Quiz: Electrical Engineering

Exam: DFCCIL

Topic: DC Motor

Each question carries 1 mark.

Negative marking: 1/4 mark

Time: 10 Minute

Q1. The field flux of a DC motor can be controlled to achieve…….

(a) The speed which is at rated speed

(b) The speed which is higher than rated speed

(c) The speed which is lower than the rated speed

(d) None of the above

Q2. The T vs I_a graph of a DC series motor is

(a) Straight line throughout

(b) Parabola throughout

(c) Parabola from no-load to overload

(d) Parabola up to full load and a straight line at overload

Q3. A series motor will over speed when…………….

(a) The load is increased

(b) The field is opened

(c) The armature circuit is opened

(d) The load is removed

Q4. A cumulatively compounded motor doses not run at dangerous speed at light loads because of the presence of ………

(a) Shunt winding

(b) Series winding

(c) Interpoles

(d) Compensating windings

Q5. D.C. series motors are used in those application where……………is required.

(a) High stating torque

(b) Constant speed

(c) Low no-load speed

(d) None of the above

Q6. A d.c. series motor is most suitable for……………

(a) Cranes

(b) Pumps

(c) Lathes

(d) Punch presses

Q7. The most suitable motor for elevators is the ……motor.

(a) Series

(b) Shunt

(c) Differentially compounded

(d) Cumulatively compounded

Q8. A 440 V shunt motor has an armature resistance of 0.8 Ω and a field resistance of 200 Ω. Find the back e.m.f when given and output of 7.46 kW at 85% efficiency.

(a) 222.4 V

(b) 425.8 V

(c) 312. 6 V

(d) 392.7 V

Q9. A 250 V d.c. shunt motor takes a total current of 20 A. Resistance of shunt field winding is 200 Ω and that of armature is 0.3 Ω. What is the current in the armature?

(a) 9.65 A

(b) 11.25 A

(c) 18.75 A

(d) 16.62 A

Q10. A d.c. shunt motor runs at 500 r.p.m. at 220 V. A resistance of 4.5 Ω is added in series with the armature of speed control. The armature resistance is 0.5 Ω. The current to stall the motor is

(a) 44 A

(b) 50 A

(c) 60 A

(d) 30 A

SOLUTIONS

S1. Ans.(b)

Sol. Speed control by flux is limited to the weakening of the field, which gives an increase in speed. This method is applicable over only to a limited range because if the field is weakened too much, there is a loss of stability.

It is constant power drive controlling scheme.

S2. Ans.(d)

Sol. T vs I_a graph of a DC series motor:

Up to full load: T α ϕI_a or T α I_a^2 (since ϕ α I_a)

So, graph will be parabolic up to full-load.

After full-load, 𝟇 is constant due to magnetic saturation.

∴T α I_a (a straight line)

S3. Ans.(d)

Sol. In a d.c. series motor, flux per pole depends upon the armature current. if the load is decreased, armature current and flux per pole are decreased. Consequently, the speed of the motor increases. If the load is small, the speed may rise to dangerously high values. For this reason, we never permit a d.c. series motor to operate on no-load.

S4. Ans.(a)

Sol. When the load from a cumulatively compounded motor is almost entirely remove the Shunt winding field prevents the speed from rising above a safe value.

S5. Ans.(a)

Sol. In a d.c. motor, T_a ∝ϕ〖 I〗_a. At first (i.e., at starting), ϕ∝I_a in a series d.c. motor so that T_a ∝ϕI_a^2

Consequently, starting torque is very high.

S6. Ans.(a)

Sol. A d.c. series motor runs at high speed at light loads and at low speed when load is heavy. It is, therefore, suitable for cranes and hoists; light loads are lifted quickly and heavy loads more slowly.

S7. Ans.(d)

Sol. The mechanical characteristic (i.e., speed/torque) of cumulatively compounded motor is intermediate between those of a series motor and a shout motor. This motor is employed where series characteristic is desired but the load may be almost entirely removed e.g., presses, shears, compressors, elevators and reciprocating tools.

When the load from a cumulatively compounded motor is almost entirely remove the Shunt winding field prevents the speed from rising above a safe value.