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AC Motors 29.1
AC Induction Motors AC Motor Construction
• Simplest and most rugged electric motor
• Consists of _____________ and ____________
• AC in the primary member (stator) ___________
current in the secondary member (rotor)
• Combined electromagnetic effects of the stator
and rotor currents produce the force (torque) to
create rotation.
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AC Motor - Stator AC Motor - Rotor
Laminate
_______________ cage
Three-Phase Electrical Power Single-Phase v. Three-Phase
• Uses three sets of armature windings to
produce three separate outputs
• Armature windings are physically
separated _______ from each other, and
therefore, each phase is _______ apart
from another
• __________ power may be generated by
a generator of a given size and weight
AC Motors 29.2
Delta and Wye Connections AC Motor Speed
• The magnetic field rotates at the
synchronous speed of the motor
• Determined by the number of poles in the
stator and the frequency of the AC power
– ns = synchronous speed (in RPM),
– f = frequency (in Hz), and
– p = the number of poles
AC Motor Speed AC Motor - Speed vs. Torque
• Synchronous speed is the absolute upper limit of 100
motor speed.
% Synchronous Speed
Service factor
• When running, the rotor always rotates slower 75
load / torque
than the magnetic field (or no torque!) Dashed line applicable to
initial acceleration only
• The speed difference, or slip, is normally 50
referred to as a % of synchronous speed: Minimum
acceleration
– s = slip (in %), 25
torque
– ns = synchronous speed Locked rotor/starting torque
– na = actual speed 0
100
% Rated Torque
Single-phase AC Motors Split-Phase AC Motor
• Single phase AC motors require a "trick" to Motor starts with both main and auxiliary winding
generate a 2nd "phase" to develop A centrifugal switch opens and removes the aux
winding switch opens
______________ _____________ switch opens
100
• Three common methods:
% Synchronous Speed
300
75
% Rated Torque
– split-phase (auxiliary winding is rotated 90°) 200
– capacitor 50
– shaded-pole 25
100
0
0
0 100 200 300
0 25 50 75 100
% Rated Torque % Synchronous Speed
AC Motors 29.3
Split-Phase AC Motor Single-Phase Capacitor Motors
Advantages Disadvantages
• Operate at ________ • Permanent split capacitor (__________)
• Repeated start/stop
speed, 4 pole, 60 Hz: cycles _________ the • Capacitor-start (later switched out)
– 1780 RPM (no load) windings (high start • Start-capacitor, run-capacitor (switched)
– 1700/1725 RPM at full resistance)
load
• Less useful for large Auxiliary winding Capacitor
• ____________ at low inertial loads
speed
• Requires ________
• Rapid acceleration Main
__________ to handle AC winding Rotor
• Relatively low cost starting currents
Permanent Split Capacitor Permanent Split Capacitor
(PSC) (PSC)
Advantages Disadvantages
• ________, smoother • More _____________
100 300 for same HP
than split phase
% Synchronous Speed
• ______________
% Rated Torque
75
200
• Reduced starting performance when
50 current starting
100 – Longer life • Need to always use
25
– Higher reliability manufacturer's
0
0 100 200 300
0
0 25 50 75 100
• Capable of ________ desired capacitor
% Rated Torque % Synchronous Speed start/stop cycles value
Shaded Pole AC Motor Shaded Pole AC Motor
Advantages Disadvantages
• _________in design • Low starting and
and construction running torque
100
• Suitable for ______ • Low _____________
300
% Synchronous Speed
__________, high • Available in sub-
% Rated Torque
75
200
volume app's fractional to ~ 1/4 hp
50 • Relatively quiet and sizes
25
100 free from vibration
• "Fail safe" design -
0
0 100 200 300
0
0 25 50 75 100
starts in only 1
% Rated Torque % Synchronous Speed direction
AC Motors 29.4
NEMA - National Electrical IEC – International
Manufacturers Association Electrotechnical Commision
• NEMA is responsible for several North • IEC is responsible for European and Asian
American electric motor industry electric motor standards
"standards" • Similar to (but not the same as!) NEMA
• ______ _________ (1/4 , 1/2 ,1 ) standards
• Frame size
– diameter, length, shaft size, etc.
• ____________ _______________
• Housing/protection types and ratings
Service Factors Motor Enclosures
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• A multiplier ( < ) applied to the rated • DP - dripproof
______________________
• DPFG - dripproof, fully guarded
• Indicate how much the motor can be
overloaded without overheating • SP - splashproof
• Generally used for • FV - forced ventilation
– handling a known, occasional overload (separate/attached fan)
– provide a _________________________ • TENV - totally enclosed, non-ventilated
where environment or service condition is not
well known • TEFC - totally enclosed, fan cooled
• TEUC - totally enclosed, unit cooled
(heat-X)
AC Motor Efficiency
• Efficiency, η
• Small universal motors have η ~ 30%
• Large 3-phase motors have η ~ 95%
• Depends on actual motor load vs. rated
load
– efficiency best near rated load
– efficiency drops rapidly for both under- and
over-load conditions
