# DC Machines Questions and Answers – Speed Control Using Field Control of Series Motor

This set of Basic DC Machines Questions and Answers focuses on “Speed Control Using Field Control of Series Motor”.

1. Which of the following DC motor has the poorest speed control?
a) Differentially compounded motor
b) Cumulatively compounded motor
c) Shunt motor
d) Series motor

Explanation: DC series motor at no load condition gives infinite speed ideally. Practically it will damage all the armature circuit. Thus, as the load is reduced speed of the motor will go on increasing rapidly. So, speed control is very poor in series motor.

2. In variable speed motor ____________
a) Stronger commutating field is needed at low speed than at high speed
b) Weaker commutating field is needed at low speed than at high speed
c) Same commutating field is needed at low speed and at high speed
d) Cannot be determined

Explanation: According to commutating fields of machine, speed and overall performance of a machine is determined. For low speeds of motor in a variable speed motor we need weaker commutating field.

3. The speed of a motor falls from 1200 rpm at no-load to 1050 rpm at rated load. The speed regulation of the motor is ____________
a) 12.36%
b) 14.28%
c) 16.77%
d) 18.84%

Explanation: Speed of regulation of DC series motor is given by ratio of difference of no-load speed and full load speed with full load speed. Thus, in this case, speed regulation = (1200-1050)/1050= 14.28%.

4. Which of the following is not the method of speed control in DC series motor?
a) Diverter
b) Tapped-field control
c) Variable resistance in series with armature
d) Series- parallel control

Explanation: Diverter means adding a variable resistance in parallel with field winding. Tapped field control is the method where field ampere turns are adjusted in steps by varying the number of turns included in the circuit. In series parallel method field winding is connected so that it will form two parts.

5. For speed reversal, field control method is suitable.
a) True
b) False

Explanation: Field control method is not suited to applications needing speed reversal; since the only way to reverse speed is to disconnect the motor from the source and reverse the field/armature polarity. The field circuit being highly inductive, it is normally the armature which is reversed.
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6. For large motors what is the ratio of compensating winding is required for increasing the speed of the motor?
a) 2:1
b) 4:1
c) 6:1
d) 8:1

Explanation: For motors requiring a wide range of speed control, the field ampere-turns are much smaller than the armature ampere-turns at high speeds causing extreme distortion of the flux density in the air-gap. This leads to poor commutation. Compensating winding can be used to increase the speed range which can be 2 to 1 for large motors.

7. In diverter resistor field control method of DC series motor, variable resistor is added ________
a) In parallel with field
b) In series with field
c) In parallel with armature

Explanation: Diverter is a small variable resistor, which is connected in parallel with field winding. By varying this resistance value, the field current and consequently the field ampere-turns can be varied and speed can be controlled.

8. Why it is advisable to use inductively wound diverter resistor?
a) To make speed control on more range
b) For long-life of machine
c) Cost efficient
d) To avoid oscillations in speed

Explanation: One precaution to be taken in this method in order to avoid oscillations in speed initiated by load changes is to use an inductively wound diverter resistor. Thus, speed controlled output will vary smoothly when load is changed.

9. For higher diverter resistance, speed-torque characteristic will lie ____________
a) Above speed-torque characteristic of lower resistance
b) Below speed-torque characteristic of lower resistance
c) On the speed-torque characteristic of lower resistance
d) Cannot say

Explanation: Speed-torque characteristic for decreasing values of diverter resistance or decreasing value of kd will lie above of the previous or lower value of diverter resistance. Thus, for higher diverter value curve will lie below the previous one.

10. In tapped field control method ____________
a) A variable resistor is connected in parallel
b) A variable resistor is connected in series
c) Field winding is made with more out pins
d) Another field winding is added with the previous one

Explanation: In tapped field control of the DC series motor a field winding is simply tapped at various points and many out pins taken, so that any out pin can be connected and particular part of field winding will be selected in the circuit, while other part is made open.

11. Which of the following method will not give many speed values for a DC series motor?
a) Diverter
b) Series-parallel
c) Field tapped
d) All field control methods will give many speed values

Explanation: In series-parallel method, field winding is equally divided in two parts which are connected in series and parallel with respect to each other. Thus, in series connection we’ll get one speed and in parallel mode we’ll get one speed. These 2 speeds are only possible.

12. In series-parallel speed control method ______________
a) Speed given by parallel connection is more
b) Speed given by series connection is more
c) Both speeds can be equal
d) Will depend on other parameters

Explanation: In parallel case, total ampere turns will be exactly half of the ampere turns of the series case. Since speed of any DC motor is inversely proportional to the MMF or flux or ampere turns developed, speed of the parallel control method will always large than the series one.

13. Where diverters are used?
a) In shunt motors
b) In series motors
c) In both motors
d) All other motors except shunt and series motors

Explanation: Diverters are used in series motors for speed control, they are not used in shunt motors. Since, shunt field winding resistance is very high, if we connect a diverter across shunt field winding, total current will flow through diverter and it will almost short circuit the shunt field winding. This will increase the motor speed to a very high value.

14. What will happen to the speed of a series motor if the temperature of armature resistance is increased?
a) Not change
b) Decreases
c) Increases
d) Cannot be determined

Explanation: In a DC series motor, when the resistance increases due to rise in temperature, the IR drop in the circuit will increase, which reduces the effective voltage applied to the armature, hence speed decreases due to the direct proportionality.

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