the mechanical stiction, backlash, and other sources of error between the motor and the

end device. Gear reducers may be used to increase resolution of positioning.

Step size repeatability is an important step motor feature and a fundamental reason for their use in

positioning.

Example: many modern hybrid step motors are rated such that the travel of every full step (example

1.8 Degrees per full step or 200 full steps per revolution) will be within 3% or 5% of the travel of

every other full step; as long as the motor is operated within its specified operating ranges. Several

manufacturers show that their motors can easily maintain the 3% or 5% equality of step travel size

as step size is reduced from full stepping down to 1/10th stepping. Then, as the microstepping

divisor number grows, step size repeatability degrades. At large step size reductions it is possible to

issue many microstep commands before any motion occurs at all and then the motion can be a

"jump" to a new position.

[edit]Theory

A step motor can be viewed as a synchronous AC motor with the number of poles (on both rotor and

stator) increased, taking care that they have no common denominator. Additionally, soft magnetic

material with many teeth on the rotor and stator cheaply multiplies the number of poles (reluctance

motor). Modern steppers are of hybrid design, having both permanent magnets and soft iron cores.

To achieve full rated torque, the coils in a stepper motor must reach their full rated current during

each step. Winding inductance and reverse EMF generated by a moving rotor tend to resist changes

in drive current, so that as the motor speeds up, less and less time is spent at full current — thus

reducing motor torque. As speeds further increase, the current will not reach the rated value, and

eventually the motor will cease to produce torque.

[edit]Pull-in torque

This is the measure of the torque produced by a stepper motor when it is operated without an

acceleration state. At low speeds the stepper motor can synchronise itself with an applied step

frequency, and this pull-in torque must overcome friction and inertia. It is important to make sure that

the load on the motor is frictional rather than inertial as the friction reduces any unwanted

oscillations.

[edit]Pull-out torque

The stepper motor pull-out torque is measured by accelerating the motor to the desired speed and

then increasing the torque loading until the motor stalls or misses steps. This measurement is taken

across a wide range of speeds and the results are used to generate the stepper motor's dynamic

performance curve. As noted below this curve is affected by drive voltage, drive current and current

switching techniques. A designer may include a safety factor between the rated torque and the

estimated full load torque required for the application.

[edit]Detent torque

Synchronous electric motors using permanent magnets have a remnant position holding torque

(called detent torque or cogging, and sometimes included in the specifications) when not driven

electrically. Soft iron reluctance cores do not exhibit this behavior.

[edit]Stepper motor ratings and specifications

Stepper motors nameplates typically give only the winding current and occasionally the voltage and

winding resistance. The rated voltage will produce the rated winding current at DC: but this is mostly

a meaningless rating, as all modern drivers are current limiting and the drive voltages greatly exceed

the motor rated voltage.

A stepper's low speed torque will vary directly with current. How quickly the torque falls off at faster

speeds depends on the winding inductance and the drive circuitry it is attached to, especially the

driving voltage.

Steppers should be sized according to published torque curve, which is specified by the

manufacturer at particular drive voltages or using their own drive circuitry.

[edit]Applications

Computer-controlled stepper motors are one of the most versatile forms of positioning systems.

They are typically digitally controlled as part of an open loop system, and are simpler and more

rugged than closed loop servo systems.

Industrial applications are in high speed pick and place equipment and multi-axis

machine CNC machines often directly driving lead screws or ballscrews. In the field of lasers and

optics they are frequently used in precision positioning equipment such as linear actuators, linear

stages, rotation stages, goniometers, and mirror mounts. Other uses are in packaging machinery,

and positioning of valve pilot stages for fluid control systems.

Commercially, stepper motors are used in floppy disk drives, flatbed scanners, computer

printers, plotters, slot machines, and many more devices.

Electrical Motors

Electrical Motors are continuous actuators that convert electrical energy into mechanical energy in the

form of a continuous angular rotation that can be used to rotate pumps, fans, compressors, wheels, etc.

As well as rotary motors, linear motors are also available. There are basically three types of conventional

electrical motor available: AC type Motors, DC type Motors and Stepper Motors.

DC Motor

AC Motors are generally used in high power single or multi-phase industrial applications were a constant

rotational torque and speed is required to control large loads. In this tutorial on motors we will look only at

simple light duty DC Motors and Stepper Motors which are used in many electronics, positional control,

microprocessor, PIC and robotic circuits.

The DC Motor

The DC Motor or Direct Current Motor to give it its full title, is the most commonly used actuator for

producing continuous movement and whose speed of rotation can easily be controlled, making them ideal

for use in applications were speed control, servo type control, and/or positioning is required. A DC motor

consists of two parts, a "Stator" which is the stationary part and a "Rotor" which is the rotating part. The

result is that there are basically three types of DC Motor available.

Brushed Motor - This type of motor produces a magnetic field in a wound rotor (the part that rotates) by

passing an electrical current through a commutator and carbon brush assembly, hence the term

"Brushed". The stators (the stationary part) magnetic field is produced by using either a wound stator field

winding or by permanent magnets. Generally brushed DC motors are cheap, small and easily controlled. 

Brushless Motor - This type of motor produce a magnetic field in the rotor by using permanent magnets

attached to it and commutation is achieved electronically. They are generally smaller but more expensive

than conventional brushed type DC motors because they use "Hall effect" switches in the stator to produce

the required stator field rotational sequence but they have better torque/speed characteristics, are more

efficient and have a longer operating life than equivalent brushed types. 

Servo Motor - This type of motor is basically a brushed DC motor with some form of positional feedback

control connected to the rotor shaft. They are connected to and controlled by a PWM type controller and

are mainly used in positional control systems and radio controlled models.

Normal DC motors have almost linear characteristics with their speed of rotation being determined by the

applied DC voltage and their output torque being determined by the current flowing through the motor

windings. The speed of rotation of any DC motor can be varied from a few revolutions per minute (rpm) to

many thousands of revolutions per minute making them suitable for electronic, automotive or robotic

applications. By connecting them to gearboxes or gear-trains their output speed can be decreased while at

the same time increasing the torque output of the motor at a high speed.

The "Brushed" DC Motor

A conventional brushed DC Motor consist basically of two parts, the stationary body of the motor called

the Stator and the inner part which rotates producing the movement called the Rotor or "Armature" for

DC machines.

The motors wound stator is an electromagnet which consists of electrical coils connected together in a

circular configuration to produce a North-pole then a South-pole then a North-pole etc, type stationary

magnetic field system (as opposed to AC machines whose stator field continually rotates with the applied

frequency) with the current flowing within these field coils being known as the motor field current. The

stators electromagnetic coils can be connected in series, parallel or both together (compound) with the

armature. A series wound DC motor has the stator field windings connected in series with the armature

while a shunt wound DC motor has the stator field windings connected in parallel with the armature as

shown.

Series and Shunt Connected DC Motor

The rotor or armature of a DC machine consists of current carrying conductors connected together at one

end to electrically isolated copper segments called the commutator. The commutator allows an electrical

connection to be made via carbon brushes (hence the name "Brushed" motor) to an external power supply

as the armature rotates. The magnetic field setup by the rotor tries to align itself with the stationary stator

field causing the rotor to rotate on its axis, but can not align itself due to commutation delays. The

rotational speed of the motor is dependent on the strength of the rotors magnetic field and the more

voltage that is applied to the motor the faster the rotor will rotate. By varying this applied DC voltage the

rotational speed of the motor can also be varied.

Conventional (Brushed) DC Motor

Permanent magnet (PMDC) brushed motors are generally much smaller and cheaper than their equivalent

wound stator type DC motor cousins as they have no field winding. In permanent magnet DC (PMDC)

motors these field coils are replaced with strong rare earth (i.e. Samarium Cobolt, or Neodymium Iron

Boron) type magnets which have very high magnetic energy fields. This gives them a much better linear

speed/torque characteristic than the equivalent wound motors because of the permanent and sometimes

very strong magnetic field, making them more suitable for use in models, robotics and servos.

Although DC brushed motors are very efficient and cheap, problems associated with the brushed DC

motor is that sparking occurs under heavy load conditions between the two surfaces of the commutator

and carbon brushes resulting in self generating heat, short life span and electrical noise due to sparking,

which can damage any semiconductor switching device such as a MOSFET or transistor. To overcome

these disadvantages, Brushless DC Motors were developed.

The "Brushless" DC Motor

The brushless DC motor (BDCM) is very similar to a permanent magnet DC motor, but does not have any

brushes to replace or wear out due to commutator sparking. Therefore, little heat is generated in the rotor

increasing the motors life. The design of the brushless motor eliminates the need for brushes by using a

more complex drive circuit were the rotor magnetic field is a permanent magnet which is always in

synchronisation with the stator field allows for a more precise speed and torque control. Then the

construction of a brushless DC motor is very similar to the AC motor making it a true synchronous motor

but one disadvantage is that it is more expensive than an equivalent "brushed" motor design.

The control of the brushless DC motors is very different from the normal brushed DC motor, in that it this

type of motor incorporates some means to detect the rotors angular position (or magnetic poles) required

to produce the feedback signals required to control the semiconductor switching devices. The most

common position/pole sensor is the Hall element, but some motors use optical sensors. Using the Hall

sensors signals, the polarity of the electromagnets is switched by the motor control drive circuitry. Then

the motor can be easily synchronized to a digital clock signal, providing precise speed control. Brushless

DC motors can be constructed to have, an external permanent magnet rotor and an internal electromagnet

stator or an internal permanent magnet rotor and an external electromagnet stator.

Advantages of the Brushless DC Motor compared to its "brushed" cousin is higher efficiencies, high

reliability, low electrical noise, good speed control and more importantly, no brushes or commutator to

wear out producing a much higher speed. However their disadvantage is that they are more expensive

and more complicated to control.

The DC Servo Motor

DC Servo motors are used in closed loop type applications were the position of the output motor shaft is

fed back to the motor control circuit. Typical positional "Feedback" devices include Resolvers, Encoders

and Potentiometers as used in radio control models such as airplanes and boats etc. A servo motor

generally includes a built-in gearbox for speed reduction and is capable of delivering high torques directly.

The output shaft of a servo motor does not rotate freely as do the shafts of DC motors because of the

gearbox and feedback devices attached.

DC Servo Motor Block Diagram

A servo motor consists of a DC motor, reduction gearbox, positional feedback device and some form of

error correction. The speed or position is controlled in relation to a positional input signal or reference

signal applied to the device.

RC Servo Motor

The error detection amplifier looks at this input signal and compares it with the feedback signal from the

motors output shaft and determines if the motor output shaft is in an error condition and, if so, the

controller makes appropriate corrections either speeding up the motor or slowing it down. This response to

the positional feedback device means that the servo motor operates within a "Closed Loop System".

As well as large industrial applications, servo motors are also used in small remote control models and

robotics, with most servo motors being able to rotate up to about 180 degrees in both directions making

them ideal for accurate angular positioning. However, these RC type servos are unable to continually

rotate at high speed like conventional DC motors unless specially modified. A servo motor consist of

several devices in one package, the motor, gearbox, feedback device and error correction for controlling

position, direction or speed. They are controlled using just three wires, Power, Ground and Signal Control.

DC Motor Switching and Control

Small DC motors can be switched "On" or "Off" by means of switches, relays, transistors or mosfet circuits

with the simplest form of motor control being "Linear" control. This type of circuit uses a

bipolarTransistor as a Switch (A Darlington transistor may also be used were a higher current rating is

required) to control the motor from a single power supply. By varying the amount of base current flowing

into the transistor the speed of the motor can be controlled for example, if the transistor is turned on "half

way", then only half of the supply voltage goes to the motor. If the transistor is turned "fully ON"

(saturated), then all of the supply voltage goes to the motor and it rotates faster. Then for this linear type of

control, power is delivered constantly to the motor as shown below.

Unipolar Transistor Switch

The simple switching circuit on the left, shows the connections for a Uni-directional(one direction only)

motor control circuit. A continuous logic "1" or logic "0" is applied to the input of the circuit to turn the motor

"ON" (saturation) or "OFF" (cut-off) respectively, with the flywheel diode connected across the motor

terminals to protect the switching transistor or MOSFET from any back emf generated by the motor when

the transistor turns the supply "OFF".

As well as the basic "ON/OFF" control the same circuit can also be used to control the motors rotational

speed. By repeatedly switching the motor current "ON" and "OFF" at a high enough frequency, the speed

of the motor can be varied between stand still (0 rpm) and full speed (100%). This is achieved by varying

the proportion of "ON" time (tON) to the "OFF" time (tOFF) and this can be achieved using a process known

as Pulse Width Modulation.

Pulse Width Speed Control

The rotational speed of a DC motor is directly proportional to the mean (average) value of its supply

voltage and the higher this value, up to maximum allowed motor volts, the faster the motor will rotate. In

other words more voltage more speed. By varying the ratio between the "ON" (tON) time and the "OFF"

(tOFF) time durations, called the "Duty Ratio", "Mark/Space Ratio" or "Duty Cycle", the average value of the

motor voltage and hence its rotational speed can be varied. For simple unipolar drives the duty ratio β is

given as:

and the mean DC output voltage fed to the motor is given as: Vmean = β x Vsupply. Then by varying the

width of pulse a, the motor voltage and hence the power applied to the motor can be controlled and this

type of control is called Pulse Width Modulation or PWM.

Another way of controlling the rotational speed of the motor is to vary the frequency (and hence the time

period of the controlling voltage) while the "ON" and "OFF" duty ratio times are kept constant. This type of

control is called Pulse Frequency Modulation or PFM. With pulse frequency modulation, the motor

voltage is controlled by applying pulses of variable frequency for example, at a low frequency or with very

few pulses the average voltage applied to the motor is low, and therefore the motor speed is slow. At a

higher frequency or with many pulses, the average motor terminal voltage is increased and the motor

speed will also increase.

Then, Transistors can be used to control the amount of power applied to a DC motor with the mode of

operation being either "Linear" (varying motor voltage), "Pulse Width Modulation" (varying the width of the

pulse) or "Pulse Frequency Modulation" (varying the frequency of the pulse).

H-bridge Motor Control

While controlling the speed of a DC motor with a single transistor has many advantages it also has one

main disadvantage, the direction of rotation is always the same, its a "Uni-directional" circuit. In many

applications we need to operate the motor in both directions forward and back. One very good way of

achieving this is to connect the motor into a Transistor H-bridge circuit arrangement and this type of

circuit will give us "Bi-directional" DC motor control as shown below.

Basic Bi-directional H-bridge Circuit

The H-bridge circuit above, is so named because the basic configuration of the four switches, either

electro-mechanical relays or transistors resembles that of the letter "H" with the motor positioned on the

centre bar. The Transistor or MOSFET H-bridge is probably one of the most commonly used type of bi-

directional DC motor control circuits which uses "complementary transistor pairs" both NPN and PNP in

each branch with the transistors being switched together in pairs to control the motor. Control

input Aoperates the motor in one direction ie, Forward rotation and input B operates the motor in the other

direction ie, Reverse rotation. Then by switching the transistors "ON" or "OFF" in their "diagonal pairs"

results in directional control of the motor.

For example, when transistor TR1 is "ON" and transistor TR2 is "OFF", point A is connected to the supply

voltage (+Vcc) and if transistor TR3 is "OFF" and transistor TR4 is "ON" point B is connected to 0 volts

(GND). Then the motor will rotate in one direction corresponding to motor terminal A being positive and

motor terminal B being negative. If the switching states are reversed so that TR1 is "OFF", TR2 is

"ON", TR3 is "ON" and TR4 is "OFF", the motor current will now flow in the opposite direction causing the

motor to rotate in the opposite direction.

Then, by applying opposite logic levels "1" or "0" to the inputs A and B the motors rotational direction can

be controlled as follows.

H-bridge Truth Table

Input A Input B Motor Function

TR1 and TR4 TR2 and TR3  

0 0 Motor Stopped (OFF)

1 0 Motor Rotates Forward

0 1 Motor Rotates Reverse

1 1 NOT ALLOWED

It is important that no other combination of inputs are allowed as this may cause the power supply to be

shorted out, ie both transistors, TR1 and TR2 switched "ON" at the same time, (fuse = bang!).

As with uni-directional DC motor control as seen above, the rotational speed of the motor can also be

controlled using Pulse Width Modulation or PWM. Then by combining H-bridge switching with PWM

control, both the direction and the speed of the motor can be accurately controlled. Commercial off the

shelf decoder IC's such as the SN754410 Quad Half H-Bridge IC or the L298N which has 2 H-Bridges are

available with all the necessary control and safety logic built in are specially designed for H-bridge bi-

directional motor control circuits.

The Stepper Motor

Like the DC motor above, Stepper Motors are also electromechanical actuators that convert a pulsed

digital input signal into a discrete (incremental) mechanical movement are used widely in industrial control

applications. A stepper motor is a type of synchronous brushless motor in that it does not have an

armature with a commutator and carbon brushes but has a rotor made up of many, some types have

hundreds of permanent magnetic teeth and a stator with individual windings.

Stepper Motor

As it name implies, a stepper motor does not rotate in a continuous fashion like a conventional DC motor

but moves in discrete "Steps" or "Increments", with the angle of each rotational movement or step

dependant upon the number of stator poles and rotor teeth the stepper motor has. Because of their

discrete step operation, stepper motors can easily be rotated a finite fraction of a rotation, 1.8, 3.6, 7.5

degrees etc. For example, assume a stepper motor completes one full revolution in 100 steps. Then the

step angle for the motor is given as 360 degrees/100 steps = 3.6 degrees per step. This is commonly

known as the motors Step Angle.

There are three basic types of stepper motor, Variable Reluctance,Permanent Magnet and Hybrid (a

sort of combination of both). AStepper Motor is particularly well suited to applications that require

accurate positioning and repeatability with a fast response to starting, stopping, reversing and speed

control and another key feature of the stepper motor, is its ability to hold the load steady once the require

position is achieved.

Generally, stepper motors have an internal rotor with a large number of permanent magnet "teeth" with a

number of electromagnet "teeth" mounted on to the stator. The stators electromagnets are polarized and

depolarized sequentially, causing the rotor to rotate one "step" at a time. Modern multi-pole, multi-teeth

stepper motors are capable of accuracies of less than 0.9 degs per step (400 Pulses per Revolution) and

are mainly used for highly accurate positioning systems like those used for magnetic-heads in floppy/hard

disc drives, printers/plotters or robotic applications. The most commonly used stepper motor being the 200

step per revolution stepper motor. It has a 50 teeth rotor, 4-phase stator and a step angle of 1.8 degrees

(360 degs/(50x4)).

Stepper Motor Construction and Control

In our simple example of a variable reluctance stepper motor above, the motor consists of a central rotor

surrounded by four electromagnetic field coils labelled A, B, C and D. All the coils with the same letter are

connected together so that energising, say coils marked A will cause the magnetic rotor to align itself with

that set of coils. By applying power to each set of coils in turn the rotor can be made to rotate or "step"

from one position to the next by an angle determined by its step angle construction, and by energising the

coils in sequence the rotor will produce a rotary motion.

The stepper motor driver controls both the step angle and speed of the motor by energising the field coils

in a set sequence for example, "ADCB, ADCB, ADCB, A..." etc, the rotor will rotate in one direction

(forward) and by reversing the pulse sequence to "ABCD, ABCD, ABCD, A..." etc, the rotor will rotate in

the opposite direction (reverse). So in our simple example above, the stepper motor has four coils, making

it a 4-phase motor, with the number of poles on the stator being eight (2 x 4) which are spaced at 45

degree intervals. The number of teeth on the rotor is six which are spaced 60 degrees apart. Then there

are 24 (6 teeth x 4 coils) possible positions or "steps" for the rotor to complete one full revolution.

Therefore, the step angle above is given as:   360o/24 = 15o.

Obviously, the more rotor teeth and or stator coils would result in more control and a finer step angle. Also

by connecting the electrical coils of the motor in different configurations, Full, Half and micro-step angles

are possible. However, to achieve micro-stepping, the stepper motor must be driven by a (quasi)

sinusoidal current that is expensive to implement.

It is also possible to control the speed of rotation of a stepper motor by altering the time delay between the

digital pulses applied to the coils (the frequency), the longer the delay the slower the speed for one

complete revolution. By applying a fixed number of pulses to the motor, the motor shaft will rotate through

a given angle and so there would be no need for any form of additional feedback because by counting the

number of pulses given to the motor the final position of the rotor will be exactly known. This response to a

set number of digital input pulses allows the stepper motor to operate in an "Open Loop System" making it

both easier and cheaper to control.

For example, assume our stepper motor above has a step angle of 3.6 degs per step. To rotate the motor

through an angle of say 216 degrees and then stop would only require 216 degrees/(3.6 degs/step) =

80 pulses applied to the stator coils.

Stepper motor controller IC's are available such as the SAA1027 which have all the necessary counter

and code conversion built-in, and automatically drives the 4 fully controlled bridge outputs to the motor in

the correct sequence. The direction of rotation can also be selected along with single step mode or

continuous (stepless) rotation in the selected direction, but this puts some burden on the controller. When

using an 8-bit digital controller, 256 microsteps per step are also possible

SAA1027 Stepper Motor Control Chip

In this tutorial we have looked at the brushed and brushless DC Motor, the DC Servo Motor and

theStepper Motor as an electromechanical actuator that can be used as an output device for position or

speed control. In the next tutorial we will continue our look at output devices called Actuators, and one

that converts a electrical signal into sound waves again using electromagnetism. The type of output device

we will look at in the next tutorial is the Loudspeaker.

http://www.electronics-tutorials.ws/io/io_7.html

Forum Repsonses From contributor E: A stepper motor is wound in such a way that the rotation has a certain number of discrete "steps". I only know of stepper motors being DC motors. These steps are where the magnetic fields cause the motor to want to settle in one of these positions. The number of steps per revolution is rather high, around two hundred or so, and varies by model and manufacturer. What this means is that the motor has effectively a resolution (smallest controlled movement) equal to the number of steps for that motor. Everything seems to have exceptions, and that applies to steppers also - there are some called micro step, with a higher resolution, but I don’t know much about them. Stepper motors may or may not have position feedback.

A servo motor can be either DC or AC, and is usually comprised of the drive section and the resolver/encoder. A servo motor is much smoother in motion than a comparable stepper, and will have a much higher resolution for position control. The servo family is further divided into AC and DC types. An AC servo had the advantage of being able to handle much higher current surges than a DC, as the DC has brushes, which are the limiting factor in this case. Therefore, for our practical considerations, you can get a lot stronger AC servo motor than you could in DC or stepper configuration. Steppers, on the other hand, have economy as an advantage, and can be incorporated into a design to produce very smooth motion also. The trend for manufacturers of “serious” CNC machinery is to use AC servos. “Entry level” machines may have DC servos, or even steppers.

A resolver/encoder is a glass disc with very fine lines on it and an optical encoder that counts those lines as it rotates with the motor. This information is couple to the controller which tracks the counts, the rate that they go by, and through a host of feedback loops, logic, and controlling the amplifiers, produces the desired motion.

Stepper systems are often “open loop” which means that the controller only tells the motors how many steps to move and how fast to move, but does not have any way of knowing where they actually are. This can lead to errors, should a situation arise where the motors are unable to comply with the commanded move. This can be very obvious, where the motion stops and it sounds like you stripped a gear, or subtle, where the motor only misses a “few” steps. The result

is the same - the controller thinks you are at X25.5, Y15.5 and in reality you might be at X25.3, Y15.4 . This can lead to a cumulative error, which may in turn lead to crashes, not to mention out of spec parts.

How the motors are controlled by the “controller” and amplifiers is a lengthy subject with a lot of technical jargon. 

From contributor B: I'm just getting ready to upgrade the steppers on my old DT902 to servos and will confess to a lack of understanding of how servos work. This upgrade is happening due to an unexpected opportunity that has left me on the short side of the technical comprehension curve. 

From contributor E: You may already know this, but the type of controller and the amplifiers for your DC steppers may not be compatible with servo drives. You should also be aware of a host of other challenges with the tuning of the servos. With steppers, all you really worry about is max speed and accel rate. With servos, you will have to consider several different gains, as well as type of feedback loop. Either velocity feedback or position feedback. Perhaps both. When upgrading to servos, you will want to consider inertial matching, and backlash. With the digital, do you have the split pinion to eliminate backlash? If not, consider upgrading that as well, otherwise I suspect you will have a dickens of a time keeping the servos from ringing or buzzing.

I hope that who-ever you are getting the servos from can help you with the tuning. Tuning is something of an art form in itself. 

From contributor B: Actually, I bought a complete 2nd DT902 that already had the servos installed. I'm pulling that gantry and putting it on my machine along with the servos and controllers. I am having someone come up and help with this, including setting up the servos properly as you described.

I don't believe DT ever put anti-backlash gearing on the 902. That was basically their first machine - a very nuts and bolts sort of setup. It's great for the type of work we do with it, but does have its drawbacks for more sophisticated or heavy duty use. 

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

From contributor T: I just finnished a course on linear control systems (servo systems), so I can give you the definitive difference between servo motors and stepper motors.

Stepper motors can lock into a fixed postion, while servo motors can not. It's that simple. A servo will compare the output (position converted to voltage) to the input (the desired position converted to voltage) and make them the same by changing the output. This is a balancing act. Any external event that changes the position of the motor will be corrected by an opposing torque produced from this balancing act. This correction takes time to settle. It will either be a slow position correction or a series of overshoots that will oscillate back and forth until a midpoint is found relatively quickly. Stepper motors have a much higher holding torque and will remain in a fixed position until overpowered. DC servo motors, however, have a higher torque *during rotation* than steppers and a much higher RPM. To match a stepper motor's holding torque, you would need an expensive high torque servo motor. Deciding wether to use a servo motor or stepper motor is based on the needed holding torque (steppers) versus torque while in motion (servo). And don't forget that servo motors have a higher RPM. 

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor M: Servos, just like steppers have varying resolutions. One common resolution is 1024 counts per revolution. Also like steppers, servos can be used to "hold" a position. One example is this: you have a controller that says I want to spin in X direction at Y speed. The servo then does whatever it must to make that happen. Then, if you spin in X direction at 0 speed, you will follow that also. In other words when a servo is sitting still, that does not mean that it is not running. It may be running at 0 speed. What this means is that you are constantly putting voltage and current in a back and forth motion to maintain a stable position. A stepper though, also uses electricity to sit still as one or more coils that brought the motor to this position must be kept on consistently to sit "still."

Now, the specifics about accuracy and performance are all dependant upon your setup. You will use a PID not unlike heat controllers. This is the "how do I get

there from here" thinking. It will be slow, but accurate and fast but risk overshoot. Then there is stability at question. Some servos are famous for "buzzing" or "chattering" at stand still - this is not necessary. This comes from the setup. If you don't care about maintaining position while stopped however, then this might not be an issue for you. The behavior while stopped is primarily an issue when you must keep the motor active at 0 speed. One reason for this is that you have multiple motors that are working with the same piece of material, or that move machine components that could "crash" if not controlled.

Much of these control factors comes from the units that you buy. The best controllers will be highly configurable. This way, you can set it up however you want, putting emphasis on the most critical performance areas.

One last item is that yes, servos are much more complex and expensive. However, by maintaining precise control over your motion, you can move much faster. If you are precise, then you can allow your machine components to get closer that would have previously been safe. Plus, then you can send a component at high speed knowing that you can still stop it on a dime, unlike the old days where you would slow down before your destination to stop things precisely.

http://www.woodweb.com/knowledge_base/Servo_vs_stepper_motors.html

(′step·ər ′mōd·ər)

(electricity) A motor that rotates in short and essentially uniform angular movements rather than continuously; typical steps are 30, 45, and 90°; the angular steps are obtained electromagnetically rather than by the ratchet and pawl mechanisms of stepping relays. Also known as magnetic stepping motor; stepping motor; step-servo motor.

Read more: http://www.answers.com/topic/stepper-motor#ixzz1AsardIZG

Electric motor - Definition

Electric motors of various sizes.

An electric motor converts electricity into mechanical motion. The reverse task, that of converting mechanical motion into electricity, is

accomplished by a generator. The two devices are identical except for their application and minor construction details.

Most electric motors work by electromagnetism, but motors based on other electromechanical phenomena, such as electrostatic forces and

the piezoelectric effect, exist. The overarching concept is that a force is generated when a current-carrying element is subjected to a magnetic

field. In a cylindrical motor, the rotor rotates because a torque is developed when this force is applied at a given distance from the axis of the

rotor.

Most electromagnetic motors are rotary, but linear types also exist. In a rotary motor, the rotating part (usually on the inside) is called

the rotor, and the stationary part is called the stator. The motor contains electromagnets that are wound on a frame. Though this frame is often

called the armature, that term is often erroneously applied. Correctly, the armature is that part of the motor across which the input voltage is

supplied or that part of the generator across which the output voltage is generated. Depending upon the design of the machine, either the rotor

or the stator can serve as the armature.

Kits for making very simple motors are used in many schools. See Westminster motor kits.

Contents [hide]

1 DC motors

1.1 Wound field DC motor

1.2 Universal motors2 AC motors

2.1 Single-phase AC induction motors

2.2 Single-phase AC synchronous motors

2.3 Three-phase AC motors3 Stepper motors4 Brushless DC motors5 Coreless dc motors6 Linear motors7 See also

DC motors

A simple DC electric motor. When the coil is powered, a magnetic field is generated around the armature. The left side of the armature is pushed away from the left magnet and drawn toward the right, causing rotation.

The armature continues to rotate.

When the armature becomes horizontally aligned, the commutator reverses the direction of current through the coil, reversing the magnetic field. The process then repeats.

One of the first electromagnetic rotary motors, if not the first, was invented by Michael Faraday in1821, and consisted of a free-hanging wire

dipping into a pool of mercury. A permanent magnetwas placed in the middle of the pool. When a current was passed through the wire, the

wire rotated around the magnet, showing that the current gave rise to a circular magnetic field around the wire. This motor is often

demonstrated in school physics classes, but brine is sometimes used in place of the toxic mercury.

The classic DC motor has a rotating armature in the form of an electromagnet with two poles. A rotary switch called a commutator reverses

the direction of the electric current twice every cycle, to flow through the armature so that the poles of the electromagnet push and pull

against the permanent magnets on the outside of the motor. As the poles of the armature electromagnet pass the poles of the permanent

magnets, the commutator reverses the polarity of the armature electromagnet. During that instant of switching polarity, inertia keeps the

classical motor going in the proper direction. (See the diagrams to the right.)

DC motor speed generally depends on a combination of the voltage and current flowing in the motor coils and the motor load or braking

torque. The speed of the motor is proportional to the voltage, and the torque is proportional to the current. The speed is typically controlled by

altering the voltage or current flow by using taps in the motor windings or by having a variable voltage supply.

As this type of motor can develop quite high torque at low speed it is often used in traction applications such as locomotives.

However, there are a number of limitations in the classic design, many due to the need for brushes to rub against the commutator. The rubbing

creates friction, and the higher the speed, the harder the brushes have to press to maintain good contact. Not only does this friction make the

motor noisy, but it also creates an upper limit on the speed and causes the brushes eventually to wear out and to require replacement. The

imperfect electric contact also causes electrical noise in the attached circuit. These problems vanish when you turn the motor inside out,

putting the permanent magnets on the inside and the coils on the outside thus designing out the need for brushes in abrushless design.

Wound field DC motor

The permanent magnets on the outside (stator) of a DC motor may be replaced by electromagnets. By varying the field current it is possible to

alter the speed/torque ratio of the motor. Typically the field winding will be placed in series (series wound ) with the armature winding to get

a high torque low speed motor, in parallel (shunt wound) with the armature to get a high speed low torque motor, or to have a winding partly

in parallel, and partly in series (compound wound) to get the best of both worlds. Further reductions in field current are possible to gain even

higher speed but correspondingly lower torque. This technique is ideal for electric traction (seeTraction motor) and many similar applications

where its use can eliminate the requirement for a mechanically variable transmission.

Universal motors

A variant of the wound field DC motor is the universal motor. The name derives from the fact that it may use AC or DC supply current,

although in practice they are nearly always used with AC supplies. The principle is that in a wound field DC motor the current in both the

field and the armature (and hence the resultant magnetic fields) will alternate (reverse polarity) at the same time, and hence the mechanical

force generated is always the same. In practice the motor must be specially designed to cope with the AC current (impedance/reluctance must

be taken into account), and the resultant motor is generally less efficient than an equivalent pure DC motor. The advantage of the universal

motor is that AC supplies may be used on motors which have the typical characteristics of DC motors, specifically high starting torque and

very compact design if high running speeds are used. The negative aspect is the maintenance and reliability problems caused by

the commutator, and as a result such motors will rarely be found in industry but are the most common type of AC supplied motor in devices

such as food mixers and power tools which are only used intermittently. Speed control of a universal motor running on ac is very easily

accomplished using a thyristorcircuit.

Unlike the other common forms of ac motors (induction motors and synchronous motors), universal motors can easily exceed one revolution

per cycle of the mains current (that is, exceed 3000 RPM on a 50Hz system or 3600 RPM on a 60Hz system). This makes them especially

useful for certain appliances such as blenders, vacuum cleaners, and hair dryers where high-speed operation is desired.

With the very low cost of semiconductor rectifiers, some applications that would have previously used a universal motor now use a pure DC

motor, usually with a permanent magnet field. This is especially true if the semiconductor circuit is also used for variable-speed control.

AC motors

AC motors generally come in two types: single phase and three phase.

Single-phase AC induction motors

Induction motors have no electrical connection between the rotor and the outside world. They operate because a moving magnetic

fieldinduces a current to flow in the rotor. This current flow in the rotor creates the second magnetic field required (along with the field from

the field coils) to produce a torque. The rotating magnetic field principle was conceived by Nikola Tesla in 1882 and he employed it to invent

the first induction motor in 1883. Introduction of the motor from 1888 onwards initiated what is known as the second industrial revolution,

making possible the efficient generation and long distance distribution of electrical energy using the alternating current transmission system,

also of Tesla's invention (1888)[1].

A common single-phase motor is the shaded pole motor, which is used in devices requiring lower torque, such as electric fans,microwave

ovens, and other small household appliances. In this motor, small single-turn copper "shading coils" create the moving magnetic field.

Another common single-phase AC motor is the split-phase induction motor, commonly used in major appliances such as washing

machines and clothes dryers. Compared to the shaded pole motor, these motors can generally provide much greater starting torque by using a

special startup winding in conjunction with a centrifugal switch and possibly a starting capacitor.

When the motor is starting, the startup winding is connected to the power source via a set of spring-loaded contacts pressed upon by the not-

yet-rotating centrifugal switch. The phase of the magnetic field in this startup winding is shifted from the phase of the mains power, allowing

the creation of a moving magnetic field which starts the motor. Once the motor reaches near design operating speed, the centrifugal switch

activates, opening the contacts and disconnecting the startup winding from the power source. The motor then operates solely on the running

winding.

In some motors, the startup winding is designed with a higher resistance than the running winding. This creates an LR circuit which slightly

shifts the phase of the current in the startup winding. In other motors, a starting capacitor is inserted in series with the startup winding,

creating an LC circuit which is capable of a much greater phase shift (and so, a much greater starting torque). The capacitor naturally adds

expense to such motors.

Another variation is the Permanent Split-Capacitor (PSC) motor. This motor operates similarly to the capacitor-start motor described above,

but there is no centrifugal starting switch and the second winding is permanently connected to the power source. PSC motors are frequently

used in air handlers, fans, and blowers and other cases where a variable speed is desired. By changing taps on the running winding but

keeping the load constant, the motor can be made to run at different speeds.

The rotors of most single-phase induction motors are of the squirrel cage design. These rotors take their name from "windings" that are in the

style of the wheel on which pet rodents exercise: a series of bars that connect conductive rings at both ends of the rotor. In fact, these

"windings" are not wound into the rotor but instead are cast aluminum that fills the slots in the stackup of steel laminations. Because each

winding is a single turn, the induced voltages in the rotor windings are very low and there is no need to insulate the cast-in-place "windings"

from the steel laminations. (The currents in the rotor are, of course, proportionally as high as the voltage is low, but the cast aluminum

windings have very little electrical resistance so I2R losses in the rotor are small.)

All induction motors are characterized by the fact that when no load is applied to the motor, the rotor rotates at a slightly slower rate than the

mains frequency (or an integer sub multiple of the mains frequency). This is because the rotor must "slip" backwards against the moving

magnetic field in order to induce any current in the rotor. The slip increases (and the motor speed decreases) as the load on the motor

increases.

Single-phase AC synchronous motors

Small single-phase ac motors can also be designed with magnetized rotors (or several variations on that idea). The rotors in these motors do

not require any induced current so they do not slip backward against the mains frequency. Instead, they rotate synchronously with the mains

frequency. Because of their highly-accurate speed, such motor are usually used to power mechanical clocks, audio turntables, andtape drives.

Because inertia makes it difficult to instantly accelerate the rotor from stopped to synchronous speed, these motors normally require some sort

of special feature to get started. Various designs use a small induction motor (which may share the same field coils and rotor as the

synchronous motor) or a very light rotor with a one-way mechanism (to ensure that the rotor starts in the "forward" direction).

Three-phase AC motors

For higher-power applications where a polyphase electrical supply is available, the three phase (or polyphase) AC induction motor is used.

The phase differences between the three phases of the polyphase electrical supply create a rotating electromagnetic field in the motor. There

are two types of rotors in use. Most motors use the squirrel cage rotor discussed above. An alternate design, called the wound rotor, is used

when variable speed is required. In this case, the rotor has the same number of poles as the stator and the windings are made of wire,

connected to slip rings on the shaft. Carbon brushes connect the slip rings to an external controller such as a variable resistor that allows

changing the motor's slip rate. Compared to squirrel cage rotors, wound rotor motors are expensive and require maintenance of the slip rings

and brushes. Transistorized inverters with variable frequency drive can now be used for speed control and wound rotor motors are becoming

less common.

Several methods of stating a polyphase motor are used. Where the large inrush current and high starting torque can be permitted, the motor

can be started across the line, by applying full line voltage to the terminals. Where it is necessary to limit the starting inrush current (where

the motor is large compared with the short-circuit capacity of the supply), reduced voltage starting using either series inductors,

an autotransformer or other devices are used. A technique sometimes used is wye-delta starting, where the motor coils are initially connected

in wye for acceleration of the load, then switched to delta when the load is up to speed.

Through electromagnetic induction, the rotating magnetic field induces a current in these conductors, which in turn sets up a counterbalancing

magnetic field that causes the rotor to turn in the direction the field is rotating. This type of motor is known as aninduction motor. In order

for it to operate, it must always rotate slower than the rotating magnetic field produced by the polyphase electrical supply; otherwise, no

counterbalancing field will be produced in the rotor.

This type of motor is becoming more common in traction applications such as locomotives, where it is known as the asynchronoustraction

motor. If the rotor coils are fed a separate field current to create a continuous magnetic field, the result is a called a synchronous motor

because the motor rotates in synchronism with the rotating magnetic field produced by the polyphase electrical supply.

A synchronous motor can also be used as an alternator.

The speed of the AC motor is determined primarily by the frequency of the AC supply, whereas the torque is determined by the amount of

slip, or difference in rotation, between the rotor and stator fields. The speed in this type of motor has traditionally been altered by having

additional sets of coils or poles in the motor that can be switched on and off to change the speed of magnetic field rotation. However,

developments in power electronics mean that the frequency of the power supply can also now be varied to provide a smoother control of the

motor speed.

Stepper motors

Another kind of electric motor is the stepper motor, where an internal rotor containing permanent magnets is controlled by a set of external

magnets that are switched electronically. A stepper motor is a cross between a DC electric motor and a solenoid.

Simple stepper motors "cog" to a limited number of positions, but proportionally controlled stepper motors can rotate extremely smoothly.

Computer controlled stepper motors are one of the most versatile forms of positioning systems, particularly when part of a digital servo-

controlled system.

Brushless DC motors

Midway between ordinary DC motors and stepper motors lies the realm of the brushless DC motor. Built in a fashion very similar to stepper

motors, these often use a permanent magnet external rotor, three phases of driving coils, one or more Hall effect devices to sense the position

of the rotor, and the associated drive electronics. The coils are activated, one phase after the other, by the drive electronics as cued by the

signals from the Hall effect sensors.

Brushless DC motors are commonly used to drive fans, the spindles within CD, CD-ROM (etc.) drives, and mechanisms within office

products such as laser printers and photocopiers. They have several advantages over conventional motors:

Compared to AC fans using shaded-pole motors, they are very efficient, running much cooler than the equivalent ac motors. This cool

operation leads to much-improved life of the fan's bearings.

Without a commutator to wear out, the life of a DC brushless motor can be significantly longer compared to a dc motor using brushes and

a commutator

The same Hall effect devices that provide the commutation can also provide a convenient tachometer signal for closed-loop control

(servo-controlled) applications. In fans, the tachometer signal can be used to derive a "fan okay" signal.

The motor can be easily synchronized to an internal or external clock, leading to precise speed control.

Modern DC brushless motors range in power from a fraction of a watt to many watts.

Coreless dc motors

A coreless DC motor is a specialized form of an ordinary DC motor. Optimized for rapid acceleration, these motors have a rotor that is

constructed without any iron core. The rotor can take the form of a winding-filled cylinder inside the stator magnets, a basket surrounding the

stator magnets, or a flat pancake (possibly formed on a printed wiring board running between upper and lower stator magnets. The windings

are typically stabilized by being impregnated with epoxy resins.

Because the rotor is much lighter in weight (mass) than a conventional rotor formed from copper windings on steel laminations, the rotor can

accelerate much more rapidly, often achieving a mechanical time constant under 1 ms. This is especially true if the windings

usealuminum rather than the heavier copper. But because there is no metal mass in the rotor to act as a heat sink, even small coreless motors

must often be cooled by forced air.

These motors were commonly used to drive the capstan(s) of magnetic tape drives and are still widely used in high-performance servo-

controlled systems.

Linear motors

A linear motor is essentially an electric motor that has been "unrolled" so that instead of producing a torque (rotation), it produces a linear

force along its length by setting up a traveling electromagnetic field.

Linear motors are most commonly induction motors or stepper motors. You can find a linear motor in a maglev (Transrapid) train, where the

train "flies" over the ground.

See also

Centrifugal switch  | Commutator (electric)  | Electrical element | Electrical generator | Electric vehicle | Flywheel energy storage | Frank J.

Sprague]] | George Westinghouse | Fan (implement) | Hybrid car | Hydrogen car | List of electronics topics | List of technologies |Maximum

power theorem | Michael Faraday | Momentum wheel | Motor | Motor controller | Nikola Tesla | Propulsion method | Single phase electric

power | slip ring | Stepper motor | Table saw | Thomas Edison | Timeline of motor and engine technology | Westminster motor kits

A self-teaching textbook that briefly covers electric motors, transformers, speed controllers, wiring codes and grounding, transistors, digital,

etc., is:

Shanefield D. J., Industrial Electronics for Engineers, Chemists, and Technicians, William Andrew Publishing, Norwich, NY, 2001.

Although this book is unusually easy to read and understand (see customer reviews at bookseller sites), it only goes up to an elementary level

on each subject, and it is not a suitable reference book for technologists already working in any of those fields.

Stepper MotorsA stepper motor is a motor controlled by a series of electromagnetic

coils. The center shaft has a series of magnets mounted on it, and the

coils surrounding the shaft are alternately given current or not,

creating magnetic fields which repulse or attract the magnets on the

shaft, causing the motor to rotate.

This design allows for very precise control of the motor: by proper

pulsing, it can be turned in very accurate steps of set degree

increments (for example, two-degree increments, half-degree

increments, etc.). They are used in printers, disk drives, and other

devices where precise positioning of the motor is necessary.

There are two basic types of stepper motors, unipolar steppers and

bipolar steppers.

Unipolar Stepper Motors

The unipolar stepper motor has five or six wires and four coils

(actually two coils divided by center connections on each coil). The

center connections of the coils are tied together and used as the

power connection. They are called unipolar steppers because power

always comes in on this one pole.

Bipolar stepper motors

The bipolar stepper motor usually has four wires coming out of it.

Unlike unipolar steppers, bipolar steppers have no common center

connection. They have two independent sets of coils instead. You can

distinguish them from unipolar steppers by measuring the resistance

between the wires. You should find two pairs of wires with equal

resistance. If you’ve got the leads of your meter connected to two

wires that are not connected (i.e. not attached to the same coil), you

should see infinite resistance (or no continuity).

Like other motors, stepper motors require more power than a

microcontroller can give them, so you’ll need a separate power supply

for it. Ideally you’ll know the voltage from the manufacturer, but if

not, get a variable DC power supply, apply the minimum voltage

(hopefully 3V or so), apply voltage across two wires of a coil (e.g. 1 to

2 or 3 to 4) and slowly raise the voltage until the motor is difficult to

turn. It is possible to damage a motor this way, so don’t go too far.

Typical voltages for a stepper might be 5V, 9V, 12V, 24V. Higher than

24V is less common for small steppers, and frankly, above that level

it’s best not to guess.

To control the stepper, apply voltage to each of the coils in a specific

sequence. The sequence would go like this:

Step

wire 1

wire 2

wire 3

wire 4

1 High low high low

2 low high high low

3 low high low high

4 high low low high

To control a unipolar stepper, you use a Darlington Transistor Array.

The stepping sequence is as shown above. Wires 5 and 6 are wired to

the supply voltage.

To control a bipolar stepper motor, you give the coils current using to

the same steps as for a unipolar stepper motor. However, instead of

using four coils, you use the both poles of the two coils, and reverse

the polarity of the current.

The easiest way to reverse the polarity in the coils is to use a pair of

H-bridges. The L293D dual H-bridge has two H-bridges in the chip, so

it will work nicely for this purpose.

Once you have the motor stepping in one direction, stepping in the

other direction is simply a matter of doing the steps in reverse order.

Knowing the position is a matter of knowing how many degrees per

step, and counting the steps and multiplying by that many degrees. So

for examples, if you have a 1.8-degree stepper, and it’s turned 200

steps, then it’s turned 1.8 x 200 degrees, or 360 degrees, or one full

revolution.

Two-Wire Control

Thanks to Sebastian Gassner for ideas on how to do this.

In every step of the sequence, two wires are always set to opposite

polarities. Because of this, it’s possible to control steppers with only

two wires instead of four, with a slightly more complex circuit. The

stepping sequence is the same as it is for the two middle wires of the

sequence above:

http://www.tigoe.net/pcomp/code/circuits/motors/stepper-motors

How Unipolar Stepper Motors Workby Contributing Writer

 

  

   

How Unipolar Stepper Motors Work

What It Is

A stepper motor is an electric motor that can use a series of electrical pulses ("steps") to create mechanical movements that rotate a motor. The shaft rotates in small steps to line up the teeth of the wheel with the electric magnet on each side sequentially. As electrical pulses continue to create magnetism, the wheel turns to line itself up with the teeth of the magnets on each side.

 

Motor Controller Tutorial  www.GaliLmc.com  Free Web Tutorials from Galil, the World Leader in Motor Controllers.

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How It WorksAn electromagnetic pulse is sent to the top electromagnet, labelled in the above picture with a 1. The wheel then rotates slightly as the teeth are drawn to the teeth of the electromagnet on top. Once this is complete, a pulse is sent to the rightmost electromagnet, which again causes the wheel to turn slightly, this time to line up with the magnet on the right. A pulse is then sent to the bottom magnet, then to the leftmost magnet, and so on and so on as the wheel continues to move incrementally to line up with each magnet in sequence. The result is a full rotation of the wheel.

Variations in the electrical pulses will naturally create variations in the rotation of the stepper motor. Unipolar motors can change the direction of rotation simply by reversing the sequence of electrical pulses. In other words, activating the magnets in the opposite order would result in the teeth moving slightly counter-clockwise rather than clockwise to line up with the magnets, thus causing the whole rotation to go in the opposite direction. Changing the speed of the pulses will also, naturally, result in a similar change in the speed of the rotation as a whole. And when the pulses stop, so does the rotation. The unipolar stepper motor operates with no brushes, and no feedback required, since the rotation can be precisely controlled through the pulses. For this reason, the stepper motor is often referred to as an "open loop", since knowledge of the rotation of the motor requires no feedback from the motor itself, only a knowledge of how the pulses are being activated.

S e p t e m b e r 2 6 , 2 0 1 0

What is a Stepper Motor? Filed under: Motors,Stepper Motors — Avayan @ 4:55 pm A stepper motor is a motor that moves in steps. Well, DUH! Isn’t that obvious?

But it may not be. Because the notion of moving in steps is not as trivial as we

may think. In fact, the ability to move in steps is so desirable an entire fleet of

motors had to be devised in order to achieve this concept. Have in mind a

brushed DC motor or an AC induction motor can not move in steps. If you were

to close the loop and add all sorts of algorithms, you may be able to emulate said

motion, but this will not be easy… So how does a stepper motor does it? 

In order to understand why a stepper motor can move in steps it is best to

understand why a Brushed DC motor can not move like that. A DC motor is an

automatic commutation machine. That is, when a voltage is applied and a

current flows through the rotor electromagnet, as the rotor moves, the inductor

which was energized will move away only for a new inductor to take its place.

Since the commutator is built such that these connections are made ad infinitum,

the motor rotates… well… ad infinitum! Stepping is then, not intrinsically

possible. 

But if we remove the commutator and we find a way for the electromagnet

commutation to be made external to the rotation, we can then control this

rotation. In other words, I can apply a magnetic field in such a way that my rotor

will move to a particular place, only to later move into the next position as I

reapply a different magnetic field. And this is what a stepper motor is. It is a

brushless dc motor (there are no brushes) in which we are given full control of the

commutation. Because we decide which electromagnet is energized and when it

is energized, we can control the position the rotor will assume and when this

happens. We can call each one of these controlled motions, a step. 

There are many ways to achieve what I just explained above. Per example, there

are variable reluctance steppers, permanent magnet steppers and a combination

of both techniques called hybrid steppers. Each technology has its advantages

when it comes to sizing and cost. Permanent magnet and variable reluctance

stepper motors will reside on the cheap side. Notice they are built using very low

cost materials and  mechanically speaking they are not necessarily very robust.

For example, the rotor is mounted in brass bushings instead of ball bearings. 

 http://robot.avayanex.com/?p=113