Air Gap Power in Induction Motor

Air gap power in an induction motor is the output power of the stator or input power to the rotor. The electrical power fed to the stator converts into the magnetic power that travels through the air gap of the motor, which is called the air gap power of the induction motor.

The motor converts electrical energy into mechanical rotational energy. The input voltage is fed to the stator of an induction motor. The stator coil opposes the applied voltage, and this opposition limits the starting current of the motor. The flux is generated in the stator, which travels through the air gap and gets linked to the rotor conductors. The air gap power is equal to the rotor input power. 

The following formula expresses the input power drawn by the induction motor.

P=√3* VL*IL* cosΦ ———-(1)


VL-Line voltage
IL- Line current
cosΦ- Power Factor

The electric power drawn is converted into the magnetic power. The air gap between the stator and the rotor is unwanted but necessary for motor operation. Without having an air gap, the rotation of the rotor is not possible. However, the losses in the motor can be minimized by keeping the air gap length as minimum as practically possible. The following losses occur in the motor at no load.

Core Losses or No Load Losses

The loss comprises the eddy current and hysteresis loss. The loss is more or less constant, called the fixed losses.

Stator I2R loss :

The loss occurs due to heat loss in the resistance of the stator because of the current flowing through the stator coils.

Thus, the total losses in the stator at no load are given by;

PstL=Ps(h+e)+Ps ————–(2)


PstL – Total losses in the stator at no load
Ps(h+e)- Core losses-Eddy current and hysteresis loss
Ps- Copper loss in the stator

The air gap power(Pg) can be calculated as;

=Total Input power – Losses in the stator

Pg=√3VL x IL cosΦ-[(Ps(h+e)+Ps)] ——(3)

The air gap power (Pg) is transferred to the input to the rotor.

The air gap Power= Rotor Input Power

The air gap power gets converted into kinetic rotational power. The following losses occur in the rotor.

1. Rotor copper loss
2. Rotor core loss- Negligible
3. Friction, windage, and stray losses

The air gap power(Pg) can also be calculated by following the formula.

Pg  = The mechanical power output(Pm) + Rotor Ohmic loss + Friction and Windage loss

Calculation of Air Gap Power

The equivalent rotor circuit of an induction motor is shown below.

equivalent circuit of induction motor rotor

The X2 and E2 are the rotor reactance per phase and EMF /Phase induced in the rotor circuit. R2/s is the rotor winding resistance.

The rotor input power  = Air Gap Power

gross power of induction motor- formula derivation

The rotor copper loss = sPg
Mechanical power output(Pm)= (1-s)Pg

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