z-learning 101 - basics of motors and motor controls

LEARNING MODULE 16:BASICS OF MOTORS AND MOTOR CONTROL

101 B

ASIC

S SE

RIES

Cutler-Hammer

1

BASICS OF MOTORS AND MOTOR CONTROL

Welcome to Module 16, which is about the basics of motors and motor control.An electric motor is a machine that converts electrical energy to mechanical energy.There are two main groups of electrical motors: DC and AC motors. This module willdiscuss both types of motors, and how to control them.

FIGURE 1. TYPICAL ELECTRIC MOTOR

Like the other modules in this series, this one presents small, manageable sectionsof new material followed by a series of questions about that material. Study thematerial carefully then answer the questions without referring back to what you’vejust read. You are the best judge of how well you grasp the material. Review thematerial as often as you think necessary. The most important thing is establishing asolid foundation to build on as you move from topic to topic and module to module.

Key points are in bold.

Glossary items are italicized and underlined the first time they appear.

You may view definitions of glossary items by clicking on terms and words that areunderlined and italicized in the text. You may also browse the Glossary by clickingon the Glossary bookmark in the left-hand margin.

WELCOME

A Note on FontStyles

Viewing theGlossary

2


We’ll step through each of these topics in detail:

Section Title Page Number

• Motor Theory 4

• Magnetic Fields 4

• Current Flow 4

• Induced Motion 5

• Commutator 8

• DC Motors 9

• Simple DC Motor 9

• Practical DC Motor 10

• Electromagnets 11

• Motor Components 12

• Reversing a DC Motor 12

• DC Motor Types 13

• Review 1 14

• AC Motors 15

• What Makes an AC Motor Different From a DC Motor? 15

• Single-Phase 15

• Three-Phase 16

• The Squirrel Cage Induction Motor 17

• Induction Principle 17

• Applying the Induction Principle to the AC Motor 17

• Three-Phase Motor 19

• Construction of Three-Phase Motors 21

• Wye and Delta 21

• Dual Voltage 22

• Review 2 23

WHAT YOUWILL LEARN

3


• Speed Control 24

• Force, Work and Torque 24

• Power and Horsepower 25

• Putting it All Together 26

• Application Types 27

• Speed Control for a DC Motor 28

• Speed Control for an AC Motor 29

• Starting the Motor 32

• Across the Line 32

• Minimizing Inrush Current 32

• Reversing the Motor 33

• Manual Reversing Starter 33

• Magnetic Reversing Starter 33

• Braking the Motor 34

• DC Injection Braking 34

• Dynamic Braking 35

• Review 3 36

• Glossary 37

• Review Answers 40

• Appendix A: Typical Multispeed Motor Connections 41

WHAT YOUWILL LEARN(CONTINUED)

4


To understand motor theory, we need to cover the underlying principles of magneticfields, current flow, and induced motion.

NOTE: There are two theories regarding the flow of current. Electron Flow Theorystates that current flows from negative to positive. Conventional FlowTheory states that current flows from positive to negative.

This module uses Electron Flow Theory. For more information on thesetheories, see Module 2, Fundamentals of Electricity.

Between the poles of a magnet, there exists a magnetic field. The direction ofthe magnetic field is called magnetic flux. Magnetic flux moves from the north poleto the south pole, as shown in Figure 2.

FIGURE 2. LINES OF MAGNETIC FLUX FLOW FROM NORTH POLE TO SOUTH POLE

Now, let’s consider a wire (conductor) with an electric current flowing through it. Amagnetic field surrounds the wire, as shown in Figure 3.

FIGURE 3. LEFT HAND FLUX RULE: LINES OF MAGNETIC FLUX SURROUND A CONDUCTOR

Understanding the direction of the magnetic flux around the conductor iscritical to understanding motor motion. The direction of the magnetic flux can bedetermined using the left hand flux rule.

Imagine grasping the wire with your left hand, making sure your thumb points in thedirection of the current flow. Your fingers will curl around the wire in the direction ofthe magnetic flux.

In Figure 3, the current is flowing into the page, so the lines of flux rotatecounterclockwise around the wire.

MOTORTHEORY

Magnetic Fields

Current Flow

⊕⊕ = CURRENTFLOWING INTOTHE PAGE

5


When this current-carrying conductor is placed between the poles of a magnet,both magnetic fields distort. In Figure 4, the conductor will tend to move upward,since the current is flowing into the page.

The force exerted upward depends on the strength of the magnetic field betweenthe poles of the magnet, and the strength of the current through the conductor.

A simple method for determining the direction of motion is the right hand motor rule.

In Figure 4, the index finger points in the direction of the magnetic flux (N to S), themiddle finger points in the direction of current flow through the conductor, and thethumb points in the direction of the conductor movement.

FIGURE 4. RIGHT HAND RULE: WIRE IS MOVED UPWARD

This means that if you know the direction the current is flowing, and theorientation the poles, you can determine which way the conductor will movethrough the magnetic field.

Applying the right hand motor rule to Figure 4, the conductor will move upwardthrough the magnetic field. If the current through the conductor were to be reversed,the conductor would move downward.

Note that the conductor current is at a right angle to the magnetic field. This isrequired to bring about motion, since no force is felt by a conductor if the currentand the field direction are parallel.

Induced Motion

TOELECTRICALDC SOURCE

+ -

DIRECTION OFCONDUCTORMOVEMENT

DIRECTION OFMAGNETIC

FLUX

DIRECTIONOF CURRENT

FLOW

6


Now, suppose we change the single conductor into a simple coil or loop of wire.This coil is called an armature, and is shown in Figure 5.

FIGURE 5. ARMATURE ROTATING

Both sections of the armature AB and CD have a force exerted on them. Why doesthe coil want to move in a counterclockwise motion?

Recall that the magnetic flux rotates around the conductors. Armature sections ABand CD have the current flowing in opposite directions. This means the magneticflux flows around them in opposite directions, as shown in Figure 6.

FIGURE 6. MAGNETIC FLUX AROUND THE ARMATURE SECTIONS

Induced Motion(continued)

DIRECTION OFROTATION

A

D

B

C

COMMUTATOR

AB CD

⊕⊕ = CURRENTFLOWING INTOTHE PAGE

¤¤ = CURRENTFLOWING OUTOF THE PAGE

ARMATURE

7


When the magnetic field of the magnets are put in the picture, the two magneticfields distort. A turning force, or torque, acts on the coil. The lines of force act likestretched rubber bands that tend to contract. The result is that the armature rotatesin a counterclockwise direction.

Figure 7 illustrates a cross-sectional view of the induced motion.

FIGURE 7. CREATING TORQUE: A CROSS SECTION

The interaction between the two magnetic fields causes a bending of the lines offorce. Where the lines straighten out, they cause the armature to rotate. The leftconductor (AB) is forced downward, and the right conductor (CD) is forced upward,causing a counterclockwise rotation.

Induced Motion(continued)

AB

CD⊕⊕ = CURRENT

FLOWING INTOTHE PAGE

¤¤ = CURRENTFLOWING OUTOF THE PAGE

8


As we mentioned earlier, when the armature is positioned so that the loop sides areat right angles to the magnetic field, a turning force is exerted. But what happenswhen the coil rotates 180°°?

A problem arises here. The magnetic field in the conductor is now opposite that ofthe field, and this will tend to push the armature back the way it came, stopping therotating motion.

To solve this problem, some method must be employed to reverse the current in thearmature every one-half rotation so that the magnetic fields will work together tomaintain a positive rotation.

A device called a commutator performs this task. Two stationary brushes, onesupplied with positive DC current, the other with negative DC current, supply currentto the two rotating commutator segments.

As the armature and commutator rotate together, the commutator reverses thedirection of the current through the armature. In this way, magnetic fields arealways running in the direction needed to contribute to a continuing turning effort.

+

-

+

-

POSITION “A” - TORQUE POSITION “B” - NEUTRAL

+

-

+

-

POSITION “C” - TORQUE POSITION “D” - NEUTRAL

FIGURE 8. THE COMMUTATOR REVERSES THE CURRENT THROUGH THE ARMATURE

Now we are getting somewhere. With the armature continuously rotating throughthe magnetic field, mechanical energy is created from electrical energy.

Commutator

TO DCPOWER

ROTATION

COMMUTATORCURRENTFLOW

BRUSH

ARMATURE

TO DCPOWER

TO DCPOWER

TO DCPOWER

9


What we have just described is a DC motor. Direct current is fed to the commutator.The commutator is connected to the armature in such a way that the currentdirection (called polarity) is switched every half-turn of the armature. This allows thearmature to continue turning in the magnetic field, creating mechanical energy fromelectrical energy.

However, this simple DC motor has a few shortcomings. Each time the armature isparallel to the magnetic field (called a neutral position), no torque is produced.(Refer back to Figure 8.)

Recall that when the armature is positioned so that the loop sides are at rightangles to the magnetic field, torque is exerted. But, as the armature turns in a circle,there are two points at which it is parallel to the magnetic field – at ¼ and ¾ of aturn – and no torque is generated. (Refer back to Figure 8.)

The change in the amount of torque is shown graphically in Figure 9. The speed ofthe motor varies because of the changes in torque. Most devices require a motorto turn at a uniform speed, so the simple DC motor just described would not besuitable.

FIGURE 9. SIMPLE DC MOTOR TORQUE AND SPEED GRAPH

Another problem with a simple DC motor is that it does not start easily. This isparticularly true if the armature is in or near a neutral position. The armature mustbe moved out of the neutral position to start the motor.

DC MOTORSSimple DC Motor

0 ¼ ½ ¾ 1REVOLUTIONS

MIN

MAX ----- TORQUE

- - - SPEED

10


In a practical DC motor, the armature is never in a neutral position, and thetorque is always at its maximum. This is accomplished by using an armature withmore than one loop. A four loop armature is shown in Figure 10. As you can see,each loop of the armature is connected to a pair of commutator segments.

FIGURE 10. FOUR-LOOP ARMATURE

When current flows through the brushes, all four loops act together, producing fulltorque at all times. There is no neutral armature position where torque is absent.

Also, notice that the brushes are larger than the gaps between the commutatorsegments. This means that contact with the commutator is maintained at everyinstant of rotation of the armature.

A DC motor of this type has uniform torque, both for running and for starting. It isa definite improvement over the simple DC motor.

This is a common cordless drillthat might be used by a buildingmaintenance person. It is run ona battery and uses a DC motor.

The small size of the DC motormakes the drill very light,portable and convenient to use.

CORDLESS DRILL USING A DC MOTOR

Practical DCMotor

TO DCSOURCE

+

-COMMUTATOR

(FOUR SEGMENTS) LOOPS

BRUSH

BRUSH

IN THE WORKPLACE

11


In the previous drawings, we have shown the armature rotating between a pair ofmagnetic poles. Practical DC motors do not use permanent magnets; they useelectromagnets instead.

Electromagnets work very similarly to permanent magnets. To make one, simplywrap an iron rod with insulated wire and run current through the wire, as shown inFigure 11. The iron rod develops a magnetic field, and North and South magneticpoles.

FIGURE 11. ELECTROMAGNET

The electromagnet has two advantages over the permanent magnet.

• By adjusting the amount of current flowing through the wire, the strength of theelectromagnet can be controlled.

• By changing the direction of current flow, the poles of the electromagnetic canbe reversed. In the diagram above, switching the leads on the battery terminalswould change the direction current flow.

(Connecting the leads to an AC source would switch the direction of current flowautomatically. We will consider AC later in this module.)

Electromagnets

DIRECTION OFCURRENT FLOW

12


We have already discussed three of the four major components that make up a DCmotor: the armature, the brushes, and the commutator. The fourth is the field coils(also called field poles or stationary windings).

Figure 12 shows a disassembled view of a typical four-pole DC motor.

FIGURE 12. A TYPICAL FOUR-POLE DC MOTOR, ASSEMBLED AND DISASSEMBLED

Note that many turns (or windings) are used to make up the field poles. The largerthe poles, the stronger the field.

The larger the number of coils used in a DC motor, the smoother the motor willrun. However, the number of field coils used must always be even. Each set of coilsconsists of a North and a South pole.

The direction of rotation of a DC motor may be reversed using one of thesemethods:

• Reversing the direction of the current through the field

• Reversing the direction of the current through the armature

The industrial standard is to reverse the current through the armature. This isaccomplished by reversing the armature connections only.

MotorComponents

Reversing aDC Motor

SHAFT

COMMUTATOR

WINDINGS

FAN

FIELD POLES

13


There are basically three DC motor types: The series motor, the shunt motor, andthe compound motor. Internally and externally they are practically the same. Thedifference between them is the way the field coil and armature coil circuits arewired.

The series motor (Figure 13) has the field coil wired in series with the armature.It is also called a universal motor because it can be used in DC or AC applications.It has a high starting torque and a variable speed characteristic. The motor can startheavy loads, but the speed will increase as the load is decreased.

FIGURE 13. DC SERIES MOTOR: SCHEMATIC AND WIRING DIAGRAM

The shunt motor (Figure 14) has the armature and field circuits wired inparallel, giving essentially constant field strength and motor speed.

FIGURE 14. DC SHUNT MOTOR: SCHEMATIC AND WIRING DIAGRAM

The compound motor (Figure 15) combines the characteristics of both theseries and the shunt motors. A compound motor has high starting torque andfairly good speed torque characteristics at rated load. Since complicated circuits areneeded to control the compound motors, this wiring arrangement is usually onlyused on large bi-directional motors.

FIGURE 15. DC COMPOUND MOTOR: SCHEMATIC AND WIRING DIAGRAM

DC Motor Types

SERIES FIELD

ARMA-TURE

DCVOLTAGE

S1S2 A2A1

S1S2 A1A2ARM

SHUNT FIELD

ARMA-TURE

DCVOLTAGE

F1F2

A1A2

F1F2

A1A2ARM

ARMA-TURE

A2A1 F1F2 S1S2

DCVOLTAGE

SERIES FIELD

SHUNT FIELDF1F2

A1A2A

S1S2

14


Answer the following questions without referring to the material just presented.Begin the next section when you are confident that you understand what you’vealready read.

1. The right hand rule is illustrated here. What does each finger indicate?

Thumb _____________________

Index ______________________

Middle _____________________

2. The 2 main problems with the simple DC motor are:

______________________________________________________

______________________________________________________

3. Label the simple DC motor’s speed/torque graph below:

4. The 2 methods for reversing a DC motor are:

______________________________________________________

______________________________________________________

5. The 3 DC motor types are:

___________________________

___________________________

___________________________

REVIEW 1

0 __ __ __ __REVOLUTIONS

MIN

MAX ----- ________

- - - ________

15


While there are only three general types of DC motors, there are many different ACmotor types. This is because each type is confined to a narrow band of operatingcharacteristics. These characteristics include torque, speed, and electrical service(single-phase or polyphase). These operating characteristics are used to determinea given motor’s suitability for a given application.

In a DC motor, electrical power is conducted directly to the armature throughbrushes and a commutator. An AC motor does not need a commutator toreverse the polarity of the current, as AC changes polarity “naturally.”

Also, where the DC motor works by changing the polarity of the current runningthrough the armature (the rotating part of the motor), the AC motor works bychanging the polarity of the current running through the stator (the stationary part ofthe motor).

The many types of AC motor may be split into two main groups: single-phase andpolyphase.

A single-phase power system has one coil in the generator. Therefore, onealternating voltage is generated. The voltage curve of a single-phase AC generatoris shown in Figure 16.

FIGURE 16. VOLTAGE CURVE OF A SINGLE-PHASE AC GENERATOR

Single-phase motors are generally motors with horsepower ratings of one orbelow. (These are generally called fractional horsepower motors.) They aregenerally used to operate mechanical devices and machines requiring a relativelysmall amount of power.

Types of single-phase motors include: shaded-pole, capacitor, split-phase,repulsion, series (AC or universal) and synchronous.

However, the single-phase motor is generally not used because it is inefficient,expensive to operate, and is not self starting.

We will not go into detail here regarding how each single-phase motor typefunctions.

AC MOTORS

What Makes anAC MotorDifferent From aDC Motor?

Single-Phase

16


Three-phase or polyphase motors run on three-phase power.

A three-phase power system has three coils in the generator. Therefore, threeseparate and distinct voltages will be generated. The voltage curve is shown inFigure 17.

FIGURE 17. VOLTAGE CURVE OF A THREE-PHASE AC GENERATOR

We will discuss how three-phase power works in more detail shortly.

Types of three-phase motors include: induction (squirrel-cage or wound), rotortypes, commutator, and synchronous.

In an AC environment, the squirrel cage induction motor is the most widelyused. We will focus only on this type of motor.

Three-Phase

17


Before we discuss the squirrel cage motor further, let’s consider the term induction.Induction refers to electrically charging a conductor by putting it near acharged body.

The principle of the induction motor was first discovered by Arago in 1824. Heobserved that if a non-magnetic metal disk and a compass are pivoted with theiraxes parallel, so that one (or both) of the compass poles are located near the edgeof the disk, spinning the disk will cause the compass needle to rotate. The directionof the induced rotation in the compass is always the same as that imparted to thedisk.

You can prove it to yourself if you like. Mount a simple copper or aluminum disk anda large compass on a vertical stem, so that each may be rotated on its own bearing,independently of the other. Spin the disk, and watch the compass needle. There isno more effective way to demonstrate the principle of induction.

FIGURE 18. DEMONSTRATING THE PRINCIPLE OF INDUCTION

So, how do we apply the concept of induction to a motor?

Recall that the AC motor works by changing the polarity of the current runningthrough the stator (the stationary part of the motor). The stator plays the role of themetallic disk described above. A rotating magnetic field is established in the stator.

The conductor, called the rotor, “follows” the rotating magnetic field by beginning torotate, just like the compass needle described above.

THESQUIRRELCAGEINDUCTIONMOTOR

InductionPrinciple

Applying theInductionPrinciple to theAC Motor

18


The induction motor uses a rotor of a special design. It resembles a cage used forexercising squirrels. This is why it is called a squirrel cage rotor.

The rotor consists of circular end rings joined together with metal bars. Notethat the metal bars are placed directly opposite each other and provide a completecircuit within the rotor, regardless of the rotor's position. Rotors normally haveseveral bars, but only a few are shown here for clarity.

FIGURE 19. THE ROTOR OF A SQUIRREL CAGE INDUCTION MOTOR

Squirrel cage motors are usually chosen over other types of motors becauseof their simplicity, ruggedness and reliability. Because of these features,squirrel-cage motors have practically become the accepted standard for AC, all-purpose, constant speed motor applications. Without a doubt, the squirrel-cagemotor is the workhorse of the industry.

The squirrel cage induction motor has certain advantages over the DC motor.

• There are only two points of mechanical wear on the squirrel cage motor: thetwo bearings.

• Since it has no commutator, there are no brushes to wear. This keepsmaintenance minimal.

• No sparks are generated to create a possible fire hazard.

Applying theInductionPrinciple to theAC Motor(Continued)

19


An induction motor depends upon an electrically rotating magnetic field, not amechanically rotating one. (A mechanically rotating field would work, but anelectrically rotating magnetic field has significant advantages.) How is an electricallyrotating field obtained? It all starts with the phase displacement of a three-phasepower system.

Three-phase power can be thought of as three different single-phase powersupplies. They are called A, B, and C. In the three-phase motor, each phase of thepower supply is provided with its own set of poles, located directly across from eachother on the stator, and offset equally from each of the other two phases’ poles.

FIGURE 20. THREE PAIRS OF FIELD COILS ON THE STATOR, SET 120°° APART

The three currents start at different times. Phase B starts 120° later than phase Aand phase C starts 120° later than phase B. This is shown on the sine wave graphin Figure 21, which indicates the way the magnetic field will point at various times inthe cycle.

FIGURE 21. MAGNETIC FIELD ROTATION PROVIDING TORQUE TO TURN THE MOTOR

Introducing these different phase currents into three field coils 120° apart on thestator produces a rotating magnetic field, and the magnetic poles are in constantrotation.

Three-PhaseMotor

PHASE C

PHASE B

PHASE A

PHASE A PHASE B PHASE C

20


The magnetic poles chase each other, simultaneously inducing electric currents inthe rotor (generally, bars of copper imbedded in a laminated iron core). The inducedcurrents set up their own magnetic fields, in opposition to the magnetic field thatcaused the currents. The resulting attractions and repulsions provide the torque toturn the motor, and keep it turning.

If each magnetic pole were to "light up" whenever it was energized, the effect wouldappear as though the lights were "running" around the stator, much as the lights onsome electric signs simulate a running border.

Let’s walk through one revolution of the motor to see how it works.

First, the A poles of the stator are magnetized by phase A. Then, the B poles aremagnetized by phase B. The rotor turns, due to the induced current. Then, the Cpoles are magnetized by phase C. The rotor turns, due to the induced current. Therotor has completed one-half turn at this point.

FIGURE 22. ROTATING MAGNETIC FIELD TURNS THE MOTOR

Now, the A poles of the stator are magnetized again, but the current flow is in theopposite direction. This causes the magnetic field to continue to rotate, and therotor follows. Then, the B poles are magnetized by phase B. The rotor turns, due tothe induced current. Then, the C poles are magnetized by phase C. The rotor turns,due to the induced current.

FIGURE 23. ROTATING MAGNETIC FIELD TURNS THE MOTOR

The rotor has completed one full revolution at this point, and the process repeatsitself.

Three-PhaseMotor(Continued)

21


The three-phase motor is probably the simplest and most rugged of all electricmotors. To get a perspective on how important the three-phase motor is, all youneed to know is that this motor is used in nine out of ten industrial applications.

All three-phase motors are constructed with a number of individually woundelectrical coils. Regardless of how many individual coils there are in a three-phase motor, the individual coils will always be wired together (series orparallel) to produce three distinct windings, which are called phases. Eachphase will always contain one-third of the total number of individual coils. As wementioned, these phases are referred to as phase A, phase B and phase C.

Three-phase motors vary from fractional horsepower size to several thousandhorsepower. These motors have a fairly constant speed characteristic but a widevariety of torque characteristics. They are made for practically every standardvoltage and frequency and are very often dual voltage motors. (We will look brieflyat dual voltage motors later.)

All three-phase motors are wired so that the phases are connected in either a Wye(Y) or Delta (∆) configuration.

In a Wye (Y) configuration (Figure 24), one end of each of the three-phases isconnected to the other phases internally. The remaining end of each phase isthen brought out externally and connected to the power line. The external leads arelabeled T1, T2 and T3, and are connected to the three-phase power lines labeledL1, L2 and L3, respectively.

FIGURE 24. WYE CONFIGURATION

Construction ofThree-PhaseMotors

Wye and Delta

INTERNALCONNECTION

OF ONE END OFEACH PHASE

PHASEC

PHASEA

PHASEB

L1 L2 L3

T1

T2

T3

MOTOR STARTER

22


In a Delta (∆∆) configuration (Figure 25), each winding is wired end to end toform a completely closed loop circuit. At each of the three points where thephases are connected, a lead is brought out externally. They are labeled T1, T2and T3, and are connected to the three-phase power lines labeled L1, L2 and L3,respectively.

FIGURE 25. DELTA CONFIGURATION

In either case, for the motor to operate properly, the three-phase line supplyingpower to the motor must have the same voltage and frequency ratings as themotor.

Many three-phase motors are made so that they can be connected to either of twovoltages. The purpose in making motors for two voltages is to enable the samemotor to be used with two different power line voltages. Usually, the dualvoltage rating of industrial motors is 230/460V. However, the nameplate mustalways be checked for proper voltage ratings.

When the electrician has the choice of deciding which voltage to use, thehigher voltage is preferred. The motor will use the same amount of power, givingthe same HP output for either high or low voltage, but as the voltage is doubled(230 to 460), the current will be cut in half. With half the current, wire size can bereduced and savings can be realized on installation.

Dual Voltage

PHASEC

PHASEA

PHASEB

L1 L2 L3

T1

T2

T3

MOTOR STARTER

23


Answer the following questions without referring to the material just presented.Begin the next section when you are confident that you understand what you’vealready read.

1. Name the two groups of AC motors.

___________________________

___________________________

2. Explain why an AC motor does not need a commutator:

________________________________________________________________

3. Three-phase power can be thought of as three different ____________

___________ _________ ____________.

4. Fill in the blanks on the diagram below.

5. Does the diagram above show a WYE or DELTA configuration?Circle the correct answer.

REVIEW 2

PHASE___

PHASE___

PHASE___

L __

T1

T2

T3

MOTOR STARTER

L __ L __

24


Speed control is essential in many applications. Mining machines, printingpresses, cranes and hosts, elevators, and conveyors, among others, all depend onspeed control.

In choosing the speed control method for an application, there are three mainfactors to consider:

• Type of equipment (load) the motor drives

• Application type

• Motor type

We will discuss each of these factors in turn.

Loads and application types are as varied as the types of motors available.However there are two fundamental motor types: AC and DC. Each type has itsown ability to control different loads at different speeds.

In order to select the correct motor type for a given application, it is necessaryto understand the load requirements first. To understand these requirements,you need to be familiar with the concepts of force, work, torque, power andhorsepower, and how they relate to speed.

Work is done when a force overcomes a resistance. Work is measured with theformula:

Work = Distance x Force

If you carry a 10-pound bag of groceries 50 feet, 500 foot-pounds (ft-lb.) of work isdone.

In the case of an electric motor, force is not exerted in a line, but in a circle, about acylindrical shaft. As you recall, turning force is called torque.

Torque = Radial Distance x Force

If you apply 100 pounds of force to a motor shaft at a radial distance of 5 feet, 500foot-pounds (ft-lb.) of torque is applied to the shaft.

FIGURE 26. TORQUE = RADIAL DISTANCE X FORCE

SPEEDCONTROL

Force, Workand Torque

FORCE

RADIALDISTANCE

25


Power takes into consideration how fast work is accomplished. Power is therate of doing work. The formula to determine power is:

Power = Work/Time

If the 10-pound bag of groceries was connected to a very small motor, it might takethe motor several minutes to move the load 50 feet. If a larger motor was used, itmight move the load in only a few seconds.

The reason for this difference is the amount of work that can be delivered in a givenamount of time. Obviously, a larger motor should be able to deliver more work in agiven time than one that is considerably smaller. It is this difference that determinesthe power rating of the motor.

Motors are rated in horsepower (HP). One horsepower is equal to 33,000 ft-lbs.per minute. (Electrical power can also be measured in watts. One horsepower isequal to 746 watts of electrical power.) Let’s figure horsepower for a motor to movethose groceries. Recall that:

Work = Distance x Force

If you carry a 10-pound bag of groceries 50 feet, 500 foot-pounds of work is done. Ifyou connect the bag to a motor that can move it 50 feet in 15 seconds, what is thehorsepower of the motor?

Power = Work/Time

Power = 500 ft-lb/.25 minutes

Power = 2000 ft-lb. per minute

And since 33,000 ft-lb. per min equals 1 HP, (2000 / 33,000) the motor has about0.06 horsepower.

Power andHorsepower

26


Torque, horsepower, and speed are all interrelated when turning a load.Horsepower is proportional to torque and speed. The following formula ties themall together:

HP = (T x N)/5252

Where:HP = the horsepower provided by the motorT = the torque of the motor in foot-poundsN = the synchronous speed of the motor in rpm

This means that if either speed or torque remains constant while the otherincreases, horsepower increases. Conversely, if either torque or speed decreaseswhile the other remains constant, horsepower will decrease.

Below is a chart that shows the relationship of horsepower, torque and speed.

SPEED INCREASES é

TORQUE CONSTANT

HORSEPOWER

INCREASES

é

SPEED

DECREASES ê TORQUE

CONSTANT

HORSEPOWER

DECREASES

ê

SPEED

CONSTANT

TORQUE INCREASES é

HORSEPOWER

INCREASES

é

SPEED

CONSTANT

TORQUE DECREASES ê

HORSEPOWER

DECREASES

ê

SPEED

INCREASES é TORQUE

DECREASES ê

HORSEPOWER

REMAINS CONSTANT

SPEED

DECREASES ê TORQUE

INCREASES é

HORSEPOWER

REMAINS CONSTANT

FIGURE 27. HORSEPOWER, TORQUE AND SPEED RELATIONSHIP

Putting it AllTogether

27


When a motor is driving a load, it will be called upon to deliver either a constant or avariable torque, and either a constant or variable horsepower. The amount of torqueand horsepower required, will depend upon the speed and size of the load.

There are three main application types. Let’s consider each briefly.

• Constant Torque/Variable Horsepower

This type of load is often found on machines that have friction-type loads, suchas conveyors, gear-type pumps, and load lifting equipment.

The horsepower required increases when the speed increases. The torquerequirement does not vary throughout the speed range except for the extrastarting torque needed to overcome the breakaway friction. The torque remainsconstant because the force of the load does not change.

• Constant Horsepower/Variable Torque

This type of load is used for loads that demand high torque at low speeds andlow torque at high speeds. Examples of these loads are machines that roll andunroll paper or metal.

Since the linear speed of the material is constant, the horsepower must also beconstant. While the speed of the material is kept constant, the motor speed isnot. At start, the motor must run at high speed to maintain the correct materialspeed while torque is kept at a minimum. As material is added to the roll, themotor must deliver more torque at a slower speed. In this application, bothtorque and speed are constantly changing while motor horsepower remains thesame.

• Variable Torque/Variable Horsepower

This type of load is used for loads that have a varying torque and horsepower atdifferent speeds. Typical applications are fans, blowers, centrifugal pumps,mixers and agitators.

As the motor speed is increased, so is the load output. Since the motor mustwork harder to deliver more output at faster speeds, both torque andhorsepower are increased.

ApplicationTypes

28


Now that you understand what factors are important in choosing a motor for anapplication, we are ready to look at how to actually control the speed of the motor.Let’s start with the DC motor.

The base speed of a motor is the speed at which the motor will run with full linevoltage applied to the armature and the field.

The speed of a DC motor is controlled by varying the applied voltage across thearmature, the field, or both. When armature voltage is controlled, the motor willdeliver a constant torque characteristic. When field voltage is controlled, the motorwill deliver a constant horsepower characteristic.

FIGURE 28. FIELD VOLTAGE VS. ARMATURE VOLTAGE IN CONTROLLING A DC MOTOR’S SPEED

DC motors are used in industrial applications that require either variable speedcontrol, high torque, or both. Since the speed of most DC motors can becontrolled smoothly and easily from zero to full speed, DC motors are used in manyacceleration and deceleration applications.

The DC motor is ideal in applications where momentarily higher torque output isneeded. The DC motor can deliver three to five times its rated torque for shortperiods of time. (Most AC motors will stall with a load that requires twice the ratedtorque.)

For these reasons, DC motors are used to run large machine tools, cranes andhoists, printing presses, cranes, elevators, shuttle cars and automobile starters.

Speed Controlfor a DC Motor

BASE SPEED OF THE MOTOR(FULL FIELD AND ARMATURE

VOLTAGE APPLIED)

DC APPLIED VOLTAGE

MO

TO

R S

PE

ED

IN R

PM

REDUCING FIELD VOLTAGEINCREASES SPEED ABOVE

BASE SPEED

REDUCING ARMATURE VOLTAGEDECREASES SPEED BELOW

BASE SPEED

29


Since each motor type has its own characteristics of horsepower, torque and speed,different motor types are more suited for different applications.

The basic characteristics of each AC motor type are determined by the design ofthe motor and the supply voltage used. These design types are classified andgiven a letter designation, which can be found on the nameplate of motortypes listed as “NEMA Design.”

NEMADesign

StartingTorque

StartingCurrent

BreakdownTorque

Full LoadSlip

Typical Applications

A Normal Normal High Low Machine ToolFan

Centrifugal Pump

B Normal Low High Low Machine ToolFan

Centrifugal Pump

C High Low Normal Low LoadedCompressor

Loaded Conveyor

D Very High Low - - - High Punch Press

The most commonly used AC NEMA Design motor is the NEMA B.

The conveyor on this beerbottling line is powered by aNEMA Design B motor.

The NEMA Design B motor is ageneral purpose AC inductionmotor. It is the most commonlyused NEMA Design motor,because it offers a good balanceof function against price.

NEMA DESIGN B MOTOR AT WORK

Speed Controlfor an AC Motor

IN THE WORKPLACE

MOTOR

30


The induction motor is basically a constant speed device. The speed at whichan induction stator field rotates is called its synchronous speed. This is because it issynchronized to the frequency of the AC power at all times. The speed of therotating field is always independent of load changes on the motor, provided the linefrequency is constant.

Synchronous speed is determined by the number of poles that the motor has,and the frequency being supplied to it. The equation for determining thesynchronous speed of a motor is:

N = 120f/P

Where:N = the synchronous speed of the motor in revolutions per minute (RPM)f = the frequency supplied to the motor in Hertz (Hz)P = the number of poles the motor has

Motors designed for 60 Hertz use (standard in the US) have synchronous speedsas follows:

Poles RPM 2 3600 4 1800 6 1200 8 900 10 720 12 600 14 514 16 450

Induction motors do not run at synchronous speed; they run at full load speed,which is the rotational speed of the rotor. Full load speed is always slower. Thepercent reduction in speed is called percent slip. The slip is required to developrotational torque. The higher the torque, the greater the slip.

The motor speed, under normal load conditions, is rarely more than 10% belowsynchronous speed. If the motor is not driving a load, it will accelerate to nearlysynchronous speed. As the load increases, the percent slip increases.

For example, a motor with a 2.8% slip and 1800 rpm synchronous speed wouldhave a slip of 50 rpm, and a full load speed of 1750 rpm (1800 - 50 = 1750 rpm). Itis this full load speed that will be found on the motor's nameplate.

From the formula, it is evident that the supply frequency and number of poles arethe only variables that determine the speed of the motor.

Varying the voltage is not a good way to change the speed of the motor. In fact, ifthe voltage is changed by more than 10%, the motor may be damaged. This isbecause the starting torque varies as the square of the applied voltage.

Speed Controlfor an AC Motor(continued)

31


Since the frequency or number of poles must be changed to change the speed ofan AC motor, two methods of speed control are available. These are:

• Changing the frequency applied to the motor

Changing the frequency requires a device called an adjustable frequency driveto be inserted upstream from the motor. This device converts the incoming 60Hz into any desired frequency, allowing the motor to run at virtually any speed.

For example, by adjusting the frequency to 30 Hz, the motor can be made to runonly half as fast.

We will look at adjustable frequency drives in much more detail in Module 20,Adjustable Frequency Drives.

• Using a multispeed motor

Multispeed AC motors are designed with windings that may be reconnectedto form different numbers of poles. They are operated at a constantfrequency.

Two-speed motors usually have one winding that may be connected to providetwo speeds, one of which is half the other.

Motors with more than two speeds usually include many windings. These can beconnected many ways to provide different speeds. Refer to APPENDIX A:Typical Multispeed Motor Connections.

Everyone is familiar with thispiece of equipment. The portablethree-speed oscillating fan canbe found in most homes.

The fan’s multispeed motorcontains many windings that canbe connected three differentways. This allows the user to setthe fan to run at any of the threepreset speeds.

THREE-SPEED OSCILLATING FAN

Speed Controlfor an AC Motor(continued)

IN THE WORKPLACE

32


A starter is a device that is used to start a motor from a stop. The across-the-linestarter is by far the most common. This type of starter places the motor directlyacross the full voltage of the supply lines, hence the name: "across-the-line.” Whenan induction motor is placed across-the-line, it will accelerate to full speed in amatter of seconds.

What applications are suitable for this type of rapid acceleration? Pumps of alltypes, fans and blowers, and most machines such as drill presses, lathes andgrinders are suitable.

We will discuss starters in much more detail in Module 19, Starter Basics.

Small DC motors are generally started by simply closing the line switch. No auxiliarystarting equipment is necessary to limit the initial rush of current. The same practiceapplies to most small (and some large) polyphase motors.

During an AC motor’s start-up accelerating period, a large amount of current isrequired to start the motor rotating and bring it up to speed. This is called inrushcurrent. Currents 6 to 8 times the full load rating of the motor are notuncommon when the motor is started across-the-line.

From this, we can see that the power company will be rather concerned, since theyhave to supply the actual current necessary to start (and also to run) the motor. So,it is desirable (if not necessary) to limit the initial rush of current to a reasonablevalue, about 1.25 to 5 times the full load rating. There are several ways of doingthis:

• (AC/DC) Inserting resistance in the line, and then cutting the resistancegradually as the motor comes up to speed.

• (AC) Using a reduced voltage starter, which we will discuss in much more detailin Module 21, Reduced Voltage Starters.

• (AC) Using a wound rotor type of motor, which employs a resistor controller forthe starting function and which may also serve as a speed control device.

• (AC) Using the Wye-Delta method, in which the stator is connected in a Wye atthe instant of starting, and in Delta after the motor has reached normal speed.

• (AC) Using an adjustable frequency drive, which we will discuss in much moredetail in Module 20, Adjustable Frequency Drives.

STARTINGTHE MOTOR

Across the Line

MinimizingInrush Current

33


In applications where it is desirable to run a motor in both forward and reverse,there are a few options for providing a reversing capability.

A manual reversing starter can be used to change the direction of rotation of athree-phase, a single-phase or a DC motor. It is made by simply connecting twomanual starters together. The electrical diagram is shown in Figure 29.

FIGURE 29. MANUAL REVERSING STARTER

This type of device is generally used to run lower horsepower motors, such asthose found on fans, small machines, pumps and blowers.

A magnetic reversing starter performs the same function as a manual reversingstarter. Electrically, the only difference between manual and magnetic startersis the addition of forward and reversing coils and the use of auxiliary contacts.

The forward and reversing coils replace the pushbuttons of a manual starter. Theauxiliary contacts provide additional electrical protection and circuit flexibility.

REVERSINGTHE MOTOR

ManualReversingStarter

MagneticReversingStarter

MOTOR TERMINAL CONNECTIONS

POWER TERMINAL CONNECTIONS

FORWARDCONTACTS

REVERSECONTACTS

MECHANICALINTERLOCK

START

STOP

START

STOP

F F F R R R

34


Two common methods used for braking a motor are DC injection braking anddynamic braking. We will look at both in detail, starting with electric braking.

DC injection braking is a method of braking in which direct current (DC) is appliedto the stationary windings of an AC motor after the AC voltage is removed. Thisis an efficient and effective method of braking most AC motors. DC injection brakingprovides a quick and smooth braking action on all types of loads, including high-speed and high-inertia loads.

Recall that opposite magnetic poles attract and like magnetic poles repel. Thisprinciple, when applied to both AC and DC motors, is the reason why the motorshaft rotates.

In an AC induction motor, when the AC voltage is removed, the motor will coast to astandstill over a period of time, since there is no induced field to keep it rotating.Since the coasting time may be unacceptable, particularly in an emergencysituation, electric braking can be used to provide a more immediate stop.

By applying a DC voltage to the stationary windings once the AC is removed, amagnetic field is created in the stator that will not change polarity.

In turn, this constant magnetic field in the stator creates a magnetic field in therotor. Since the magnetic field of the stator is not changing in polarity, it will attemptto stop the rotor when the magnetic fields are aligned (N to S and S to N).

FIGURE 30. DC INJECTION BRAKING

The only thing that can keep the rotor from stopping with the first alignment is therotational inertia of the load connected to the motor shaft. However, since thebraking action of the stator is present at all times, the motor is braked quickly andsmoothly to a standstill.

Since there are no parts that come in physical contact during braking, maintenanceis kept to a minimum.

BRAKING THEMOTOR

DC InjectionBraking

STATOR

ROTOR

35


Dynamic braking is another method for braking a motor. It is achieved byreconnecting a running motor to act as a generator immediately after it isturned off, rapidly stopping the motor. The generator action converts themechanical energy of rotation to electrical energy that can be dissipated as heat ina resistor.

Dynamic braking of a DC motor may be needed because DC motors are often usedfor lifting and moving heavy loads that may be difficult to stop.

There must be access to the rotor windings in order to reconnect the motor to act asa generator. On a DC motor, access is accomplished through the brushes on thecommutator.

In this circuit, the armature terminals of the DC motor are disconnected from thepower supply and immediately connected across a resistor, which acts as aload. The smaller the resistance of the resistor, the greater the rate of energydissipation and the faster the motor slows down.

The field windings of the DC motor are left connected to the power supply. Thearmature generates a voltage referred to as “counter electromotive force” (CEMF).This CEMF causes current to flow through the resistor and armature. The currentcauses heat to be dissipated in the resistor, removing energy from the systemand slowing the motor rotation.

The generated CEMF decreases as the speed of the motor decreases. As themotor speed approaches zero, the generated voltage also approaches zero. Thismeans that the braking action lessens as the speed of the motor decreases. As aresult, a motor cannot be braked to a complete stop using dynamic braking.Dynamic braking also cannot hold a load once it is stopped, because there is nomore braking action.

For this reason, electromechanical friction brakes are sometimes used along withdynamic braking in applications that require the load to be held, or in applicationswhere a large heavy load is to be stopped. This is similar to using a parachute toslow a race car before applying the brakes.

FIGURE 31. DYNAMIC BRAKING IS OFTEN USED WITH ELECTROMECHANICAL FRICTION BRAKING

Dynamic braking for AC motors can be handled with an adjustable frequency drive.We will discuss adjustable frequency drive in much more detail in Module 20,Adjustable Frequency Drives.

Dynamic Braking

36


Answer the following questions without referring to the material just presented.

1. Fill in the blanks for the following formulas:

Work = _________ x_________ Power = _________ / __________

2. Work out the horsepower rating of a motor that moves a load of 1000 pounds adistance of 330 feet in one minute.

Answer: _________ HP

3. A conveyor is an example of a ________ Torque / _________ Horsepowerapplication.

4. Name the two devices that can be used to reverse the direction of a motor.

________________________________

________________________________

5. Reducing the voltage supplied to the field of a DC motor will cause the motorspeed to INCREASE or DECREASE. Circle the correct answer.

6. Using the synchronous speed formula, calculate the full load speed of a motorwith 8 poles running on 60 Hz with a slip of 2.2%.

Answer: _________ RPM

REVIEW 3

37


AdjustableFrequency Drive

This device converts the incoming 60 Hz power into anydesired frequency, allowing an AC motor to run at virtuallyany speed.

Armature The turning conductor in a DC motor.

Base Speed The speed at which a DC motor will run with full voltageapplied to the armature and the field

Brushes The stationary components of the commutator, providingcurrent to the rotating commutator segments.

Coils The stationary windings of the DC motor that generate anelectromagnetic field.

Commutator A device used in a DC motor to reverse the current in thearmature every one-half rotation so that the magnetic fieldswill work together to maintain rotation.

Compound Motor A DC motor that combines the characteristics of both theseries and the shunt motors.

Conventional FlowTheory

A theory regarding the flow of current. It states that currentflows from positive to negative.

DC InjectionBraking

A method of braking an AC motor in which direct current(DC) is applied to the stationary windings of an AC motorafter the AC voltage is removed.

Delta A motor connection arrangement where each winding iswired end to end to form a completely closed loop circuit.

Dual VoltageMotor

A motor made for two voltages. It enables the same motorto be used with two different power line voltages.

Dynamic Braking A method of braking a DC motor by reconnecting a runningmotor to act as a generator immediately after it is turnedoff. Reconnecting the motor in this way makes the motoract as a loaded generator that develops a retarding torque,rapidly slowing the motor.

Electron FlowTheory

A theory regarding the flow of current which states thatcurrent flows from negative to positive.

Full Load Speed The true speed at which a motor turns, found on thenameplate. To calculate, take Synchronous Speed minusPercent Slip. It is the speed of the rotor.

Horsepower A unit of power measurement, used for rating the amount ofWork a motor can do. One horsepower equals 33,000 foot-pounds per minute of Work.

GLOSSARY

38


Induction The process of producing a current by the relative motion ofa magnetic field across a conductor.

Left HandFlux Rule

The relationship of the factors used to determine is whichdirection the magnetic flux moves around a conductor.Imagine grasping the wire with your left hand, making sureyour thumb points in the direction of the current flow. Yourfingers will curl around the wire in the direction of themagnetic flux.

Magnetic Flux The direction of a magnetic field.

MagneticReversing Starter

A device that performs the same function as a manualreversing starter. Electrically, the only difference betweenmanual and magnetic starters is the addition of forward andreversing coils and the use of auxiliary contacts.

Manual ReversingStarter

A device used to change the direction of rotation of a three-phase, a single-phase or a DC motor. It is made by simplyconnecting two manual starters together.

Neutral Position The position at which the armature in a DC motor is parallelto the magnetic field, where no torque is produced.

Percent Slip The percentage difference between a motor’s SynchronousSpeed and its Full Load Speed.

Polarity Direction of current flow through a conductor.

Poles The stationary windings of the DC motor that generate anelectromagnetic field.

Power A measure of work done per unit of time.

Reduced VoltageStarter

A type of starter that ramps up the power to a motorgradually to cut down on current draw at start-up.

Right HandMotor Rule

The relationship between the factors involved indetermining the movement of a conductor in a magneticfiled. The index finger points in the direction of the magneticfield (N to S), the middle finger points in the direction ofelectron current flow in the conductor, and the thumb pointsin the direction of the force on the conductor.

Rotor The rotating part of an AC motor.

Series Motor A DC motor with the field coil wired in series with thearmature coil. It is also called a universal motor.

Shunt Motor A DC motor with the field coil wired in parallel with thearmature coil.

39


Starter A device that is used to start a motor from a stop

StationaryWindings

The stationary windings of the DC motor that generate anelectromagnetic field.

Stator The stationary part of an AC motor.

Squirrel CageInduction Motor

The most common AC motor type, named for the rotor’sresemblance to a cage used for exercising squirrels.

SynchronousSpeed

The rotational speed of the stator, defined by the formula:

N = 120f/P

Where:N = the synchronous speed of the motor in revolutions

per minute (RPM)f = the frequency supplied to the motor in Hertz (Hz)P = the number of poles the motor has

Torque Turning or rotational force.

Work Applying a force over a distance.

Wye A motor connection arrangement where one end of each ofthe three-phases is connected to the other phasesinternally. The remaining end of each phase is then broughtout externally.

In preparing this training module, some material was taken from the publicationlisted below:

Gary Rockis and Glenn A. Mazur, Electrical Motor Controls. (Homewood, IL:American Technical Publishers, Inc., 1997).

REFERENCE

40


1. Thumb: Direction of the conductor movementIndex: Direction of the magnetic fluxMiddle: Direction of current flow through the conductor

2. When the armature is parallel to the magnetic field, no torque is produced.They are hard to start.

3. Blanks on the bottom of the graph, from left to right: “1/4”, “1/2”, “3/4”.Blanks on the side of the graph, from top to bottom: “Torque”, “Speed”.

4. Reversing the direction of the current through the field.Reversing the direction of the current through the armature.

5. Series, shunt and compound

1. Single phase and polyphase

2. AC changes polarity “naturally.”

3. single-phase power supplies

4. Blanks from left to right: “L1”, “L2”, “L3”, “B”, “C”, “A”.

5. Delta

1. Work = Distance x Force Power = Work/Time

2. 10

3. Constant Torque / Variable Horsepower4. Manual reversing starter; Magnetic reversing starter

5. Increase

5. About 800 RPM

REVIEW 1ANSWERS

REVIEW 2ANSWERS

REVIEW 3ANSWERS

41


Common motor connection arrangements, conforming to NEMA standards, areused when connecting motors. The diagrams on these two pages are typicalarrangements, but do not depict all possible arrangements.

APPENDIX A:TYPICALMULTISPEEDMOTORCONNECTIONS

42


Publication No. TR.90.06.T.EFebruary 1999Printed in U.S.A. (GSP)

101 Basics Series and 201 Advanced Series are trademarks of Cutler-Hammer University, Cutler-Hammer and Eaton Corp.©1999, Eaton Corp.

Cutler-HammerMilwaukee, Wisconsin U.S.A.

z-learning 101 - basics of motors and motor controls

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