Question

Directions: It consists of two statements, one labelled as the ‘Statement (I)’ and the other as ‘Statement (II)’. You are to examine these two statements carefully and select the answer using the codes given below:

Statement (I): In large dc motors which drive reversing rolls of a steel mill and in the generators which supply them, large fluctuations of currents cause higher voltages being induced in the coils located between adjacent commentator segments. This may result in the so called ‘flash-over’ between adjacent brushes, causing a heavy short-circuit.

Statement (II): To counteract the possibilities of a ‘flash-over’, a ‘compensating winding’, which is embedded in the slots on the pole faces, is connected in series with armature winding, and which supplies an mmf that magnetizes in the opposite direction to that of the armature mmf.
1. Both Statement (I) and Statement (II) are individually true and Statement (II) is the correct explanation of Statement (I)
2. Both Statement (I) and Statement (II) are individually true but Statement (II) is NOT the correct explanation of Statement (I)
3. Statement (I) is true but Statement (II) is false
4. Statement (I) is false but Statement (II) is true

Answer 1

Correct Answer - Option 2 : Both Statement (I) and Statement (II) are individually true but Statement (II) is NOT the correct explanation of Statement (I)

Flashover in DC machines:

• Armature reaction causes the flux density wave to be so badly distorted. Because of this when a coil is passing the region of peak flux densities, the emf induced in it far exceeds the average coil voltage.
• If this emf is higher than the breakdown voltage across the adjacent segment, a spark over could result which can easily spread over.
• This spark can envelop the whole commutator as the environment near the commutator is always ionized and conditions are favourable for flashover.
• This causes the complete short circuit of the armature. Hence the maximum allowable voltage between adjacent segments should be 30-40 V to limit the average voltage between them.

 

Compensating windings:

• The cross-magnetizing effect of armature reaction may cause trouble in dc machines subjected to large fluctuations in load.
• In order to neutralize the cross magnetizing effect of armature reaction, a compensating winding is used. The compensating windings consist of a series of coils embedded in slots in the pole faces. These coils are connected in series with the armature.
• The series-connected compensating windings produce a magnetic field, which varies directly with armature current. Because the compensating windings are wound to produce a field that opposes the magnetic field of the armature, they tend to cancel the cross magnetizing effect of the armature magnetic field.

 

Both Statement (I) and Statement (II) are individually true but Statement (II) is NOT the correct explanation of Statement (I).