Question

Consider the table given:

Constructional feature	Machine type	Mitigation
(P) Damper bars	(S) Induction motor	(X) Hunting
(Q) Skewed rotor slots	(T) Transformer	(Y) Magnetic locking
(R) Compensation       winding	(U)Synchronous   machine	(Z) Armature reaction
	(V) DC machine

The correct combination that relates the constructional feature, machine type and mitigation is
1. P - V - X, Q - U - Z, R - T - Y
2. P - U - X, Q - S - Y, R - V - Z
3. P - T - Y, Q - V - Z, R - S - X
4. P- U - X, Q - V - Y, R - T - Z

Answer 1

Correct Answer - Option 2 : P - U - X, Q - S - Y, R - V - Z

Hunting:

Oscillations of the rotor about its new equilibrium position, due to sudden application or removal of load is called swinging or hunting in the synchronous machine.

Causes for Hunting:

• Sudden change in load.
• Sudden change in the supply system or in the field system.
• Load containing harmonic torque.

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Methods for eliminating hunting:

1. By designing the machine with a suitable synchronizing power coefficient.
2. By using the flywheel.
3. By using damper winding.

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Damper winding or bars:

• Damper winding is made with low resistance copper, aluminum, or brass.
• They are inserted in the slots made under the pole shoes.
• With respect to damper winding, the rotor behaves like a squirrel cage rotor of an induction motor.

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Functions of damper winding:

1. In alternator to eliminate hunting and to suppress the negative sequence field.
2. In synchronous motor to eliminate hunting and for starting purpose.

 

Skewing:

• ​In the squirrel cage induction motor, the rotor slots in lamination or rotor core are not made parallel to the rotor shaft. A slight angle is maintained between the rotor slots and the rotor shaft due to some operational advantages. This is called rotor skewing.
• As the rotor slot of the induction motor skewed through some angle so that the bars lie under alternate harmonic poles of the same polarity.​

 

​The functions of skewed rotor slots in induction motor:

1. The skewed rotor slot increases the length of the copper bar thereby increases the resistance of the rotor bars hence starting torque of the machine can be improved and also the starting current is drawn by the machine can be reduced.
2. It makes the air gap flux distribution uniform thereby reduces harmonic torque produced by the machine.
3. As harmonic torque is reduced, the cogging or magnetic locking phenomenon due to harmonic torque can also be reduced. Here the magnetic locking tendency occurs when rotor teeth remain directly under stator teeth thus they might be magnetically locked.
4. And we can also eliminate a particular harmonic by selecting a proper skew angle.
5. The humming noise can be reduced.

 

Compensating windings:

• The cross-magnetizing effect of armature reaction may cause trouble in dc machines subjected to large fluctuations in load.
• In order to neutralize the cross magnetizing effect of the armature reaction, a compensating winding is used. The compensating windings consist of a series of coils embedded in slots in the pole faces. These coils are connected in series with the armature.
• The series-connected compensating windings produce a magnetic field, which varies directly with armature current. Because the compensating windings are wound to produce a field that opposes the magnetic field of the armature, they tend to cancel the cross magnetizing effect of the armature magnetic field.