Construction and working of single phase induction motor.
Single phase motors are very widely used in home, offices, workshops etc. as power delivered to
most of the houses and offices is single phase. In addition to this, single phase motors are
reliable, cheap in cost, simple in construction and easy to repair.
Single phase electric motors can be classified as:
1. Single phase induction motor (Split phase, Capacitor and shaded pole etc)
2. Single phase synchronous motor
3. Repulsion motor etc.
This article explains basic construction and working of single phase induction motor.
Similar to a d.c. motor, single phase induction motor has basically two main parts.one rotating
and other stationary. The stationary part in single phase induction motors is called stator while
the rotating part is called rotor.
The stator has laminated construction, made up of stampings. The stampings are slotted on
its periphery to carry the winding called stator winding or main winding. This is excited by a
single phase a.c. supply. The laminated construction keeps iron losses to minimum. The
stampings are made up of material like silicon steel which minimises the hysteresis loss. The
stator winding is wound for certain definite number of poles means when excited by single phase
a.c. supply, stator produces the magnetic field which creates the effect of certain definite number
of poles. The number of poles for which stator winding is wound, decides the synchronous speed
of the motor. The synchronous speed is denoted as Ns and it has a fixed relation with supply
frequency f and number of poles P. The relation is given by,
The induction motor never rotates with the synchronous speed but rotates at a speed which is
slightly less than the synchronous speed.
The rotor construction is of squirrel cage type. In this type, rotor consists of uninsulated
copper or aluminium bars, placed in the slots. The bars are permanently shorted at both the ends
with the help of conducting rings called end rings. The entire structure looks like cage hence
called squirrel cage rotor. The construction and symbol is shown in the Fig..1
As the bars are permanently shorted to each other, the resistance of the entire rotor is very very
small. The air gap between stator and rotor is kept uniform and as small as possible. The main
feature of this rotor is that it automatically adjusts itself for same number of poles as that of the
stator winding.
The schematic representation of two pole single phase induction motor is shown in the Fig.2.
Single Phase Induction Motor
Construction of a single phase induction motor similar to the construction of three phase
induction motor having squirrel cage rotor, except that the stator is wound with single phase
motor. Stator winding is also provided with a 'starting winding' which is used only for starting
purpose. This can be understood from the schematic of single phase induction motor at the left.
Working Principle Of Single Phase Induction Motor
When stator of a single phase motor is fed with single phase supply, it produces alternating flux
in the stator winding. The alternating current flowing through stator winding causes induced
current in the rotor bars (as squirrel cage rotor is used) according to Faraday's law of
electromagnetic induction. This induced current in the rotor will also produce alternating flux.
Even after both alternating fluxes are set up, the motor fails to start (the reason is explained
below). However, if the rotor is given a initial start by external force in either direction, then
motor accelerates to its final speed and keeps running with its rated speed. This behavior of a
single phase motor can be explained by double-field revolving theory.
Why Single Phase Induction Motor Is Not Self Starting?
The stator of a single phase induction motor is wound with single phase winding. When the
stator is fed with a single phase supply, it produces alternating flux (which alternates along one
space axis only). Alternating flux acting on a squirrel cage rotor can not produce rotation,
only revolving flux can. That is why a single phase induction motor is not self starting.
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'. Hence, stator of a single phase motor has two
windings: (i) Main winding and (ii) Starting winding (axillary winding). These two windings are
connected in parallel across a single phase supply and are spaced 90 electrical degrees apart.
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.