Answer / karuna

no power factor do not vary with frequency because pf
depends only on v& i

Answer / roysayan01

I think pf doesnt vary wid speed. PF mainly depends on
excitation / field current. When overexcited the pf becomes
leading while it lags when underexcited in a synchronous motor.

BUT

cos(phi)=R/Z;
& z=(x^2 + R^2)^1/2
& x=2*pi*f*L
so if speed changes(n=120f/p), frequency changes &
accordingly x changes & as a result pf changes wid change in
speed.

Really am confused.wat do u think?

Answer / srinivasa rao talasila

PF will not vary with speed.
For speed control of induction motors we will follow these
techniques 1) By varying the frequency
2) By varying the input voltage
3) By adding the rotor resistance
4) By consequent pole changing technique
So, power factor is cosine of the angle between the voltage
and current.it will be lag in inductive loads and Lead in
capacitive loads.

Answer / sivakumar cv

First we need to understand that what are the way the speed of an AC motor can be changed.
only two way either by changing the no of pole or by frequency.For a given machine only option available is changing the frequency.

Now the next question is which factor determine the power factor.
It is the amount of magnetizing current which determine the powerfactor of the machine at a particular load.So magnetizing current is controlled by adjusting the machine reluctance.

The Only variable which affects the reactance of the machine is frequency of the supply for given machine (Sice the geometry of the machine is invariable).

So if there is any change in speed by adjusting the frequency of the supply definitely there would be change in powerfactor of the machine for a given load.

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