Interview Questions - Electrical Machines

What is “Hunting” in synchronous machines? 
The synchronous machine oscillates about the operating point, due to
  • A sudden change in load.  
  • A fault in the supply system.
  • A load containing harmonic torques.  
  • A sudden change in field current.  
This oscillatory behaviour of the synchronous machines is known as hunting.

What are the effects of hunting in synchronous machines?
The effects of hunting are as follows-
  • It produces severe mechanical stress and fatigue in the shaft.  
  • It causes great surges in current and power flow.  
  • It increases machine losses and thus the temperature rise of the machine.  
Does “Hunting” occur in synchronous generators only or in synchronous motors also? 
Hunting occurs in both synchronous generators and synchronous motors.

What are the conditions under which a shunt excited dc generators may fail to self-excite? 
A shunt excited dc generator may fail to self-excite for any of the following reasons-
  • Residual magnetism is absent. This difficulty can be overcome by exciting the field winding from a separate dc source for a few seconds with the armature at rest.  
  • The field connection to the armature is such that the induced emf due to the residual magnetism tends to destroy the residual magnetism. This condition can be remedied by reversing the field connection to the armature.   
  • The field circuit resistance is more than the critical value. For critical resistance, Rf line coincides with the linear portion of the magnetization curve.  
  • Speed is less than critical speed. At critical speed, the open-circuit characteristic is tangential to the Rf line. 
What is the purpose of providing dummy coils in the armature of a dc machine? 
The purpose of providing dummy coils in the armature of a dc machine is to provide mechanical balance for the rotor.

Where capacitor-start motors are generally used? 
Capacitor-start motors are used where high starting torque is required and where high starting period may be, long.

What is the use of interpoles in dc machines? 
In practice, the interpoles of appropriate polarity are strengthened so that in the interpolar zone, the armature crosses flux is neutralized and in addition, some flux is produced there. This additional flux in the interpolar zone induces rotational emf in the commutated coil in such a direction as to oppose the reactance voltage. So, the resultant emf in the commutated coil would be zero and therefore zero current in that coil would amount to sparkles commutation. This is the reason why interpoles are designed to provide more mmf than the armature mmf in the commutating zone. In practice, the inter-pole mmf may be  1.2 to 1.3 times the armature mmf per pole.

Why the air gap between the yoke and armature in a dc motor is kept small? 
To achieve a stronger magnetic field, the air gap between the yoke and the armature is kept small in rotating machines.

When the motor is said to be crawling? 
An induction motor when started on load does not accelerate up to full speed but runs at 1/7th of the rated speed, then, the motor is said to be crawling.

What is the cause of crawling in the induction motor? 
The crawling in the induction motor is caused by harmonics developed in the motor.

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