Electrical Machines
Module 3: AC Machines
PREPARED BY
Academic Services Unit
April 2012
© Institute of Applied Technology, 2012
ATE1230: Electrical Machines
Module 3: AC Machines
Module Objectives:
Upon successful completion of this module, students should be able to:

Describe the basic construction and applications of AC
induction motors and discuss the various types and
applications of single phase induction motors.

Describe the basic construction and working principle of three
phase induction motors and calculate the synchronous speed.

Define the term slip and calculate the percentage slip

Discuss the working principles , construction , applications of
synchronous motors.

Describe the principle of magnetic induction as it applies to ac
generators and state the differences between the two basic
types of ac generators.

Build a basic dynamo using a DC motor.
Module Contents:
Topic
Page No.
3.1
3.2
3.3
Introduction to AC Machines
AC Induction Motors Fundamentals
Single-Phase Induction Motors
3.4
Three-Phase Induction Motors
14
3.5
3.6
3.7
Synchronous Motors
AC Generators
Lab Activity 1
15
16
20
3.8
Review Exercise
21
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3
4
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3.1 Introduction to AC Machines
Alternating Current (AC) is the world standard for driving motors and
other electrical equipment. Nowadays, apart from lighting devices,
electric motors represent the largest loads in industry and commercial
installations. Their function, to convert electrical energy into mechanical
energy, means they are particularly significant in economic terms, and
hence, they cannot be ignored by installation or machinery designers,
installers or users.
There are many types of motor in existence, but 3-phase asynchronous
or induction motors, and in particular squirrel cage motors, are the
most
commonly
used
in
industry
and
in
commercial
buildings
applications above a certain power level. Moreover, although they are
ideal for many applications when controlled by contactor devices, the
increasing use of electronic equipment is widening their field of
application. Simple and rugged design, low-cost, low maintenance and
direct connection to an AC power source are the main advantages of AC
induction motors
On the other hand the use of synchronous motors, known as brushless
or permanent magnet motors, combined with converters is becoming
increasingly common in applications requiring high performance levels,
in particular in terms of dynamic torque (on starting or on a change of
duty), precision and speed range.
In this course, you will be studying the main parts and working
principles of the following machines:
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1. Induction Motors
2. Synchronous Motors.
3. AC Generators
Task 1:
Watch the following videos to help you better understand the
construction and principle of operation of AC machines:
http://www.youtube.com/watch?v=HWrNzUCjbkk&feature=rel
ated
http://www.youtube.com/watch?v=Q4FlUP-kJe8
http://www.youtube.com/watch?v=uYfTzCa71SE
http://www.youtube.com/watch?v=nRMZ3K2pzcE
3.2 AC Induction Motors Fundamentals
BASIC CONSTRUCTION
Like most motors, an AC induction motor has a fixed outer portion,
called the stator and a rotor that spins inside with a carefully
engineered air gap between the two.
Virtually all electrical motors use magnetic field rotation to spin their
rotors. A three-phase AC induction motor is the only type where the
rotating magnetic field is created naturally in the stator because of the
nature of the supply. DC motors depend either on mechanical or
electronic commutation to create rotating magnetic fields. A singlephase AC induction motor depends on extra electrical components to
produce this rotating magnetic field.
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Two sets of electromagnets are formed inside any motor. In an AC
induction motor, one set of electromagnets is formed in the stator
because of the AC supply connected to the stator windings. The
alternating nature of the supply voltage induces an Electromagnetic
Force (EMF) in the rotor (just like the voltage is induced in the
transformer secondary) as per Lenz’s law, thus generating another set
of electromagnets; hence the name – induction motor. Interaction
between the magnetic field of these electromagnets generates twisting
force, or torque. As a result, the motor rotates in the direction of the
resultant torque. The three basic parts of an AC motor are the rotor,
stator, and enclosure.
Stator
The stator is made up of several thin laminations of aluminum or cast
iron. They are punched and clamped together to form a hollow cylinder
(stator core) with slots as shown in Figure 3.1. Coils of insulated wires
are inserted into these slots as shown in Figure 3.2. Each grouping of
coils, together with the core it surrounds, forms an electromagnet (a
pair of poles) on the application of AC supply. The number of poles of
an AC induction motor depends on the internal connection of the stator
windings. The stator windings are connected directly to the power
source. Internally they are connected in such a way, that on applying
AC supply, a rotating magnetic is created.
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Figure 3.1: Stator construction
Figure 3.2: Stator windings
Rotor
There are two types of induction motor rotor – the wound rotor or
slip ring rotor and the squirrel cage rotor. The cage rotor consists
of a laminated cylinder of silicon steel with copper or aluminium bars
slotted in holes around the circumference and short-circuited at each
end of the cylinder as shown in Figure 3.3 a, b and c . In small motors
the rotor is cast in aluminium. Better starting and quieter running are
achieved if the bars are slightly skewed.
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Figure 3.3a: A typical squirrel cage rotor
Figure 3.3b: Skewed rotor conductors
Figure 3.3c: Arrangement of conductor bars in a cage rotor
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Enclosure
The enclosure consists of a frame (or yoke) and two end brackets (or
bearing housings). The stator is mounted inside the frame. The rotor
fits inside the stator with a slight air gap separating it from the stator.
There is no direct physical connection between the rotor and the stator.
The enclosure also protects the electrical and operating parts of the
motor from harmful effects of the environment in which the motor
operates. Bearings, mounted on the shaft, support the rotor and allow
it to turn. A fan, also mounted on the shaft, is used on the motor
shown below for cooling. Exploded views of squirrel cage rotor motor
and slip ring motor rotor are shown in Figures 3.4 and 3.4 respectively.
.
Figure 3.4: Exploded view of a squirrel cage rotor motor
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Figure 3.5: Exploded view of a slip-ring rotor motor
Types of AC Induction Motors
Generally, induction motors are categorized based on the number of
stator windings. They are:
• Single-phase induction motor
• Three-phase induction motor
3.3 Single-Phase Induction Motors
There are probably more single-phase AC induction motors in use today
than the total of all the other types put together. It is logical that the
least expensive, lowest maintenance type motor should be used most
often. The single-phase AC induction motor best fits this description.
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As the name suggests, this type of motor has only one stator winding
(main winding) and operates with a single-phase power supply. In all
single-phase induction motors, the rotor is the squirrel cage type.
The single-phase induction motor is not self-starting. When the motor
is connected to a single-phase power supply, the main winding carries
an alternating current. This current produces a pulsating magnetic field.
Due to induction, the rotor is energized. As the main magnetic field is
pulsating, the torque necessary for the motor rotation is not generated.
This will cause the rotor to vibrate, but not to rotate. Hence, the singlephase induction motor is required to have a starting mechanism that
can provide the starting “kick” for the motor to rotate.
The starting mechanism of the single-phase induction motor is mainly
an additional stator winding (start/auxiliary winding) as shown in
Figure 3.6. The start winding can have a series capacitor and/or a
centrifugal switch. When the supply voltage is applied, current in the
main winding lags the supply voltage due to the main winding
impedance. At the same time, current in the start winding leads/lags
the supply voltage depending on the starting mechanism impedance
(i.e. the total impedance of the capacitor and starting coil). Interaction
between magnetic fields generated by the main winding and the
starting mechanism generates a resultant magnetic field rotating in one
direction. The motor starts rotating in the direction of the resultant
magnetic field.
Once the motor reaches about 75% of its rated speed, a centrifugal
switch disconnects the start winding. From this point on, the singlephase motor can maintain sufficient torque to operate on its own.
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Except for special capacitor start/capacitor run types, all single-phase
motors are generally used for applications up to 3/4 hp (horsepower)
only. Depending on the various start techniques, single-phase AC
induction motors are further classified as described in the following
sections.
Figure 3.6: single-phase ac induction motor with and without a start
mechanism
Split-Phase AC Induction Motor
The split-phase motor is also known as an induction start/induction
run motor. It has two windings: a start and a main winding as shown
in Figure 3.7. The start winding is made with smaller gauge wire and
fewer turns, relative to the main winding to create more resistance,
thus putting the start winding’s field at a different angle than that of
the main winding which causes the motor to start rotating. The main
winding, which is of a heavier wire, keeps the motor running the rest of
the time. Good applications for split-phase motors include small
grinders, small fans and blowers and other low starting torque
applications with power needs from 1/20 to 1/3 hp.
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Figure 3.7: Typical split-phase ac induction motor
Capacitor Start AC Induction Motor
This is a modified split-phase motor with a capacitor in series with the
start winding as shown in Figure 3.8. The capacitor provides a start
“boost” by increasing the phase shift between the start winding and the
main winding. Like the split-phase motor, the capacitor start motor also
has a centrifugal switch which disconnects the start winding and the
capacitor when the motor reaches about 75% of the rated speed.
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Figure 3.8: Typical capacitor start AC induction motor
Permanent Split Capacitor (Capacitor Run) AC Induction Motor
A permanent split capacitor (PSC) motor as shown in Figure 3.9 has a
run type capacitor permanently connected in series with the start
winding. The typical starting torque of the PSC motor is low, from 30%
to 150% of the rated torque. Permanent split-capacitor motors have a
wide variety of applications depending on the design. These include
blowers with low starting torque needs, and intermittent cycling uses
such as adjusting mechanisms, gate operators and garage door
openers.
Figure 3.9: Typical PSC motor
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Capacitor Start/Capacitor Run AC Induction Motor
This motor as shown in Figure 3.10 has a start type capacitor in series
with the auxiliary winding, like the capacitor start motor, for high
starting torque. Like a PSC motor, it also has a run type capacitor that
is in series with the auxiliary winding after the start capacitor is
switched out of the circuit. This allows high overload torque.
It is able to handle applications too demanding for any other kind of
single-phase
motor.
These
include
woodworking
machinery,
air
compressors, high-pressure water pumps, vacuum pumps and other
high torque applications requiring 1 to 10 hp.
Figure 3.10: Typical capacitor start/run induction motor
3.4 Three- Phase Motors
If a three-phase supply is connected to three separate windings
equally distributed around the stationary part or stator of an electrical
machine, an alternating current circulates in the coils and establishes a
magnetic flux.
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Working principles of three phase induction motors:
When a three-phase supply is connected to insulated coils set into slots
in the inner surface of the stator or stationary part of an induction
motor as shown in Figure 3.11, a rotating magnetic flux is produced.
The rotating magnetic flux cuts the conductors of the rotor and induces
an emf in the rotor conductors. This induced emf causes rotor currents
to flow and establish a magnetic flux which reacts with the stator flux
and causes a force to be exerted on the rotor conductors, turning the
rotor.
Figure 3.11: Three phase windings
3.5 Synchronous Motors
Another type of AC motors is the synchronous motor. One type of
synchronous motor is constructed somewhat like a squirrel cage rotor
as shown in Figure 3.12. In addition to rotor bars, coil windings are
added. The coil windings are connected to an external DC power supply
by slip rings and brushes. On start, AC is applied to the stator and the
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synchronous motor starts like a squirrel cage rotor. DC is applied to the
rotor coils after the motor reaches maximum speed. This produces a
strong constant magnetic field in the rotor which locks in step with the
rotating magnetic field. The rotor turns at the same speed as
synchronous speed (speed of the rotating magnetic field). There is no
slip. Variations of synchronous motors include a permanent magnet
rotor. The rotor is a permanent magnet and an external DC source is
not required. These are found on small horsepower synchronous
motors.
Rotor Bar
Brush
External DC
Power Supply
Coil
Slip
Ring
Figure 3.12: AC motor as the synchronous motor
3.6
AC Generators
Synchronous generators or alternators are used to convert mechanical
power derived from steam, gas, or hydraulic-turbine to ac electric
power. They are the primary source of electrical energy we consume
today.
Regardless of size, all electrical generators, whether dc or ac, depend
upon the principle of magnetic induction. An emf is induced in a coil as
a result of:
1. a coil cutting through a magnetic field, or
2. a magnetic field cutting through a coil.
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As long as there is relative motion between a conductor and a magnetic
field, a voltage will be induced in the conductor. The part of a generator
that produces the magnetic field is called the field, while the part in
which the voltage is induced is called the armature. For relative
motion to take place between the conductor and the magnetic field, all
generators must have two mechanical parts — a rotor and a stator.
There are two basic types of AC generators:

ROTATING-ARMATURE ALTERNATORS

ROTATING-FIELD ALTERNATORS
1. ROTATING-ARMATURE ALTERNATORS
The rotating-armature alternator is similar in construction to the dc
generator in that the armature rotates in a stationary magnetic field as
shown in Figure 3.13. In the dc generator, the emf generated in the
armature windings is converted from ac to dc by means of the
commutator. In the alternator, the generated ac is brought to the load
unchanged by means of slip rings. The rotating armature is found only
in alternators of low power rating and generally is not used to supply
electric power in large quantities.
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Figure 3.13: Rotating armature alternators
2. ROTATING-FIELD ALTERNATORS
The rotating-field alternator has a stationary armature winding and a
rotating-field winding as shown in Figure 3.14. The advantage of having
a stationary armature winding is that the generated voltage can be
connected directly to the load.
Figure 3.14: Rotating field alternators
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The stators of all rotating-field alternators are about the same. The
stator consists of a laminated iron core with the armature windings
embedded in this core as shown in Figure 3-15. The core is secured to
the stator frame.
Figure 3-15: The stator
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3.7 Lab Activity 1
Objective:
In this activity you are required to convert a simple DC motor into a
dynamo which can be used to provide a small current to energize an
LED or a small bulb.
Components used:
a) Small DC motor
b) Plastic shaft turned by hand
c) LED/ low wattage small bulb
d) Wires
e) Gears/pulleys
f) Pliers, needle-nose with wire cutters
Procedure:
Visit the websites below and learn how to build your own dynamo using
a DC motor.
http://www.metacafe.com/watch/424127/generate_elecricity_using_a_
dc_motor/
Make/use a shaft and gears to allow you to spin the motor at greater
speeds. Observe the effect on the amount of energy produced.
Record your comments and observations
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3.6 Review Exercise
A-Complete the following
1. The ____________ and the ____________ are two parts
of an electrical circuit that form an electromagnet.
2. The ____________ is the stationary electrical part of an
AC motor.
3. The ____________ is the rotating electrical part of an AC
motor.
4. The ____________ ____________ rotor is the most
common type of rotor used in AC motors.
B. Very briefly describe how a three-phase supply produces a rotating
magnetic flux in an induction motor.
C. Briefly describe why a centrifugal switch is required in a single-phase
a.c. motor.
D. List three types of single-phase a.c. motors and give one application
for each type.
References:
Siemens – Step 2000-AC Motors
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