Design of Magnetic Circuit Interview Question Answer - 2

Important Questions of Magnetic Circuit

In this post, some of important question of design of magnetic circuits are given. It includes loss of rotating machines, effect of specific magnetic loading on loss, relation between linear dimensions and output, relation between linear dimensions and losses, factor affecting specific magnetic loading, peripheral speed, cooling coefficient and specific electrical loading, cooling coefficient of machine, round conductors, Space factor for round conductors, machine frame and machine frame size and Pole section of the DC Machine.

How the core loss of rotating machine is related to flux density?        

Core Losses in the rotating machines

• The specific core loss (Core loss per unit volume) of rotating machine is directly proportional to square of flux density. The core loss of rotating machine is directly proportional to flux density.

Describe the effect of specific magnetic loading on iron loss.      

Effect of Specific magnetic loading on iron loss

• The iron loss in the rotating machine is directly proportional to specific magnetic loading. 
• Therefore, the higher value of specific magnetic loading means higher iron loss and vice versa.

Describe the effect of linear dimensions on the iron loss and output of machine.        

Iron loss and Output of Machine

• The two machines having linear dimension in the ratio of x: 1, the iron loss of the two machines in the ratio of x³ : 1 but the output in the ratio of x⁴ : 1.

Which factors greatly influence due to specific electrical loading?     

Factor affecting specific electrical loading

• Temperature rise
• Voltage
• Size of machine
• Current density

Describe the relation between cooling coefficient and specific electrical loading.        

Cooling coefficient and specific electrical loading

• The specific electrical loading of the machine is inversely proportional to cooling coefficient. 
• The cooling coefficient depends upon cooling condition of the machine.
•  Lower value of cooling coefficient for better ventilation and vice versa.
• Therefore, the higher value of specific electrical loading allows for better cooling condition of any machine.

Define: Space factor 

Space factor

• It is defined as the ratio of bare conductor area to the total slot area.

On which factors the cooling coefficient of machine depends?   

• Ventilation condition

Give reason: The specific electrical loading for high voltage machine is less than the low voltage machine.  

Specific Electrical Loading

• The specific electrical loading of a given machine is directly proportional to space factor. 
• The value of space factor is low for high voltage machine due to higher insulation thickness. 
• Therefore, the specific electrical loading for high voltage machine is lower than that of low voltage machine.

Why the round conductors are used in place of rectangular conductors in the rotating machines?  

Space factor for round conductors

• The value of space factor depends upon shape of the conductor. 
• The space factor for round conductor is lower than that of rectangular conductor therefore lower value of specific electrical loading is used for round conductors.

Describe the relation between efficiency and linear dimension of rotating machines.

Efficiency – Linear dimensions

• The efficiency of rotating machine increases with its linear dimension.
• Therefore, the efficiency of large machine is much higher than that of fractional HP motor.

Describe the relation between heat dissipating surface and linear dimensions of rotating machine. 

• The heat dissipating surface of rotating machine is directly proportional to square of linear dimension of machine. 

Describe the effect of temperature rise on linear dimension of rotating machines.

Effect of temperature rise on Linear dimension of Machines

• The temperature rise of given machine is directly proportional to its linear dimensions but this is not strictly true. 
• As the size of the machine increases, it requires better cooling condition in order to keep temperature rise within limits. 
• Therefore, the fractional HP motors are self cooled than the large dimension machines.

Why the pole section of the DC machine should be kept circular?      

Pole section of the DC Machine

• The pole section of the DC machine should be kept circular in order to kept mean length of field winding smallest. 
• However, the circular pole requires solid pole structure.

What should be ratio of L / τ in the rotating machine?      

• The ratio of stator core length to pole pitch is in the range of 0.7 to 0.9.

Whether the peripheral speed of DC machine is limited?  

Peripheral speed

• The peripheral speed of the rotating machine is limited by considering diameter of rotor. 
• It should not exceed 30 m/s. 
• If the peripheral speed increases, the rotor may distort due to excessive centrifugal forces.

On which factors the rotor diameter of the machine depends?  

• Peripheral speed of machine

Define: Frame and Frame size  

Frame

• It is mechanical structure provided to cover stator for diameter D, core length L, bearings, end covers and terminal box.

Frame size

• The frame size number indicates its canter height from shaft canter which is expressed in mm. 
• The first number indicates shaft canter and second letter indicates frame length.
• The letter S indicates small core length; M stands for medium core length and L for large core length.

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