# DC Machines Questions and Answers – Operating Characteristics of DC Compound Motor – 2

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This set of DC Machines Multiple Choice Questions & Answers focuses on “Operating Characteristics of DC Compound Motor – 2”.

1. Net field current in a compound motor is given by ______________
a) If (net)= If + (Nse/Nf) Ia
b) If (net)= If – (Nse/Nf) Ia
c) If (net)= If + (Nf/Nse) Ia
d) If (net)= If – (Nf/Nse) Ia

Explanation: Net flux in a compound motor is determined by addition of shunt field flux and series field flux. Thus, for calculating effective flux, net field current is find out. If is the constant value given for shunt field, while If net takes the presence of series field also.

2. With increase in armature current, speed of the cumulative compound motor _________
a) Reduces slowly compare to shunt motor
b) Reduces sharply compare to shunt motor
c) Remains constant
d) Increases in proportionality with armature current

Explanation: As armature current increases, the back emf decreases similar to shunt motor. By the same time series field resistance carries more armature current producing more amount of flux which is inversely proportional speed.

3. Speed current characteristic of cumulative DC compound motor at larger values of armature current is similar to that of _____________
a) DC shunt motor
b) Induction motor
c) DC differential compound motor
d) DC series motor

Explanation: At higher values of current speed of the DC cumulative compound motor starts decreasing due to effect of series field flux, which is also seen in DC series motor. Thus series motor and DC cumulative motor follow same characteristics.

4. Differential compound motor is used in large number of applications.
a) True
b) False

Explanation: A differentially compound motor has flux/pole φ = (φse – φf). It is seen that on over-load φ reduces sharply and so the motor acts like a series motor on no load. This is why the differential compound motor is not used in practice.

5. Where speed-current characteristic of DC shunt motor lies with respect to DC cumulative compound motor (assume current greater than full load current)?
a) Can be represented by same curve
b) Above
c) Below
d) Cannot be determined

Explanation: Because of φse increasing with Ia, the speed of the compound motor falls much more sharply than the shunt motor. Therefore, the n – armature current characteristic of the compound motor lies above that of the shunt motor for Ia < Ia ( fl) and lies below for Ia > Ia ( fl).

6. Where speed-current characteristic of DC shunt motor lies with respect to DC cumulative compound motor (assume current smaller than full load current)?
a) Can be represented by same curve
b) Above
c) Below
d) Cannot be determined

Explanation: At start, speed of DC cumulative motor is more than the speed of shunt motor because of series characteristic effect. Because of φse increasing with Ia beyond full load current, the speed of the compound motor falls much more sharply than the shunt motor.

7. Only motor listed below whose speed increases with increase in armature current is ___________
a) DC series motor
b) DC cumulative compound motor
c) DC shunt motor
d) DC differential compound motor

Explanation: Shunt motor speed though decreases by slight value as armature current increases, it is assumed to be constant. But series motor and cumulative motor speed decreases as armature current increases. Only differential motor flux reduces with increase in armature current thus, speed increases.

8. Torque of the cumulative compound DC motor is _____________
a) Addition of Ia and Ia2 term
b) Subtraction of Ia from Ia2 term
c) Subtraction of Ia2 from Ia term
d) Addition of -Ia and -Ia2 term

Explanation: DC cumulative compound motor is the addition of best of series and best of shunt field motor. For shunt type torque is proportional to armature current while in series motor torque is proportional to square of armature current.

9. Torque-current characteristic of DC cumulative compound motor _____________
a) Lies above shunt motor characteristic
b) Lies below shunt motor characteristic
c) Lies above of series and shunt motor characteristic both
d) Lies between of series and shunt motor characteristic both

Explanation: Starting from zero armature current the characteristic of cumulative motor lies above the series characteristic and shunt characteristic, because it is the sum of series and shunt characteristic.

10. For strong series field, speed-torque characteristic of cumulative compound motor lies __________
a) Above series motor characteristic if current is less than full load current
b) Above shunt motor characteristic if current is less than full load current
c) Below series motor characteristic if current is more than full load current
d) Below series motor characteristic if current is less than full load current

Explanation: Due to strong series field, speed-torque characteristic lies above of shunt field characteristic but below of series field characteristic, as no-load speed is finite for cumulative compound motor, at armature current less than full load current.

11. Which of the following DC motor has negative speed regulation?
a) Shunt motors
b) Series motors
c) Differential compound motors
d) Series and differential compound motors  