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Chapter 19
AC Motors
Objectives
After studying this chapter, you will be able
to:
• Describe the construction of various types
of AC motors
• Explain the principle of operation of
various types of three-phase motors and
single-phase motors
Objectives (cont’d.)
• Discuss the reasons for the difference in
the values of starting and running currents
in AC motors
AC Motor Construction
• Induction motor is most common AC motor
– Two main components: stator and rotor
– Stator has electromagnets secured to frame
– Rotor made of steel lamination in shape of
cylinder
• Windings placed in slots on rotor surface
• Squirrel-cage rotor winding
– Heavy copper bars welded to end rings
AC Motor Construction (cont’d.)
• Wound-rotor
motor
– Copper wire
wound into rotor
slots
– Connected in wye
– Ends connected to
slip rings on shaft
Three-Phase Motor Theory
• Two-pole, three phase stator winding
– When energized from three-phase source,
three-phase currents are 120 degrees apart
and continuously changing in value and
direction
– The effect of this variation in strength and
direction produces a rotating field
– See Figure 19-4C
Three-Phase Motor Theory (cont’d.)
• Speed of the rotating magnetic field
– Synchronous speed of two-pole induction
motor is 60 revolutions per second when
supplied with 60 Hz.
• Lower speeds obtained with more poles
Three-Phase Motor Theory (cont’d.)
• Rotor speed
– Depends on synchronous speed and load
– Rotor speed lags behind synchronous speed
one percent (no load) to five percent (full load)
• Direction of rotor rotation
– Depends on the phase sequence
– Revolves in same direction as stator field
Three-Phase Motor Theory (cont’d.)
• Torque
– Varies with the strength of the stator and rotor
fields
– Also affected by phase relationship between
the two fields
Three-Phase Motor Theory (cont’d.)
• Slip
– Difference between synchronous speed and
rotor speed
– May be stated in rpm or a percentage
Three-Phase Motor Starting and
Running Current
• Starting current is limited to four to six
times the full load current
• Loading a squirrel-cage motor
– For large motors, reduced voltage starters are
used to limit the starting current to a lower
value that will not cause line drops
Three-Phase Motor Starting and
Running Current (cont’d.)
• Double squirrel-cage rotor
– Has two squirrel-cage windings
– Inner winding has low resistance and high
inductance
– Outer winding has high resistance and low
inductance
– Results in large current and high power factor
in outer winding; high starting torque
Types of Three-Phase Motors
• Multispeed squirrel-cage motors
– Number of poles may be varied by changing
external connections
• Wound-rotor induction motors
– Provides high starting torque at low current
value
– Accelerates smoothly under heavy loads
– Poor speed regulation when operating with
resistance in the rotor circuit
Types of Three-Phase Motors
(cont’d.)
• Adjustable-speed induction motor (brushshifting motor)
– Provides a wide range of speeds, depending
on position of the brushes
• High-frequency motors
– Operate at speeds greater than can be
obtained from a 60Hz supply
Types of Three-Phase Motors
(cont’d.)
• Synchronous motors
– Operates at constant speed from no load to
full load
– Power factor can be controlled by varying
amount of current in the exciter winding
– Generally used for driving loads requiring
constant speeds and infrequent starting and
stopping
Single-Phase Motors
• Disadvantages
when compared to
polyphase motors
– Torque developed is
not as smooth
– Larger in physical
size and less
efficient
Starting Single-Phase Motors
• Phase-splitting principle: one method of
making a single-phase motor self-starting
– Stator has two windings: main and auxiliary
– Two currents out of phase by less than 90
degrees
• Resistance split-phase motor
– Resistance connected in series with auxiliary
winding, resulting in better starting torque
Starting Single-Phase Motors
(cont’d.)
• Capacitor split-phase motors
– Three classes: 1) low starting torque, 2)
capacitor start, induction run, 3) capacitor
start, capacitor run
• Reversing split-phase motors
– Reverses direction of rotor rotation by
reversing the polarity of one of the fields
Starting Single-Phase Motors
(cont’d.)
• Shaded-pole motors
– Started by a low-resistance, short-circuited
coil placed around one tip of each pole
– Have poor starting torque
• Repulsion motors
– Starts based on the principle that like poles
repel
Starting Single-Phase Motors
(cont’d.)
• Series AC motors
– More complex structure and heavier per
horsepower
– More expensive than DC motors of the same
rating
– Operating characteristics very similar to DC
motors
Universal Motors
• Series motor that will operate on both AC
and DC
– Manufactured in small sizes with fractional
horsepower ratings
– Used for vacuum cleaners, portable electrical
tools, and small household appliances
Selection of Single-Phase Motors
• Shaded-pole motors adequate for small
fans, measuring instruments and other
applications requiring little starting torque
• Split-phase motors can be used for large
machines that start without a load (small
lathes, drills, grinders)
• Capacitor and repulsion motors for
machines that start under load
Torque Motors
• Have greatest torque output when stalled
• Applications: opening and closing valves,
dampers, doors, gates, windows, etc.
• Large torque motors operate on threephase
– Small ones versions of universal motor
• Not intended for continuous duty
Dual-Voltage Windings
• Motors that can
operate on either of
two voltages
– Accomplished by
dividing each phase
into two sections
(either delta or wye)
Multispeed Induction Motors
• Synchronous speed of induction motor
depends on supply frequency and number
of poles
• Changing speed by varying frequency
requires use of variable frequency drive
and means of adjusting motor current
• Solid-state controller can be used to
accomplish this
Summary
• Induction motor made up of stator and
rotor
• Types of three-phase motors include
multispeed squirrel-cage, wound-rotor
induction, adjustable speed induction,
high-frequency and synchronous motors
• Single-phase motors are larger in size and
less efficient than polyphase motors
Summary (cont’d.)
• Universal motors can operate on either AC
or DC
• Dual voltage windings can operate on
either of two voltages
• Multispeed induction motors offer varying
speeds by changing frequency or number
of poles