Dear all,

I have come to know from some one that when an induction motor spins, it draws reactive power from system for half cycle and then returns this reactive power to the system (Traffo/Generator) for the next half cycle. This returned reactive power can’t be stored without capacitor. So we connect capacitor which stores that reactive power which is generated by motor and then provides that power to the motor when it demands. Is this explanation true?

Dear Ab2005,


I think what you have com to know about reactive power flows explains the effect but does not really shows the phenomena.
Reactive power does not flow. It is only a concept of fluctuating power in AC circuits.When power is absorbed and subsequently delivered back such that the average power is zero it is called reactive power
We have to consider three electrical quantities namely voltage, current and power and also three circuit elements namely resistance ,inductance and capacitance.
In an ac circuit the three electrical quantities are periodically changing in magnitude and direction.Voltage and current fluctuate at 60 cycles and power at 120 cycles per second.
In a resistance voltage and current change in the same manner in the same time and
hence there is a net absorption of power and no reverse flow of power at any instant.
In case of inductor and capacitor voltage and current change do not in same time.The average of product of voltage and current is zero and there is no net power flow.
The time relation ship of voltage and current in inductance and capacitance are exactly opposite.
During half cycle rather quarter cycle when inductor is absorbing power capacitor delivers power
and vice versa hence reduces reactive power.
I only hope I am not confusing.
subrao

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