61. Flywheel is used with DC compound motor to reduce the peak demand by the motor, compound motor has to be ______
Level compounded
Under compounded
Cumulatively compounded
Differentially compounded
Answer:3. Cumulatively compounded
Explanation:
A flywheel is a big wheel that is mounted on the same shaft of the motor if the speed of the motor is not to be reversed. It is used in the load equalization process.
To operate the flywheel efficiently, the driving motor should have drooping characteristics
The flywheel is not used with motors having constant speed motors. Hence, we can’t use a flywheel along with a synchronous motor as it is a constant speed motor.
In the 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.
The flywheel will smooth out the load on the motor and will also reduce peaks on the power system. So flywheels may be used with a Cumulatively Compounded motor during heavy loads.
62. Torque Current characteristics of the cumulative compound DC motor is ________
Lies above shunt motor characteristic
Lies below shunt motor characteristic
Lies above of series and shunt motor characteristic both
Lies between of series and shunt motor characteristic both
Answer: 4. Lies between of series and shunt motor characteristic both
Explanation:
The figure shows the torque—load current characteristics of the shunt, series, and cumulative compound motors.
Except for lighter loads, the series motor has much greater torque than the shunt or the cumulative compound motor for a given armature current.
In the case of a cumulative compound motor, as the armature current increases, the series flux increases, so flux per pole increases.
But T α Iaφ
Consequently, the torque also increases; however, this increase in torque is greater than the shunt motor but less than that of the series motor.
62. Torque of the cumulative compound DC motor is ________
Lies above shunt motor characteristic
Lies below shunt motor characteristic
Lies above of series and shunt motor characteristic both
Lies between of series and shunt motor characteristic both
Answer: 3. Lies above of series and shunt motor characteristic both
Explanation:
A compound wound DC motor is said to be cumulatively compounded when the shunt field flux produced by the shunt winding assists or enhances the effect of main field flux, produced by the series winding.
Because the torque varies directly as the product of the armature current and the field flux, it is evident that the cumulative compound motor has greater starting torque than does the shunt motor for equal values of armature current and shunt field strength.
Cumulative motors give high starting torque like a series motor and reasonable good speed regulation at high speeds like a shunt DC motor. It can work with huge loads and run smoothly after that. It is practically suitable for most common applications like elevators.
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.
63. What will happen if shunt field suddenly opens, in a differential compound DC motor?
Motor will first stop and then run in opposite direction as series motor
Motor will not work and come to stop
Motor will work as series motor and run at infinitely high speed
Motor will work as series motor and run at slow speed
Answer: 1. 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.
64. For strong series field, speed-torque characteristic of cumulative compound motor lies ______
Above series motor characteristic if current is less than full load current
A
Below series motor characteristic if current is more than full load current
Below series motor characteristic if current is less than full load current
Answer: 2. Above shunt motor characteristic if current is less than full load current
Explanation:
The figure shows the speed-torque characteristics of the shunt, series, and cumulative compound motors.
The speed of a shunt motor changes very little (in fact, less than 5% in large motors or less than 8% in small motors). Because of this, shunt motors are classified as constant-speed motors.
The speed of a series motor changes drastically as the load or the load torque changes.
However, the cumulative compound motor combines the operating characteristics of the shunt and series motors.
Unlike a series motor, a compound motor has a definite no-load speed and can be safely operated at no load.
As the load or load torque is increased, the growth in the field flux decreases the speed more in a series motor than in a shunt motor.
Due to the strong series field, the 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 the full load current.
For strong series field, speed-torque characteristic of cumulative compound motor lies above shunt motor characteristic if current is less than full load current
65. What will happen if the series field of a differential compound motor is short-circuited?
Net flux will decrease
Speed will increase
Speed will remain constant
Speed will decrease
Answer: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.
66. DC differential compound motor is used in _____
Traction
Punches
Experimental and Research work
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.
67. 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 _____
Series motor
Shunt motor
Compound motor
Any DC motors
Answer:3. Compound motor
Explanation:
The figure shows the speed-torque characteristics of the shunt, series, and cumulative compound motors.
The speed of a shunt motor changes very little (in fact, less than 5% in large motors or less than 8% in small motors). Because of this, shunt motors are classified as constant-speed motors.
The speed of a series motor changes drastically as the load or the load torque changes.
However, the cumulative compound motor combines the operating characteristics of the shunt and series motors. Unlike a series motor, a compound motor has a definite no-load speed and can be safely operated at no load.
As the load or load torque is increased, the growth in the field flux decreases the speed more in a series motor than in a shunt motor.
The cumulative compound motor is provided with a flywheel so that this machine can deal with peak value.
68. What will happen if the series field of a cumulative compound motor is short-circuited?
Net flux will increase
Speed will increase
Speed will remain constant
Speed will decrease
Answer: 2. Speed will increase
Explanation:
The motor will behave as a shunt motor. As the series field is shorted, the shunt field will be only working. However, the overall torque generation will be low because of the loss of cumulative behavior of flux. But speed will increase slightly and would remain almost constant only if the shunt current remains constant.
Total MMF created in Cumulative Compound motor is due to series field MMF and Shunt Field MMF.
MMF = MMF(shunt) + MMF(series)
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 less than the previous case, which results in a total decrease in the Flux created.
As Electromagnetic Torque (Te) is proportional to armature current and total Flux, the Te by the motor will be reduced.
Also, as flux decreases, the new operating speed will be more than its operating speed.
69. ______ motors have applications with loads that require Pulsating Load.
DC Series
DC Shunt
Cumulative Compound
Differentially compound
Answer: 3. Cumulative Compound
Explanation:
In Cumulative-compound motor the series field aids with the shunt field winding, the flux is increased, as load is applied to the motor, and due to this reason, the motor speed slightly decreases.
Such machines are used where series characteristics are required. Due to the shunt field. the winding speed will not become excessively high, but due to the series field winding, it will be able to take heavy loads.
Compound wound motors have the greatest application with loads that require high starting torque and Pulsating Load.
70. _______ motors also have a tendency to start in the reverse direction if under a heavy load.
DC Series
DC Shunt
Cumulative Compound
Differentially compound
Answer: 4. Differentially compound
Explanation:
Differentially compound motors also have a tendency to start in the reverse direction if under a heavy load. Once again, damage may result. For these reasons, the differential compound motor is seldom used in industry.
Note:-
Almost 0% speed regulation can be obtained with a differentially compounded motor.
When a motor is connected as a differential compound machine, the series field opposes the shunt field so that the field fluxes decrease as a load is applied. As a result, the speed remains substantially constant with an increase in load.
With over compounding, a slight increase in speed is possible with an increase in load. This speed characteristic is achieved only with a loss in the rate at which torque increases with load.
Because the field decreases with a load increase, a differential compound motor has a tendency for load instability. When starting a differential motor, it is recommended that the series field be shorted because the great starting current in this field may overbalance the shunt field and cause the motor to start in the opposite direction.
