Download Single Phase AC Motors

Single Phase AC
Motors
Operation
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A 3 phase motor self starts due to a rotating
magnetic field in the windings
If a single phase was applied to one winding,
it would simply pulse and produce no torque
If we could simply ‘push’ the rotor, it would
begin to spin for the same reasons in a 3
phase motor
Operation
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To make the spin happen, we add a start
winding to the motor
A start winding usually has the same number
of turns as the run winding but a smaller
diameter wire
This creates an inductive reactance causing
the current to lag
Split Phase Motor
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A split phase motor has 2 windings:
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Run winding
Start winding
Voltage
Run Winding
Run (I)
Start (I)
Cage Rotor
Automatic or centrifugal
switch
L
Start Winding
N
Run Winding
Cage Rotor
Automatic or centrifugal
switch
L
Start Winding
N
Split Phase Motor
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The two windings cause the current to lag
depending on the inductive reactance of each
This mimics the effect of a changing magnetic field
Once started, the switch cuts out to stop burning out
Voltage
Run Winding
Run (I)
Start (I)
Cage Rotor
Automatic or centrifugal
switch
L
Start Winding
N
Split Phase Motor
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Can be reversed by changing connections to
any one winding
Has a high starting current
Used in simple domestic machines
Capacitor Start Motor
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Similar in effect to the split phase motor
Capacitor causes current in start winding to
lead voltage
Start and Run windings are approximately
90° out of phase
Capacitor Start Motor
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High torque
Used in compressors, refrigerators and when high
torque starting is needed
Voltage
Run Winding
Run (I)
Start (I)
Cage Rotor
Automatic or centrifugal
switch
L
Start Winding
N
Capacitor Start – Capacitor
Run Motor
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2 capacitors are
connected in parallel
with the start winding
Once started, the first
capacitor cuts out and
leaves the second to
continue to maintain an
out of balance
magnetic field
Run Winding
Cage Rotor
Automatic or centrifugal
switch
C2
Start Winding
C1
L
N
Capacitor Start – Capacitor
Run Motor
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This type is similar to
the last but has:
Better torque
Improved power factor
Run Winding
Cage Rotor
Automatic or centrifugal
switch
C2
Start Winding
C1
L
N