DC Motor-1

Q. State different types of rotating machines?

Ans:

i)Direct current machines: DC current in both stator and rotor winding.

ii) Synchronous machines: AC current in one winding and DC in other.

iii) Induction machines: AC current in both stator and rotor. 

Q. State the types of winding in DC machine?

Ans: Lap winding and Wave winding. The number of parallel paths of a lap wound machine is equal to the number of poles. However, a wave wound machine always has two parallel paths, regardless of the number of poles.

Lap winding is used for low voltage high current machines where as wave winding is used for high voltage low current machines.

Q. What is the function of commutator?

Ans:In a dc motor commutator reverses the direction of current in armature coil to produces rotating magnetic flux.

In a dc generator commutator is used to provide unidirectional current to the external circuit.

Q. Why brushes are used in DC machine?

Ans: Brushes are used to make connection between the external circuit and commutator.

Q. Define commutation?

Ans: The process of reversal of current in a commutating coil is known as commutation.

Q.Why starter necessary for DC motor?

Ans: When d.c. motor is at standstill the back emf generated in the

armature is zero and hence Eb= 0 and V = Ia*Ra for a shunt motor and V = Ia(Ra+ Rse) for a series and compound motor(value of Ra and Rse is low).

 

Therefore, with the rated applied voltage, the standstill armature current is very high and may damage the armature winding and commutator.
To limit the armature current to a safe value during starting, external resistance is inserted in the armature circuit. As the motor accelerates, the induced emf is developed in the armature, and armature current gradually reduces to a small value. Thus the resistance inserted in the armature circuit should be decreased as the motor accelerates. This may be done manually or automatically with the help of starters.

Figure shows three point dc motor starter:

Q. Explain methods of speed control of DC motor?

Ans: The two methods that are commonly used to secure speed control are:

i)  Armature control: In this method, the speed control is achieved by inserting a resistance in the armature circuit of a shunt, series or a compound motor. This method is suitable to operate the motor at a speed lower than the rated speed while delivering the same torque.

ii) Field control: The field current in a shunt motor can be controlled by inserting an external resistance (called the field regulator) in series with the field winding. Because the field current is a very small fraction of the total current intake of a shunt motor, the power dissipated by the external resistor is relatively small. Therefore, the flux control method is economically better than the armature resistance-control method.

To control the flux in a series motor, a field diverter resistor can be connected in parallel with the series field winding. Since the speed of the motor is inversely proportional to its flux, a decrease in it’s flux results in an increase in its speed. Thus, the flux-control method makes a motor operate at a speed higher than its rated speed.

 

Q.What are the losses in DC motor?

Ans: Losses in a D.C. Machine:

A. Copper losses

i) Armature copper loss

ii) Field copper loss

iii)  Brush contact resistance and commutator losses

B.  Magnetic losses

i)  Hysteresis loss

ii) Eddy current loss

C.  Mechanical losses

i) Friction losses in bearings and commutator

ii) Air friction and windage loss of rotating armature

Usually Magnetic losses, Mechanical losses and Shunt field copper loss together is taken as the “constant” losses.

Whereas the armature series field and compole and compensating

winding copper loss is called the “variable” loss.

Therefore, Total losses in a d.c. machine

  = Constant loss + Variable loss

= Wc+Ia²R where R includes armature, series, field, compole and

compensating winding, brush contact, commutator resistances.

Further Study See: Motor Part-II