3: Alternating current electric motors

6



Motors: magnetic

fields make the

world go around

AC motors have the same basic components as DC motors but lack the

need for others. They all contain a rotor and stator. Their magnetic fields can be

generated by current-carrying coils. They utilise the motor effect to transform

electrical potential energy into rotational kinetic energy. Let’s explore examples

of the AC induction motor that exist both in industry and in the home.



Three-phase AC induction motors

PRACTICAL

EXPERIENCES

Activity 6.3



Activity Manual, Page

51



The simplest induction motor is the three-phase AC induction motor

(Figure 6.3.3). These motors are used in industry for their efficiency and

In Figure 6.3.5a,

reliability. Three-phase AC is fed to the coils in the stator.

each pair of magnetic poles in the stator is fed one phase of the AC signal.

The peak current of each phase is reached sequentially around the stator

(Figure 6.3.5b), creating a magnetic field (the stator field) that rotates.



electromagnets



rotor



Figure 6.3.3 In this cutaway image of a three-phase AC induction

motor, you can see the stator, consisting of electromagnets

arranged to form a hollow cylinder. Within the stator sits

the rotor, which is mounted on the motor’s shaft.



Universal motors



T



he most common type of motor in appliances

around the house is the universal motor. This

type of motor is capable of using both AC and

DC. Figure 6.3.4 shows that coils act as

electromagnets in the stator of these motors.

These provide stronger fields and are lighter

than permanent magnets. The coils are

connected in series with the rotor coils and use

a single-phase alternating current. These motors

are found in many household appliances in

which a variable speed is required, such as drills

and blenders.



armature coils



carbon brushes

shaft



stator coils

segmented

commutator



spring-loaded

brush holders



Figure 6.3.4 A typical universal electric motor, showing the main components.

Some motors would have additional stator coils. The commutator

feeds current to the armature coils in the position where most

torque will be experienced.



122



motors and

generators

a



b



conducting bars



Current



+

0



Time



–

i

rotor



ii



1



iii

2



end ring



3



Figure 6.3.5 (a) In a three-phase motor, as the current in each pair of opposite



1



2



coils peaks, the field appears to rotate, dragging the rotor around

with it. (b) A ‘squirrel cage’ rotor. The rotor is made of iron

laminations to cut down undesirable eddy currents. The induced

currents flow lengthwise in copper or aluminium rods which are

joined at the ends (as in a squirrel cage).



3



stator pole



These motors contain a ‘squirrel cage’ rotor (Figure 6.3.5b) that does not

Around the circumference of these

require the input of an external current.

rotors are a number of parallel conducting bars. These bars are joined at the ends

by an end ring that allows current to flow from one bar to another. The rotating

stator field induces a current in these bars and a magnetic field is induced in

accordance with Lenz’s law. This induced magnetic field interacts with the rotating

field from the stator and the resulting forces cause a torque on the rotor. This

causes the rotor to spin without the need for a commutator as in a DC motor.

As these motors do not need brushes and therefore have fewer moving parts, they

are more efficient and more reliable.

Another way to understand the operation of an AC induction motor is to

consider a positive particle within one of the conducting bars in the stator.

Let’s consider the bar marked A in Figure 6.3.6a. As the rotating magnetic field

moves upwards past bar A this is equivalent to the bar moving downwards in a

stationary magnetic field. Figure 6.3.6b shows this equivalent situation in which

bar A moves relative to the magnetic field. Using the right-hand palm rule

(see section 4.2) we see that a positive particle in bar A would experience a force

into the page. This is equivalent to a current being induced into the page in bar A.

Now we must use the right-hand palm rule (see section 4.3) to deduce the direction

of the motor effect on a current-carrying conductor. This indicates that a force is

exerted upwards on bar A and this is in the same direction as the rotating field in

Figure 6.3.6a. This force on bar A is the same as the force experienced by each bar

as the magnetic field rotates. These forces, in the same direction as the rotating

stator field, exert a torque on the rotor and are responsible for its rotation.

a



stator



b



2



force on bar A due

to motor effect



B

‘squirrel cage’

conducting bars



A



rotating

magnetic

field



B

I

motion of bar A

relative to the

magnetic field



2



Figure 6.3.6 (a) The magnetic field due to

one phase of an AC inductor

motor. (b) The force acting on

squirrel cage rotor bar A in

part (a) due to the rotating

magnetic fields

123



6



Motors: magnetic

fields make the

world go around



Single phase AC induction motors



Try this!

Induction exerts forces

Suspend a small piece of

a lightweight conductor (e.g.

aluminium foil) from fishing line.

Use your right-hand palm rule to

predict the force acting on the foil

as you move a strong magnet

vertically past the foil. Can you

observe any movement? If not,

use the scientific method to

discover why it’s not working

and try again.



Describe the main features of

an AC electric motor.

Gather, process and analyse

information to identify some of

the energy transfers and

transformations involving the

conversion of electrical energy

into more useful forms in the

home and industry.



A more complicated, but widely used, AC induction motor is the ‘shaded-pole’

AC induction motor shown in Figure 6.3.7. These motors require only a single

phase of AC and can be found in most household electric fans. In these motors

the alternating current is passed through a coil wrapped around the soft iron

casing. The stator field induced by the alternating current passes through the

casing and through the squirrel-cage rotor. Figure 6.3.7a shows the rotor

removed and leaning on the motor. A cross-section of a squirrel cage rotor is also

shown (Figure 6.3.7b) and clearly shows the conducting bars within the rotor.

Four small copper shading rings can be seen within the stator in Figure

6.3.7a. These are inserted into the stator on each side of the rotor on opposite

poles. The currents induced in these shading rings in accordance with Lenz’s law

act to delay the magnetic flux passing through the rotor. This produces an

asymmetric magnetic field passing through the rotor shown in Figure 6.3.7c.

This leads to a changing magnetic field in each cycle of the alternating current

that sweeps across each pole of the stator. The sweeping change in magnetic field

strength across the rotor is essentially the same as the rotating magnetic field we

studied in the three-phase AC induction motor. This rotating magnetic field

causes a torque on the rotor in the same way as outlined for the three-phase

induction motor.

b



a



conducting bar



squirrel cage rotor

shading rings



c



rotor

shading rings delay

the phase of part of

the field to produce

a rotating field



thick, copper

shading ring



Figure 6.3.7 (a) A shaded-pole AC induction motor taken from a small household fan, with

(b) a cross-section of the rotor. The conducting bars can be seen clearly within

the laminated rotor. (c) The principle of a simple single-phase, ‘shaded pole’

induction motor. The distorted (or ‘shaded’) field causes the rotor to turn in

one direction in preference to the other.



124



motors and

generators



The right motor for the job

Now that we have seen a few examples of some common electric motors, let’s

consider for a moment why they are chosen for their common applications.

The initial price and operating cost are key factors in making a decision.

Operating costs depend on factors such as energy efficiency and replacement of

parts due to wear and tear. The amount of torque required and how often a

motor will be put under load are also critical factors. These will determine how

much current is needed, the strength of the magnetic field required and

numerous other parameters. Of course there are other constraints including

portability, size and weight to consider. All these factors lead to a lot of

homework for an engineer who is trying to design a machine or appliance

driven by a motor.

A summary of the characteristics of the different types of motors discussed

in this chapter is provided in Table 6.3.1. Can you see why they are used in

their common applications?

Table 6.3.1  Characteristics of motors

Type



Advantages



Disadvantages



Common applications



Simple (brushed) DC

motor



Efficiency (%)

40–90



Low cost, battery powered,

speed easily controlled



Toys, power tools, treadmill

exercisers, automotive

starters



Brushless DC motor



30–90



AC universal motor



40–60



Three-phase AC

induction motor



70–90



Single-phase (shadedpole) AC induction motor



20–35



Long working life, low

maintenance, high efficiency

High starting torque,

compact design, high

running speeds

High starting torque, high

power, high efficiency, good

power to weight ratio

Inexpensive, long working

life, high power, multi-speed



Short working life, high

maintenance (brushes),

sparking and ozone

production

High cost of some designs,

requires a controller

Less efficient than

equivalent DC motor

Requires three-phase power



Inefficient, low starting

torque



CD and DVD players,

computer hard drives

Blenders, vacuum cleaners,

hair dryers, portable power

tools, sewing machines

Industrial machinery, pumps

and compressors

Fans



Checkpoint 6.3

1

2

3



Compare the features of an AC induction motor and a simple DC motor.

Construct a flow chart to account for the operation of a three-phase AC induction motor.

Justify the choice of a three-phase AC induction motor for use in industrial machinery.



125



6



Motors: magnetic

fields make the

world go around



PRACTICAL EXPERIENCES

CHAPTER 6



This is a starting point to get you thinking about the mandatory practical

experiences outlined in the syllabus. For detailed instructions and advice, use

in2 Physics @ HSC Activity Manual.



Activity 6.1: Applications of the motor effect

Identify data sources, gather

and process information to

qualitatively describe the

application of the motor

effect in:

• the galvanometer

• the loudspeaker.



Discussion questions

1 Outline how the motor effect is used to make music in a loudspeaker.

2 Explain how a loudspeaker differs from a motor in its use of the

motor effect.

3 Determine the difference between the way in which the motor effect

is used in a loudspeaker and in a galvanometer.



armature



N



S



Part A: Make a loudspeaker and determine how the motor effect is used to make

it work.

Equipment: two horseshoe magnets, strong sticky tape, thin insulated wire,

cardboard, power supply, alligator clips and wires.

Part B: Make a working galvanometer and determine the differences in this

application of the motor effect.

Equipment: PVC-covered copper wire (150 cm) with bare ends, wooden base

board, armature block, magnets, split pins, knitting needle, rivets, wire

strippers, drinking straw, rheostat (10–15 ohms, rated at 5 A or more).



brush



Activity 6.2: Motors and torque

commutator



Figure 6.4.1 A simple motor



Solve problems and analyse

information about simple

motors using: 

τ = nBIA cos θ



Make a motor like the one shown and note what factors change its performance.

Calculate the torque of your motor.

Equipment: insulated wire, magnets, magnetic field sensor and data logger

(if available), paperclips, Blu-Tack, connecting wires with alligator clips,

power supply.

Discussion questions

1 Investigate the factors that determine the effectiveness of the motor.

2 Calculate the amount of torque in your motor and list ways in which

torque can be increased.



Activity 6.3: AC induction motors

Perform an investigation to

demonstrate the principle of

an AC induction motor.



Using the equipment supplied, make a model of an AC induction motor and relate

each part to the parts in a real AC motor.

Equipment: aluminium foil, fishing line, retort stand and clamp, ceramic

magnet.

Discussion questions

1 Outline how the metal is made to move.

2 Explain why the AC induction motor is so efficient.



126



Chapter summary

•



•



•



•



•



The main features of a DC motor are the rotor (coils,

shaft and frame), stator (permanent or electromagnets),

commutator split ring and commutator brushes.

The roles of these components are summarised in

Table 6.1.1.

Torque τ is the turning effect of a force F. τ = Fd where

d is the distance from the pivot to the point where the

force is applied.

A current-carrying loop in an external magnetic field

experiences forces due to the motor effect that generate

a torque.

The torque τ on a current-carrying coil can be

quantified by the relation τ = nBIA cos θ where n is the

number of turns in the coil, B is the strength of the

magnetic field, I is the strength of the current in the

coil, A is the area of the coil and θ is the angle between

the plane of the coil and the magnetic field lines.

The torque on a coil in an external magnetic field varies

as the coil rotates. The torque is at a maximum when

the plane of the coil is parallel to the magnetic field



•



•



•



•



motors and

generators



and zero when the coil is perpendicular to the

magnetic field.

The current-carrying coil of a galvanometer experiences

a torque due to the motor effect. This torque is balanced

by a spring and this causes the needle to deflect by an

amount proportional to the current flowing. This allows

the determination of the magnitude and direction of the

current being measured.

When a DC motor rotates, its coils experience an

induced emf (back emf ) set up in accordance with

Lenz’s law. This back emf opposes supply emf and

reduces the current flowing through the motor’s coils.

AC induction motors contain a squirrel-cage rotor

(conducting bars, shaft and frame) and stator

electromagnets.

AC induction motors generate a rotating magnetic field

that induces currents in the squirrel-cage rotor. These

current-carrying conductors in the rotor experience

a force due to the motor effect, which exerts a torque

on the rotor.



Review questions

Physically Speaking



Reviewing



The terms in the following list belong to two distinct groups.

Group these terms into the two groups and add the definition

of each term. Create a diagram to display the relationship

between them.



1 a Identify the type of motor in Figure 6.4.2.



•

•

•

•



Split-ring commutator

Squirrel cage

Brushes

Stator



•

•

•

•



b Identify each of the labelled features.

c Construct a table to list the parts you have

identified and the role each plays.



2 Explain why radial (curved) magnets in a motor allow



Fan

Bearings

Armature

Magnets



for greatest efficiency.



3 Describe the differences and similarities in the way

permanent and current-carrying coils produce

magnetic fields within a DC motor.



D

E



4 Define a galvanometer and outline what it is used for.

5 A galvanometer has a spring attached to the centre

of it, distinguishing it from a simple DC motor. Give

reasons for its presence.



6 Recall Lenz’s law and explain how Lenz’s law accounts

for the conservation of energy.



A



Figure 6.4.2



B



C



7 Describe what is meant by back emf.

8 Determine how back emf is produced in a motor.

9 Explain what a manufacturer does to a motor to

account for back emf.



127



6



Motors: magnetic

fields make the

world go around



10 a Label the parts of the AC motor in Figure 6.4.3.

b Explain what you could do to this motor to make it

into a DC motor.

B



C



18 Students undertook to measure the torque produced

by a simple DC motor.



The motor contained 100 turns and the square

armature was 0.03  m in length. The motor is attached

to a piece of string holding a mass (Figure 6.4.4).



D

A



motor



E



motor shaft



r

edge of table

string



F



mass



Figure 6.4.3



11

12

13

14



Figure 6.4.4



Give examples of where an AC motor would be used.

Compare and contrast DC and AC motors.



The motor was turned on and allowed to wind up the

mass until it stalled and stopped. At this point the

radius of the windings r was recorded. The current

supplied to the motor was gradually increased and

the process repeated.



Explain how an AC induction motor works.

Explain how a single-phase induction motor gets

started.



15 Distinguish between situations in which AC universal

motors and AC induction motors would be best

suited.



The table of results is shown below.





solving Problems

16 Calculate the maximum torque that is generated by a

force of 460 N applied to an object at a distance of

3 m from its axis of rotation.



17 Calculate the torque in a square coil with sides of

length 3 cm. The current in the coil is 2 A and it is

placed in a magnetic field of 0.3 T.







Re



128



Q uesti o



n



s



v



a

b

c

d



iew



τ = F × d = mg × d



Mass = 0.5 kg



Torque (Nm)



Current a



Radius (m)



0.27



0.1



0.055



0.54



0.2



0.11



0.81



0.3



0.165



1.08



0.4



0.22



1.35



0.5



0.276



1.62



0.6



0.331



Draw a graph of torque versus current.

Determine the gradient of the line.

What quantity does this value represent?

From this value, determine the magnetic field

in which the armature is spinning.



motors and

generators



PHYSICS FOCUS

Linear motors



F



igure 6.4.5 is of a maglev train—a train that floats

on its rail and moves at very high speeds. You will

study the floating mechanism in detail in Module 3

‘From Ideas to Implementation’, but let’s have a look

at how the train actually moves forward.

The maglev train operates solely on electric power.

The propulsion method is via an electric motor—but

one with a difference—it is a linear motor. This is, in

principle, an electric motor that has been unwrapped

and flattened. Magnetic fields on the train and rail

are continually created to attract and repel each other.

It is these interactions that apply the forces to propel

the train forward.

Linear motors produce motion by moving in a

straight line rather than the traditional rotational

motion. There are two main parts: the stator unrolled

(the primary) and laid flat on the rail, and the

secondary, which is the glider on the train that floats

over the rail.



3. Applications and uses of physics



Why use this type of motor? Reasons include:

• It has no moving parts so there is no wear and tear.

• The train rides on an air cushion, so less energy is

lost due to friction.

• Electromagnets are used for braking, so the train

is a lot quieter.

1 Outline how a DC motor works.

2 Outline how an AC induction motor works.

3 Compare the uses for an AC induction motor with

that of an AC universal motor.

4 Draw a diagram of a DC motor and explain what

can be done to it to make it an AC motor.

5 Determine how torque is calculated in a motor.

6 Justify the use of linear motors in such applications

as the maglev train.

7 Evaluate the cost associated with maglev trains and

standard trains.



Research

8 Find out exactly how a linear motor works.

9 Compare the torque produced in a standard motor

with the linear force in a linear motor.



Figure 6.4.5 A linear motor propels this maglev train.



129



7



Generators and

electricity supply:

power for the

people

Technology that changed our lives



generator, transformer, step-down

transformer, step-up transformer,

flux leakage, magnetic hysteresis,

power stations, substation,

transmission towers, insulators,

lightning protector



Widespread access to affordable electricity has arguably been the

single greatest catalyst for change in modern society. It has had an

impact on every aspect of our lives, from our health to our wealth and

even what we do in our leisure time. Some key developments that

enabled this revolution were the invention of the AC generator

and the transformer. Their significance is far beyond

the reaches of the power lines they service and, as

we shall see, they involve more than meets the eye.



7.1 AC and DC generators



Permanent magnets

or electromagnets

provide an external

magnetic field.



In chapter 6, we saw that the real value of electric motors was that they convert

electrical potential energy into rotational kinetic or mechanical energy. A logical

question that follows is ‘how do we get the electrical potential energy that turns

these motors?’. For many of our household applications, the answer is ‘we use

a generator’. When you switch on an electrical appliance at home, you may not

realise where the energy comes from. In some distant power plant there is a huge

rotating machine as big as a building that looks like a huge electric motor. The

difference is that this generator is producing electricity instead of using it.

The simplest AC motor design is the synchronous AC

motor (Figure 7.1.1). If this motor was supplied with 50 Hz

AC from the power point, it would spin at 50 revolutions

per second in synchronisation with the electrical signal. This

S

design has no practical application, but more complicated

designs are used in clocks and tape drives, due to their

constant speed.



N



AC supply

Slip-ring commutator continuously

connects the rotating coil to the AC supply.



130



Figure 7.1.1 A synchronous motor uses a slip-ring commutator

to feed AC to the motor.



motors and

generators

Although it lacks applications as a motor, this design would produce an

electric current if you turned the coil with your hands. In this way it is acting as

When we turn the motor ourselves, its coil experiences a

a generator.

changing magnetic field and an emf is induced (recall Faraday’s law). If the loop

is connected by a circuit, a current flows, and we have turned the motor into

a generator.

The parts of a generator are essentially the same as those of a motor (see

Table 7.1.1). The difference is that we physically turn a generator and it produces

electrical potential energy. So the operation of a generator is the opposite of that

of a motor. In this section we will only consider simple generators; we will see a

more complicated version in section 7.3.



Describe the main components

of a generator.



Table 7.1.1  The parts of a simple generator and their function

Part



Description and role



Armature



The armature is the part of a generator that contains current-carrying coils. These carry an induced

current caused by a changing magnetic field. For simple generators these are the coils in the rotor, but

in other generators the armature is in the stator.

These are the many loops of wire that carry electrical current. In many generators there are two sets of

coils. One set is the electromagnets that provide the magnetic field (in simple generators this field is

provided by permanent magnets). The other set is in the armature and these electromagnets carry the

current produced by the generator.

The rotor generally consists of coils of wire wound around a laminated iron frame. The frame is attached

to an axle or shaft that allows it to rotate. The iron frame is laminated to reduce heating and losses due

to eddy currents. The iron itself acts to intensify the magnetic field passing through the coils.

In simple generators the stator is the stationary permanent magnets or electromagnets that provide an

external magnetic field around the rotor. These magnets are curved to maximise the amount of time the

sides of the rotor coil are travelling perpendicular to the magnetic field. In some generators the stator

contains the armature coils and a magnet turns as part of the rotor to produce a changing magnetic field.

A simple DC generator contains semicircular metal contacts (a split ring) that reverse the direction of the

current flowing out of the rotor coil every half rotation. This reversal of the current ensures that the

current being produced is DC.

A simple AC generator has two circular metal contacts. Each slip ring is connected to one end of the

coils in the rotor. These provide an alternating current that changes direction every half rotation. In more

complicated examples these provide a current to an electromagnet in the rotor and a current is produced

in the stator.

Brushes are conducting contacts (generally of graphite or metal) that connect the commutator to the

external circuit.



Coils



Rotor



Stator



Commutator

split ring

Commutator

slip ring



Commutator

brushes



Figure 7.1.2a shows a typical hand-operated generator found in schools. This

generator has the features of both an AC and a DC generator. With the flick of

a switch you can connect to the components for either type of generator and

produce a current by winding the handle. If you have access to one of these make

sure you try it out and try to identify all its parts.



A simple AC generator



PRACTICAL

EXPERIENCES

Activity 7.1



Activity Manual, Page

54



Figure 7.1.3a shows a simple model of an AC generator. This generator

therefore contains a slip-ring commutator that simply connects each end of the

rotating coil to an external circuit. The rotor of the generator shown is being

turned by hand. Let’s follow a full rotation of this generator as shown in Figure

7.1.3b and use the graph (Figure 7.1.3c) to gain an understanding of its

operation. Figure 7.1.3c shows the amount of flux ΦB passing within the coil

WXYZ. It also shows the induced emf ε caused by the rate of change of flux

∆ΦB/∆t in accordance with Faraday’s law (see section 5.1). This emf would result

in a current flowing within the coil, if the output terminals were connected.

131