Ac Induction Motor
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AC Induction Motors ► Simplest
and most rugged electric motor
► Consists
of ___________ 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 ns = 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!) speed difference, or ___, is normally referred to as a % of synchronous speed:
► The
s = slip (in %), ns = synchronous speed na = actual speed
AC Motor - Speed vs. Torque % 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
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
Speed
ns
Split-Phase AC Motor Motor starts with both main and auxiliary winding A centrifugal switch opens and removes the aux switch opens winding 100
300
75
% Rated Torque
% Synchronous Speed
switch opens
50
25
0 0
100
200
% Rated Torque
300
200
100
0 0
25
50
75
% Synchronous Speed
100
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
AC
Main winding
Rotor
100
300
% Rated Torque
% Synchronous Speed
Permanent Split Capacitor (PSC) 75
50
25
0
200
100
0 0
100
200
% Rated Torque
300
0
25
50
75
% Synchronous Speed
100
Permanent Split Capacitor (PSC) Advantages ► Quieter, smoother than split phase ► Reduced starting current Longer life Higher reliability ► Capable
of frequent start/stop cycles
Disadvantages ► More expensive for same HP ► Lower performance when starting ► Need to always use manufacturer's desired capacitor value
100
300
75
% Rated Torque
% Synchronous Speed
Shaded Pole AC Motor
50
25
0
200
100
0 0
100
200
% Rated Torque
300
0
25
50
75
% Synchronous Speed
100
Shaded Pole AC Motor 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
Disadvantages ► Low starting and running torque ► Low efficiency ► Available in subfractional to ~ 1/4 hp sizes
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 η = _______________________ ► Small universal motors have η ~ ____ ► Large 3-phase motors have η ~ ____ ► Depends on actual motor load vs. rated load ► Efficiency,
efficiency best near rated load efficiency drops rapidly for both under- and over-load conditions
and most rugged electric motor
► Consists
of ___________ 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 ns = 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!) speed difference, or ___, is normally referred to as a % of synchronous speed:
► The
s = slip (in %), ns = synchronous speed na = actual speed
AC Motor - Speed vs. Torque % 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
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
Speed
ns
Split-Phase AC Motor Motor starts with both main and auxiliary winding A centrifugal switch opens and removes the aux switch opens winding 100
300
75
% Rated Torque
% Synchronous Speed
switch opens
50
25
0 0
100
200
% Rated Torque
300
200
100
0 0
25
50
75
% Synchronous Speed
100
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
AC
Main winding
Rotor
100
300
% Rated Torque
% Synchronous Speed
Permanent Split Capacitor (PSC) 75
50
25
0
200
100
0 0
100
200
% Rated Torque
300
0
25
50
75
% Synchronous Speed
100
Permanent Split Capacitor (PSC) Advantages ► Quieter, smoother than split phase ► Reduced starting current Longer life Higher reliability ► Capable
of frequent start/stop cycles
Disadvantages ► More expensive for same HP ► Lower performance when starting ► Need to always use manufacturer's desired capacitor value
100
300
75
% Rated Torque
% Synchronous Speed
Shaded Pole AC Motor
50
25
0
200
100
0 0
100
200
% Rated Torque
300
0
25
50
75
% Synchronous Speed
100
Shaded Pole AC Motor 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
Disadvantages ► Low starting and running torque ► Low efficiency ► Available in subfractional to ~ 1/4 hp sizes
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 η = _______________________ ► Small universal motors have η ~ ____ ► Large 3-phase motors have η ~ ____ ► Depends on actual motor load vs. rated load ► Efficiency,
efficiency best near rated load efficiency drops rapidly for both under- and over-load conditions
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