12/1/2012
Overview
• Single Phase Induction Motors
• Universal Motors
36-Single Phase Motors
ECEGR 450
Electromechanical Energy Conversion
Text:10.1-10.2, 10.4, 10.7-10.8
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Introduction
Introduction
• Majority (~90%) of motors are powered by single
phase supplies
 Households
 Small businesses
 Specialty locomotive applications
Basic Types of Single-Phase AC Motors
• Usually <1hp (746W)
• Disadvantages: pulsating torque
Induction
Universal
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Single Phase Induction Motors
Synchronous
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Single Phase Induction Motors
• Recall: poly-phase induction motors use rotating magnetic
field to cause rotation
• Single phase supplies produce stationary fields
 Non-rotational
 Pulsates with time
• Stator windings are distributed
1
• Squirrel-cage rotor
1
0.8
0.6
0.4
0.2
a’
1350, 2250
900, 2700
450 ,3150
a
0
1800
00, 3600
-0.2
-0.4
a
flux
-0.6
a’
a
0
-0.8
-1
-1
stationary, pulsating field
-1
0
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90
-0.5
0
0.5
1
Flux magnitude and direction at t =0, 45, 90, … 360 deg
180 270 360 450 540 630 720
time
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Single Phase Induction Motors
Single Phase Induction Motors
• Single phase induction motors do not inherently
self start
• Force experienced by conductors cancel
• Once a single-phase induction motor begins
rotating, it will continue
 See text 10.2 and “cross-field theory”
• We will focus on methods of starting the motor
• Need to create a temporary pseudo-revolving
field
 Behaves like a stationary transformer with
secondary shorted





x
x
x
F
a’
a
a
F
assume
a
is increasing
Split-phase motor
Capacitor start motor
Capacitor start, capacitor run motor
Capacitor run motor
Shaded pole motor
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Single Phase Induction Motors
Single Phase Induction Motors
Does this arrangement create a revolving field?
Does this arrangement create a revolving field?
a
1
a
a
0.8
0.6
0.4
0.2
a
a
a
a
a
a
a
0
-0.2
rotor
-0.4
rotor
-0.6
-0.8
-1
-1
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-0.5
0
0.5
1
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Split-Phase Motors
Split-Phase Motors
• Idea: use impedance to create two circuit branches with
differing phases
• Main winding: lower resistance, high inductance
• Auxiliary winding: higher resistance, lower inductance
• Disconnect auxiliary winding via centrifugal switch at
approx. 75% of rated speed (avoid copper loss)
•
•
Auxiliary and main winding currents not necessarily equal
Phase difference ( ) ~25o
•
•
Revolving field is unbalanced
Rotation direction can be reversed by reversing connections
to auxiliary winding
 Small value, lower starting torque
Auxiliary winding
V
Iaux
V
Zaux
Iaux
Zmain
Imain
aux
aux
Imain
main
Main winding
main
Note: do not confuse
direction with phase
switch
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Split-Phase Motors
Split Phase Motors
Flux magnitude and position
(both windings connected)
1
0.8
Auxiliary winding
0.6
flux rotation
0.4
switch opened
0.2
aux
0
main
Main winding
-0.2
-0.4
-0.6
Note: do not confuse
direction with phase
-0.8
-1
-1
-0.5
0
0.5
1
Unbalanced rotation, small starting torque
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Capacitor Start
Capacitor Start
• Same concept as split-phase motor, except use
electrolytic capacitor to obtain greater phase shift
(~80o) in windings
 higher starting torque than split phase
 more expensive than split phase
Zmain
• Less current than split-phase
• Main winding identical to split phase
• Applications:
Imain
Iaux
 120W to 7.5kW
 Compressors, large fans, pumps, high-inertia loads
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Capacitor Start Motors
1
Zaux
V
aux
Iaux
V
Imain
switch
main
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Capacitor Start Motors
Flux magnitude and position
(both windings connected)
0.8
Auxiliary winding
0.6
0.4
flux rotation
0.2
aux
main
0
Main winding
-0.2
-0.4
Note: do not confuse
direction with phase
-0.6
-0.8
-1
-1
-0.5
0
0.5
1
Nearly-balanced rotation, higher starting torque
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Capacitor Start Capacitor Run
Capacitor Start Capacitor Run
• Split phase and capacitor start induction motors
have low power factor at rated speed (aux.
winding disconnected)
 Low efficiency (50% – 60%)
• Solution: improve power factor by utilizing two
capacitors
Zmain
 Start capacitor: sized based on desired starting
torque
 Run capacitor: sized based on desired running
characteristics
aux.
winding
V
start
cap.
• Higher cost motor (switch, two capacitors)
run
cap.
switch
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Imain
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Capacitor Start Capacitor Run
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Capacitor Run Motors
• Idea: leave capacitor and auxiliary winding
permanently connected to source
• Trade-off between starting and running
characteristics
 Optimized for running characteristics
• Two windings permanently connected leads to:
 Consistent torque
 Quite operation
 Greater efficiency
• Applications: fans, air conditioners, refrigerators
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Capacitor Run Motors
Shaded Pole Motor
•
•
•
•
•
Zmain
V
Zaux
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Idea: shift flux using transformer action to create rotating
field
Simple, inexpensive construction
Low starting torque, power factor, efficiency
Rotation cannot be changed
Often used in small horsepower applications (~40W)
 Turntables, projectors, small fans
Imain
Iaux
Shading coil
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Shaded Pole Motor
•
•
•
Shaded Pole Motor
Main winding connected to single-phase source
Induced current in shaded pole creates flux that opposes change
in flux
Slight phase difference between main and shaded poles
Main winding
Shorted winding
(shaded coil)
main
shade
main
shade
rotation
shaded flux lags main flux
only one pole shown
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Universal Motors
Universal Motors
• Powered by AC or DC
• Circuit Model
 Similar torque-speed characteristic in either case
Ea
 Torque proportional to I2a
jIaXa
• Similar construction as DC Series motor
Vs
IaRa
 Brushes, commutator
jIaXs
Ra
 fractional horsepower motors
 power tools (e.g. dremmel)
 vacuum cleaners
Rs
jXs
Ia
Ia
I aR s
jXa
vs
Ea
torque (Nm)
• Similar torque-speed characteristic as DC series
motor
• Applications:
speed (rpm)
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Universal Motors
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Drawbacks of Universal Motors
• Poles and yoke should be laminated
 AC fields present, minimize core loss
ia
b
 Decrease number of windings and increase number
of armature conductors
b
AC
Source
F
a
F
ia positive
AC
Source
x
x
a
• Voltage drops and reactive power consumption
due to field and armature inductances
F
x
x
F
ia
• Additional armature conductors increases
armature reaction
• Additional sparking of commutator brushes due
to transformer action
ia negative
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Universal Motors Advantages
• Suitable for AC or DC operation
• AC motor with potential for high speed operation
(>3600 rpm)
• Speed adjusts to load
 Large load: low speed
 Small load: high speed
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