# Recent questions and answers in DC motor

Why shouldn’t a DC shunt motor start on the heavy load setting?

What will happen when a DC shunt motor is started with a load?

Why does the speed of a DC shunt motor decrease as the load increases?

With the increase in load on a d.c. shunt motor, speed of the motor (a) increases proportionately (b) remains constant (c) increase slightly (d) reduces slightly

A DC Motor develops a torque of 150 N-m. A 10 percent reduction in the field flux causes a 50 percent increase in armature current. The new value of torque is: A) 102.5 N-m B) 202.5 N-m C) 172.5 N-m D) 232.5 N-m

If a d.c. motor is disconnected from the source and armature circuit is short circuited through a suitable resistance, the process is called (a) regenerative breaking (b) rheostatic breaking (c) dynamic breaking (d) no breaking is possible

A separately excited d.c. motor fed from single phase full converter with firing angle 60° runs at 1000 r.p.m. If motor is connected to single phase semiconverter with the same firing angle of 60°, it would run at (a) 2000 rpm (b) 1850 rpm (c) 1500 rpm (d) 1000 rpm

The widely used series-parallel system of speed control of d.c. series motors in traction work gives a speed range of about (a) 1 : 2 (b) 1 : 3 (c) 1 : 4 (d) 1 : 6

Retardation test on d.c. shunt motors is performed for (a) frictional losses (b) stray losses (c) copper losses (d) iron loss

A d.c. shunt motor runs at no load speed of 1140 r.p.m. At full load armature reaction weakens the main flux by 5%. Whereas the armature circuit voltage drops by 10%. The full load speed of the motor is (a) 1200 r.p.m. (b) 1080 r.p.m. (c) 1040 r.p.m. (d) 1000 r.p.m.

The purpose of inserting external resistance at the time of starting a d.c. motor is to (a) increase the starting torque. (b) increase the armature flux. (c) reduce the starting current. (d) reduce the armature flux.

The function of carbon brush and commutator arrangement in a d.c. motor is (a) to produce unidirectional torque. (b) to produce unidirectional current in the armature. (c) to help in changing the direction of rotation. (d) to reduce sparking.

A dc series motor driving and electric train faces a constant power load. It is running at rated speed and rated voltage. If the speed has to be brought down to 0.25 p.u. the supply voltage has to be approximately brought down to  (A) 0.75 p.u (B) 0.5 p.u (C) 0.25 p.u (D) 0.125 p.u

To conduct load test on a dc shunt motor, it is coupled to a generator which is identical to the motor. The field of the generator is also connected to the same supply source as the motor. The armature of generator is connected to a load resistance. The armature resistance is 0.02 p.u. Armature reaction and mechanical losses can be neglected. ... rated armature current in both motor and generator. The p.u value of this load resistance is  (A) 1.0 (B) 0.98 (C) 0.96 (D) 0.94

The dc motor, which can provide zero speed regulation at full load without any controller is  (A) series (B) shunt (C) cumulative compound (D) differential compound

The chief advantage of Ward-Leonard system D.C. motor speed control is that is  1. Can be used even for small motors 2. has high overall efficiency at all speeds 3. Gives smooth, sensitive and wide speed control 4. Uses a flywheel to reduce fluctuations in power demand

Under which of the following conditions is a DC motor provided with compensating winding used?  1. Wide range of speed control above normal  2. Wide range of steady load variation with no speed control  3. Wide range of rapid variation in load.   Select the correct answer using the codes given below:   (a) 1, 2 and 3 (b) 2 and 3 (c) 1 and 2 (d) 1 and 3

A 220 V shunt motor develops a torque of 54 N-m at armature current of 10 A. The torque produced when the armature current is 20 A, is?

In a d.c. motor, unidirectional torque is produced with the help of

A 200 V, 10 A motor could be rewound for 100 V, 20 A by using .......... as many turns per coil of wire, having .......... the cross-sectional area.

A shunt d.c. motor works on a.c. mains?

The d.c. series motor should never be switched on at no load because?

A series motor is best suited for driving?

When load is removed, .......... motor will run at the highest speed.

Unlike a shunt motor, it is difficult for a series motor to stall under heavy loading because?

As compared to shunt and compound motors, series motor has the highest torque because of its comparatively .......... at the start.

The Ta/Ia graph of a d.c. series motor is a?

A d.c. shunt motor is found suitable to drive fans because they require?

If the field circuit of a loaded shunt motor is suddenly opened?

If the pole flux of a d.c. motor approaches zero, its speed will?

If the load current and flux of a d.c. motor are held constant and voltage applied across its armature is increased by 10 per cent, its speed will?

If load on a d.c. shunt motor is increased, its speed is decreased due primarily to?

Neglecting saturation, if current taken by a series motor is increased from 10 A to 12 A, the percentage increase in its torque is ........ percent

A d.c. motor develops a torque of 200 N-m at 25 rps. At 20 rps it will develop a torque of .......... N-m.

The shaft torque of a d.c. motor is less than its armature torque because of .......... losses.

The current drawn by a 120 - V d.c. motor of armature resistance 0.5 Ω and back e.m.f. 110 V is .......... ampere.

It is possible to increase the field flux and, at the same time, increase the speed of a d.c. motor provided its .......... is held constant.

Under constant load conditions, the speed of a d.c. motor is affected by?

The maximum torque of d.c. motors is limited by?

In a d.c. motor, the mechanical output power actually comes from?

A d.c. motor can be looked upon as d.c. generator with the power flow?

The induced e.m.f. in the armature conductors of a d.c. motor is?

The mechanical power developed by the armature of a d.c. motor is equal to?

The Eb/V ratio of a d.c. motor is an indication of its?