# Question and Answers on DC Motors

ByAPSEEE

### Question and Answers on DC Motors

1. What is a DC Motor?

Ans:- A machine that converts DC electric energy into mechanical energy is called as a DC motor.

1. What is the working principle of DC motor?

Ans:- Its working depends upon the basic principle that when a current carrying conductor placed in the magnetic field, a force is exerted on it and torque develops.

1. 3. How can the direction of rotation of a DC motor be determined?

Ans:- By applying Fleming’s left hand rule.

1. What is back e.m.f.?

Ans: – When the armature of DC motor rotates under the influence of the driving torque, the armature conductors move through the magnetic field and e.m.f. is induced in them as in a generator. It is seen that induced e.m.f. are in direction opposite to the flow of conductor current. Since this generated emf opposes the flow of current, it is called back e.m.f. or counter e.m.f.

1. What are the different types of DC motors?

Ans:-  The different types of DC motors are as following;

• Separatetly excited DC motors
• Self excited DC motors
1. DC shunt wound DC motors
2. DC series wound DC motors
• Compound wound DC Motors
1. Long shunt compound wound DC motors
2. Short shunt compound wound DC motors
3. What is the condition for developing maximum mechanical power in a DC motor

Ans:- The mechanical power developed by the motor is maximum when back emf is equal to half the applied voltage.

Relation:    Eb = V/2

1. Why is the speed of a DC shunt motor is practically constant?

Ans :- In DC shunt motors, the shunt field current remains constant since the supply voltage is constant. Hence, the flux in DC shunt motors is practically constant. Therefore, speed of a shunt motor will remain constant as the armature current varies.

1. The emf induced in the armature of a DC motor is called the back emf why?

Ans:- Since the generated emf in a DC motor opposes the voltage applied to the armature of the motor it is called back or counter e.m.f.

1. What is the chief advantage of DC series motor?

Ans:- The characteristics of a series motor is that it has high torque at low speed and vice versa. So that, acceleration is rapid because the torque is high at low speeds. Due to this advantage the application of DC series motor is found in traction system.

1. DC series motor is a highly variable speed motor, why?

Ans:- In a DC series motor, the field winding is connected in series with the armature winding , therefore, field flux varies approximately linearly with load and thus the motor speed N brings inversely proportional to the field flux, varies. That is why a DC series motor is said to be a highly variable speed motor.

1. What will happen if the windings of a DC shunt motor gets disconnected while in normal operation?

Ans: – At this condition, the flux will drop to practically zero values, and, therefore speed will increase to an extremely high value and the motor may get damaged due to heavy centrifugal forces. (Note: the speed of a DC motor is inversely proportional to the magnetic flux)

1. Can a DC series motor be operated on AC supply?

Ans: – If the current direction through a DC 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. Due to this, a series motor will operate on AC supply, its direction of rotation being the same for both half cycles.

1. Will a DC shunt motor operate on an AC supply?

Ans: – DC shunt motor will not usually run on AC supply.

1. How can the direction of rotation of a series motor be changed?

Ans:-  By reversing the supply terminals

1. Why should the series motor not be run at no-load?

Ans: – On no-load, the series motor will attain dangerously high speed and the motor may get damaged due to heavy centrifugal forces set up in the rotating part.

1. What is the effect of connecting additional resistance in series with the armature circuit?

Ans: – Speed will decrease

1. Why DC shunt motor should not be started direct on line?

Ans: If a motor is directly started on line, it will draw heavy current and may get damaged.

1. The speed of a DC shunt motor is to be increased above the rated speed, which method must be adopted?

Ans: – Field controlled method.

1. Belt load are never recommended for a DC series motor, why?

Ans: – 1. Belt may slip over the pully, then machine may pick-up very high speed and damage self.   2. Belts are weak and may break, this may cause damage to the machine due to high speed.

1. For punching, DC compound motor is best suited, why?

Ans: – DC compound motors, particularly cumulative compound wound motors are used in driving machines, which are subjected to sudden applications of heavy loads such as in punching and shearing machines. The other advantage of cumulative compound wound motor is that it has high starting torque.

1. What will happen if the shunt field winding is connected in series with armature?

Ans: – A shunt field winding has relatively high resistance, if it were connected in series with the armature; its high resistance would prevent the passage of normal armature current.

1. What are the causes of sparking at the brushes of a DC machine?

Ans: – Sparking at the brushes of a DC machine may be caused due to the following reasons;

1. Insufficient contact surface of the brush II) too short a brush III) too little spring tension IV) wrong brush setting V) dirt on commutator VI) open armature coil
2. What will happen if a DC motor is directly switched on to the supply, without any starter?

Ans:  – The motor will draw heavy current, about 10 to 12 times of rated current, and the motor armature winding may get burnt.

1. Why large variable speed DC motors are fitted with compensating winding?

Ans: – To reduce sparking in commutation and prevent distortion owing to armature reactance.

1. In a DC motor starter is necessary, why?

Ans:- To protect the DC motor against the flow of excessive current during starting period, it is imperative to insert high resistance in series with the armature circuit.

1. What are the various types of electric braking?

Ans: – Electric braking is divided into three types; i) regenerative braking ii) plugging or counter current braking iii) Dynamic braking

1. Why do copper losses occur in DC machine?

Ans: – Because energy consumed in forcing a current against the resistance of windings.

1. When does the maximum efficiency of a DC machine take place?

Ans: – When variable losses become equal to copper losses.

1. 29. Why Swimberne’s test is performed?

Ans: – It is performed to determine constant losses of a DC machine, it is performed at no-load.