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Thursday 29 September 2011

THREE PHASE INDUCTION MOTOR INTERVIEW Questions 2

Why the exciting current of induction motor is so high as compared to the power transformer?
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.

How transformation ratio of induction motor is different as compared to power transformer?
In a transformer, the windings are concentrated whereas in an induction motor, the windings are distributed. This affects the transformation ratio.
Why it is required to start 3-Phase Induction Motors?
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.
What is the effect of staring of induction motor on connected line?
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.
Please describe the Methods of Starting 3-Phase Induction Motors?
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



How slip ring motors are started?
Slip ring motors are invariably started by rotor resistance starting.
What is Direct-on-line starting of induction motor?
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) machines.
Does starting torque is higher than full load torque?
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.
What is Stator resistance starting method?
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.
Why Stator resistance starting is not recommended?
 
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.
What is Autotransformer starting method?
This method also aims at connecting the induction motor to a reduced supply at starting and then connecting it to the full voltage as the motor picks up sufficient speed. Fig. shows the circuit arrangement for autotransformer starting.
clip_image002
The tapping on the autotransformer is so set that when it is in the circuit, 65% to 8O% of line voltage is applied to the motor.
At the instant of starting, the change-over switch is thrown to “start” position. This puts the autotransformer in the circuit and thus reduced voltage is applied to the circuit. Consequently, starting current is limited to safe value. When the motor attains about 8O% of normal speed, the changeover switch is thrown to “run” position. This takes out the autotransformer from the circuit and puts the motor to full line voltage.

 
What are the advantages of Autotransformer starting?
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.
What is Star-delta starting method of starting of 3 phase induction motor?
The stator winding of the motor is designed for delta operation and is connected in star during the starting period. When the machine is up to speed, the connections are changed to delta. The circuit arrangement for star-delta starting is shown below:
clip_image002[7]
The six leads of the stator windings are connected to the changeover switch as shown. At the instant of starting, the changeover switch is thrown to “Start” position which connects the stator windings in star. Therefore, each stator phase gets  image volts where V is the line voltage. This reduces the starting current. When the motor picks up speed, the changeover switch is thrown to “Run” position which connects the stator windings in delta. Now each stator phase gets full line voltage V.
Explain in details the Starting of Slip-Ring Motors?
Slip-ring motors are invariably started by rotor resistance starting. In this method, a variable star-connected rheostat is connected in the rotor circuit through slip rings and full voltage is applied to the stator winding as shown in Fig.
clip_image002[10]
At starting, the handle of rheostat is set in the OFF position so that maximum resistance is placed in each phase of the rotor circuit. This reduces the starting current and at the same time starting torque is increased.
As the motor picks up speed, the handle of rheostat is gradually moved in clockwise direction and cuts out the external resistance in each phase of the rotor circuit. When the motor attains normal speed, the change-over switch is in the ON position and the whole external resistance is cut out from the rotor circuit.
What are the advantages of slip-ring induction motors over the squirrel cage 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

Is there any disadvantage of slip-ring motors?
(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