Losses in Induction Motor

In any electrical machine losses are the most important factor to be considered. They help us to determine the efficiency and performance of any electrical machine. 

In similar context, losses in induction motor are the important factor to determine the efficiency of induction motor. 

To understand the losses of a three phase induction motor, first of all we have to understand the process of conversion of electrical energy into mechanical energy. We have discussed this process in detail in the working of three phase induction motor.

An induction motor converts electrical energy into mechanical energy. During this process of electromechanical energy conversion some part of input power is lost in the form of Electrical Losses and Mechanical Losses in different parts of the induction motor.

 

Electrical losses in Induction Motor

 

As the name implies, electrical losses are the loss of electric power in copper windings and in the core of stator as well as rotor. There are two types of electrical losses in induction motor

 

• Copper loses
• Core loses

 

Copper loses in Induction Motor

As the name implies, Copper losses are the loss of power in copper windings of stator and rotor. These losses are due to the electrical resistance offered by the windings to the electrical current flowing through it.

 

Core loses in Induction Motor

Core losses are the loss of magnetic flux in the magnetic core during the electromechanical and energy conversion. 

 

Core losses are for the divided into Eddy current Losses and Hysteresis Losses. These two types of losses are due to the high frequency harmonic current and depend on the frequency supply voltage.

 

Mechanical Losses in Induction Motor

Mechanical losses are the loss of power in different mechanical parts of the induction motor due to friction and other physical obstructions. These losses are further divided into Friction Losses and Windage Losses.

 

Friction losses in induction motor are caused by the friction between different mechanical parts like shaft, bearing and brushes.

Windage Losses in induction motor occur due to the aerodynamic pressure on the rotating part of the motor.

 

Power Stages in Induction Motor

 

To ease the study of electromechanical energy conversion the stages of electrical the power flow in induction motor is bifurcated as follows.

In the working of 3 phase induction motor we studied that the three phase power is supplied to the stator winding. This is the first stage of power. Let us say the input power to the induction motor is Pi.

 

These stator windings are made up of copper wire and mounted on a steel laminated core. Due to the presence of these copper and core losses occur in the stator. Let us say

P_Scu  =  Stator Copper Losses

P_Sco  =  Core Losses

After the occurrence of these losses the rest of the power is supplied to the stator output. 

Stator Power Output (P_S,O/p)   =    Pi  -  (P_Scu   +    P_Sco)

 

This output power of the stator acts as the input to the rotor and this power is also known as air gap power. This is the second stage of power.

 

Stator Power Output (P_S,O/p)   =   Rotor Power Input (P_R,i/p)   =  Air gap power (P_ag)

 

Again rotor contains copper winding and steel laminated core and due to the presence these copper and core losses occur in the rotor. Let us say

 

P_Rcu   =   Rotor Copper Losses

P_Rco   =   Rotor Core Losses

 

After these losses in rotor the rest of the electrical power output is fed to the rotor shaft and also known as the rotor power output.

 

Shaft input power (P_Sh,i/p)  =  Rotor Power Ouput (P_R,o/p)   =   P_R,i/p -   (P_Rcu  +   P_Rco))

 

This rotor power output or input power of the shaft is responsible for the gross torque developed in the motor. The conversion of electrical power output of the rotor into the torque occurs according to the formula given below.

 

     P_R,o/p   =   P_Sh,i/p   =    ω.Tg

  

      ω    =    2.π.N / 60

 

Tg      =   Gross torque developed in the induction motor

ω       =    Angular frequency of the rotor

 

During the development of gross torque in induction motor some power is lost in mechanical parts of the induction motor due to the friction and aerodynamic pressure.  Let us say

P_mech  =  Mechanical losses

P_f       =   Frictional Losses

P_w     =    Windage Loses

 

 P_mech      =       P_f     +     P_w

After these losses the net power transferred to the shaft output is responsible for the net torque development in induction motor and this is the actual output power of the induction motor. 

 

Net Power Output (P_net,o/p)   =    P_Sh,i/p   -   P_mech

Constant losses in Induction Motor

 

It is discussed earlier that in stator core losses and copper losses occur. As the core losses are frequency dependent and the supply frequency is constant so the stator core losses are categorized as constant losses. And the rotor core losses are neglected because during the operation of induction motor slip is very low and the frequency supplied to the rotor is slip dependent hence the frequency supplied to the rotor is very low.

 

Variable losses in Induction Motor

 

The stator copper loss, rotor copper loss and the mechanical losses are load dependent; they varies according to the load. So they combinedly categorized as variable losses.