Electric Traction Motors Interview Questions and Answers:

1. Why the magnetic circuit of a traction motor is not made of cast iron ?

Ans. Because cast iron cannot withstand continuous vibrations but traction motors operate in extremely severe service conditions.

2. Why the traction motor must be small in overall dimensions specially in its overall diameter ?

Ans. The traction motor is located underneath a motor coach (except in few electric locomotives) and the space underneath a motor coach is limited by the size of the driving wheels.

3. Why the traction motor must be of totally enclosed con­struction ?

Ans. The traction motor must be totally enclosed type, particularly when mounted beneath the locomotive or motor coach, to provide protection against ingress of dirt, dust, water, mud etc.

4. Why the traction motor must be light in weight, robust and capable to withstand continuous operation ?

Ans. The traction motor must be light in weight in order to increase the payload capacity of the vehicle.

A traction motor must be robust and capable to withstand continuous vibrations since service conditions are extremely severe.

5. Why series motors are preferred for electric traction ?

Ans. The series motors are ideally suited for traction purposes because of the following reasons :

• The dc series motor develops high torque at start which is an essential requirement of traction service.
• Simple speed control.
• Power drawn from supply mains varies as the square of the load torque and so series motors are self relieving ones.
• Commutation is excellent up to twice full load.
• The series motors are less susceptible to sudden change in supply voltage.
• The series motors are capable of withstanding exces­sive overloads.
• The dc series motors, when operated in parallel to drive a vehicle by means of different driving axles, share load almost equally even there is unequal wear of different driving wheels.
• The dc series motor is simple and robust in construction.

6. Why are the ac single phase series motors not suitable for urban and suburban services ?

Ans. Single phase ac series motors develop low starting torque and poor power factor at start and for urban and suburban services large starting torque is an essential feature to provide high acceleration and frequent starting is involved.

7. Why ac series motors used for traction services are designed to operate at low frequency i.e., 16 2/3 or 25 Hz ?

Ans. Single phase ac series motors have better performance (improved pf, higher efficiency, improved commutation, better starting etc.) if operated on low frequency (16 2/3 or 25 Hz).

8. Why ac series motors used in traction services are designed to operate at low voltage (300 to 400 V) ?

Ans. The operating voltage is kept low (300 to 400 V) in order to reduce inductance. Because high voltage motor with proportionately low current would require a large number of turns to produce the required flux.

9. Why 3-phase induction motors are not used in electric traction ?

Ans. Because of flat speed-torque characteristics, constant speed operation, developing low starting torque, drawing higher current at start, complicated speed control system and complicated overhead feeding system, 3-phase induction motors are not suitable for electric traction.

10. Traction motors are given one-hour rating as well as con­tinuous rating ?

Ans. The motors are designed for intermittent loading as well as continuous loading. The motors employed in traction work are subject to intermittent loading particularly in case of tramway and suburban services, therefore, traction motors are given one-hour rating as well as continuous rating.