PAM
(pulse amplitude modulation) see variable voltage (VVI) inverter.

part winding starter
A special type of motor starter that limits current inrush by initially energizing one set of stator coils in the motor, and then energizing a second set as the motor reaches normal running speed. This requires a special motor. An adjustable speed drive should not drive a motor through a part winding starter. When run from an adjustable frequency drive, the motor must be wired so that the drive energizes all coils.

phase angle
Relationship between two wave forms having the same frequency. This describes the amount of offset (generally measured in degrees of a complete cycle) between corresponding parts of the two waves.

phase controlled rectifier
A device that converts alternating current to direct current through the use of SCRs, transistors or other controllable electronic devices. This is one common type of input stage used in adjustable frequency drives. The DC bus that is produced has an adjustable voltage level.

phase shift
A measure of the lead or lag in voltage or current in relation to another voltage or current in the same or a related circuit.

phase shift power factor
see displacement power factor

pickup voltage or current
The minimum voltage or current required to operate a magnetically operated device, such as a relay or contactor.

PLC
(programmable logic controller) A solid state controller for processes. This establishes a sequence of operations which can be changed easily with programming.

plugging
A type of motor braking provided by reversing the voltage polarity or phase sequence so that the motor develops a counter-torque and brakes the load.

point of common coupling
(pcc) The location in an electrical distribution system where all devices of concern are connected together. From a building's point of view, this is often at the main distribution panel near the high voltage transformer. From a utility's point of view, this is often where the high voltage transformer ties into the rest of the utilityís system. This point is used to measure the voltage and current distortion. Also called point of common coupling.

potentiometer
A rheostat with three terminals or a resistor with one or more sliding contacts that functions as an adjustable voltage divider.

power
Work done per unit of time. Usually expressed in Watts (W), kilowatts (kW) or break horsepower (BHP),

power factor
The ratio between the actual power delivered by an electronic circuit and the apparent power in the circuit. This can be divided into two components: displacement power factor (or phase shift power factor) and distortion power factor. Often, only the phase shift aspect of power factor is meant when power factor is mentioned. Power factor values can range from 0 to 1. The ideal power factor is 1.

power factor correction capacitor
A capacitor that is often used in conjunction with AC induction motors to correct the displacement power factor caused by the inductance of the motorís coils. When harmonic distortion is present, power factor correction capacitors can cause resonance problems.

preset speed
A fixed speed at which the drive will operate when so commanded.

primary coil
The coil of a transformer that is connected to the source of power.

proportional controller
Simple controller used in control circuity when the quantity being controlled responds quickly to the device that is doing the controlling. An example is controlling the pressure in a fan system. The output signal from the controller is made proportional to the difference between the actual pressure and the set point pressure. eg: The lower the pressure compared to the set point, the faster the fan will run.

proportional integral controller
(PI controller) A controller that in addition to measuring the difference between the actual pressure (or temperature) and the set point, also measures the length of time that the difference has been in existence. This will tend to eliminate a dead band and improve system stability.

proportional integral derivative controller
(PID controller) A controller that, in addition to having proportional and integral control, also measures the rate of change of the quantity being measured. This is generally used in fast-response robotic control systems.

PWM
(pulse-width modulation) A type of adjustable frequency drive that uses a diode bridge rectifier input stage and controls the output voltage to the motor by pulsing the inverter power components on and off at a fast rate.