Thursday, June 25, 2015

Power factor variation with load;Why Power factor low during Starting of Motor?

During starting of Induction motor , motor draws very high current as power factor at the start is very low. Power factor is very low because the magnetizing component of current  is very high at starting. At Start Magnetizing component current is very high as it has to overcome the reluctance offered by air gap between stator and rotor. This leads to higher current withdrawal during starting.

You can also say that angle between core-loss component and net current increases. As power factor is angle between core-loss component and net current, then there will be decrease in power factor.
During starting Power factor starts from zero and keeps on increasing and will be maximum at full load current. Rated Power factor of motor will be achieved at Rated current of motor. Usually it is power factor at start is considered as 0.2.

Motors usually have rated Power factor between 0.80 -0.90.

Power factor varies as per load connected to the motor.

Chart for the same is shown below:-

Power factor variation wirh load
From above you will see that there is variation in Power factor depending upon the load. 
Circle diagram for the power factor is shown as below:-

Power factor circle diagram

Power factor circle diagram