## Short Notes Of DC Motor | SSC, GATE & ESE

Contents

### Q. 1. Why is shaft torque less than armature torque in a d.c. motor?

Ans. It is because some percentage of torque developed in the armature is used up to overcome mechanical losses, i.e., iron and friction losses.

### Q. 2. In a generator, the main flux is distorted in the direction of rotation due to an armature reaction. Why is it distorted in the opposite direction of rotation in a d.c. motor?

Ans. In, a generator, the armature current flows in the direction of induced e.m.f wheat in a motor, the armature current flows against the induced e.m.f. (i.e., back e.m.f.). It should, therefore, be expected that for the same direction of rotation and field polarity, the armature flux of the motor will be in opposite direction to that of the generator. Hence, d of the main flux being distorted in the direction of rotation, as in the generator, it is distorted in the opposite direction of rotation.

### Q. 3. What is the speed of a shunt motor practically constant?

Ans Thy speed of a d.c. the motor is given by;
N ∝ V – Ia Ra
In a shunt motor, V, Ra, and Φ are practically constant. At no-load, the armature current is very small and Ia Ra drop is negligible as compared to V. At full-load, Ia Ra drop is ordinarily about 5% of V. Consequently, the full-load speed of the motor is about 95% of the no-load value. This is sufficient speed is reduced slightly due to armature reaction. In n some cases, armature reaction s sufficient to cause the speed to remain nearly constant. For these reasons, a shunt motor is considered a constant speed motor, though the speed decreases slightly with the increase in load.

### Q .4. What is the chief advantage of a d.c. series motor?

Ans. The outstanding characteristic of a series motor is that it has high torque at low speeds and vice-versa. It is well-suited for traction purposes such as in electric trains. Acceleration is rapid because the torque is high at low speeds. Furthermore, the series motor automatically slows down as the train goes up an inclined surface, yet turns at top speed on the flat ground. The power of a series motor tends to be constant because high torque is accompanied by low speed and vice-versa. Series motors are also used in electric cranes and hoists; light loads are lifted quickly and heavy loads more slowly.

### Q. 5. Why is the actual operating efficiency of a series motor less than that of a shunt motor?

Ans. Although the efficiency of a series motor varies about the same as that of a shunt motor, the actual operating efficiency is usually considerably lower. This is due to the fact that a series motor seldom runs at its rated load, but is continuously accelerating and decelerating.

### Q. 6. Can a d.c. the series motor is operated on a.c. supply?

Ans. If the current direction through a series motor is reversed, the polarities of the field poles and the armature conductors are both reversed, resulting in a pushing effort between them in the same direction. Because of this, a series motor will operate on a.c. supply, its direction of rotation is the same for both half cycles. The lower the frequency of a.c., the greater the torque that can be produced. If the frequency is high, the inductive reactance of the coil limits the current that can flow and the torque of the motor. Note. When d.c. is applied to a series motor, the current flowing has nothing to oppose it except the resistance in the circuit. This results in a high starting current and torque.

### Q. 7. Will a d.c. shunt motor operates on an a.c. supply?

Ans. The shunt winding has a large number of turns so that it has appreciable inductance, When a.c. is applied to a shunt motor, the large inductive reactance of shunt wind will reduce the field current too much. Consequently, the shunt motor will not usually be on a.c. supply.

### Q.8 . Why is the torque of a cumulatively-compounded motor more than of a shunt motor?

Ans. The current-torque characteristic of a cumulatively-compounded motor is intermediate between those of a shunt motor and a series motor. When the armature current is zero, the field strength is only due to shunt winding. as the load is applied, the series turns to increase the flux, causing the torque TA ∝ Φ Ia for any given current to be greater than it would be for a shunt motor.

### Q.9. Why does not cumulatively compounded motor run at dangerous speed at light loads?

Ans. At no-load, the flux due to the series field of a cumulatively compounded motor is almost zero. However, the flux due to shunt winding prevents the motor from being at dangerously high speed.

### Q.10.What will happen if a shunt motor running at no-load has its shunt field winding owed if high speed?

Ans. If a shunt motor, running at no-load, has its shunt field suddenly opened or burnt out, the back e.m.f. will fall to nearly zero. The armature current will continuously increase the speed of the motor until its parts fly apart.

### Q. 11. What will happen if the shunt field winding of a loaded shunt motor accidentally breaks?

Ans. If the shunt winding of a loaded shunt motor accidentally breaks, the loss of shunt field current would result in the stopping of the motor or bum out the armature or fuse.

### Q. 12 How is the speed of a compound wound motor controlled?

Ans. The speed of a compound motor is controlled in the following ways :
(i) In order to reduce the speed of a compound motor, a resistance is inserted in the armature.
(ii) The speed can be increased either by resistance in series with the shunt winding or resistance in parallel with the series winding.

### Q. 13. what is the chief advantage of a cumulatively-compounded motor?

Ans. The speed-torque characteristics of a cumulatively-compounded motor may approach those of the shunt or series motors, depending upon the strength of the two fields.
(i) If the motor contains a light series winding, it will have better torque characteristics than a shunt motor, but will largely retain the good regulation or efficient speed control of the shunt motor.
(ii) If the motor has a light shunt winding (one sufficient to keep the no-load speed, safe value) and a heavy series winding, it will have the characteristics of a series motor, but will not run at a dangerous speed with light loads.

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