1. In a cumulative compound DC motor with rated voltage applied with rated load, if the series winding part is shorted, then speed will be:
Less than full speed
More than rated speed
Same
Zero
Answer: 2. More than rated speed
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.
In cumulative compound dc motor, field flux is given by
ϕ = ϕse + ϕsh
ϕse = series field
ϕsh = shunt field
The relation between induced emf and speed and series field is given by
Eb = knϕN
Where kn = constant
N = Speed in rpm
Now if the series field is short-circuited
Then total field flux get reduced
N ∝ 1/φ
Hence, speed N gets increased above rated speed.
2. Net field current in a compound motor is given by _______
If (net)= If + (Nse/Nf) Ia
If (net)= If – (Nse/Nf) Ia
If (net)= If + (Nf/Nse) Ia
If (net)= If – (Nf/Nse) Ia
Answer: 2. Series
Explanation:
Compound motors are so named because they contain both a series and shunt fields. Compound motors are the most used DC motors in the industry.
They combine the high torque characteristics of series motors with the speed regulation of shunt motors. Although their speed regulation is not as good as a shunt motor, it is much better than the series motor.
Net flux in a compound motor is determined by the addition of shunt field flux and series field flux.
Thus, for calculating effective flux, the net field current is found out. It is the constant value given for shunt field, while If net takes the presence of series field also.
If (net)= If + (Nse/Nf) Ia
Nf = number of turns per pole of the shunt field winding
Nse = number of turns per pole of the series field winding
3. A 4-pole, long shunt lap wound generator supplies 20 kW at a terminal voltage of 400 V. The armature resistance is 0.02 Ω, series field resistance is 0.04 Ω and shunt field resistance is 100 Ω. The brush drop may be taken as 1.0 V. Determine the induced EMF.
405.24 V
400 V
380 V
505 V
Answer: 1. Interchanging field terminals
Explanation:
Given
Vt = Terminal Voltage (V) = 400 V
IL = Line Current ( = 50 A
Rsf = Series field resistance (Ω) = 0.04 Ω
Ra = Armature resistance (Ω)= 0.02 Ω
Rshf = Shunt field resistance (Ω) = 100 Ω
Ish = Shunt current ( = Vt / Rshf = 400 / 100 = 4 A
Ia = Armature current (
Eg = Generated EMF (V)
Vb = Brush voltage drop ( = 2 Vb1 = 2 × voltage drop per brush)
The induced EMF in long-shunt compound generator:
Eg = Vt + Ia(Ra + Rsf) + 2Vb1
P = Vt × Il ⇒ 20000 = 400 × Il
Il = 50 A
Ia = Ish + Il
Ia = 4 + 50 = 54 A
Eg = Vt + Ia(Ra + Rsf) + 2 Vb1
Eg = 400 + 54(0.02 + 0.04) + 2 × 1
Eg = 405.24 V
4. Which DC motor is more preferred for elevators?
Shunt motor
Series motor
Differential compound motor
Cumulative compound motor
Answer: 4. Cumulative compound motor
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.
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.
5. With increase in armature current, speed of the cumulative compound motor _________
Reduces slowly compare to shunt motor
Reduces sharply compare to shunt motor
Remains constant
Increases in proportionality with armature current
Answer: 4. Cumulative compound motor
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.
In the cumulatively compounded 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. Series field strength increases as in a series motor.
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.
As in the series motor, the decrease in speed is necessary to allow the counter emf to decrease at the same time the field increases.
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.
6. The motor used for punching machine is subjected to
No-load
Continuous part load
Continuous full load
Intermittent load
Answer: 4. Intermittent load
Explanation:
Intermittent loads: The load on the motor occurs periodically in an identical duty cycle, each duty cycle having a period of application of load and rest.
Cumulative compound motors develop good starting torque and also its no-load speed is limited. Therefore these motors are used for intermittent load applications. I.e. the load operation consists of load and no-load operation intervals continuously.
Cumulative compound motors develop good starting torque and also its no-load speed is limited. Therefore these motors are used for intermittent load applications. I.e. the load operation consists of load and no-load operation intervals continuously.
8. Speed current characteristic of cumulative DC compound motor at larger values of armature current is similar to that of _______
DC shunt motor
Induction motor
DC differential compound motor
DC series motor
Answer:3. Cumulative compound motor
Explanation:
In a cumulative compound-wound motor the series-field winding is connected in such a way that the flux produced by it helps the flux produced by the main field winding,
Speed current characteristic of the cumulative DC compound motor at larger values of armature current is similar to that of DC series Motor.
In the cumulatively compounded 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. Series field strength increases as in a series motor.
As in the series motor, the decrease in speed is necessary to allow the counter emf to decrease at the same time the field increases.
9. In which type of DC generator, one part of the field winding is in parallel with the armature winding while the other part is in series?
Series
Independently active
Shunt
Compound
Answer:4. Compound
Explanation:
In a compound-wound DC Machine, there are two sets of field windings on each pole, one is in series and the other in parallel with the armature.
A compound wound DC Machine may be:
Short Shunt in which only shunt field winding is in parallel with the armature winding.
Long Shunt in which shunt field winding is in parallel with both series field and armature winding.
10. 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
11. 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.
12. 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.
13. 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: 2. Interchanging field terminals
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.
14. 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.
15. 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.
16. For strong series field, speed-torque characteristic of cumulative compound motor lies __________
Above series motor characteristic if current is less than full load current
Above shunt motor characteristic if current is less than full load current
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. Interchanging field terminals
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.
17. DC motor is to drive a load that is almost zero for a 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: 2. Interchanging field terminals
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 flywheel so that this machine can deal with peak value.
18. 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.
Edit:
Also, as flux decreases, the new operating speed will be more than its operating speed.
19. ______ motors have applications with loads that require Pulsating Load.
DC Series
DC Shunt
Cumulative Compound
Differentially compound
Answer: 2. Interchanging field terminals
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.
