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Shafique · Aug 3, 2007, 01:04 PM

Hi,

I understand that the power factor of Single Phase generators/motors is 1 or Unity. Is it always so ? If so, why so; if not, why not? Please elaborate.

Nosnosna · Aug 3, 2007, 01:12 PM

A single phase generator or motor will only have a power factor of 1 if the capacitance and inductance of the load are balanced properly.

Remember that for any inductive load, the current lags the voltage, and for any capacitive load, voltage lags the current. Since power factor is the ratio of P(cos x) / P (where P is the apparent power, and x is the angle between the voltage and the current) any reactive load will lead to a non-unity value for the power factor.

tkrussell · Aug 3, 2007, 01:30 PM

Originally Posted by Nosnosna

A single phase generator or motor will only have a power factor of 1 if the capacitance and inductance of the load are balanced properly.

Remember that for any inductive load, the current lags the voltage, and for any capacitive load, voltage lags the current. Since power factor is the ratio of P(cos x) / P (where P is the apparent power, and x is the angle between the voltage and the current) any reactive load will lead to a non-unity value for the power factor.

Nosnosna,

Can you put this explanation into laymen's terms?

Also, please reviiew your explanation for an error.

Nosnosna · Aug 3, 2007, 11:31 PM

Ack, it's not P(cos x) / P, it's P |cos x| / P... power factor is between 0 and 1.

Not sure how well I can do putting it into layman's terms... I've never explained it to anybody but engineering students before.

Power factor is the ratio between the power generated by a generator and the power delivered to the motor.

The power generated is simply P = VI, as the voltage and current are in phase at the time of generation. This means that the maximum value of V occurs at the same time as the maximum value of I.

The power delivered to the motor is almost always less than P, as motors are inductive loads, which have the effect of delaying the current. This puts the voltage and current out of phase, meaning that the maximum current is no longer experienced at the same time as the maximum voltage.

Power delivered is therefor not always equal to power generated, so the ratio of power delivered over power generated is not always 1. It will always take a value between 0 and 1 (inclusive). The lower the power factor, the harder a generator has to work to power a given motor.