DC Machines Multiple Choice Questions on “Operating Characteristics of DC Compound Motor – 1”.
1. Differentially compound DC motors are used in applications requiring _______
a) High starting torque
b) Low starting torque
c) Variable speed
d) Frequent on-off cycles
Answer: b
Clarification: Compound motor shows combine effect of shunt and series field windings. 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 motor.
2. Which DC motor is more preferred for elevators?
a) Shunt motor
b) Series motor
c) Differential compound motor
d) Cumulative compound motor
Answer: d
Clarification: Cumulative wound DC motors give high starting torque like a series motor and reasonable good speed regulation at high speeds like a shunt dc motor. As this type of motor offers the best of both series and shunt motor, it is practically suitable for most common applications like elevators.
3. Which DC motor is used in shears and punches?
a) Shunt motor
b) Series motor
c) Cumulative compound motor
d) Differential compound motor
Answer: c
Clarification: Cumulative compound DC motor provides high starting torque which is required at instant of starting in shears and punches. It also provides good speed regulation in order to vary pressure. So, this motor suits well to shears and punches application.
4. Which DC motor has got maximum self-loading property?
a) Series motor
b) Shunt motor
c) Cumulative compound motor
d) Differential compound motor
Answer: d
Clarification: A differentially compound DC motor, flux reduces so sharply at small increase in load at higher values of load. It is advisable that motor should not be used beyond some load value, as it may damage itself by self-loading.
5. For the same H.P. rating and full load speed, which of the following motor has poor starting torque?
a) Series motor
b) Shunt motor
c) Cumulative compound motor
d) Differential compound motor
Answer: d
Clarification: In differential compound motor, series field opposes shunt field. It has poor starting torque as the resultant flux is minimized by this opposition. The flux starts decreasing with increase in load. The decrease in flux cause the starting torque to be less than any other DC motor.
6. Unlike DC series motor, DC compound motor can be started on no-load.
a) True
b) False
Answer: a
Clarification: Cumulative motor takes best from shunt motor and series motor as well. Due to shunt field flux component, DC cumulative motor can be started on no load. No-load speed of DC cumulative motor is greater than that of DC shunt motor but it’s not equal to infinity like series motor.
7. Flywheel is used with DC compound motor to reduce the peak demand by the motor, compound motor has to be _____________
a) Level compounded
b) Under compounded
c) Cumulatively compounded
d) Differentially compounded
Answer: c
Clarification: In case of cumulative compound DC motor, whenever a mechanical load is subject to fluctuations for short durations, a flywheel is generally provided for carrying peaks. Flywheel will smooth out the load on the motor and will also reduce peaks on power system.
8. What will happen if shunt field suddenly opens, in a differential compound DC motor?
a) Motor will first stop and then run in opposite direction as series motor
b) Motor will not work and come to stop
c) Motor will work as series motor and run at infinitely high speed
d) Motor will work as series motor and run at slow speed
Answer: a
Clarification: If shunt field is suddenly opened, flux due to shunt field will become 0. As, net flux is equal to shunt field flux – series field flux, the direction of flux will be reversed and thus motor will run in opposite directions.
9. What will happen if the series field of a differential compound motor is short circuited?
a) Net flux will decrease
b) Speed will increase
c) Speed will remain constant
d) Speed will decrease
Answer: d
Clarification: As the series field is short circuited, current through series field becomes zero and MMF due to series field becomes zero. Hence, net MMF in motor becomes greater than previous case, which results in total increase in the flux created and decrease in speed.
10. What will happen if the series field of a cumulative compound motor is short circuited?
a) Net flux will increase
b) Speed will increase
c) Speed will remain constant
d) Speed will decrease
Answer: b
Clarification: As the series field is short circuited now, current through series field becomes zero and MMF due to series field becomes zero. Hence, total MMF in motor becomes less than previous case, which results in total decrease in the flux created and increase in speed.
11. DC motor is to a drive a load which is almost zero for certain part of the load cycle and peak value for short duration. We will select __________
a) Series motor
b) Shunt motor
c) Compound motor
d) Any DC motors
Answer: c
Clarification: We can’t use series motor as our load is almost zero at some points. Thus, we’ll use compound motor which can work on no load also. Cumulative compound motor is provided with flywheel so that this machine can deal with peak value.