The number of individual voltages applied to an AC motor. A
single-phase motor has one voltage in the shape of a sine wave applied to
it. A three-phase motor has three individual voltages applied to it. The
three phases are at 120 degrees with respect to each other so that peaks
of voltage occur at even time intervals to balance the power received and
delivered by the motor throughout its 360 degrees of rotation.
Plugging:
A method of braking a motor that involves applying partial or
full voltage in reverse to bring the motor to zero speed.
Polarity:
As applied to electric circuits, polarity indicates which terminal
is positive and which is negative. As applied to magnets, it indicates which
pole is north and which pole is south.
Poles:
Magnetic devices set up inside the motor by the placement and
connection of the windings. Divide the number of poles into 7200 to determine
the motor’s normal speed. For example, 7200 divided by 2 poles
equals 3600 RPM.
Power Factor:
The ratio of “apparent power” (expressed in kVA) and
true or “real power” (expressed in kW).
Power Factor =
Real Power/
Apparent Power
Apparent power is calculated by a formula involving the “real power,” that
which is supplied by the power system to actually turn the motor, and
“reactive power,” which is used strictly to develop a magnetic field within
the motor. Electric utilities prefer power factors as close to 100% as
possible, and sometimes charge penalties for power factors below 90%.
Power factor is often improved or “corrected” using capacitors. Power
factor does not necessarily relate to motor efficiency, but is a component
of total energy consumption.
Prime Mover:
In industry, the prime mover is most often an electric
motor. Occasionally engines, hydraulic or air motors are used. Special
application considerations are called for when other than an electric motor
is the prime mover.
Pull Out Torque:
Also called breakdown torque or maximum torque, this
is the maximum torque a motor can deliver without stalling.
Pull Up Torque:
The minimum torque delivered by a motor between
zero and the rated RPM, equal to the maximum load a motor can
accelerate to rated RPM.
Pulse Width Modulation:
Abbreviated PWM, the most common
frequency synthesizing system in AC drives; also used in some DC drives
for voltage control.
