HomeDefinitionPower factor (PF)

Power factor (PF)

Power factor will increase vs. load. The minimum value is obtained at no-load and is around 0.2 because absorbed active power is minimum (only losses) and reactive power is high. As you increase the motor load, the p.f. will increase because active power increases and reactive power remains constant (it's not affected by load). As an average you can consider a 0.75 pf for 1/2 rated load, a 0.85 pf for 3/4 rated load and 0.9 pf for rated load. All the above characteristics are likely to change with motor characteristics (speed, voltage, power) and work environment or motor usage so use them only to make an idea of pf vs motor load curve. If you need more accurate figures please provide more data about the motor.

When you talk of cyclic duty like cranes or compressors the duty cycle should be known. Further as the rating of motor increases the pf is better. For example a 75 kw motor pf at full load will be less than that of say 750 kw of same speed. As polarity increases pf drops for the same kw. There are many other factors like quality of lamination, air gap, etc.

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