101 basic series - inw- · pdf file101 basic series learning module 16: ... practical dc motor...

Learning Module 16:

Fundamentals of Motors and Motor Control
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What You Will Learn We’ll step through each of these topics in detail:

Motor Theory 5Magnetic Fields 5Current Flow 5Induced Motion 5Commutator 7

DC Motors 8Simple Motors 8Practical DC Motor 9Electromagnets 10Motor Components 10Reversing a DC Motor 11DC Motor Types 11

Review 1 12

AC Motors 13What Makes an AC Motor Different From a DC Motor? 13Single-Phase 13Three-Phase 13

The Squirrel Cage Induction Motor 14Induction Principle 14Applying the Induction Principle to the AC Motor 14Three-Phase Motor 15Construction of Three-Phase Motors 16Wye and Delta 17Dual Voltage 17

Review 2 18

Speed Control 19Force, Work and Torque 19Power and Horsepower 19Putting It All Together 20Application Types 21Speed Control for a DC Motor 22Speed Control for an AC Motor 23

Starting the Motor 25Across the Line 25Minimizing Inrush Current 25

Reversing the Motor 26

Braking the Motor 26DC Injection Braking 26Dynamic Braking 27

Review 3 29

Glossary 30


Review 1 Answers 33

Review 2 Answers 33

Review 3 Answers 33

Appendix A: Typical Multispeed Motor Connections 34

Reference 35


Welcome Welcome to Module 16, which is about the fundamentals of motors and motor control. An electric motor is a machine that converts electrical energy to mechan-ical energy. There are two main groups of electrical motors: DC and AC motors. This module will discuss 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 sec-tions of new material followed by a series of questions about that material. Study the material carefully then answer the questions without referring back to what you’ve just read. You are the best judge of how well you grasp the material. Review the material as often as you think necessary. The most important thing is establishing a solid foundation to build on as you move from topic to topic and module to module.

A Note on Font Styles Key points are in bold.

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

Viewing the Glossary Printed versions have the glossary at the end of the module. You may also browse the Glossary by clicking on the Glossary bookmark in the left-hand margin.


Motor Theory To understand motor theory, we need to cover the underlying principles of mag-netic fields, current flow, and induced motion.

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

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

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

Figure 2. Lines of Magnetic Flux Flow from North Pole to South Pole

Current Flow Now, let’s consider a wire (conductor) with an electric current flowing through it. A magnetic 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 is critical to understanding motor motion. The direction of the magnetic flux can be determined using the Left Hand Flux Rule.

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

In Figure 3, the current is flowing into the page, so the lines of flux rotate counter-clockwise around the wire.

Induced Motion 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 because the current is flowing into the page.

The force exerted upward depends on the strength of the magnetic field between the 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), the middle finger points in the direction of current flow through the conductor, and the thumb 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 the ori-entation the poles, you can determine which way the conductor will move through the magnetic field.

Applying the right hand motor rule to Figure 4, the conductor will move upward through 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 is required to bring about motion because no force is felt by a conductor if the cur-rent and the field direction are parallel.

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 does the coil want to move in a counterclockwise motion?

Recall that the magnetic flux rotates around the conductors. Armature sections AB and CD have the current flowing in opposite directions. This means the mag-netic flux flows around them in opposite directions, as shown in Figure 6.


Figure 6. Magnetic Flux Around the Armature Sections

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

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

Figure 7. Creating Torque: A Cross Section of the induced motion

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

Commutator As we mentioned earlier, when the armature is positioned so that the loop sides are at right angles to the magnetic field, a turning force is exerted. But what hap-pens when the coil rotates 180°?

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

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

A device called a Commutator performs this task. Two stationary Brushes, one supplied with positive DC current, the other with negative DC current, supply cur-rent to the two rotating commutator segments.

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


Figure 8. The Commutator Reverses the Current Through the Armature

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

DC MotorsSimple Motors What we have just described is a DC motor. Direct current is fed to the commuta-

tor. The commutator is connected to the armature in such a way that the current direction (called Polarity) is switched every half-turn of the armature. This allows the armature to continue turning in the magnetic field, creating mechanical energy from electrical energy.

However, this simple DC motor has a few shortcomings. Each time the armature is parallel to the magnetic field (called a Neutral Position), no torque is pro-duced. (Refer back to Figure 8.)

Recall that when the armature is positioned so that the loop sides are at right angles to the magnetic field, torque is exerted. But, as the armature turns in a cir-cle, there are two points at which it is parallel to the magnetic field – at ¼ and ¾ of a turn – 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 of the motor varies because of the changes in torque. Most devices require a motor to turn at a uniform speed, so the simple DC motor just described would not be suitable.

Figure 9. Simple DC Motor Torque and Speed Graph

Position “A” – Torque Position “B” – Neutral

Position “C” – Torque Position “D” – Neutral


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

Practical DC Motor In a practical DC motor, the armature is never in a neutral position, and the torque is always at its maximum. This is accomplished by using an armature with more 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 full torque 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 commutator segments. This means that contact with the commutator is maintained at every instant of rotation of the armature.

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

In the WorkplaceThis is a common cordless drill that might be used by a building maintenance per-son. It is run on a battery and uses a DC motor.

Cordless Drill Using a DC Motor

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


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

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

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 the electromagnet can be controlled

• By changing the direction of current flow, the poles of the electromagnetic can be reversed. In Figure 11, switching the leads on the battery terminals would change the direction current flow

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

Motor Components We have already discussed three of the four major components that make up a DC motor: 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 larger the poles, the stronger the field.

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


Reversing a DC Motor The direction of rotation of a DC motor may be reversed using one of these meth-ods:

• 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 is accomplished by reversing the armature connections only.

DC Motor Types There are basically three DC motor types: The Series Motor, the Shunt Motor, and the Compound Motor. Internally and externally they are practically the same. The difference between them is the way the field coil and armature coil circuits are wired.

The series motor (Figure 13.) has the field coil wired in series with the arma-ture. It is also called a universal motor because it can be used in DC or AC appli-cations. It has a high starting torque and a variable speed characteristic. The motor can start heavy 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 in par-allel, 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 the series and the shunt motors. A compound motor has high starting torque and fairly good speed torque characteristics at rated load. Because complicated cir-cuits are needed to control the compound motors, this wiring arrangement is usu-ally only used on large bi-directional motors.

Figure 15. DC Compound Motor: Schematic and Wiring Diagram


Review 1 Answer the following questions without referring to the material just presented. Begin the next section when you are confident that you understand what you’ve already 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:_________________________________________________________________________________


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

What Makes an AC Motor Different From a DC Motor?

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

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

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

Single-Phase A single-phase power system has one coil in the generator. Therefore, one alternating voltage is generated. The voltage curve of a single-phase AC genera-tor is 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 or below. (These are sometimes called fractional horsepower motors.) They are generally used to operate mechanical devices and machines requiring a relatively small amount of power.

Types of single-phase motors include: shaded-pole, capacitor, split-phase, repul-sion, 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 type func-tions.

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

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

Figure 17. Voltage Curve of a Three-Phase AC Generator

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

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


The Squirrel Cage Induction Motor

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

Induction Principle The principle of the induction motor was first discovered by Arago in 1824. He observed that if a non-magnetic metal disk and a compass are pivoted with their axes parallel, so that one (or both) of the compass poles are located near the edge of the disk, spinning the disk will cause the compass needle to rotate. The direction of the induced rotation in the compass is always the same as that imparted to the disk.

You can prove it to yourself if you like. Mount a simple copper or aluminum disk and a 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. This is a very effective way to demonstrate the principle of induction.

Figure 18. Demonstrating the Principle of Induction

Applying the Induction Principle to the AC Motor

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 running through the stator (the stationary part of the motor). The stator plays the role of the metallic disk described above. A rotating magnetic field is established in the stator.

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

The induction motor uses a rotor of a special design. It resembles a cage used for exercising squirrels, which is why it is called a squirrel cage rotor.

The rotor consists of circular end rings joined together with metal bars. Note that the metal bars are placed directly opposite each other and provide a com-plete circuit within the rotor, regardless of the rotor's position. Rotors normally have several 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 because of their simplicity, ruggedness and reliability. Because of these fea-tures, squirrel-cage motors have practically become the accepted standard for AC, all-purpose, constant speed motor applications. Without a doubt, the squirrel-cage motor 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, the two bearings

• Because it has no commutator, there are no brushes to wear, this keeps main-tenance minimal

• No sparks are generated to create a possible fire hazard

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

Three-phase power can be thought of as three different single-phase power supplies. They are called A, B, and C. In the three-phase motor, each phase of the power supply is provided with its own set of poles located directly across from each other 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 A and phase C starts 120° later than phase B. This is shown on the sine wave graph in Figure 21, which indicates the way the magnetic field will point at various times in the cycle.


Figure 21. Magnetic Field Rotation Providing Torque to Turn the Motor

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

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

If each magnetic pole were to “light up” whenever it was energized, the effect would appear as though the lights were “running” around the stator, much as the lights on some 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 are magnetized by phase B. The rotor turns, due to the induced current. Then, the C poles are magnetized by phase C. The rotor turns, due to the induced current. The rotor 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 the opposite direction. This causes the magnetic field to continue to rotate, and the rotor follows. Then, the B poles are magnetized by phase B. The rotor turns, due to the 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 repeats itself.

Construction of Three-Phase Motors

The three-phase motor is probably the simplest and most rugged of all elec-tric motors. To get a perspective on how important the three-phase motor is, all you need to know is that this motor is used in nine out of ten industrial applica-tions.

All three-phase motors are constructed with a number of individually wound elec-trical coils. Regardless of how many individual coils there are in a three-phase motor, the individual coils will always be wired together (series or


parallel) to produce three distinct windings, which are called phases. Each phase will always contain one-third of the total number of individual coils. As we mentioned, these phases are referred to as phase A, phase B and phase C.

Three-phase motors vary from fractional horsepower size to several thousand horsepower. These motors have a fairly constant speed characteristic but a wide variety of torque characteristics. They are made for practically every standard voltage and frequency and are very often Dual Voltage Motors.

Wye and Delta 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 is connected to the other phases internally. The remaining end of each phase is then brought out externally and connected to the power line. The external leads are labeled T1, T2 and T3, and are connected to the three-phase power lines labeled L1, L2 and L3, respectively.

Figure 24. Wye Configuration

In a Delta (∆) configuration (Figure 25.), each winding is wired end to end to form a completely closed loop circuit. At each of the three points where the phases are connected, a lead is brought out externally. They are labeled T1, T2 and 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 supplying power to the motor must have the same voltage and frequency ratings as the motor.

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

When the electrician has the choice of deciding which voltage to use, the higher voltage is preferred. The motor will use the same amount of power, giv-ing the same HP output for either high or low voltage, but as the voltage is dou-bled (230 to 460), the current will be cut in half. With half the current, the wire size can be reduced and a cost savings can be realized on installation.


Review 2 Answer the following questions without referring to the material just presented. Begin the next section when you are confident that you understand what you’ve already 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.


Speed Control Speed control is essential in many applications. Mining equipment, printing presses, cranes, hoists, elevators, and conveyors are just a few examples of the many applications that depend on speed control.

In choosing the speed control method for an application, there are three main factors 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. How-ever there are two fundamental motor types: AC and DC. Each type has its own ability to control different loads at different speeds.

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

Force, Work and Torque Work is done when a force overcomes a resistance. Work is measured with the formula:

Work = Distance x Force

If you push a 10-pound bag of sand 50 feet, 500 foot-pounds (ft.-lb.) of work is done. (Please note that the force has to be exerted in the same direction as the work.)

In the case of an electric motor, force is not exerted in a line, but in a circle, about a cylindrical 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, 500 foot-pounds (ft.-lb.) of torque is applied to the shaft.

Figure 26. Torque = Radial Distance X Force

Power and Horsepower Power takes into consideration how fast work is accomplished. Power is the rate of doing work. The formula to determine power is:

Power = Work/Time

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


The reason for this difference is the amount of work that can be delivered in a given amount of time. Obviously, a larger motor should be able to deliver more work in a given time than one that is considerably smaller. It is this difference that determines the 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 is equal to 746 watts of electrical power.) Let’s figure horsepower for a motor to move that sand. Recall that:

Work = Distance x Force

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

Power = Work/Time

Power = 500 ft.-lb. / .25 minutes

Power = 2000 ft.-lb. per minute

And because 33,000 ft.-lb. per minute equals 1 HP, (2000 / 33,000) the motor has about 0.06 horsepower.

Putting It All Together Torque, horsepower and speed are all interrelated when turning a load. Horse-power is proportional to torque and speed. The following formula ties them all together:

HP = (T x N) / 5252

Where:

HP = the horsepower provided by the motor

T = the torque of the motor in foot-pounds

N = the synchronous speed of the motor in revolutions per minute (RPM)

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


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

Figure 27. Horsepower, Torque and Speed Relationship

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

There are three main application types:

• Constant Torque / Variable Horsepower

• Constant Horsepower / Variable Torque

• Variable Torque / Variable Horsepower

Let’s consider each briefly.

Constant Torque / Variable HorsepowerThis type of load is often found on machines that have friction-type loads, such as conveyors, gear-type pumps, and load lifting equipment.

The horsepower required increases when the speed increases. The torque requirement does not vary throughout the speed range except for the extra start-ing torque needed to overcome the breakaway friction. The torque remains con-stant because the force of the load does not change.

Constant Horsepower / Variable TorqueThis type of load is used for loads that demand high torque at low speeds and low torque at high speeds. Examples of these loads are machines that roll and unroll paper or metal.

Because the linear speed of the material is constant, the horsepower must also be constant. While the speed of the material is kept constant, the motor speed is not. At start, the motor must run at high speed to maintain the correct material speed while torque is kept at a minimum. As material is added to the roll, the motor must deliver more torque at a slower speed. In this application, both torque and speed are constantly changing while motor horsepower remains the same.

Speed Increases Horsepower IncreasesTorque Constant

Speed Decreases Horsepower DecreasesTorque Constant

Speed Constant Horsepower IncreasesTorque Increases

Speed Constant Horsepower DecreasesTorque Decreases

Speed Increases Horsepower IncreasesTorque Decreases

Speed Decreases Horsepower DecreasesTorque Increases


Variable Torque / Variable HorsepowerThis type of load is used for loads that have a varying torque and horsepower at different speeds. Typical applications are fans, blowers, centrifugal pumps, mixers and agitators.

As the motor speed is increased, so is the load output. Because the motor must work harder to deliver more output at faster speeds, both torque and horsepower are increased.

Speed Control for a DC Motor

Now that you understand what factors are important in choosing a motor for an application, 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 line voltage applied to the armature and the field.

The speed of a DC motor is controlled by varying the applied voltage across the armature, the field, or both. When armature voltage is controlled, the motor will deliver a constant torque characteristic. When field voltage is controlled, the motor will 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 speed control, high torque, or both. Because the speed of most DC motors can be controlled smoothly and easily from zero to full speed, DC motors are used in many acceleration and deceleration applications.

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

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


Speed Control for an AC Motor

Because 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 of the motor and the supply voltage used. These design types are classified and given a letter designation, which can be found on the nameplate of motor types listed as “NEMA Design.”

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

In the WorkplaceThe conveyor on this beer bottling line is powered by a NEMA Design B motor.

NEMA Design B Motor at Work

The NEMA Design B motor is a general purpose AC induction motor. It is the most commonly used NEMA Design motor because it offers a good balance of function against price.

The induction motor is basically a constant speed device. The speed at which an induction stator field rotates is called its Synchronous Speed. This is because it is synchronized to the frequency of the AC power at all times. The speed of the rotating field is always independent of load changes on the motor, provided the line frequency is constant.

Synchronous speed is determined by the number of poles in the motor and the frequency being supplied to it. The equation for determining the synchro-nous 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)

NEMA Design

Starting Torque

Starting Current

Breakdown Torque

Full Load Slip

Typical Applications

A Normal Normal High Low Machine ToolFan

Centrifugal PumpB Normal Low High Low Machine Tool

FanCentrifugal Pump

C High Low Normal Low Loaded CompressorLoaded Conveyor

D Very High Low — High Punch Press


P = the number of poles in the motor

Motors designed for 60 Hertz (standard in the United States) have synchronous speeds as follows:

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. The percent reduction in speed is called Percent Slip. The slip is required to develop rotational torque. The higher the torque, the greater the slip.

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

For example, a motor with a 2.8% slip and 1800 rpm synchronous speed would have a slip of 50 rpm, and a full load speed of 1750 rpm (1800 - 50 = 1750 rpm). It is 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 are the 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, if the voltage is changed by more than 10%, the motor may be damaged. This is because the starting torque varies as the square of the applied voltage.

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

• Changing the frequency applied to the motor

• Using a multispeed motor

• Changing the frequency applied to the motor

Changing the frequency requires a device called an Adjustable Frequency Drive to be inserted upstream from the motor. This device converts the incoming 60 Hz 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 run only half as fast.

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

• Using a multispeed motor

Poles RPM2 36004 18006 12008 900

10 72012 60014 51416 450


Multispeed AC motors are designed with windings that may be reconnected to form different numbers of poles. They are operated at a constant frequency.

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

Motors with more than two speeds usually include many windings. These can be connected many ways to provide different speeds. Refer to “Appendix A: Typical Multispeed Motor Connections” on page 34.

In the WorkplaceEveryone is probably familiar with this portable three-speed oscillating fan that can be found in most homes.

Three-Speed Oscillating Fan

The fan’s multispeed motor contains many windings that can be connected three different ways. This allows the user to set the fan to run at any of the three preset speeds.

Starting the Motor A Starter is a device that is used to start a motor from a stop. The across-the-line starter is by far the most common. This type of starter places the motor directly across the full voltage of the supply lines, hence the name, “across-the-line.” When an induction motor is placed across-the-line, it will accelerate to full speed in a matter of seconds.

Across-the-Line What applications are suitable for this type of rapid acceleration? Pumps of all types, fans and blowers, and most machines such as drill presses, lathes and grinders 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 auxil-iary starting equipment is necessary to limit the initial rush of current. The same practice applies to most small (and some large) polyphase motors.

Minimizing Inrush Current

During an AC motor’s start-up accelerating period, a large amount of current is required to start the motor rotating and bring it up to speed. This is called inrush current. Currents 6 to 8 times the full load rating of the motor are not uncom-mon when the motor is started across-the-line.

From this, we can see that the power company will be rather concerned because they have 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


reasonable value, about 1.25 to 5 times the full load rating. There are several ways of doing this:

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

• (AC) Using a Reduced Voltage Starter, which we will discuss in much more detail in Module 21, Reduced Voltage Starters.

• (AC) Using a wound rotor type of motor, which employs a resistor controller for the 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 at the 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 more detail in Module 20, Adjustable Frequency Drives.

Reversing the Motor To reverse a motor all we need to do is reverse the order in which the Line Power is fed to the motor. This wiring change is accomplished by “swapping” two of the phases of power. In short, A motor wired with phases ABC to run forward, would have its phases wired CBA to run in reverse.

In applications were 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

• A Magnetically Reversing Starter

Both methods involve two sets of contacts. One set of contacts is wired ABC. The second set has its contact phases wired CBA. The two contact blocks are mechanically interlocked so that only one set of contacts can be engaged at a given time.

Figure 29. Reversing the Motor

A Manual Reversing Starter uses two interlocked manual motor starters. The operator has to physically (manually) engage the starters to put the motor in for-ward or reverse. A Magnetically Reversing Starter uses interlocked electromag-netic starters. The motor can be reversed at the control panel (selector switches and pushbuttons) or remotely.

Braking the Motor Two common methods used for braking a motor are DC Injection Braking and Dynamic Braking. We will look at both in detail.

DC Injection Braking DC injection braking is a method of braking in which direct current (DC) is applied to the stationary windings of an AC motor after the AC voltage is removed. This is an efficient and effective method of braking most AC motors. DC injection braking provides 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. This principle, when applied to both AC and DC motors, is the reason why the motor shaft rotates.

In an AC induction motor, when the AC voltage is removed, the motor will coast to a standstill over a period of time because there is no induced field to keep it rotat-ing. Because the coasting time may be unacceptable, particularly in an emer-gency situation, 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, a magnetic 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 the rotor. Because the magnetic field of the stator is not changing in polarity, it will attempt to 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 the rotational inertia of the load connected to the motor shaft. However, because the braking action of the stator is present at all times, the motor is braked quickly and smoothly to a standstill.

Because there are no parts that come in physical contact during braking, mainte-nance is kept to a minimum.

Dynamic Braking Dynamic braking is another method for braking a motor. It is achieved by recon-necting a running motor to act as a generator immediately after it is turned off, rapidly stopping the motor. The generator action converts the mechanical energy of rotation to electrical energy that can be dissipated as heat in a resistor.

Dynamic braking of a DC motor may be needed because DC motors are often used for 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 as a generator. On a DC motor, access is accomplished through the brushes on the commutator.

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

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

The generated CEMF decreases as the speed of the motor decreases. As the motor speed approaches zero, the generated voltage also approaches zero. This


means that the braking action lessens as the speed of the motor decreases. As a result, 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 no more braking action.

For this reason, electromechanical friction brakes are sometimes used along with dynamic braking in applications that require the load to be held, or in applications where a large heavy load is to be stopped. This is similar to using a parachute to slow 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.


Review 3 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 1,000 pounds a distance of 330 feet in one minute.Answer: _________ HP

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

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 motor speed to INCREASE or DECREASE. Circle the correct answer.

6. Using the synchronous speed formula, calculate the full load speed of a motor with 8 poles running on 60 Hz with a slip of 2.2%.Answer: _________ RPM


GlossaryAdjustable Frequency Drive

A device that converts the incoming 60 Hz power into any desired frequency allowing an AC motor to run at virtually any speed.

Armature The turning conductor in a DC motor.Base Speed The speed at which a DC motor will run with full voltage

applied to the armature and the field.Brushes The stationary components of the commutator, providing

current to the rotating commutator segments.Coils The stationary windings of the DC motor that generate

an electromagnetic field.Commutator A device used in a DC motor to reverse the current in

the armature every one-half rotation so that the magnetic fields will work together to maintain rotation.

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

Conventional Flow Theory

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

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

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

Dual Voltage Motor A motor made for two voltages. It enables the same motor to be used with two different power line voltages.

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

Electron Flow Theory A theory regarding the flow of current which states that current flows from negative to positive.

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

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

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

Left Hand Flux Rule The relationship of the factors used to determine is which direction the magnetic flux moves around a conductor. Imagine grasping the wire with your left hand, making sure your thumb points in the direction of the current flow. Your fingers will curl around the wire in the direction of the magnetic flux.


Magnetic Flux The direction of a magnetic field.Magnetically Reversing Starter

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

Manual Reversing Starter

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

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

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

Polarity Direction of current flow through a conductor.Poles The stationary windings of the DC motor that generate

an electromagnetic field.Power A measure of work done per unit of time.Reduced Voltage Starter

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

Right Hand Motor Rule

The relationship between the factors involved in determining the movement of a conductor in a magnetic filed. The index finger points in the direction of the magnetic field (N to S), the middle finger points in the direction of electron current flow in the conductor, and the thumb points in 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 the

armature coil. It is also called a universal motor.Shunt Motor A DC motor with the field coil wired in parallel with the

armature coil.Starter A device that is used to start a motor from a stop.Stationary Windings The stationary windings of the DC motor that generate

an electromagnetic field.Stator The stationary part of an AC motor.Squirrel Cage Induction Motor

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

Synchronous Speed 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

of the three-phases is connected to the other phases internally. The remaining end of each phase is then brought out externally.


Review 1 Answers 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”, “1”. Blanks on the side of the graph, from top to bottom: “Torque”, “Speed”. (See Figure 9.)

4. Reversing the direction of the current through the field. Reversing the direc-tion of the current through the armature.

5. Series, shunt and compound

Review 2 Answers 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”. (See Figure 25.)

5. Delta

Review 3 Answers 1. Work = Distance x Force and Power = Work/Time

2. 10

3. Constant Torque / Variable Horsepower

4. Manual reversing starter; Magnetic reversing starter

5. Increase

6. About 880 RPM


Appendix A: Typical Multispeed Motor Connections

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


Reference The publication listed below assisted in preparing this training module:

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

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