1. EET 306 ELECTRICAL MACHINE
Single-Phase Induction Motor
Baharuddin Ismail
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2. Types of Single Phase Motor
The main types of single-phase motor :
a. Single-phase induction motors
- Split-phase type
- Shaded pole type
- Reluctance start type
b. Single-phase synchronous motors
- Reluctance motor
- Hysteresis motor
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3. c. Single-phase commutator type motors
- Repulsion motor
- repulsion induction motor
- ac series motor
- Universal motor
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4. Single Phase Induction Motor
The single-phase induction machine is the most frequently
used motor for refrigerators, washing machines, clocks,
drills, compressors, pumps, and so forth.
The single-phase motor stator has a laminated iron core
with two windings arranged perpendicularly.
One is the main and
The other is the auxiliary winding or starting winding
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5. Single Phase Induction Motor
Stator
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6. Single Phase Induction Motor
This “single-phase”
motors are truly two-
phase machines.
The motor uses a
squirrel cage rotor,
which has a laminated
iron core with slots.
Aluminum bars are
molded on the slots
and short-circuited at
both ends with a ring.
Cross-sectional view of a
single-phase induction motor
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7. Cross-section view of a single phase motor
4-pole run winding
4-pole start winding
Rotor (squirel cage)
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8. Single Phase Induction Motor
Figure 3 Squirrel cage rotor
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9. Single Phase Induction Motor
The single-phase induction motor operation can be described by two methods:
Double revolving field theory; and
Cross-field theory.
Double revolving theory is perhaps the easier of the two explanations to understand
Learn the double revolving theory only
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10. 360 Chapter 7 Induction Motor
Double revolving field theory
A single-phase ac current supplies the main winding that produces a pulsating magnetic field.
Mathematically, the pulsating field could be divided into two fields, which are rotating in opposite directions.
The interaction between the fields and the current induced in the rotor bars generates opposing torque
Under these conditions, with only the main field energized the motor will not start
5/18/2019
360 Chapter 7 Induction Motor

11. 360 Chapter 7 Induction Motor
Single-phase motor main winding generates two rotating fields, which oppose and counter-balance one another.
However, if an external torque moves the motor in any direction, the motor will begin to rotate.
5/18/2019
360 Chapter 7 Induction Motor

12. Single Phase Induction Motor
Double revolving field theory :
The pulsating filed is divided a forward and reverse
rotating field
Motor is started in the direction of forward rotating field
this generates small (5%) positive slip
Reverse rotating field generates a larger (1.95%) negative
slip
cont...
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13. Each of the rotating fields induces a voltage in
cont...
Each of the rotating fields induces a voltage in
the rotor, which drives current and produces
torque.
An equivalent circuit, similar to the equivalent
circuit of a three phase motor, can represent
each field
The parameters of the two circuits are the same
with the exception of the slip.
The two equivalent circuits are connected in
series.
cont...
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14. Figure 4 shows the equivalent circuit of a single
cont...
Figure 4 shows the equivalent circuit of a single
phase motor in running condition.
The current, power and torque can be calculated
from the combined equivalent circuit using the
Ohm Law
The calculations are demonstrated on a numeri-
cal example
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15. Single Phase Induction Motor
Figure 4 Equivalent circuit of a single-phase motor in running condition.
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16. The results of the calculations are:
Single-phase IM
The results of the calculations are:
- Input power:
- Developed or output power:
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17. Single Phase Induction Motor
Figure 5 Single-phase motor mechanical output power and electrically developed power versus speed.
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18. The single-phase motor starting torque is zero because
of the pulsating single-phase magnetic flux.
The starting of the motor requires the generation of a
rotating magnetic flux similar to the rotating flux in a three
phase motor.
Two perpendicular coils that have currents 90° out-of-
phase can generate the necessary rotating magnetic fields
which start the motor.
Therefore, single-phase motors are built with two
perpendicular windings.
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19. The axes of the two windings are 90o with respect to each other.
Split-Phase Motor
This machine is constructed with two separate stator windings, called main (run) and auxiliary (start) windings
The axes of the two windings are 90o with respect to each other.
cont…
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20. cont…
Vsource
Irun
The start winding is out of phase with the main (run) winding, as a result of the different reactances of the two windings.
Once the motor started, centrifugal switch (CS) is used to disconnected the start winding
Istart
Isource
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21. Starting torque
The single-phase motor starting torque is zero because of the pulsating single-phase magnetic flux.
The starting of the motor requires the generation of a rotating magnetic flux similar to the rotating flux in a three-phase motor.
Two perpendicular coils that have currents 90° out-of-phase can generate the necessary rotating magnetic fields which start the motor.
Therefore, single-phase motors are built with two perpendicular windings.
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22. why a single-phase single-winding IM produces no starting torque?
The single winding of stator of a single phase IM is distributed in space so that space fundamental of mmf is the most dominant component of the actual mmf distribution. When the winding carries a sinusoidal current, it produces a sinusoidally space-distributed mmf whose peak value pulsates with time.
The mmf at any time is
F = Fpeak cos θ
where θ is the angle measured from the winding axis
Now Fpeak = Fmax cos ωt
so that the mmf has both space and time distribution expressed as
F = Fmax cos θ cos ωt
F = ½ Fmax cos (θ-ωt) + ½ Fmax cos (θ+ωt)
This equation tell us that pulsating single-phase field can be considered as superposition of two rotating field rotates at synchronous speed in opposite directions (Fig.2),
the forward rotating field, Ff = ½ Fmax cos (θ- ωt)
the backward rotating field, Fb = ½ Fmax cos (θ+ωt)
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23. phase shift The phase shift is achieved by connecting
a resistance,
an inductance, or
a capacitance
in series with the starting winding.
Most frequently used is a capacitor to generate the starting torque.
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24. Capacitor-Start Motor
Figure 6 shows the connection diagram of a motor using a capacitor to generate the starting torque.
When the motor reaches the operating speed, a centrifugal switch turns off the starting winding.
The centrifugal switch is necessary because most motors use a cheap electrolytic capacitor that can only carry ac current for a short period.
Figure 6 Capacitor- start motor
A properly selected capacitor produces around 90° phase shift and large starting torque.
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25. Capacitor-Start Motor (cont..)
By addition of a Capacitor in series with the start winding, Istart to lead Vsource .
A properly selected capacitor produces around 90° phase shift between Irun and Istart.
This results in a large starting torque and less source current.
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26. Torque–speed characteristic of a small single-phase induction motor.
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27. Permanent Split-Capacitor
Lrun
Rrun
Lstart
Rstart
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28. Capacitor-Start Capacitor-Run Motor (CSCR)
or Two-Value-Capacitor Motor
Start capacitor
Switch
Run capacitor
120 V
50 Hz
Rotor
Run
winding
Start
winding
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29. Capacitor-Start Capacitor-Run Motor (cont…)
The run capacitor is an oil-filled, low-value capacitor that gives the motor the run characteristics of the permanent-spilt capacitor motor.
The start capacitor, being a high-value, ac electrolytic, provides high starting torque when the start switch is closed by a centrifugal switch actuator.
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30. The advantage of a capacitor run over a capacitor start motor are:
It has a better power factor
(ii) It has more efficiency
(iii) It has quieter and smoother operation
(iv) It used for both easy and hard to start loads
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31. Single-Phase shaded pole Motor
The motor has two salient poles excited by ac current.
Each pole includes a small portion that has a short-circuited winding.
This part of the pole is called the shaded pole
Figure Concept of single-phase shaded pole motor.
cont…
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32. Figure Shaded pole motor for household fan.
cont…
Figure Shaded pole motor for household fan.
cont…
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33. A less effective but more economical method using shaded pole motors
cont…
A less effective but more economical method using shaded pole motors
The motor has two salient poles excited by ac current.
Each pole includes a small portion that has a short-circuited winding. This part of the pole is called the shaded pole.
The main winding produces a pulsating flux that links with the squirrel cage rotor.
This flux induces a voltage in the shorted winding
cont…
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34. The induced voltage produces a current in the shorted winding.
cont…
The induced voltage produces a current in the shorted winding.
This current generates a flux that opposes the main flux in the shaded pole (the part of the pole that carries the shorted winding).
The result is that the flux in the unshaded and shaded parts of the pole will be unequal.
Both the amplitude and the phase angle will be different.
cont…
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35. The motor efficiency is poor but it is cheap
cont…
These two fluxes generate an unbalanced rotating field. The field amplitude changes as it rotates.
Nevertheless this rotating field produces a torque, which starts the motor in the direction of the shaded pole.
The starting torque is small but sufficient for fans and other household equipment requiring small starting torque.
The motor efficiency is poor but it is cheap
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36. Universal Motor
The universal motor is a rotating machine similar to a DC motor but designed to operate either from DC or single phase AC. The stator and rotor windings of the motor are connected in series through the rotor commutator. Therefore the universal motor is also known as an AC series motor or an AC commutator motor. The Universal motor can be controlled either as a phase-angle drive or as a chopper drive The universal motor has a sharply drooping torque-speed characteristics of a DC series motor Typical applications is in vacuum cleaners, drills, and kitchen aplliances
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37. Stepper Motors
A Stepper motor is special type of synchronous motor which is designed to rotate a specific number of degrees for every electric pulse received by its control unit. Typical steps are 7.5 to 15o per pulse. It moves in accurate angular increments known as steps, in response to the application of digital pulses to the electric drive circuit. Two basic types of stepper motor (differing only in rotor construction) : - permanent-magnet type - reluctance-type (has a ferromagnetic rotor).
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38. Motor Selection Criteria
Available power (DC or AC)
Operating condition
Starting characteristics (torque and current)
Operating speed
Forward/reverse operation
Acceleration characteristics (depending on load)
Efficiency at rated load
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39. Motor Selection Criteria (cont…)
Overload capability
Electrical and thermal safety
Life span and maitenance
Mechanical aspects(size, weight, noise level, environment)
EMC and EMI
Control complexity and cost
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