MMF & Flux Density Waveforms in DC Machines - GATE EE Electrical Free MCQ

Since field excitation remains substantially constant, while aarmature mmf reaches higher values at loading.

It is cross magnetizing in nature which happens due to interaction between stator and field mmf.

The flux density increases under one pole tip while it decreases under the other due to the saturation of iron magnetic circuit.

Since field excitation remains substantially constant, while aarmature mmf reaches higher values at loading.

Correct Answer :- a

Explanation : By ultimately increasing the reluctance of the magnetic circuit at the pole tips so that the armature reaction effect is neutralized.

When the brushes are shifted from GNA, there will either be magnetizing or demagnetizing effect will be observed.

Series field current = Is = I(l)+I(f)
= 400+4.7
= 405 A(approx)
Main field mmf = I(f)+(Ns/Nf)*I(s)
= 4.7+(3/1000)*405
= 5.9 AT

Compensating windings are embedded in pole faces and having an opposite polarity as that of adjoining armature winding.

Correct Answer :- a

Explanation : A compensation winding in a DC shunt motor is a winding in the field pole face plate that carries armature current to reduce stator field distortion. Its purpose is to reduce brush arcing and erosion in DC motors that are operated with weak fields, variable heavy loads or reversing operation such as steel-mill motors. When flux from the armature current is about equal to the flux from the field current, the flux at the field pole plate is shifted.

Leaving the field winding open, the machine will not start as the exciting flux is zero.