Mechanical Systems 397
watts.) Then,
P
pf = ——
VA
where:
pf = the power factor of the circuit,
P = the true power in watts, and
VA = the apparent power in volt-amperes.The maximum pf value is 1.0, or 100%, which would be obtained in
a purely resistive circuit. This is referred to as unity power factor.Effect of Load
Since electrical power will probably become more expensive and less
abundant, the efficiency and power factor of electric motors will become
increasingly important. The efficiency of a motor shows mathematically
just how well a motor converts electrical energy into mechanical energy. A
mechanical load placed on a motor affects its efficiency. Thus, it is particu-
larly important for industrial users to load motors so that their maximum
efficiency is maintained.
Power factor is also affected by the mechanical load placed on a mo-
tor. A higher power factor means that a motor requires less current to pro-
duce a given amount of torque or mechanical energy. Lower current levels
mean that less energy is being wasted (converted to heat) in the equip-
ment and circuits connected to the motor. Penalties are assessed on indus-
trial users by the electrical utility companies for having low system power
factors (usually less than 0.8 or 0.85 values). By operating at higher power
factors, industrial users can save money on penalties, and can help, on a
larger scale, with more efficient utilization of electrical power. Motor load
affects the power factor to a much greater extent than it does the efficiency.
Therefore, motor applications should be carefully studied to ensure that
motors (particularly very large ones) are not overloaded or underloaded,
so that the available electrical power will be used more effectively.Effect of Voltage Variations
Voltage variation also has an effect on the power factor and efficien-
cy. Even slight changes in voltage produce a distinct effect on the power
factor. However, a less distinct effect results when the voltage causes a
variation in the efficiency. Because proper power utilization is becoming
more and more important, motor users should make sure that their mo-