Interview Questions - Electrical Machines

What could be the worst type of load on a supply system?
Arc furnace load is the worst type of load on a supply system.

What will be the effect of armature mmf on the main field flux and its consequences?
The armature mmf produces two undesirable effects on the main field flux and these are
  • The net reduction in the main field flux per pole.
  • Distortion of the main field flux wave along the air gap periphery.
Reduction in the main flux per pole reduces the generated voltage and torque, whereas the distortion of the main field flux influences the limits of successful commutation in d.c machines.

What do you mean by armature reaction?
When the armature of a d.c machine carries current, the distributed armature winding produces its own mmf (distributed) known as armature reaction.

What are the methods for starting of 3-phase squirrel-cage induction motors?
There are primarily two methods of starting squirrel cage induction motors-
  • Full-voltage starting, and
  • Reduced-voltage starting.
The full-voltage starting consists of DOL starting, only reduced voltage starting is the most popular method of starting 3-phase squirrel-cage induction motors and consists of-
  • Stator impedance starting.
  • Auto-transformer starting.
  • Star-delta starting.
Which characteristic of a dc series motor makes it be used in electric locomotives, cranes etc?
Due to high starting torque dc series motor is used in electric locomotives, cranes etc.

What are slot wedges in a dc machine, made of?
Slot wedges are used to reduce the air gap so made of silicon steel.

Why sparking occurs at the brushes during commutation in a dc machine running with a heavy load and with the brushes located on the geometrical neutral axis?
When zero-crossing of the flux density wave shifted from GNA to MNA, the coils undergoing commutation do not have zero emf induced in them. The induced emf in the commutated coils delays the reversal of armature current in the short-circuited coils; this may result in detrimental sparking.

State Faraday’s law.
Faraday’s law states that a time-varying magnetic field produces an electromotive force (emf) which may establish a current in a suitable closed circuit.

When a slip ring induction motor will run at super synchronous speed?
A slip ring induction motor run at super synchronous speed when an emf is injected in the rotor circuit in phase with the rotor induced emf.

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