Electrical Engineering Dictionary - part 132

permanent magnet DC motor

See

permanent magnet machine

a machine

that uses permanent magnets to establish the
field. In DC machines, the permanent mag-
nets are placed on the stator, while on AC
synchronous machines they are placed on the
rotor.

permanent magnet stepper motor

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 distribu-
tion of stator phase windings. The field wind-
ings in the rotor are replaced by permanent
magnets to provide the field excitation in
these machines.

permanent split-capacitor (PSC) motor
a induction motor that operates from a single-
phase supply. The motor contains two phase
windings in quadrature; however, one of
them has a capacitor in series with it to cre-
ate a phase shift between the winding cur-
rents. Both windings and the capacitor op-
erate continuously so the machine acts like a
two-phase machine when running at its op-
erating speed, producing less vibration and
noise than a single-phase motor. Since the ca-
pacitor runs continuously, it is sized smaller
than the capacitor used in a capacitor-start
induction motor (CSIM). Thus, the PSC mo-
tor produces a lower starting torque than the
CSIM.

permeability

tensor relationship between

the magnetic field vector and the magnetic
flux density vector in a medium with no hys-
teresis; flux density divided by the magnetic
field in scalar media. Permeability indicates
the ease with which a magnetic material can
be magnetized.

An electromagnet with a

higher permeable core material will produce
a stronger magnetic field than one with a

lower permeable core material. Permeability
is analogous to conductance, when describ-
ing electron flow through a material. See also

reluctance

permeameter

making use of Hall effect

gaussmeters, search coils, and flux meters,
the permeameter, or hysteresigraph, records
the major hysteresis loop of a material, from
which its basic material properties can be de-
termined: residual induction, coercivity, en-
ergy product, saturation flux density, and re-
coil permeability.

permeance

the magnetic analog for con-

ductance, indicating the ease with which
magnetic flux will follow a certain path,
which can be approximated by calculations
based purely on magnetic circuit geometry.

permeance coefficient

the slope of the

load line for a magnetic circuit, determined
solely by physical geometry of the magnet
and permeable materials around it; the ratio
of magnetic induction (

B) and applied field

(

H) at the operating point.

permission

See

access right

permittivity

See

electric permittivity

persistent current

a current circulating in

a closed structure without applied potential.
Examples are the supercurrent in a supercon-
ducting magnet and the current in a closed
mesoscopic ring in a magnetic field.

persistent spectral hole burning

spectral

hole burning with a long lifetime, usually on
the scale of seconds or longer.

personal computer

a general term for

a microcomputer used for such purposes as
word processing, email, financial manage-
ment, and game playing. While, the acronym
“PC” is applied to microcomputers used both
for business and personal use, the term “per-
sonal computer” is more usually applied to
those used by individuals or families at home.

2000 by CRC Press LLC

personal communications network (PCN)

a telecommunications network designed to

provide services to a person rather than a geo-
graphic location. The network may comprise
a range of different technologies from end to
end and contains within it intelligence to en-
able the communication to be directed to the
appropriate terminal or device carried with
the person.

personal communications services (PCS)

a mobile telephone service with an essential

urban and suburban coverage characterized
by low cost pocket terminals, communica-
tions at a price comparable to a cable tele-
phone, and distribution of the services and
products to the general public. This defini-
tion is independent of the technology used.

PCS is the proposed next generation of

wireless network services, providing voice
communication services similar to today’s
cellular services, only with smaller cells,
lower power, and cheaper rates.

personal digital cellular (PDC)

one type

of digital cellular phone system.

PDC is

800 MHz/1.5 GHz band, FDD, TDMA sys-
tem, and it handles 1/4p shift QPSK modu-
lated signal with 32 kbit/s. This cellular sys-
tem was developed and operated in Japan.

personal handy phone system (PHS)

digital microcell system designed in Japan
that operates in the 1.9 GHz band. PHS pro-
vides cordless telephone or telepoint services
similar to other digital cordless technologies,
such as CT2. The spectrum allocation for
PHS is 1895–1918 MHz. There are 77 ra-
dio channels, each 300 kHz wide and divided
into 2

× 4 time slots. This provides a total of

308 duplex traffic channels, with duplexing
via TDD. The carrier bit rate is 384 kb/s, and
PHS terminals have a peak transmit power of
80 mW.

perspective distortion

a type of object

distortion that results from projecting 3-D
shapes onto 2-D image planes by conver-
gence of rays towards a center of projection:

See also

perspective projection

. This type of

distortion is also called foreshortening.

perspective inversion

a property of per-

spective projection in which planar (typically
silhouetted) objects that are not perpendicu-
lar to the axis of projection will appear, in the
absence of additional information, to have ei-
ther of two possible orientations in space.

perspective projection

projection of a

3-D object on a plane, termed “projection
plane,” as if it were imaged by an ideal pin-
hole camera located in a point termed “center
of perspective.”

perspective transformation

a matrix

transformation that represents the perspec-
tive projection of objects in 3-D scenes into
2-D images or the projection of one 2-D im-
age into another 2-D image.

Perspective

transformations are conveniently carried out
using 4

× 4 matrices representing homoge-

neous co-ordinate transformations.

perturbations of controllable systems
the set of controllable linear stationary
continuous-time finite-dimensional dynami-
cal systems is dense and open in the set of
all linear stationary continuous-time finite-
dimensional dynamical systems with the
same state and input dimensions.

PET

See

positron emission tomography

Petersen coil

another term for a ground

fault neutralizer.

petticoat

another name for shed, a feature

of an insulator.

PFN

See

pulse forming network

PGA

See

field-programmable gate array

phantom

an artificial target, sometimes

designed to mimic the size, shape, and atten-
uation characteristics of actual tissue, that is

2000 by CRC Press LLC

used to test and calibrate imaging hardware
and software.

phase

(1) a notion used extensively in in-

terpreting complex quantities such as Fourier
series, Fourier transforms etc. Given a com-
plex number

c = x +iy = r cos φ +ir sin φ,

then

r represents the magnitude of c and φ the

phase.

(2) a horizontal translation parameter of

the signal. Given a sinusoidal signal

s(t) =

Asin(2πf t + φ), then f represents the fre-
quency (in Hz) of

s and φ the phase.

phase alternate line (PAL)

a color televi-

sion system that inverts the (R-Y) color signal
on alternate lines. The color burst is located
on the back porch of the composite video sig-
nal and the phase alternates every line for
identification of the (R-Y) difference signal
phase. The color demodulator uses the shift
in the color burst phase and the (R-Y) phase
reversals to eliminate any hue changes caused
by phase errors. The PAL color demodula-
tion method eliminates the tint control but
does cause desaturated colors.

The vertical scan parameters for the PAL

television system are based on the European
power line frequency of 50 Hz. The different
PAL standards in existence in different coun-
tries allow for a slightly wider video band-
width. The primary PAL system parameters
(with the allowed bandwidth variance) are

Vertical Fields Frequency

50 Hz

Number of Interlaced Fields

Vertical Frame Frequency

25 Hz

Number of Horizontal Lines/Frame

625

Horizontal Scan Frequency

15,625 Hz

Video Bandwidth

5 to 6 MHz

Color Subcarrier Frequency

4.433618 MHz

Sound Modulation

RF Channel Bandwidth

7 to 8 MHz

[* Alternative definitions for the 3 televi-

sion systems that you may enjoy]

[NTSC - Never Twice the Same Color, SE-

CAM - System Essentially Contrary to the
American Method, PAL - Perfect At Last]

phase angle

the angle in the complex

plane of a complex value

x = a + jb cal-

culated as

x = arctan

b
a

phase angle meter

meter used to measure

the phase angle difference between two AC
quantities. In power systems, typical meters
use perpendicular moving coils to measure
the phase angle between an AC current and
an AC voltage. More accurate devices typi-
cally measure the time interval between zero
crossings of the two input signals.

phase breaking

the process by which the

quantum mechanical phase, which is related
to energy, of a particle is destroyed. The
most common example is the inelastic scat-
tering due to electron–electron and electron–
phonon interactions.

phase coded modulation (PCM)

a form

of angle modulation. The modulated wave,
s(t), is given by:

s(t) = A

cos

[2πf

t + k

m(t)],

where

m(t) is the message signal, f

is the

carrier frequency, and

is the phase sensi-

tivity of the modulator.

phase comparator

often referred to as

a phase detector; a three-port device that
produces an analog output proportional to
the phase difference between its two inputs.
Since both inputs are periodic, the relative
output voltage (or current) as a function of
input phase difference (i.e., the transfer func-
tion) is also periodic; the shape of the trans-
fer function (sometimes called the “output
characteristic” of the phase detector) depends
upon the particular technique used to accom-
plish the phase detection.

These include

sinusoidal, triangula, and sawtooth shape
factors. Analog/digital implementation, re-
quired linearity, and range of input phase dif-
ference are primary factors in determining a
suitable output characteristic for a specific
phase comparator application.

2000 by CRC Press LLC

phase comparison relay

a phase com-

parison relay is a protective relay used on
transmission lines which operates by com-
paring phase angles of signals generated at
opposite ends of the line. They employ a
dedicated communications channel to make
the comparison. The signals compared are
typically corresponding phase currents or se-
quence currents.

phase conjugate mirror

See

phase

conjugator

phase conjugation

a technique for pro-

viding dispersion compensation that relies on
spectral inversion of the optical signal us-
ing a nonlinear process known as four-wave-
mixing.

phase conjugator

an optical set-up or

system that can generate the time-reversed
replica of the incident wave. Phase conjuga-
tors play an important role in many optical
systems that require the transmission of op-
tical waves through scattering media such as
atmosphere, optical fibers. Photorefractive
crystals such as BaTiO

, SBN, BSO are by

far the most efficient media for the generation
of phase conjugate waves.

phase constant

a constant, which is gen-

erally complex, that is important in the study
of electromagnetic waves. The phase con-
stant is equal to the frequency of excitation
of the wave times 2 times pi times the square
root of the product of the permeability and
the permittivity of the medium that the wave
is traveling in. Also called propagation con-
stant or wavenumber.

phase control

a method for controlling

the amount of power delivered to a load by
varying the delay angle. This controls the
delay between the instant when the voltage
across the power semiconductor goes posi-
tive and the actual start in conduction of the
device.

phase delay

the difference in the abso-

lute angles between a point on a wavefront at
the device output and the corresponding point
on the incident input wavefront, expressed in
seconds or degrees. The delay can exceed
360 degrees.

phase detector gain

the ratio of the DC

output voltage of the phase detector to the
input phase difference. This is usually ex-
pressed in units of volts per radian.

phase deviation

the peak difference be-

tween the instantaneous angle of the modu-
lated wave and the angle of the carrier. In
the case of a sinusoidal modulating function,
the value of the phase deviation, expressed in
radians, is equal to the modulation index.

phase discriminator

a device for detect-

ing the phase deviation of a phase modulated
signal.

phase distortion

problem that occurs

when the phase shift in the output signal of an
amplifier is not proportional to the frequency.

phase error

the difference in phase be-

tween two sinusoidal waveforms having the
same frequency.

phase grating

an optical grating charac-

terized by a spatially periodic variation in the
refractive index of a medium.

phase interrupting collision

a collision

that interrupts the phase of the wavefunctions
of lasing atoms and consequently broaden
their emission spectra.

phase locked loop

a device containing a

voltage controlled oscillator, a synchronous
detector and a low-pass filter, arranged in a
feedback loop so that the oscillator output
can be made to lock onto a weak incoming
periodic signal such as a radio wave. Also of
use in frequency synchronization.

2000 by CRC Press LLC

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