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Ans. An induction motor derives its name from the current in the rotor conductor is induced by the motion of rotor conductor relative to the magnetic field developed by the stator currents.
A. I- Squirrel cage induction motor
II-Wound rotor/slip-ring motor
Squirrel cage induction motor is generally preferred.
A. Because its run at a speed other than the synchronous speed of rotating magnate field developed by the starter current.
A. the air gap is made as small as possible so as to produced required flux with a minimum exciting current and give leakage reluctance as small as possible. this result in improved power factor.
A. The flux set up by the stator winding is a rotating flux with a definite no. of pole ; operation is only possible if the no. of poles are same in both.
Unlike that of a power transformer, the magnetic circuit of the induction motor has an air-gap. Therefore, the exciting current of induction motor (3O to 4O% of full-load current) is much higher than that of the power transformer. Consequently, the exact equivalent circuit must be used for accurate results.
In a transformer, the windings are concentrated whereas in an induction motor, the windings are distributed. This affects the transformation ratio.
The induction motor is fundamentally a transformer in which the stator is the primary and the rotor is short-circuited secondary. At starting,
the voltage induced in the induction motor rotor is maximum ( s = 1).
Since the rotor impedance is low, the rotor current is excessively large. This large rotor current is reflected in the stator because of
transformer action. This results in high starting current (4 to 1O times the full-load current) in the stator at low power factor and consequently the value of starting torque is low. Because of the short duration, this value of large current does not harm the motor if the motor accelerates normally.
Large starting current will produce large line-voltage drop. This will adversely affect the operation of other electrical equipment connected
to the same lines. Therefore, it is desirable and necessary to reduce the magnitude of stator current at starting and several methods are
available for this purpose.
The common methods used to start induction motors are:
(i) Direct-on-line starting
(ii) Stator resistance starting
(iii) Autotransformer starting
(iv) Star-delta starting
(v) Rotor resistance starting
Slip ring motors are invariably started by rotor resistance starting.
This method of starting in just what the name implies the motor is started by connecting it directly to 3-phase supply. The impedance of
the motor at standstill is relatively low and when it is directly connected to the supply system, the starting current will be high (4 to
1O times the full-load current) and at a low power factor. Consequently, this method of starting is suitable for relatively small (up to 7.5 kW)
No, starting current is as large as five times the full-load current but starting torque is just equal to the full-load torque. Therefore, starting current is very high and the starting torque is comparatively low. If this large starting current flows for a long time, it may overheat the motor and damage the insulation.
In this method, external resistances are connected in series with each phase of stator winding during starting. This causes voltage drop across
the resistances so that voltage available across motor terminals is reduced and hence the starting current. The starting resistances are gradually cut out in steps (two or more steps) from the stator circuit as the motor picks up speed. When the motor attains rated speed, the resistances are completely cut out and full line voltage is applied to the rotor.
This method suffers from two drawbacks. First, the reduced voltage applied to the motor during the starting period lowers the starting torque and hence increases the accelerating time. Secondly, a lot of power is wasted in the starting resistances. Therefore, this method is used for starting small motors only.
Autotransformer starting has several advantages like low power loss, low starting current and less radiated heat. For large machines (over 25
H.P.), this method of starting is often used. This method can be used for both star and delta connected motors.
(i) High starting torque with low starting current.
(ii) Smooth acceleration under heavy loads.
(iii) No abnormal heating during starting.
(iv) Good running characteristics after external rotor resistances are cut out.
(v) Adjustable speed
(i) The initial and maintenance costs are greater than those of squirrel cage motors.
(ii) The speed regulation is poor when run with resistance in the rotor circuit.