Polyphase induction motors produce torque by themselves. This is because the interaction of the multiple fields gives the torque on the rotor. This is due to the multiple phases. So they are self starting.
1. Squirrel cage motors:
a) In Induction motors the starting current is very large for a short time duration. This large current and the resulting voltage drops affect the devices in the ciruits. The starting torque is reduced only 1/4th by increasing the rotor resistance, so that the current becomes 1/2.
b) Auto-transformers /auto-starters : A transformer is used for each phase. At the start the switch on the transformer allows only a reduced part of the voltage to affect the rotor. When the motor is near full speed, the auto-transformer automatically disconnects from the circuit. Then the fulloutput voltage of the transformer is applied to the 3 phases.
c) Star delta started method:
A two way switch is used to connect the stator winding in star when the moror is in the starting phase.. When the motor is running at normal speed, the stator windings are connected in delta (triangle) form. In the star network form, only a reduced voltage is affects the rotor. So the torque will be 1/square root(3) of the full voltage when the motor is operating.
2) Slip-ring motors:
These motors can be started off with full input line voltage. There is an external resistance connected on each phase to the brushes at the slop rings. A star connected rheostat can be used for this purposes to add resistance in series with the rotor winding. Thus the starting current in rotor and thus in the stator.
The rheostat can be automatically switched off or manually operated.
Single phase split phase induction motor
(a). a separate
starting circuitry is needed. This provides a rotation in the proper
direction. A single phase motor results in the rotation of rotor in
any of the two directions. So the starting circuitary causes the rotor
to rotate in one direction only by providing the magnetic field in the
A second stator winding is fed with
an out-of-phase current (by phase difference created by using an
inductor or by a capacitance). Thus the interaction of the two fields
gives a start to the rotation. As soon as the motor starts rotating
and reaches the intended speed, the second winding is disconnected.
This is automatically achieved by a centrifugal switch or a thermistor.
(b). A shaded pole:
A coppe wire wound around part of the pole. Not complete pole.. The
current induced in this turn (of the wire) lags behind the input
current. So there is a delayed magnetic field due to delayed current,
around that part (shaded pole) of the pole face. The interaction of
the two magnetic fields causes rotor to rotate. The rotational field
energy to just start the motor is given by this shaded pole.
3. Power electronics methods:
Thyristors, MOSFET controlled thyristor, Insulated gate Bipolar transistor, BJT are used.