Question 1
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
View Answer
Answer: Option C
Explanation:
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.
Question 2
Differential compound motor is used in large number of applications.
A. TRUE
B. FALSE
View Answer
Answer: Option B
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.
Question 3
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
View Answer
Answer: Option B
Explanation:
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.
Question 4
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
View Answer
Answer: Option C
Explanation:
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.
Question 5
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
View Answer
Answer: Option B
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.
Question 6
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
View Answer
Answer: Option D
Explanation:
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.
Question 7
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
View Answer
Answer: Option A
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.
Question 8
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
View Answer
Answer: Option D
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.
Question 9
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
View Answer
Answer: Option D
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.
Question 10
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
View Answer
Answer: Option A
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.
Question 11
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
View Answer
Answer: Option C
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.
Question 12
Unlike DC series motor, DC compound motor can be started on no-load.
A. TRUE
B. FALSE
View Answer
Answer: Option A
Explanation:
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.
Question 13
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
View Answer
Answer: Option A
Explanation:
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.
Question 14
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
View Answer
Answer: Option B
Explanation:
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.
Question 15
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
View Answer
Answer: Option D
Explanation:
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.
Question 16
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
View Answer
Answer: Option B
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).
Question 17
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
View Answer
Answer: Option C
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.
Question 18
Which DC motor has got maximum self-loading property?
A. Series motor
B. Shunt motor
C. Cumulative compound motor
D. Differential compound motor
View Answer
Answer: Option D
Explanation:
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.
Question 19
Which DC motor is more preferred for elevators?
A. Shunt motor
B. Series motor
C. Differential compound motor
D. Cumulative compound motor
View Answer
Answer: Option D
Explanation:
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.
Question 20
Which DC motor is used in shears and punches?
A. Shunt motor
B. Series motor
C. Cumulative compound motor
D. Differential compound motor
View Answer
Answer: Option C
Explanation:
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.
Question 21
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
View Answer
Answer: Option C
Explanation:
In differential compound motors flux will reduce as load is increased because series flux opposes shunt field flux. This will increase speed upon addition of load. Therefore, it has negative speed regulation means speed will increase due to increase in load.
Question 22
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
View Answer
Answer: Option B
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.