AC Motors
AC Induction Motors
29.1
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
• 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
Single-Phase v. Three-Phase
AC Motors
29.2
AC Motor Speed
Delta and Wye Connections
• 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
motor speed.
• When running, the rotor always rotates slower
than the magnetic field (or no torque!)
• The speed difference, or slip, is normally
referred to as a % of synchronous speed:
– s = slip (in %),
– ns = synchronous speed
– na = actual speed
% Synchronous Speed
100
75
Service factor
load / torque
Dashed line applicable to
initial acceleration only
50
25
Minimum
acceleration
torque
Locked rotor/starting torque
0
100
% Rated Torque
Split-Phase AC Motor
Single-phase AC Motors
switch opens
100
300
75
% Rated Torque
– split-phase (auxiliary winding is rotated 90°)
– capacitor
– shaded-pole
Motor starts with both main and auxiliary winding
A centrifugal switch opens and removes the aux
switch opens
winding
% Synchronous Speed
• Single phase AC motors require a "trick" to
generate a 2nd "phase" to develop
______________ _____________
• Three common methods:
50
25
0
0
100
200
% Rated Torque
300
200
100
0
0
25
50
75
% Synchronous Speed
100
AC Motors
Split-Phase AC Motor
Advantages
• Operate at ________
speed, 4 pole, 60 Hz:
– 1780 RPM (no load)
– 1700/1725 RPM at full
load
• ____________ at low
speed
• Rapid acceleration
• Relatively low cost
Single-Phase Capacitor Motors
Disadvantages
• Repeated start/stop
cycles _________ the
windings (high start
resistance)
• Less useful for large
inertial loads
• Requires ________
__________ to handle
starting currents
75
50
25
0
200
100
0
0
100
200
300
0
% Rated Torque
25
50
75
100
% Synchronous Speed
75
50
25
0
200
100
0
0
100
200
% Rated Torque
300
0
25
50
Auxiliary winding Capacitor
AC
Main
winding
Rotor
Disadvantages
Advantages
• ________, smoother • More _____________
for same HP
than split phase
• ______________
• Reduced starting
performance when
current
starting
– Longer life
• Need to always use
– Higher reliability
manufacturer's
• Capable of ________ desired capacitor
value
start/stop cycles
Shaded Pole AC Motor
300
% Rated Torque
% Synchronous Speed
Shaded Pole AC Motor
100
• Permanent split capacitor (__________)
• Capacitor-start (later switched out)
• Start-capacitor, run-capacitor (switched)
Permanent Split Capacitor
(PSC)
300
% Rated Torque
% Synchronous Speed
Permanent Split Capacitor
(PSC)
100
29.3
75
% Synchronous Speed
100
Advantages
• _________in design
and construction
• Suitable for ______
__________, high
volume app's
• Relatively quiet and
free from vibration
• "Fail safe" design starts in only 1
direction
Disadvantages
• Low starting and
running torque
• Low _____________
• Available in subfractional to ~ 1/4 hp
sizes
AC Motors
NEMA - National Electrical
Manufacturers Association
• NEMA is responsible for several North
American electric motor industry
"standards"
• ______ _________ (1/4 , 1/2
,1
• Frame size
29.4
IEC – International
Electrotechnical Commision
)
• IEC is responsible for European and Asian
electric motor standards
• Similar to (but not the same as!) NEMA
standards
– 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
______________________
• Indicate how much the motor can be
overloaded without overheating
• Generally used for
– handling a known, occasional overload
– provide a _________________________
where environment or service condition is not
well known
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
•
•
•
•
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)