# AC Induction Motors AC Motor Construction

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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.

http://www.sea.siemens.com/step/templates/lesson.mason?motors:3:1:1

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
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

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
• 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)
• 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)
• ________, 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

• _________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
http://www.sea.siemens.com/step/templates/lesson.mason?motors:4:3:1

• 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