Size of DC motor is larger compared to the AC motor for same capacity

Electrical Engineering Electrical Machines
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Size of DC Motor Comparison with Size of AC Motor:

I have noticed that for running Jock Oil Pump two motors are using in my plant one is A.C for normal operation and the other one is D.C in case of emergency.

The ratings of these motors are as follows.

  1. A.C motor

Rating (KW): 55, Full load current (Amps):88

  1. D.C motor

Rating (KW): 55, Full load current (Amps):289

From the above we can observe that to develop the same torque D.C motor needs more current but why the size of the D.C motor is larger than A.C motor is explained here.

There are two factors which decide the size of the motor as follows.

  1. Area of the winding.

  2. The torque produced by the motor.

  3. Area of the winding

The cross sectional area of the conductor is more in d.c motor than a.c motor.

As resistance is inversely proportional to the area

one

To decrease the resistance of the winding, hence to increase the current carrying capacity of d.c motor need to use more cross-sectional area of conductor.

whereas in a.c motor it is not possible to increase the current carrying capacity to a great extent like in case of d.c motor by increasing the cross-sectional area due to skin-effect.

This is the reason for limit the value of current in a.c motor.

  1. The torque produced by the motor

In DC motor torque is proportional to flux and current as

two

Here to increase the torque, the only alternative is to increase the current as the flux become constant after saturation of the magnetic field. To increase the current we have to use low resistance conductor means more cross sectional area so its winding occupies more space and hence more size.

Whereas in AC three phase induction motor torque depends on rotor resistance, square of the induced e.m.f and inversely proportional to square of the rotor impedance as

three

Hence to obtain more torque we can design the rotor with low impedance by using less no. of turns so that winding requirement is less.

Also by increasing the rotor resistance we can get the more torque. If the rotor resistance is high it means the cross section of the conductor is less. If conductor cross section is less obviously A.C motor winding occupies less volume and hence less size.

The other factors which effect the size of DC motor are

  1. Presence of commutator and brushes.
  2. High thickness of insulation