## 07/03/2020

### DC Generator Interview Question Answer - 2

Which are the different losses taken places in the DC Generator?

Losses in the DC Generator

Iron loss: (1) Hysteresis loss and (2) Eddy current loss

Armature copper loss

Field copper loss

Mechanical losses: (1) Friction loss and (2) Windage loss

Which losses are considered as iron losses?

The hysteresis loss and eddy current loss are considered as iron loss.

Why the hysteresis loss occurs in the core of the armature?

Hysteresis loss

When the armature core rotates in the magnetic flux set up by the field poles, the portion of the armature core passes under the N and S field poles respectively, thereby attaining S and N poles respectively.  Its magnetism reverses after passing through pair of field poles. Therefore hysteresis loss is due to reversal of the magnetism of the core material.

Which parameters affect the value of hysteresis loss?

The hysteresis loss is given by

Wh  α Bmax2 f  V  watt

W= η Bmax2 f  V  watt

Therefore its value is affected by (a) type of the core material (b) maximum value of flux density and (c) frequency.

State the unit of Steinmetz hysteresis coefficient.

The unit of Steinmetz hysteresis coefficient is joule / meter3.

Which material has lowest Steinmetz hysteresis coefficient for same flux density? Cast iron, cast steel or silicon steel

Silicon steel

How to reduce hysteresis loss?

Hysteresis loss

The hysteresis loss depends upon the B-H curve of the material. (The B – H loop is a measure of hysteresis loss per unit volume per unit cycle.)  Therefore the core material should be selected such that area enclosed by the core material is as small as possible. The silicon steel has small B – H loop therefore it is used as core material to reduce hysteresis loss.

What is eddy current?

Eddy current

When the armature rotates in the magnetic field, an emf is induced in it according to the faradays’ law of electromagnetic induction. As the core resistance is very small, it sets up large current in the core. This current is known as eddy current.

How to reduce eddy current loss?

The eddy current loss is given by

We  α Bmax2 f 2 t2 V2 watt

Therefore the eddy current loss is reduced by laminating the armature core at right angle to its axis.

Which losses are known as stray losses (rotational losses)?

Stray loss

The core loss and mechanical loss are collectively known as stray loss.

What are constant losses?

Constant losses

The combined iron and mechanical losses are known as constant losses. The shunt field copper loss is constant in the DC shunt and compound motor.

What is condition for maximum efficiency in the DC Generator?

Condition for maximum efficiency

The variable loss is equal to the constant loss is the condition for the maximum efficiency in the DC Generator.

Variable loss = Constant loss

What is armature reaction? Describe its effects.

Armature reaction

The effect of the armature flux on the main (field) flux is called as armature reaction in the DC machine.

Effect of armature reaction

The armature flux weakens the main flux and

It distorts the main flux.

Define: Magnetic neutral axis

Magnetic neutral axis

The axis along which no emf is produced in the armature conductor is called as magnetic neutral axis.

OR

The axis which is perpendicular to the armature flux is called as magnetic neutral axis.

Define: Geometric neutral axis

Geometric neutral axis

The axis which is perpendicular to the magnetic flux lines passing through centre of the armature is known geometric neutral axis. The armature conductor along this axis does not cut flux therefore no emf is produced in the armature conductors.

Why the magnetic neutral axis is called as the ‘axis of commutation’?

Axis of commutation

The magnetic neutral axis is called as axis of commutation because brushes are always placed on this axis and reversal of current takes place on this axis.

What is direction of brush shift due to armature reaction?

Brush Shift

The direction of brush shift always is in the same direction as that of direction of armature rotation in the case of generator whereas the direction of brush shift is opposite to that of direction of rotation of the motor.

What do you mean by the leading pole tips and trailing pole tips?

The pole tips which is first met during the rotation of the armature is known as leading pole tips and other pole tips is known as trailing pole tips.

What is direction of flux for demagnetizing conductors in the DC Generator?

The direction of flux for demagnetizing conductors is opposite to that of main flux or field flux.

What is direction of flux for cross - magnetizing conductors in the DC Generator?

The direction of flux for cross – magnetizing conductors is vertically downward right angles to the main flux if main flux flows left to the right.

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