Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

pad-mount transformer
a heavily-enclosed distribution transformer mounted at grade level upon a concrete slab or pad.
panelboard
an assembly of one or more panel units containing power buses, automatic overcurrent protective devices, that is placed in a cabinet or cutout box located in or flush on a wall. The assembly can only be accessed from the front and may contain switches for operation of light, heat, or power circuits.
See switchboard
parallel paths
the number of separate paths through the armature winding that exist between the brushes of a DC machine.

In a DC machine's armature, the conductors and coils are placed in their slots and connected to the commutator using either the lap winding method or the wave winding method. The number of conductors that are connected in parallel depend on the number of poles the machine has, and whether the winding connections are lap or wave. For the lap wound armature, the number of parallel paths is found by multiplying the number of poles by the number of revolutions it takes to
fill all the slots of the armature. The number of revolutions it takes to fill the slots is known as the machine's "plex" value. In a simplex wound armature, the "plex" value is 1, duplex has a "plex" value of 2, triplex has a "plex" value of 3, and so on. For the wave wound armature, the number of parallel paths is two times the "plex" value. This same concept
can also be applied to AC machinery.
parasitic capacitance
the generally undesirable and not-designed-for capacitance between two conductors in proximity of one another.
parasitic inductance
the generally undesirable and not-designed-for inductance associated with a conductor, or path of current on a conductor.
parasitic reactance
the generally undesirable and not-designed-for reactance associated with one or more conductors in a circuit.
parasitic resistance
the generally undesirable and not-designed-for resistance associated with a conductor, or path of current on a conductor.
peak let-through current
the maximum value of the available short-circuit current that is let through a current-limiting fuse.
See current limiting fuse
per-unit system
a dimensionless system for expressing each quantity in terms of a fractional part of a "base" value, often the nominal or rated value of the system. Typical electrical calculations require four base quantities (voltage, current, impedance, and apparent power), any two of which may be chosen arbitrarily. The per-unit system greatly simplifies calculations in electrical systems containing transformers with non-unity turns ratios, making the voltage differences transparent.
percent impedance
the per-unit impedance expressed as a percentage on a certain MVA and voltage base.
percent system
a variation of the per-unit system in which the ratios expressing system quantities are expressed as a percentage of the base quantity.
permanent fault
a fault that remains in existence indefinitely if no corrective actions are taken.
permanent magnet (PM)
a magnet that produces an external magnetic field by virtue of the alignment of domains inside the material and retains its magnetism after being subjected to demagnetizing fields.
permanent magnet AC motor
a generic term used to describe both permanent magnet synchronous motors and brushless DC motors.
permanent magnet brushless DC machine
a machine that is similar in structure to a permanent magnet synchronous machine, containing armature windings on the stator and permanent magnets on the rotor. The permanent magnet brushless DC machine, however, is characterized by a trapezoidal flux density distribution in the airgap instead of the sinusoidal distribution of the synchronous machine. In operation, a DC voltage is applied sequentially to the stator coils to create a rotating field that pulls the rotor with it. To correctly operate, the brushless DC machine requires sensors to determine the rotor position so that the proper stator phases may be excited.
permanent magnet DC machine
a DC machine in which the field excitation in the stator is provided by permanent magnets instead of electromagnets.
permanent magnet DC motor

See permanent magnet DC machine
permanent magnet machine
a machine that uses permanent magnets to establish the field. In DC machines, the permanent magnets are placed on the stator, while on AC synchronous machines they are placed on the rotor.
permanent magnet stepper motor
a stepper motor that has a permanent magnet assembly on the rotor.
permanent magnet synchronous machine
a polyphase AC motor with rotor mounted permanent magnets and sinusoidal distribution of stator phase windings. The field windings in the rotor are replaced by permanent magnets to provide the field excitation in these machines.
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