All a-c induction motors operate at a power factor less than unity. Power factor is simply a measure of the phase displacement or time lag between the current through the motor and the voltage that’s applied. With everything else being equal, the closer the power factor is to 100% (meaning zero phase displacement between current and voltage) the lower the amount of current drawn from the line to supply a given horsepower output.
But that extra current does not necessarily mean that the motor is less efficient. Much of the motor’s internal energy loss is unrelated to current (the iron or core loss, for example).
Looking at the other side of this relationship: Motor design changes made to increase the efficiency, by reducing losses, may or may not change the power factor. For example, reducing the air gap between stator and rotor lowers the “magnetizing current” the motor draws to maintain the magnetic field across that gap. That means lower “copper loss” in the motor winding through which that current flows, and hence a higher efficiency. But the power factor will be increased too, because of the lower magnetizing current. However, a smaller air gap increases the stray load loss in the motor, which tends to drive efficiency down. To keep the efficiency high, the air gap must be increased, which lowers the power factor.
Which of the conflicting trends will predominate? That depends upon the particular design, and upon the remaining losses that aren’t affected by the air gap change.
Don’t be fooled, then, by claims that “energy efficient” motors will also have “higher power factor”. Some of them will. Many others will not. Examination of published data from five manufacturers of the same motor rating showed that the lower factor for the “energy efficient” version was higher in two of the designs, lower in two others and unchanged for the fifth.
More importantly … we don’t really care about this. By adding relatively inexpensive capacitors to the motor circuit, an unacceptably low power factor can be raised as far as the power system is concerned. In many instances, there’s no need for this, because the higher line current resulting from low power factor is insignificant to the supply system capacity, and represents negligible energy loss. (Improving a motor circuit power factor from 85% to 90% in a typical industrial plant effects only a 0.1% saving in current energy usage, caused by the lower motor current flowing through the supply circuit.)
But if the system capacity is being overtaxed, if a utility power factor penalty applies, or if the motor is a large one having a major influence on plant load, the “capacitor correction” is easy to adopt. Motor efficiency, in contrast, can’t be boosted by adding circuit accessories. And efficiency directly influences the energy bill; power factor does not.
Richard L. Nailen, P.E.