Thursday, May 1, 2014

Induction motor losses and Power flow diagram

In Case of transformers there were only two types of losses i.e. constant losses and fixed losses same is true for Induction motors but as transformer don’t have any rotating part so there were not any mechanical associated with transformers. But since in motors there is rotating parts so they have mechanical losses which are also covered in constant losses. Lets discuss about these losses one by one.

To know about Transformer losses visit link below:-

Induction motor losses are divided as below:-

1.    Constant losses also known as fixed losses
These are known as constant losses as they will remains constant even if induction motor kept running without load. This means that these losses remains fixed irrespective of load on motor. These are further divided as below:-
(a)  Core losses known as iron losses
(b)  Mechanical losses or Friction losses
      2.     Variable losses
These losses are also called copper losses as These losses occur due to current flowing in stator and rotor windings. As the load changes, the current flowing in rotor and stator winding also changes and hence these losses also changes. Therefore these losses are called variable losses. These losses are occurred in both stator and rotor as current flows in both.

(a)  Stator copper losses or Stator ohmic losses
(b)  Rotor copper losses or rotor ohmic losses
(c)  Stray Load losses

Let’s discuss in details about these losses:-

Constant or Fixed Losses

1.    Core Losses Or Iron losses

As in case of transformers core losses or iron losses are further divided into two parts :-
(a)   Hysteresis losses
(b)  Eddy current losses
Eddy current losses can be minimized by using lamination of core. By laminating the core area decreases and hence resistance increases, which results in decrease in eddy currents.
Hysteresis losses are minimized by using high grade silicon steel.
The core losses depend upon frequency.
Stator frequency is always equal to supply frequency and rotor frequency is equal to slip multiplied by frequency so usually rotor core losses are very small and can be neglected.

2.    Mechanical losses or Friction losses

Mechanical losses occur at the bearing and brush friction loss occurs in wound rotor induction motor. These losses vary slightly with the change in speed.

Fixed losses = Power I/P at no load- Stator I2 R loss at no load

Variable Losses:-

The copper losses or Ohmic losses are obtained by performing blocked rotor test on three phase induction motor. The stator and rotor ohmic losses or copper losses can be calculated directly if stator and rotor winding losses are known.

Stray load losses occur in iron as well as in conductors. They are usually taken as 0.5% of motor efficiency.

Power flow diagram for 3-phase induction motor is given below:-

Induction Motors power flow diagram; losses in Induction motors

Power flow diagram has shown how these losses takes place during conversion of electrical power to mechanical power. Under normal running conditions rotor core losses are neglected.  Power developed at shaft differs from mechanical power developed by friction and windage losses.