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Electrical Interview Questions With Answers DC Machines Part 1

ElecticalQuestion 1: What is Electrical machine?

Electrical machine is an electro mechanical device which converts input motive power ( prime mover like turbine in case of generator or electrical supply in case of dc motor) to output motive power (Electrical power in case of the generator or mechanical output in case of motor). This uses the principle of electro magentic induction principle.

Electrical Question 2: What is the advantage of DC motors over AC motors?

Answer:

With the advert of the ac power generation and transmission over long distances with less losses using electrical transformers, dc machines were out ruled by the ac machines in industrial and home applications. However dc motors still exist in the industries because of some special properties of the dc machines where ac drives have failed to attain.

DC motors have some special properties such as:

  1. High Starting Torque
  2. Excellent speed control capability
For traction applications in electrical trains and cranes motor drives require to have high starting torque. DC series motor has very good starting torque capabilities compared to induction motors and synchronous motors. So still in present traction applications DC series motors are used
In some applications such as paper mills and roll mills speed control of the motor drive should be accurate else the process will be failure. In such applications of finer speed control DC shunt machines are widely used.

Electrical Question 3: What is the use of Commutator in DC machine?

Answer:
Commutator is the part in DC machine that makes the difference from the ac machine. In DC machine the current flow in the armature is ac. This current is made uni directional with the help of commutator.
In ac machines slip rings are used to collect the armature current from the armature and supply to the load. In DC machines this slip rings are replaced by the split rings. Split rings consists of 2 conductor materials separated by insulating material.
In first half cycle the induced current will be taken by the split rings or commutator which is in one position.During the other half cycle when the induced current in the armature changes the split rings position also changes results in the delivering the unidirectional current to the external load.

Electrical Question 4: Explain essential parts of DC machine?

Essential parts of the dc machine are:
  1. Field System
  2. Armature
  3. Commutator
  4. Brushes
  5. Brearings and Shaft

Electrical Question 5: Why field poles are laminated in DC machine ?

Field poles of a dc machine is laminated in order to reduce the eddy currents. Eddy currents are the circulating currents that are induced in the iron by the alternating magnetic field. These eddy currents produced will create a flux which opposes the main flux. So this eddy current production is considered as loss in the dc machine.Hence in order to reduce this eddy current losses thin laminations are considered and insulating coating is provided in between the laminations.

Electrical Questions 6: Do commutator convert ac to dc or dc to ac ?

Commutator in a dc machine converts both ac to dc current and dc to ac current. Consider the dc generator the current produced in the armature due to the electro magnetic induction is ac. However commutator converts the bi directional current to uni directional current and supplies the current the external load circuit.
In DC motor the voltage we apply at the terminals is dc. This dc current is converted to ac and supply to the armature circuit so as to produce the rotational torque.
Hence Commutator converts both ac to dc variables and dc to ac variables.

Electrical Question 7: Why armature teeth of DC machine is skewed ?

The air gap length changes between the slot and teeth of the armature when the armature is rotating under constant magnetic field. This results in the production of small variation in the torque hence this results can be observed as small vibrations in the frame of the machine. So in order to avoid this vibration effect armature teeth is skewed.
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