Direct Current Motors

They are very popular because:

- They are usually very fast, spinning at several thousand revolutions per minute (rpm);

- They are simple to operate.

- Their starting torque is large, which is the main reason for using them in several traction applications;

- In a special form, they can be used with either an a.c. or d.c. supply.

Theory of Operation

DC Armature

Wound-Field DC Motors

Permanent Magnet Motor

PM DC-Motor Model

• If we apply the law of conservation of angular momentum we have

• The motor torque is given by

• and the load in this case is the rotor inertia. The only torque generated by the load is the friction (or damping) torque expressed as

• Applying Kirchof's voltage law on the electrical system we have

• Neglecting the inductance La

Control of DC Motors

• Switch ON-OFF

• Speed:

- Analog

- Pulse With Modulation (PWM)

- Continuous Control

• Direction

Switch • Diod Switch

• BJT

• BJT Switching Characteristics

When the transistor is saturated, it acts as a closed switch. When a transistor is in the cutoff region, it acts as an open switch. When it is in the active region, it acts as a current (iB) controlled current (iC) amplifier. Realistically, transistor switching is not instantaneous. The turn-on time tON of the transistor is the sum of the delay time tD and the rise time tR. Similarly, the turn-off time tOFF is the sum of the storage time tS and the fall time tF . The turn-on and turn-off time of a transistor limits the maximum switching frequency. Typical switching frequency for a power BJT is between 2 and 20 kHz. BJTs can switch at a higher frequency than thyristors but can handle less power. Power BJTs can handle currents up to several hundred amperes and VCE up to about 1 kV.

• MOSFET

When operating in the enhancement mode, a MOSFET behaves very similar to a BJT. Instead of base current, the MOSFET behavior is determined by the gate voltage. When carefully controlling the gate voltage of a MOSFET, the transistor can be made to operate as a voltage controlled switch that operates between the cutoff (point A) and the Ohmic (point B) region. One advantage of a MOSFET device is that the MOSFET has significantly larger input impedance as compared to BJT. This simplifies the circuit that is needed to drive the MOSFET since the magnitude of the gate current is not a factor. This also implies that a MOSFET is much more efficient than BJTs as well as it can be switching at a much higher frequency. Typical MOSFET switching frequency is between 20 and 200 kHz, which is an order of magnitude higher than BJTs. Power MOSFETs can carry drain currents up to several hundreds of amperes and VDS up to around 500 V.

Control of Direction

Reversing the PM Motor • To reverse the rotation direction of the PM motor, the polarity of the applied

voltage must be reversed. One way to accomplish this is to have a motor-driver amp capable of outputting a positive and negative voltage.

• When the drive voltage is positive with respect to ground, the motor turns clockwise (CW). When the drive voltage is negative with respect to ground, the voltage polarity at the motor terminals reverses, and the motor rotates counterclockwise (CCW). The LM12 power op-amp is capable of providing positive and negative output voltages.

Relays • In many applications, the drive amplifier cannot output both positive and

negative voltages, in which case a switching circuit must be added to reverse the motor. One approach is to use a double-pole relay . When the relay contacts are up the positive voltage is connected to terminal A of the motor, and terminal B is connected to the negative voltage. When the relay contacts are down, the positive voltage is connected to terminal B, and terminal A goes to the negative voltage, thus effectively reversing the polarity.

H-Bridge • Forward-reverse switching can also be done with solid-state devices using four

FETs.

• When Q1 and Q4 are on, the current I1,4 causes the motor to turn clockwise. When Q2 and Q3 are on, the current I3,2 flows in the opposite direction and causes the motor to turn counterclockwise. The entire

• switching operation can be performed by a single IC, such as the Allegro A3952 .

• This IC contains four separate driver transistors that are controlled by

• internal logic to operate in pairs . The A3952 controls a motor-supply voltage of up to 50 V with up to 2 A of output current.

L293 H-bridge chip

DC Motor Drive Speed Control

DC Motor Analog Drive Speed Control

Using a single power transistor: • The circuit could be either:

1- the common emitter (CE) configuration, which gives current and voltage gain, or

2- the common collector (CC) configuration, which gives only current gain.

• When the base voltage (VB) is increased (beyond the forward-bias voltage), the transistor begins to turn on and let the collector current (IC) flow. The collector current is 30–100 times greater than the base current, depending on the gain of the transistor.

• Once the transistor starts to conduct, IC increases with VB more or less linearly.

• Note that all of IC goes through the motor, providing the drive current

Heating Problem with BJT • Power transistors are physically bigger than signal transistors and are designed to carry large

currents. In control systems, they are used to provide the drive current for motors and other electromechanical devices.

• When a transistor has a large current and voltage at the same time, the resulting power (VCIC) must be dissipated in the form of heat. A typical power transistor is designed to operate up to 200°C (360°F) above ambient temperature. However, its power capacity is derated proportionally for temperatures above 25°C .

• The power transistor case has a flat metal surface to provide a thermal escape path for the heat. Therefore, to operate at anywhere near the rated power, the transistor must be mounted firmly to the chassis or a metal heat sink—a piece of metal with cooling fins to dissipate the heat into the air

• Many times the case itself is the collector terminal. If the collector must be kept electrically insulated from the mounting chassis, then a special mica insulator is used, together with a thermally conducting white grease.

Power IC Drive (LM12)

• The power IC driver is a single-package DC amplifier with a relatively high current output. An example is the LM12 (National Semiconductor)

• The high-power operational amplifier can supply up to 13 A with a maximum voltage of ±30 V. As in any op-amp circuit, feedback resistors are added to adjust the gain to any desired value.

Darlington Power Transistor Drive • The Darlington configuration consists of two CC amplifiers connected in such a way

that the first transistor directly drives the second. Although the voltage gain is only 1 (maximum), the current gain can be very high. The transistor shown in the Figure is a TIP 120, which has a current gain of 1000 and a maximum output current of 5 A. The motor must be placed in the emitter path of the output transistor. A separate small-signal amplifier, probably an op-amp, would be needed to provide any voltage gain required.

Power MOSFET

• Notice the output current (ID) is 0 A when the input voltage (VGS) is in the 0-5-V range but then climbs to 12 A when VGS rises to 13 V.

• Using a power MOSFET, the motor is in series with the drain, which means the FET will provide both voltage and current gain.

• The gate voltage is supplied from an op-amp circuit that is

designed to interface the controller with the FET.

DC Motor Control Using Pulse-Width Modulation

• Pulse-width modulation is an entirely different approach to controlling the torque and speed of a DC motor. Power is supplied to the motor in a square wavelike signal of constant magnitude but varying pulse width or duty cycle.

• Duty cycle refers to the percentage of time the pulse is high (per cycle).

PWM Control Circuits

DC-DC Converters

• The purpose of a DC-DC converter is to supply a regulated DC output voltage to a variable-load resistance from a fluctuating DC input voltage. In many cases the DC input voltage is obtained by rectifying a line voltage that is changing in magnitude.

• DC-DC converters are commonly used in applications requiring regulated DC power, such as computers, medical instrumentation, communication devices, television receivers, and battery chargers . DC-DC converters are also used to provide a regulated variable DC voltage for DC motor speed control applications.

• The output voltage in DC-DC converters is generally controlled using a switching concept, as illustrated by the basic DC-DC converter.

• Early DC-DC converters were known as choppers with silicon-controlled rectifiers (SCRs) used as the switching mechanisms.

• Modern DC-DC converters classified as switch mode power supplies (SMPS) employ insulated gate bipolar transistors (IGBTs) and metal oxide silicon field effect transistors (MOSFETs).

• The switch mode power supply has several functions :

1. Step down an unregulated DC input voltage to produce a regulated DC output voltage using a buck or step-down converter.

2. Step up an unregulated DC input voltage to produce a regulated DC output voltage using a boost or step-up converter.

3. Step down and then step up an unregulated DC input voltage to produce a regulated DC output voltage using a buck–boost converter.

4. Invert the DC input voltage using a Cúk converter.

5. Produce multiple DC outputs using a combination of SMPS topologies.

• DC-DC conver

• DC-DC conver

DC-DC converter voltage waveforms.

Pulse width modulation concept.

Choppers

• Choppers are DC-DC converters that are used for transferring electrical energy from a DC source into another DC source, which may be a passive load. These converters are widely used in regulated switching power supplies and DC motor drive applications.

• Choppers are one-quadrant, two-quadrant, and four-quadrant

• Step-down (buck) converter and step-up (boost) converters are basic one-quadrant converter.

A step-down converter produces an average output voltage, which is lower than the DC input voltage

• A step-up converter produces output voltage always greater than the input voltage.

Step-down buck converter.

Step-up boost converter.

• The two-quadrant chopper, which, in fact, is a current reversible converter, is the combination of the two basic topologies. It has the ability to operate in two quadrants of the (v– i) plane. Therefore, input and output voltages are positive; however, input and output currents can be positive or negative. These converters are also named current reversible choppers.

• In four-quadrant choppers, not only can the output current be positive and negative, but the output voltage also can be positive and negative. These choppers are full-bridge DC-DC converters. The main advantage of these converters is that the average of the output voltage can be controlled in magnitude as well as in polarity. A four-quadrant chopper is a combination of two quadrant choppers in order to achieve negative average output voltage and/or negative average output current.

DC Motor Control for Larger Motors

• For larger motors—say, 20 A or more—the hardware needed to supply pure DC becomes bulky and expensive.

• An alternative solution is to drive the DC motor with rectified AC, where no attempt is made to smooth the waveform.

• A device that is frequently used in this application to provide both rectification and some measure of control is the silicon-controlled rectifier (SCR).

Basic SCR motor control circuit • the power source is single-phase AC and that the DC motor is connected in series

with the SCR.

• The gate of the SCR is driven by a trigger circuit that provides one pulse for each cycle of the AC.

• The free-wheeling diode (D) across the motor provides an escape path for the energy stored in the motor windings when the SCR switches off.

Half – wave rectifier

Full-wave Rectifier • SCR1 is triggered during the positive half of the AC cycle, and SCR2 is triggered

during the negative half cycle. The result? The motor receives two power pulses per cycle.

• In this case, four diodes are used for the full-wave rectifier, and a single SCR controls the delay of each half cycle.

• The SCR circuits described thus far are triggered somewhere in the middle of the AC positive half of the AC cycle. The resulting abrupt voltage rise generates high-frequency harmonics known as electrical “noise,” which can cause interference with other circuits, such as with radio and TV. A solution to this problem is called zero-voltage switching.

• With zero-voltage switching, the SCR is triggered on only at the very beginning of the cycle, when the voltage is zero anyway; consequently, there is no quick voltage change. If less than full power is desired, then, for example, only three out of four cycles would be triggered on (or some other ratio). Zero-voltage switching requires a more sophisticated trigger circuit than the phase-shift circuit discussed so far.

• Electric motors have a large starting current that is many times more than the running current. For smaller motors, this may not present a problem; for larger motors (over the range of 1-2 hp), however, special reduced voltage-starting circuits are used.

• A reduced voltage-starting circuit will limit the armature current to some acceptable value when the motor starts. One way to do this is to have a resistor in series with the armature. After the motor comes up to speed, a relay is used to bypass the resistor, allowing the full line voltage to the motor.

Single- Phase Full Converters

Single-Phase Dual Converter

Braking the DC Motor • Dynamic braking, uses the fact that a spinning motor becomes a generator

when the power is removed.

• when the armature windings are switched to a resistor as the motor is coasting down, the “generated” current from the motor delivers power to the resistor, which dissipates the power as heat.

• The power to heat the resistor has to come from somewhere, and in this case it is coming from the mechanical inertia of the spinning motor shaft.

• Plugging braking is by reversing the polarity of the armature windings

and thereby causing the motor to apply a reversing torque to the load.

• The problem is that when the voltage is reversed, it becomes the same polarity as the CEMF, so they add. The sudden large voltage will cause a large in-rush of current, which could damage the armature. To prevent this problem, a series resistor is put in the reversing voltage circuit . Also, when the motor finally comes to a stop, the reversing voltage should be switched off so that the motor doesn’t start to run backwards.

• This switching could be accomplished with a centrifugal switch.

BRUSHLESS DC MOTORS • The brushless DC motor (BLDC) operates without brushes by taking

advantage of modern electronic switching techniques.

• In three-phase BLDC, the armature (called the rotor) is a permanent magnet, and it is surrounded by three field coils. Each field coil can be switched on and off independently. When a coil is on, such as coil A, the north pole of the rotor magnet is attracted to that coil. By switching the coils on and off in sequence (A, B, C), the rotor is “dragged” around clockwise—that is, the field has rotated electronically.

• The three-phase BLDC has three optical slotted couplers and a rotating shutter (Hall-effect sensors can also be used for this application). These position sensors control the field windings. When the shutter is open for sensor P1 , field coil A is energized. When the rotor actually gets to field coil A, sensor P1 is turned off and P2 is turned on, energizing field coil B and pulling the rotor on around to coil B, and so on. In this manner, the rotor is made to rotate with no electrical connection between the rotor and the field housing. These signals are passed directly on to solid-state switches that drive the motor coils.

• A more sophisticated motor-control system would provide for the motor to reverse direction (by reversing the sequencing) and would control the speed by using PWM techniques.

• BLDC motors exhibit excellent speed control. In fact, some models come with a built-in tachometer that feeds back to the control unit, allowing a speed regulation of 0% (perfect). When used in a variable-speed motion-control system, BLDCs can vary their speed in the range of 100:1.

• However, unlike a brushed DC motor, the BLDC has a minimum operating speed (around 300 rpm) below which the individual power pulses can be felt (called cogging).

• Besides being more reliable, modern BLDC motors have performance advantages over brushed DC motors and even AC induction motors. Specifically, BLDC motors have higher power efficiency (they use less power for the same horsepower) and are smaller and lighter than other types of motors with the same horsepower.

Selecting a Motor

Expensive and

complicated drives

Maintenance free, long

lifetime, no sparking,

high speeds, clean

rooms, quiet, run cool

Bru

sh

less

Mo

tors

Maintenance required, no

clean rooms, sparking of

brushes causes EMI and

danger in explosive

environments

Inexpensive, moderate

speed, good high end

torque, simple drives

Bru

sh

ed

DC

Noisy and resonant, poor

high speed torque, not for

hot environments, not for

variable loads

Inexpensive, can be run

open loop, good low-end

torque, clean rooms

Ste

pp

er

Robotics

Pick and place

Very high torque

applications

Velocity control

High speed control

Positioning

Micro movement

Disadvantages Advantages Applications

DC Servomoters

Open-loop block diagram for a field controlled DC motors

Position and velocity feedback control of a dc motor

Phase-locked control

Main components of a PWM drive system for a dc motor