Losses and Efficiency of Induction Motor

There are two types of losses occur in three phase induction motor. These losses are,

1. Constant or fixed losses,
2. Variable losses.

Constant or Fixed Losses

Constant losses are those losses which are considered to remain constant over normal working range of induction motor. The fixed losses can be easily obtained by performing no-load test on the three phase induction motor. These losses are further classified as-

1. Iron or core losses,
2. Mechanical losses,
3. Brush friction losses.

Iron or Core Losses

Iron or core losses are divided into hysteresis and eddy current losses. Eddy current losses are reduced by laminating the core, which increases resistance and decreases eddy currents. Hysteresis losses are minimized using high-grade silicon steel. Core losses depend on supply voltage frequency. The stator frequency is the supply frequency, while the rotor frequency is slip times the supply frequency, usually much lower. For a 50 Hz stator frequency, the rotor frequency is about 1.5 Hz due to a typical slip of 3%. Thus, rotor core loss is usually negligible compared to stator core loss during operation.

Mechanical and Brush Friction Losses

Mechanical losses occur at the bearings, and brush friction losses occur in wound rotor induction motors. These losses are minimal at start-up but increase with speed. In three phase induction motors, the speed generally stays constant, so these losses also remain nearly constant.

Variable Losses

Variable losses, also known as copper losses, occur due to the current in the stator and rotor windings. As the load changes, the current and thus these losses change. These losses are determined by performing a blocked rotor test on a three-phase induction motor. The main function of an induction motor is to convert electrical power into mechanical power, which involves different stages of power flow.

This power flowing through different stages is shown by power flow diagram. As we all know the input to the three phase induction motor is three phase supply. So, the three phase supply is given to the stator of three phase induction motor.
Let, P_in = electrical power supplied to the stator of three phase induction motor,
V_L = line voltage supplied to the stator of three phase induction motor,
I_L = line current,
Cosφ = power factor of the three phase induction motor.
Electrical power input to the stator, P_in = √3V_LI_Lcosφ
A part of this power input is used to supply stator losses which are stator iron loss and stator copper loss. The remaining power i.e (input electrical power – stator losses) are supplied to rotor as rotor input.
So, rotor input P₂ = P_in – stator losses (stator copper loss and stator iron loss).
Now, the rotor has to convert this rotor input into mechanical energy but this complete input cannot be converted into mechanical output as it has to supply rotor losses. As explained earlier the rotor losses are of two types rotor iron loss and rotor copper loss. Since the iron loss depends upon the rotor frequency, which is very small when the rotor rotates, so it is usually neglected. So, the rotor has only rotor copper loss. Therefore the rotor input has to supply these rotor copper losses. After supplying the rotor copper losses, the remaining part of Rotor input, P₂ is converted into mechanical power, P_m.

Let P_c be the rotor copper loss,
I₂ be the rotor current under running condition,
R₂ is the rotor resistance,
P_m is the gross mechanical power developed.
P_c = 3I₂²R₂
P_m = P₂ – P_c
Now this mechanical power developed is given to the load by the shaft but there occur some mechanical losses like friction and windage losses. So, the gross mechanical power developed has to be supplied to these losses. Therefore the net output power developed at the shaft, which is finally given to the load is P_out.
P_out = P_m – Mechanical losses (friction and windage losses).
P_out is called the shaft power or useful power.

Efficiency of Three Phase Induction Motor

Efficiency is defined as the ratio of the output to that of input,

Rotor efficiency of the three phase induction motor ,

= Gross mechanical power developed / rotor input

Three phase induction motor efficiency,

Three phase induction motor efficiency