# Differentially compound DC motors MCQ || Differentially compound DC motors Questions and Answers

1. Differentially compound DC motors are used in applications requiring _______

1. High starting torque
2. Low starting torque
3. Variable speed
4. Frequent on-off cycles

Explanation:

A compound wound DC motor is said to be deferentially compounded when the flux due to the shunt field winding diminishes the effect of the main series winding.

Differential type compound DC motor is never practically used, as it is unstable for all possible applications. Its speed increases with the increase in the load, leading to unitability.

The differential compound series motor gives low starting torque, examined by torque current characteristic. Hence, applications with low starting torque are called in differentially compound DC motors.

1. Series motor
2. Shunt motor
3. Cumulative compound motor
4. Differential compound motor

Explanation:

In a differential compound motor, because of the opposition of the series field to the shunt field, the flux decreases as the load and armature current increase. A decrease in flux causes an increase in speed. Under heavy load the speed of the differential compound motor is unstable; and if the overload current is very heavy.

At higher values of load. flux reduces so sharply at a small increase in load.  It is advisable that the motor should not be used beyond some load value, as it may damage itself by self-loading.

At high load flux decrease very sharply even for a small change in load.

Speed = back EMF / flux

Flux = main flux – armature flux

Armature flux depends on load current.

3. ______ DC motor Practically found no application.

1. Series motor
2. Shunt motor
3. Cumulative compound motor
4. Differential compound motor

Explanation:

In the differential compound motor, an increase in load creates an increase in current and a decrease in total flux in this type of motor.

These two tend to offset each other and the result is a practically constant speed. However, since an increase in load tends to decrease the field strength, the speed characteristic becomes unstable. This is why the differential compound motor is not used in practice.

4. For the same H.P. rating and full load speed, which of the following motor has poor starting torque?

1. Series motor
2. Shunt motor
3. Cumulative compound motor
4. Differential compound motor

Explanation:

In the differential compound motor, an increase in load creates an increase in current and a decrease in total flux in this type of motor.

More armature current is required of the differential compound motor than of the shunt motor for the same increase in torque.

This results from the fact that an increase in the armature and series-field current reduces the field flux.  It has poor starting torque as the resultant flux is minimized by this opposition. The flux starts decreasing with an increase in load. The decrease in flux causes the starting torque to be less than any other DC motor.

Since the torque acting on the armature is proportional to the product of armature current and field strength, a decrease in field strength must be accompanied by a disproportionate increase in armature current in order that the product will increase.

5. Differential compound D. C. motors can find applications requiring _______

1. High starting torque
2. Low starting torque
3. Variable speed
4. Frequent on-off cycles

Explanation:

In the differential compound motor, an increase in load creates an increase in current and a decrease in total flux in this type of motor.

More armature current is required of the differential compound motor than of the shunt motor for the same increase in torque.

DC Differentially compound motor has low starting torque and has a constant speed, but the dangerously high speed at no load under the circumstances it is seldom used.

6. Only motor listed below whose speed increases with increase in armature current is ___________

1. DC series motor
2. DC cumulative compound motor
3. DC shunt motor
4. DC differential compound motor

Explanation:

In the differential compound motor, an increase in load creates an increase in current and a decrease in total flux in this type of motor.

More armature current is required of the differential compound motor than of the shunt motor for the same increase in torque.

The differential compound motor has its series and shunt coils wired so that an increase in armature current will cause a decrease in the flux field.

This allows the motor to increase in speed as the motor’s load is increased. Because the field flux varies at a slower rate than the armature current in this motor, the torque will exceed the demand of the load. This is the only type of motor that provides for negative regulation.

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

7. Which of the following motor has negative speed Regulation?

1. DC series motor
2. DC cumulative compound motor
3. DC shunt motor
4. DC differential compound motor

Answer: 4. DC differential compound motor

Explanation:

The differential compound motor has its series and shunt coils wired so that an increase in armature current will cause a decrease in the flux field.

This allows the motor to increase in speed as the motor’s load is increased. Because the field flux varies at a slower rate than the armature current in this motor, the torque will exceed the demand of the load.

This is the only type of motor that provides for negative regulation.

8. ______ motor is sometimes referred to as a “suicide motor.”

1. DC series
2. DC cumulative compound
3. DC shunt
4. DC differential compound

Answer: 4. DC differential compound motor

Explanation:

The differential compound motor has its series and shunt coils wired so that an increase in armature current will cause a decrease in the flux field.

This allows the motor to increase in speed as the motor’s load is increased. Because the field flux varies at a slower rate than the armature current in this motor, the torque will exceed the demand of the load. This is the only type of motor that provides for negative regulation.

DC differential compound motor is sometimes referred to as a “suicide motor.”

While the motor is operating, if the shunt field suddenly was to open, the series field would then take control. This would cause the polarity of the field poles to reverse.

The motor will come to a sudden stop and begin rotating in the opposite direction as a series motor. Under certain conditions, this can cause extreme damage to the driven parts.

Under other conditions of coupling, the load could be removed with reverse rotation. The motor would accelerate in speed and destroy itself.

9. What will happen if shunt field suddenly opens, in a differential compound DC motor?

1. Motor will first stop and then run in opposite direction as series motor
2. Motor will not work and come to stop
3. Motor will work as series motor and run at infinitely high speed
4. Motor will work as series motor and run at slow speed

Answer: 2. Motor will first stop and then run in opposite direction as series motor

Explanation:

• The differential compound motor has its series and shunt coils wired so that an increase in armature current will cause a decrease in the flux field.
• While the motor is operating, if the shunt field suddenly was to open, the series field would then take control. This would cause the polarity of the field poles to reverse.
• The motor will come to a sudden stop and begin rotating in the opposite direction as a series motor. Under certain conditions, this can cause extreme damage to the driven parts.
• Under other conditions of coupling, the load could be removed with reverse rotation. The motor would accelerate in speed and destroy itself.

10. What will happen if the series field of a differential compound motor is short-circuited?

1. Net flux will decrease
2. Speed will increase
3. Speed will remain constant
4. Speed will decrease

Explanation:

The differential compound motor has its series and shunt coils wired so that an increase in armature current will cause a decrease in the flux field.

Total MMF created in Differential Compound motor is due to series field MMF and Shunt Field MMF.

NET MMF = MMF(Shunt) – MMF(series)

• Shunt field is always stronger than series field so it always dominates the series field MMF
• As the series field is short-circuited, the current through the series field is zero and MMF due to the series field becomes zero.
• Hence total MMF in motor becomes greater than the previous case, which results in a total increase in the Flux created.
• As Electromagnetic Torque is proportional to armature current and total Flux, the Te produced by the motor will be increased by a small amount.
• Also, as flux increases, the new operating speed will be less than its rated speed.
•  As the series field is short-circuited, the current through the series field becomes zero, and MMF due to the series field becomes zero. Hence, net MMF in motor becomes greater than the previous case, which results in a total increase in the flux created and a decrease in speed.

11. DC differential compound motor is used in _____

1. Traction
2. Punches
3. Experimental and Research work
4. All of the above

Answer: 3. Experimental and Research work

Explanation:

In the differential compound motor, an increase in load creates an increase in current and a decrease in total flux in this type of motor.

More armature current is required of the differential compound motor than of the shunt motor for the same increase in torque.

DC Differentially compound motor has low starting torque and has a constant speed, but the dangerously high speed at no load under the circumstances it is seldom used.

DC differential compound motor is used in Experimental and Research work.

12. _______ motors also have a tendency to start in the reverse direction if under a heavy load.

1. DC Series
2. DC Shunt
3. Cumulative Compound
4. Differentially compound