Design of Electrical Apparatus Important Questions and answers

1. What is peripheral speed?


The peripheral speed is a translation speed that may exist at the surface of the rotor while it is rotating.

Peripheral speed Va = π Dr in m/sec.

2. What is meant by unbalanced magnetic pull?


The unbalanced magnetic pull is the radial force acting on the rotor due to non-uniform air-gap around armature periphery.

3. List the advantage of Hydrogen cooling.


  1. Increased efficiency.
  2. Increasing rating.
  3. Increase is life.
  4. Smaller size of coolers.
  5. Eliminating of Fire Hazards.
  6. Less noise.

4. Define SCR? What are the effects of SCR on machine performance.


It is defined as the ratio of field current required to produced rated voltage on open circuit to field current required to circulate rated current at sheet circuit.

SCR = Field c.t for OC Volt/Field c.t for SC Volt

With higher values of SCR the machine has higher stability limit and low value of regulation.

5. What is runaway speed?


It is defined as the speed which the prime mover would have, if it is suddenly unloaded, when working as its rated load.

6. What is the use of damper windings in Synchronous Generators and Synchronous Speed.


The damper windings is used to reduce the oscillation developed in the rotor of alternator when it is suddenly loaded.

The damper winding is used to start the synchronous motor as an induction motor.

7. Why is length of air gap in an induction motor kept at minimum possible range?


  1. To reduce in armature reaction.
  2. Small value of regulation.
  3. Higher value of stability.
  4. A higher synchronizing power which makes the machine less sensitive to load variations.

8. What are the effects of open slots and ducts on magnetic circuit of a machine?


  • Due to open slots, the flux will be crowd over the teeth.
  • Reluctance of slotted armature with ducts will be increased.

9. What is window space factor?


It is defined as the ratio of copper area in window to total area of window.

10. What are the advantages of having circular coil in a transformer?


  • These circular coil in a transformer will be used in high KVA rating.
  • It used 1 and 2 strips only in parallel.
  • Coil deformation will be avoided.

11. Give the methods adopted to reduce the effect of armature reaction in a DC machine.


  • Increase in length of air gap at pile tips
  • Increase reluctance of pile tips
  • Compensating winding (d) interpoles.

12. List the advantages of lap winding in a DC machine.


  • The total number of brushes is equal to the number of poles.
  • It provides large current and low voltage. Hence it is used for large machines.

13. What is dispersion coefficient in a induction motor?


It is defined as the ratio of magnetizing current to ideal short circuit current.

14. Give the methods to reduce harmonic torques in an induction motor.


The methods to reduce harmonic torques are (a) chording (b) Integral slot windings (c) Skewing

15. What are the merits of computer aided design?


It has capability to sore amount of data, count integers and other data in advance.

It makes it possible to select an optimized design with a reduction in cost.

16. What is run away speed?


It is defined as the speed which the prime mover would have, it it is suddenly unloaded when working at its rated load.