Why does the power factor of an induction motor increase with
the load?
induction motor and transformer works on same principle, except it
has an rotating part.
Hence at light load or no load current drawn by the motor
is magnetising current (reactive current) ,used to magnetise the
motor’s core.
And this magnetising current (reactive current) lags the voltage by a
large angle. This results to very low power factor but when the load
increases more current is drawn by the motor, thus active power
increases and also the power factor increases.
The power factor at no load is low because the magnetizing
component of input current is a large part of the total input current of
the motor. When the load on the motor is increased, the in-phase
current supplied to the motor increases, but the magnetizing
component of current remains the same. This means that the resultant
line current is more nearly in phase with the voltage and the power
factor is improved when the motor is loaded, compared with an
unloaded motor which has its magnetizing current as a major
component of the input current.
At the starting condition, the induction motor draws higher
magnetisation component of current in order to overcome the
reluctance offered by air gap between stator and rotor. Then the total
current increases. In the phasor representation, the angle between
coreloss component and net current increases. As the power factor is
the cos of angle between net current and core loss component,
therefore the power factor decreases.