Standard Motor Catalog

Section TR Technical Reference Guide

POWER FACTOR In a sense, motors are electromagnets and power factor is a measure of the amount of magnetizing current required. Power factor is an important consideration when selecting a motor for particular application since low power factor may result in power factor penalty charges from the utility company. Power companies must supply KVA but normally meters only kilowatts used, low motor power factor requires additional KVA with low return on kW utilized; hence, power factor penalties.

There are two basic methods for improving the power factor of a motor for a particular application: 1. Purchase a motor with an inherently high power factor. 2. Install power factor correction capacitors. Capacitors draw leading current as opposed to the lagging current drawn by induction motors. Placing capacitors in parallel with the motor windings will result in leading current offsetting some of the lagging current, increasing power factor as shown in Figure 21:

Following is the equation for power factor in a three-phase system:

WITHOUT CAPACITORS

l P

Watts input √3 x Volts x PF

Amps =

l M

The equation below is a numerical method of expressing the phase difference between voltage and current in a motor circuit. The current in an induction motor lags the applied voltage, and only the component that is in phase with the voltage varies with motor power. The relationship expressed in the above equation can be show as a vector relationship in which the numerical expression is actually the cosine of the angle L.

l L

WITH CAPACITORS lC (CAPACITOR CURRENT)

l P l L

l M

Figure 21: Power Factor Correction

For many applications the use of capacitors for power factor correction is the most economical method and one that also works at partial loads. Capacitors should be used to improve full load power factor to approximately 95% maximum. WARNING: IN NO CASE SHOULD POWER FACTOR IMPROVEMENT CAPACITORS BE APPLIED IN RATINGS EXCEEDING THE MAXIMUM SAFE VALUE SPECIFIED BY THE MOTOR MANUFACTURER, EXCESSIVE IMPROVEMENT MAY CAUSE OVER EXCITETATION RESULTING IN HIGH TRANSIENT VOLTAGES, CURRENTS AND TORQUES THAT CAN INCREASE SAFETY HAZARDS TO PERSONNEL AND CAUSE POSSIBLE DAMAGE TO THE MOTOR OR TO THE DRIVEN EQUIPMENT.

Watts input √3 x Volts x Amps

PF =

TECHNICAL REFERENCE GUIDE

Figure 19

As seen from their relationship, line current required for a given motor output varies inversely with power factor. Increasing power factor will reduce the required line current, thus reducing voltage drop in power lines and transformers. The lagging current shown above is actually motor magnetizing current, which is dependent upon motor design. This magnetizing current is independent of motor load; i.e., just as much required at no-load as at full-load. Thus power factor at partial loads is never as high as at full load, and at no-load power factor is essentially zero.

FULL-LOAD

1/2 LOAD

1/4 LOAD

NO-LOAD

l P

l P

l P

l P

l M

l M l

l M l L

l M l L

l L

L

l P = In-phase Current l M = Magnetizing Current l L = Total Current

Figure 20

TR.19

Data subject to change without notice. 03/24 • www.wolongamerica.com