___________ and ____________ AC in the primary member (stator) produces a
________ field The magnetic field induces ______ in the
secondary member (rotor) another ________ Combined fields produce the force (torque) to
create rotation. AC Induction Motors Rotors typically consist of a laminated, cylindrical iron core
with slots for receiving the conductors. Common type of rotor has cast-aluminum conductors and
short-circuiting end rings. AC Induction Motors The "squirrel cage" rotates when the moving
magnetic field induces a current in the shorted
conductors. AC Motor Speed The magnetic field rotates at the
__________ speed of the motor Determined by the number of _____ in
the stator and the frequency of the AC
power
n
s
= synchronous speed (in RPM),
f
= frequency (in Hz), and
p
= the number of poles AC Motor Speed Synchronous speed is the absolute upper limit
of motor speed. When running, the rotor always rotates _____
than the magnetic field (or no torque!) The speed difference, or
___
, is normally
referred to as a % of synchronous speed:
s
= slip (in %),
n
s
= synchronous speed
n
a
= actual speed AC Motor - Speed vs. Torque
0
25
50
75
100
% Rated Torque
% Synchronous Speed
100
Locked rotor/starting torque
Minimum
acceleration
torque
Service factor
load / torque
Dashed line applicable to
initial acceleration only Single-phase AC Motors Single phase AC motors require a "trick" to
generate a 2nd "phase" to develop starting
torque Three common methods:
split-phase (auxiliary winding is rotated 90°)
capacitor
shaded-pole Split-Phase AC Motor
- Motor starts with both main and auxiliary winding
- A centrifugal switch opens and removes the
auxiliary winding
Centrifugal switch opens
Torque
Rated full load
n
s
Speed Split-Phase AC Motor
switch opens
switch opens
Motor starts with both main and auxiliary winding
A centrifugal switch opens and removes the aux
winding
300
% Rated Torque
0
25
50
75
100
0
100
200
300
% Rated Torque
% Synchronous Spee
d
200
100
0
0
25
50
75
100
% Synchronous Speed Split-Phase AC Motor
Advantages Operate at ~ constant
speed, 4 pole, 60 Hz:
1780 RPM (no load)
1700/1725 RPM at full
load Reversible at low
speed Rapid acceleration Relatively low cost
Disadvantages Repeated start/stop
cycles heat the
windings (high start
resistance) Less useful for large
inertial loads Requires large wiring
to handle starting
currents Single-Phase Capacitor Motors Permanent split capacitor (PSC) Capacitor-start (later switched out) Start-capacitor, run-capacitor (switched)
Auxiliary winding Capacitor
Main
winding
AC
Rotor Permanent Split Capacitor (PSC)
0
25
50
75
100
0
100
200
300
% Rated Torque
% Synchronous Speed
0
100
200
300
0
25
50
75
100
% Synchronous Speed
% Rated Torque Permanent Split Capacitor (PSC)
Disadvantages More expensive for
same HP Lower performance
when starting Need to always use
manufacturer's desired
capacitor value
Advantages Quieter, smoother
than split phase Reduced starting
current
Longer life
Higher reliability Capable of frequent
start/stop cycles Shaded Pole AC Motor
0
25
50
75
100
0
100
200
300
% Rated Torque
% Synchronous Speed
0
100
200
300
0
25
50
75
100
% Synchronous Speed
% Rated Torque Shaded Pole AC Motor
Disadvantages Low starting and
running torque Low efficiency Available in sub-
fractional to ~ 1/4 hp
sizes
Advantages Simple in design and
construction Suitable for low cost,
high volume app's Relatively quiet and
free from vibration "Fail safe" design -
starts in only 1
direction NEMA - National Electrical
Manufacturers Association
NEMA is responsible for several electric
motor industry "standards" Motor ratings (1/4 hp, 1/2 hp, 1 hp,
etc.) Frame size
diameter, length, shaft size, etc. Service factors Housing/protection types and ratings Service Factors A multiplier applied to the rated horsepower Indicate how much the motor can be
overloaded without overheating Generally used for
handling a known, occasional overload
provide a factor of safety where environment or
service condition is not well known Motor Enclosures DP - dripproof DPFG - dripproof, fully guarded SP - splashproof FV - forced ventilation
(separate/attached fan) TENV - totally enclosed, non-ventilated TEFC - totally enclosed, fan cooled TEUC - totally enclosed, unit cooled
(heat-X) AC Motor Efficiency Efficiency,
= _______________________ Small universal motors have
~ ____ Large 3-phase motors have
~ ____ Depends on actual motor load vs. rated load
efficiency best near rated load
efficiency drops rapidly for both under- and
over-load conditions