Ans:- A machine that converts DC electric energy into mechanical energy is called as a 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.
Ans:- By applying Fleming’s left hand rule.
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.
Ans:- The different types of DC motors are as following;
Ans:- The mechanical power developed by the motor is maximum when back emf is equal to half the applied voltage.
Relation: Eb = V/2
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.
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.
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.
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.
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)
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.
Ans: – DC shunt motor will not usually run on AC supply.
Ans:- By reversing the supply terminals
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.
Ans: – Speed will decrease
Ans: If a motor is directly started on line, it will draw heavy current and may get damaged.
Ans: – Field controlled method.
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.
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.
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.
Ans: – Sparking at the brushes of a DC machine may be caused due to the following reasons;
Ans: – The motor will draw heavy current, about 10 to 12 times of rated current, and the motor armature winding may get burnt.
Ans: – To reduce sparking in commutation and prevent distortion owing to armature reactance.
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.
Ans: – Electric braking is divided into three types; i) regenerative braking ii) plugging or counter current braking iii) Dynamic braking
Ans: – Because energy consumed in forcing a current against the resistance of windings.
Ans: – When variable losses become equal to copper losses.
Ans: – It is performed to determine constant losses of a DC machine, it is performed at no-load.