Basic Units of Measurement and Conversions - Basic Mathematics for Water and Wastewater Operators - page 48

electrical devices such as space heaters and old fluorescent or high-discharge lamps
also have poor power factor. Treatment plants have several motors, numerous lamps,
and often electric heaters, which, combined, lower the facility's overall power factor.
Power factor may be leading or lagging. Voltage and current waveforms are in
phase in a resistive alternating current (AC) circuit; however, reactive loads, such
as induction motors, store energy in their magnetic fields. When this energy gets
released back to the circuit it pushes the current and voltage waveforms out of phase.
The current waveform then lags behind the voltage waveform.
Improving power factor is beneficial, as it improves voltage, decreases system
losses, frees capacity to the system, and decreases power costs where fees for poor
power factor are billed. Power factor can be improved by reducing the reactive power
component of the circuit. Adding capacitors to an induction motor is perhaps the
most cost-effective means to correct power factor as they provide reactive power.
Synchronous motors are an alternative to capacitors for power factor correction.
Like induction motors, synchronous motors have stator windings that produce a
rotating magnetic field; however, unlike the induction motor, the synchronous motor
requires a separate source of DC from the field. It also requires special starting
components. These include a salient-pole field with starting grid winding. The rotor
of the conventional type of synchronous motor is essentially the same as that of the
salient-pole AC generator. The stator windings of induction and synchronous motors
are essentially the same.
In operation, the synchronous motor rotor locks into step with the rotating mag-
netic field and rotates at the same speed. If the rotor is pulled out of step with the
rotating stator field, no torque is developed and the motor stops. Because a synchro-
nous motor develops torque only when running at synchronous speed, it is not self-
starting and hence requires some device to bring the rotor to synchronous speed.
For example, the rotation of a synchronous motor may be started with a DC motor
on a common shaft. After the motor is brought to synchronous speed, AC current is
applied to the stator windings. The DC starting motor now acts as a DC generator,
supplying DC field excitation for the rotor. The load then can be coupled to the motor.
Synchronous motors can be run at lagging, unity, or leading power factor by
controlling their field excitation. When the field excitation voltage is decreased, the
motor runs in lagging power factor. This condition is referred to as under-excitation .
When the field excitation voltage is made equal to the rated voltage, the motor runs
at unity power factor. The motor runs at leading power factor when the field excita-
tion voltage is increased above the rated voltage. This condition is over-excitation .
When over-excited, synchronous motors can provide system power factor correction.
The feasibility of adding capacitors depends on whether the electric utility charges
for low power factor. Corrective measures are infrequently installed, as many electric
utilities do not charge small customers for poor power factor but rather price it into the
electrical rates as a cost of business. A cost evaluation is necessary to determine the
type of correction equipment to use. The evaluation should include motor type, motor
starter, exciter (for synchronous motors), capacitors, switching devices (if needed),
efficiency, and power factor fees (IEEE, 1990). Manufacturers should be consulted
before installing capacitors to reduced voltage solid-state starters and variable fre-
quency drives, as there can be problems if they are not properly located and applied.