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 = ϕ_{se} + ϕ_{sh}

ϕ_{se} = series field

ϕ_{sh} = shunt field

The relation between induced emf and speed and series field is given by

E_{b} = k_{n}ϕN

Where k_{n} = 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. Differentially compound DC motors are used in applications requiring _______

High starting torque

Low starting torque

Variable speed

Frequent on-off cycles

Answer: 2. Low starting torque

Explanation:

A compound wound DC motor is said to be deferentially compounded when the flux due to the shunt field winding diminishes the effect of the main series winding.

Differential type compound DC motor is never practically used, as it is unstable for all possible applications. Its speed increases with the increase in the load, leading to unitability.

The 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 motors.

3. Net field current in a compound motor is given by _______

I_{f} (net)= I_{f} + (N_{se}/N_{f}) I_{a}

I_{f} (net)= I_{f} – (N_{se}/N_{f}) I_{a}

I_{f} (net)= I_{f} + (N_{f}/N_{se}) I_{a}

I_{f} (net)= I_{f} – (N_{f}/N_{se}) I_{a}

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.

I_{f} (net)= I_{f} + (N_{se}/N_{f}) I_{a}

N_{f} = number of turns per pole of the shunt field winding

N_{se} = number of turns per pole of the series field winding

4. 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

I_{L} = 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 ( = V_{t} / R_{shf} = 400 / 100 = 4 A

Ia = Armature current (

Eg = Generated EMF (V)

V_{b} = Brush voltage drop ( = 2 V_{b1} = 2 × voltage drop per brush)

The induced EMF in long-shunt compound generator:

E_{g} = V_{t} + Ia(Ra + Rsf) + 2Vb1

P = V_{t} × I_{l} ⇒ 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

5. 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.

6. 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.

7. 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.

8. Which DC motor is used in shears and punches?

Shunt motor

Series motor

Cumulative compound motor

Differential compound motor

Answer:3. Cumulative compound motor

Explanation:

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.

9. 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.

10. 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.