Page 333 - CITS - Electronic Mechanic - TT - 2024

ELECTRONICS MECHANIC - CITS

How To Make Single Phase Induction Motor Self Starting?
As explained above, single phase induction motor is not self-starting. To make it self-starting, it can be temporarily
converted into a two-phase motor while starting. This can be achieved by introducing an additional ‘starting
winding’ also called as auxillary winding.
Hence, stator of a single phase motor has two windings:
i Main winding
ii Starting winding (auxillary winding).
These two windings are connected in parallel across a single phase supply and are spaced 90 electrical degrees
apart. Phase difference of 90 degree can be achieved by connecting a capacitor in series with the starting
winding.

Hence the motor behaves like a two-phase motor and the stator produces revolving magnetic field which causes
rotor to run. Once motor gathers speed, say upto 80 or 90% of its normal speed, the starting winding gets
disconnected form the circuit by means of a centrifugal switch, and the motor runs only on main winding.

Starting Methods of Single Phase Induction Motor

Single -phase induction motor doesn’t have starting torque, so external circuitry is needed to provide this starting
torque. The stator of these motors contains Auxiliary winding for this purpose. The Auxiliary winding is connected
in parallel to a capacitor. When the capacitor is turned on, similar to main winding, revolving two magnetic fields
of the same magnitude but opposite direction are observed on Auxiliary winding.
From these two magnetic fields of Auxiliary winding, one cancel outs one of the magnetic fields of main winding
whereas the other adds up with another magnetic field of main winding. Thus, resulting in a single revolving
magnetic field with high magnitude. This produces force in one direction, hence rotating the rotor. Once the rotor
starts rotating it rotates even if the capacitor is turned off.
There are different stating methods of single-phase induction motors. Usually, these motors are chosen based on
their starting methods. These methods can be classified as

• Split-phase starting.
• Shaded-pole starting.
• Repulsion motor starting
• Reluctance starting

In the split -phase starts, the stator has two types of windings – main winding and Auxiliary winding, connected in
parallel. Motors with this type of starting methods are
• Resistor split -phase motors

• Capacitor split -phase motors
• Capacitors start and run motors
• Capacitor-run motor

Applications of Single Phase Induction Motor

These motors find use in fans, refrigerators, Air-conditioners, Vacuum cleaners, washing machines, centrifugal
pumps, tools, small farming appliances, blowers etc…
So, basically, we use single phase induction motors in our daily life frequently. These motors are easy to repair.
How to calculate single phase motor speed?

Speed of an induction motor depends on the motor design itself, the synchronous speed determined by the
frequency and amount of AC input power poles in the stator.

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CITS : E & H - Electronics Mechanic - Lesson 205 - 222