Electrical Interview Questions: DC machines 1

Electrical Question: Explain the Importance of the Commutator in DC Machine?

Answer: Commutator in the DC machine is the component which converts the ac induced current in the armature to the uni directional current or dc current to the load circuit.In the DC Generator the due to the Electro-magnetic Induction principle the current induced in the armature coils are alternative current in nature.Commutator of the DC machine contains segments separated by the insulation medium. 

It happens that when the alternating current in the armature varies from one half cycle ( say positive half cycle) and enters in to other half cycle ( say negative half cycle), at the same time the polarity of the commutator segments changes i.e, initially the commutator segment which is in contact with the positive polarity of the brush when the induced currents in the armature changes at that instant the segment of the commutator which is earlier in contact with the positive polarity brush will now will be in contact with the negative polarity brush results in the same direction of current flow in the external circuit.

Electrical Question: Commutator in a DC machine converts AC to DC or DC to AC quantity of the current ?

Answer: In DC Generator the commutator converts from AC induced current produced in the armature windings is converted to DC and gives to the external load connected.

On the Other hand, in case of DC Motor when the supply given is DC it is converted from DC to AC and provided to the armature windings so that in order to produce the rotational torque.

Thus Commutator in a DC machine converts both AC to DC and DC to AC.

Electrical Question: What are Eddy currents and where they are present in DC machine and how they are minimized ?

Answer: Eddy currents are the circulating currents produced in the iron because of the alternating magnetic field. This eddy currents will cause local circulating currents which produces flux and magnetic field which apposes the main magnetic field. This results in the loss to the machine. 

Eddy currents are produced in the field poles and armature of the DC machine

Eddy Currents are minimized by providing thin laminations separated by the insulating coating This laminations reduces the area for producing the circulating currents. Hence eddy current losses are minimized.

Electrical Question: What happen when the poles of the DC Generator increases?

Answer: When the poles of the DC Generator increases the rate at which the coils cut the magnetic flux increases ( For two pole machine the rate at which the flux cut by the conductor is less compared to four pole machine), at constant speed this results in the increase in the production of the output voltage.

More number of poles in DC Machine have some advantages such as Overall diameter and length of the machine reduces, reduction in weight of the core and the yoke. Reduction in the copper usage for the field and armature windings,  and reduction in the length of the commutator.

It also have some disadvantages such as Frequency of flux reversals increases results in increase of the iron loss, labor charges increases and sparking at the brushes takes place and commutation failure occurs.