11. Which DC motor has got maximum self-loading property?
Series motor
Shunt motor
Cumulative compound motor
Differential compound motor
Answer: 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.
So it is advised not to load it beyond a certain limit. It may damage itself due to self-loading.
12. ______ DC motor Practically found no application.
Series motor
Shunt motor
Cumulative compound motor
Differential compound motor
Answer: 2. 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.
13. The voltage and current in various branch in a long-shunt compound generator is given in the following network. Determine the generated EMF.
510 V
502.2 V
504.16 V
512.2 V
Answer:3. 504.16 V
Explanation:
Given:
Ish = 2 A
Il = 50 A
Vt = 500 V
Ra = 0.05 Ω
Rsf = 0.03 Ω
Rshf = 250 Ω
Ia = Ish + Il
Ia = 2 + 50 = 52 A
Eg = Vt + Ia(Ra + Rsf)
Eg = 500 + 52(0.05+0.03)
Eg = 504.16 V
14. For the same H.P. rating and full load speed, which of the following motor has poor starting torque?
Series motor
Shunt motor
Cumulative compound motor
Differential compound motor
Answer: 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.
15. . Where speed-current characteristic of the DC shunt motor lies with respect to DC cumulative compound motor (assume current greater than full load current)?
Can be represented by the same curve
Above
Below
Cannot be determined
Answer: 2. Interchanging field terminals
Explanation:
For current greater than full load current, The speed-current characteristic of DC shunt motor lies above with respect to DC cumulative compound motor.
In the cumulative compound motor the speed decreases (when a load is adde more rapidly than it does in a Shunt motor, but less rapidly than in a series motor.
The speed of the cumulative compound motor falls much more sharply than the shunt motor. Therefore, the Speed- armature current characteristic of the Shunt motor lies above that of the compound motor.
16. Differential compound D. C. motors can find applications requiring _______
High starting torque
Low starting torque
Variable speed
Frequent on-off cycles
Answer: 2. Interchanging field terminals
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.
17. DC compound motor can be started on ______
Full Load
No-Load
Medium Load
None of the above
Answer: 2. No-Load
Explanation:
A compound motor runs on ‘no-load’ at a speed determined by its shunt winding since the series field contributes little to the total flux in this condition.
In a cumulative compound motor at ‘no-load’, the series field strengthens the shunt winding so that the speed falls as in a series machine.
Since the flux at any load is equal to the shunt and series fluxes, the speed is less than it would be if running on either field alone.
Such a characteristic has two important advantages.
(i) The machine has the compensating action of reducing its speed on heavy loads, as is the case with the series machine.
(ii) The maximum speed on no-load is limited by the shunt winding since this produces a maximum flux even on no-load.
18. Only motor listed below whose speed increases with increase in armature current is ___________
DC series motor
DC cumulative compound motor
DC shunt motor
DC differential compound motor
Answer: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.
19. Which of the following motor has negative speed Regulation?
DC series motor
DC cumulative compound motor
DC shunt motor
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.
20. ______ motor is sometimes referred to as a “suicide motor.”
DC series
DC cumulative compound
DC shunt
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.