Fig. 1,353.—Example of wattless current showing that the power factor is zero when the phase difference between current and pressure is 90°. For zero power factor the current may lead 90° as in fig. 1,352, or lag 90° as here shown. Since the shaded or negative areas = the plus areas, the average power (indicated by WW' which coincides with the zero line) is zero, that is the circuit is carrying current under pressure yet delivering no power, hence, the power factor is zero.
Ques. Is the condition as just described met with in practice?
Ans. No.
Ques. Why not?
Ans. The condition just described involves that the circuit have no resistance, all the load being reactance, but it is impossible to have a circuit without some resistance, though the resistance may be made very small in comparison to the reactance so that a close approach to wattless current is possible.
Ques. Give some examples where the phase difference is very nearly 90°.
Ans. If an alternator supplies current to a circuit having a very small resistance and very large inductance, the current would lag nearly 90° behind the pressure. The primary current of a transformer working with its secondary on an open circuit is a practical example of a current which represents very little energy.
Fig. 1,354.—Performance curves of General Electric single phase repulsion induction motor.
Ques. When the phase difference between the current and pressure is 90°, why is the current called "wattless"?