Design of Magnetic Circuit Interview Question Answer - 1

Magnetic Circuit Important Questions

In this post, some of the important questions of design of magnetic circuits are given. It includes dimensions of rotating machines, electrical loading, magnetic loading, specific electric loading, specific magnetic loading, output coefficient, flux density in the teeth, magnetic circuit definition, magnetizing current, magnetic loading on induction motor and DC machines.

Electrical loading & Magnetic loading

Which are the main dimensions of rotating machine?

• Armature diameter and armature length

Explain the terms: Electrical loading, Magnetic loading

Electrical loading

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

Magnetic loading

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

Specific electrical loading & Specific magnetic loading

Explain the terms : Specific electrical loading, Specific magnetic loading

Specific electrical loading

• It is defined as the total number of armature ampere conductors per meter of armature periphery.
• Specific electrical loading = IZ / πD
• Where D = Diameter of armature

Specific magnetic loading

• It is defined as the average flux density over the air gap in the machine.
• Specific magnetic loading = pФ / πDL
• 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.

On which factors the output of rotating machine depends?

• The output of the rotating machine depends upon specific electrical loading and specific magnetic loading.

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

Size of machine

• The output coefficient of the rotating machine is directly proportional to specific electrical loading and specific magnetic loading.  
• 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?

The specific magnetic loading of rotating machine depends upon

• 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.

Specific Magnetic Loading

• The tapering of teeth is generally used in the small machine having small diameter. 
• The ratio of B_t / B_av 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 (B_t).

Output Coefficient

What is output coefficient?

Output coefficient

The output coefficient of the rotating machine is given by

• C_o = π²(specific electrical loading)(specific magnetic loading) × 10^-3
• C_o = π²( ac )( B_av ) × 10^-3

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

Output coefficient equation

DC machine

• C_o = π²( ac )( B_av ) × 10^-3

AC machine

• C_o = 11( ac )( B_av )( K_w ) × 10^-3

Where K_w = 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.

Describe the effect of specific magnetic loading on magnetizing current.

Effect of specific magnetic loading on magnetizing current

• The magnetizing current of a machine is directly proportional to specific magnetic loading. 
• The flux density in the iron part is low for lower value of specific magnetic loading therefore iron parts work in linear or knee portion of the B – H curve. 
• The iron parts work in saturation region for higher value of specific magnetic loading. 
• Therefore, the magnetizing current low for low value of specific magnetic loading and vice versa.

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