Test: MMF & Flux Density Waveforms in DC Machines


10 Questions MCQ Test Electrical Machines | Test: MMF & Flux Density Waveforms in DC Machines


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QUESTION: 1

The flux distortion caused by cross magnetizing armature reaction is more pronounced in a

Solution:

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

QUESTION: 2

Armature reaction in a dc machine is

Solution:

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

QUESTION: 3

The air-gap flux density waveform, has decreased flux under one pole tip and measured under the other is due to

Solution:

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

QUESTION: 4

The flux distortion caused by cross magnetizing armature reaction is more pronounced in a

Solution:

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

QUESTION: 5

The method needed to limit cross-magnetizing effect is

Solution:

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.

QUESTION: 6

The brushes of a dc motor, are shifted by 5′, from the main field axis, then

Solution:

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

QUESTION: 7

A 100KW, 250 V, 400 A, a long shunt compound generator has an armature resistance of 0.025 ohms. There are 1000 shunt fields turns per pole and 3 series field turns per pole. The series field is connected in a such a fashion that positive armature current produces direct-axis MMF which adds to that of the shunt field. Compute the gross MMF at the rated terminal current when shunt field current is 4.7A and speed is 1150 rpm.

Solution:

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

QUESTION: 8

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

Solution:

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

QUESTION: 9

 Compensating winding has advantage of

Solution:

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.

QUESTION: 10

A dc shunt motor is connected to the source through 3-point starter. If the field id kept open and starter handle is moved from off to on position, then

Solution:

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