Control of Traction Motors Interview Questions and Answers:
1. The resistors employed for control of traction motors are designed for short-time rating. Why ?
Ans. Because control resistors are required only during starting of traction motors.
2. Under what conditions is field weakening adopted for controlling the speed of a dc series traction motor ?
Ans. The field weakening method of speed control is used for increasing the speed of traction motors when they have attained maximum possible speed by series-parallel control system.
3. Under what circumstances series-parallel method of speed control of dc series traction motors adopted ?
Ans. The series-parallel method of speed control of dc series motors is adopted when motors employed are two, four, six or more than this even.
4. What is the main difficulty associated with series-parallel control ?
Ans. The main difficulty associated with series-parallel control is in obtaining a satisfactory method of transition from series to parallel without interrupting the torque or permitting any heavy rush of current.
5. What are the advantages of thyristor control of traction motors ?
Ans. The advantages of thyristor control of traction motors are accurate control and fast response, elimination of bulky on-load tap changer and electromagnetic devices, saving of energy, notch less control, increase in pulling ability of the motive power, and minimum wear and tear.
6. Enumerate the methods employed for speed control of 3-phase induction motors used in electric traction.
Ans. The methods employed for speed control of 3-phase induction motors used in electric traction are (i) rheostatic control (ii) pole-changing control, (iii) cascade control (iv) combination of cascading and pole-changing control method and (v) thyristorised control.
7. In ac locomotive tap-changer is provided on high voltage side of transformer. Give suitable reason.
Ans. The transformer tappings are usually provided on low voltage side of the transformer to avoid insulation difficulties but 11,000 V ac locomotives have been built with high voltage tappings, which enables a lighter construction to be used on account of much lower currents to be dealt with.