# Magnetic Circuit Important Questions

### Which are the main dimensions of rotating machine?

• Armature diameter and armature length

• It is defined as total number of ampere conductors in the armature periphery.
• Where Z = Number of conductors

• It is defined as the total flux around armature periphery at the air gap.
• Where Ф = flux per pole

• It is defined as the total number of armature ampere conductors per meter of armature periphery.
• Where D = Diameter of armature

• It is defined as the average flux density over the air gap in the machine.
• Where D = Diameter of armature and
• L = Length of armature

### Why the specific magnetic loading on induction motor is kept low as compared to DC machines?

DC machine

• The magnetizing current is not serious in the case of DC machine because there is enough space is provided for salient pole field turns.

Induction Motor

• The higher value of magnetizing current means lower power factor therefore the specific magnetic loading of the induction motor is kept low as compared to DC machine.

### Describe the effect of specific electrical and specific magnetic loading on the size of machine.

Size of machine

• The specific electrical loading and specific magnetic loading should be as high as possible in order to reduce weight and cost of rotating machine.

### On which factor the specific magnetic loading of rotating machine depends?

• Maximum flux density in the iron part of machine ( specific magnetic loading )
• Core loss
• Magnetizing current

### Give reason : The small machines have low specific magnetic loading.

• The tapering of teeth is generally used in the small machine having small diameter.
• The ratio of Bt / Bav is large at the smallest width of teeth therefore the lower value of specific magnetic loading is permissible for a given value of teeth flux density (Bt).

## Output Coefficient

### What is output coefficient?

Output coefficient

The output coefficient of the rotating machine is given by

• Co = π2( ac )( Bav ) × 10-3

### Write the output coefficient equation for DC machine and AC machine.

Output coefficient equation

DC machine

• Co = π2( ac )( Bav ) × 10-3

AC machine

• Co = 11( ac )( Bav )( Kw ) × 10-3

Where Kw = Winding factor

### Describe the relation between volume of machine and output coefficient.

Relation between volume and output coefficient

• The volume of machine is inversely proportional to output coefficient.
• The output coefficient of rotating machine should as high as possible in order to reduce volume or weight of the machine.

## Flux density in the teeth

On which factor the flux density in the teeth depends?

• The flux density in the teeth depends upon the specific magnetic loading.

At which portion of the tooth the maximum value of flux density occurs?

Flux density in the tooth

• If the tooth is tapered, the width of the tooth is not same for entire height. Therefore, the flux density is different for entire length of tooth.
• The highest value of flux density occurs at the smallest width of tooth ( root of the tooth in the DC machine and section near air gap in the synchronous machine ).

## Magnetizing Current

### Explain the term : Magnetic current

Magnetizing current

• It is defined as the mmf required for forcing flux through air gap and ironing part of a given machine.