P
“P” BASE
A special mounting similar to “D” flange except with a machine fit tenon recessed instead of protruding. Usually found on pumps.
PARALLELING
When two or more DC motors are required to operate in parallel – that is, to drive a common load while sharing the load equally among all motors – they should have speed-torque characteristics which are identical. The greater the speed droop with load, the easier it becomes to parallel motors successfully. It follows that series motors will operate in parallel easier than any other type. Compound motors, which also have drooping speed characteristics (high regulation), will generally parallel without special circuits or equalization. It may be difficult to operate shunt or stabilized-shunt motors in parallel because of their nearly constant speed characteristics. Modifications to the motor control must sometimes be made before these motors will parallel within satisfactory limits.
PART WINDING START MOTOR
It is arranged for starting by first energizing part of the primary winding and subsequently energizing the remainder of this winding in one or more steps. The purpose is to reduce the initial value of the starting current drawn or the starting torque developed by the motor. A standard part winding start induction motor is arranged so that one-half of its primary winding can be energized initially and subsequently the remaining half can be energized, both halves then carrying the same current.
PERMANENT MAGNET SYNCHRONOUS (PMR)
(HYSTERESIS SYNCHRONOUS)
A motor with magnets embedded into the rotor assembly, which enables the rotor to align itself with the rotating magnetic field of the stator. These motors have zero slip (the constant speed with load) and provide higher torque, efficiency and draw less current than comparable reluctance synchronous motors.
PHASE
Indicates the space relationships of windings and changing values of the recurring cycles of A.C. voltages and currents. Due to the positioning (or the phase relationship) of the windings, the various voltages and currents will not be similar in all aspects at any given instant. Each winding will lead or lag another, in position. Each voltage will lead or lag another voltage, in time. Each current will lead or lag another current, in time. The most common power supplies are either single (10) or three-phase (with 120 electrical degrees between the 3 phases).
PLUG REVERSAL
Reconnecting a motor’s winding in reverse to apply reverse braking torque to its normal direction of rotation while running. Although it is an effective dynamic braking means in many applications, plugging produces more heat than other methods and should be used with caution.
POLARIZATION TEST
A ratio of a one-minute megger test to a ten-minute megger test. Used to detect contaminants in winding insulation done typically on high voltage, V.P.I. motors that are tested by water immersion.
POLES
In an AC motor, refers to the number of magnetic poles in the stator winding. The number of poles is a determinant of the motor’s speed. (See Synchronous Speed)
In a DC motor, refers to the number of magnetic poles in the motor. It creates the magnetic field in which the armature operates. (Speed is not determined by the number of poles).
POLYPHASE MOTOR
Two or three-phase induction motors have their windings, one for each phase, evenly divided by the same number of electrical degrees. Reversal of the two-phase motor is accomplished by reversing the current through either winding. The reversal of a three-phase motor is accomplished by interchanging any two of its connections to the line. Polyphase motors are used where a polyphase (3-phase) power supply is available and is limited primarily to industrial applications.
Starting and reversing torque characteristics of polyphase motors are exceptionally good. This is due to the fact that the different windings are identical and, unlike the capacitor motor, the currents are balanced. They have an ideal phase relation which results in a true rotating field over the full range of operation from locked rotor to full speed.
POWER CODE
Identifies the type of power supply providing power to a DC motor. Frequency, voltage, and type of rectifier configuration.
POWER FACTOR
A measurement of the time phase difference between the voltage and current in an AC circuit. It is represented by the cosine of the angle of this phase difference. For an angle of 0 degrees, the power factor is 100% and the volt/amperes of the circuit are equal to the watts. (This is the ideal and an unrealistic situation.) The power factor is the ratio of Real Power-KW to total KVA or the ratio of actual power (watts) to apparent power (volt-amperes).
POWER SUPPLY
An electrical device used to convert an electric current from an outside source to the necessary frequency, voltage, and current needed to power a load. These devices are sometimes known as electric power converters. A good example of an industrial power supply is the Reliance Automax 57C493.
PRIMARY WINDING
That winding of a motor, transformer or another electrical device that is connected to the power source.
PROTECTIVE RELAY
A relay, the principal function of which is to protect service from interruption, or to prevent or limit damage to apparatus.
PULL-IN TORQUE
The maximum constant torque in which a synchronous motor will accelerate into synchronism at rated voltage and frequency.
PULL-UP TORQUE
The minimum torque developed by an AC motor during the period of acceleration from zero to the speed at which breakdown occurs. For motors that do not have a definite breakdown torque, the pull-up torque is the minimum torque developed during the process of getting up to the rated speed.
R – R (r bar) is the per-unit armature circuit resistance using counter emf as a base.
| R = Hot IR voltage drop | where |
| —————————- | |
| Terminal volts – (Hot IR voltage drop) |
Hot IR voltage drop = (Rated lax Hot Arm. Cir. Resistance) + 2.0 (Brush drop) volts.