Electrical Interview Questions With Answers: DC Machine 2

Updated on January 2, 2014

Electrical Question 1: Explain Armature reaction in DC machine ?

Armature reaction is the effect of the magnetic field set up by the armature current on the distribution of the flux under the main poles of the DC machine.

Emf will be induced in the armature of the dc machine by the Fleming's electromagnetic induction principle (Conductor and magnetic field required to produce emf). As the conductors of the armature are shorted current starts flowing and this current will produce a counter magnetic filed which opposes the main field. This effect of armature flux on the main flux so as to distort or weaken the main field flux is called armature reaction.

Electrical Question 2: What is the effects of the armature reaction in a dc machine?

Due the armature reaction the armature flux will have two impacts on the main field flux

1. It partly weakens or distorts the main field flux
2. It cross magnetises or distorts the main field flux

Electrical Question 3: Explain the methods to reduce the armature reaction in DC machine ?

1. Armature reaction causes the distortion in the main field flux. This can be reduced if the reluctance of the path of cross magnetizing filed is increased.The armature teeth and air gap at the pole tips offer high reluctance to armature flux. Thereby increasing the air gap armature reaction can be reduced.
2. By special arrangements such that leading and trailing pole tips of the poles are alternatively omitted.
3. By providing compensating winding. Compensating winding will be in series with the armature winding
4. By providing Inter poles which are placed at the geometrical neutral axis at the mid way between the main poles.

Electrical Question 4: Why brushes of a DC machine is slightly shifted from the neutral axis by small angle?

During no load Geometrical neutral pole (GNP) coincides with the magnetic neutral pole(MNP). This poles are required to place the brushes in neutral poles to collect the current from the armature with out the sparking effect. When the machine is loaded the Magnetic neutral pole shifts slightly by small angle due to armature reaction. So brushes of the dc machine is placed by slightly shifting from the neutral axis by an angle.

Electrical Question 5: Explain Commutation in DC machine ?

Commutator and the brushes in a dc machine are plays a very important role:

1. It receives electrical current from the rotating armature and deliver it to the static external load in dc generator and vice verse in case of dc motor they collect the current from the terminals and deliver to the rotating armature.
2. They converts generated alternative current in the armature to dc current (Bi directional variable in to a uni directional variable) to deliver to the load and vise verse in the case of motor operation.

Rotating armature coil is connected to the commutator segments (commutator Segment is a part of the commutator which is separated with other segments by insulator). Brushes will be in contact with rotating commutator segments and are stationary.

When the armature is rotating under the magnetic field the current direction in the armature coil changes when coil of the armature passes from one pole ( say North pole) to one pole (South pole)

So in order to get uni directional current the commutator segment shifts from one brush to other when the armature current in the coil changes its direction. Under this condition the armature coil of the dc machine will be in short circuit condition for some period.

Commutation is defined as the process of reversal of current in the conductors of the armature as they pass through the brush axis is called commutation.

Commutation period is defined as period in which the armature coil of the dc machine is under short circuit condition as the commutator segment moves over the brush axis.

Good commutation means no sparking at the brushes and the commutator surface remaining un effected.Bad commutation is one such where sparks occur at the brushes and wear and tear of the commutator material is much high.

Electrical Questions 6: Reasons for poor commutation ?

Emf of the armature coil undergoing commutation will not be zero because of

1. Due to the use of short pitched or fractional pitched winding
2. Due to the variation the brush contact resistance
3. Due to the presence of the self inductance of the coil

Short pitched winding:

When the fractional pitch coil or short pitched coil is used the emf induced in coil at the time of commutation will not be zero and also will induce short circuit current when it is undergoing commutation results in wear out of the commutator and sparks at the brushes.

Brush Resistance variation:

Brush contact resistance variation results in the non linear variation in the coil which is undergoing the commutation.

Self Inductance of coil:

For a good commutation the current direction the coil should be changed when the coil crosses the brush axis. Due to Inductance in the coil which oppose the change in the direction of the current which results in increasing the current density at the leaving edge of the brush so sparking will occur.

Electrical Question 7: Explain how commutation in DC machine is improved ?

1. By using high resistance brushes
2. By shift of brushes
3. By use of inter poles or commutation poles

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