Table Of ContentEngineering Principles
for Electrical Technicians
BY
Κ. M. SMITH, CENG., M.I.E.E.
AND
P. HOLROYD, CENG., M.I.MECH.E., A.M.I.STRUCT.E.
PERGAMON PRESS
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First edition 1969
Library of Congress Catalog Card No. 68-26947
Printed in Great Britain by Thomas Nelson (Printers) Ltd., London and Edinburgh
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08 012985 4 (flexicover)
08 012986 2 (hardcover)
A cknowledgements
THE authors wish to acknowledge permission given by Edward
Arnold (Publishers) Ltd. for the Abridged Callendar Steam Tables
printed at the end of Chapter 8. They also wish to express their
gratitude to the following bodies who have given permission for
their examination questions to be included :
City and Guilds of London Institute. (C.G.L.I.)
Northern Counties Technical Examinations Council.
(N.C.T.E.C.)
Union of Lancashire and Cheshire Institutes. (U.L.C.I.)
Union of Educational Institutions. (U.E.I.)
East Midland Educational Union. (E.M.E.U.)
They also wish to acknowledge the assistance given by colleagues
at the Huddersfield College of Technology and Ramsden Technical
College, Huddersfield.
xi
Introduction
THE rapid development of courses for technicians has shown the
need for a different type of textbook suited to their special require-
ments. Ideas and principles must be emphasised by demonstrations
and laboratory work so that the technician can develop his skills
and techniques. This book has been written for the technician to
use in lectures and in the laboratory. By using typical results taken
from simple laboratory experiments and with worked examples the
authors have provided the necessary introduction to fundamental
engineering principles.
This book was written specifically for Part I of the Electrical
Technicians Course (C.G.L.I. No. 57) and the Telecommunica-
tions Technicians Course (C.G.L.I. No. 49). It will also be useful
for the Mechanical Technicians Course (C.G.L.I. No. 293), and
will cover Part I of the new Electrical Installation Technicians
Course.
N. HILLER
Cambridge Editor
xiii
Symbols y Units and Abbreviations
Quantity Symbol Units Abbreviations
Length I inch, foot, metre in., ft, m
Mass m pound, kilogramme lb, kg
Time t second, minute, hour s or sec, min, h
Force F,P pound-force, newton Ibf, Ν
kilogramme-force kgf
Torque Τ pound-force-feet lbf-ft
newton-metre N-m
Power Ρ foot-pound-force per ft-lbf/s
second
newton-metre per second Nm/s
(or Watt) (or W)
horsepower hp
Work, energy W foot-pound-force ft-lbf
Displacement, linear s foot ft
Displacement, angular Θ radian rad
Velocity, linear ν foot per second ft/s
metre per second m/s
Velocity, angular ω radian per second rad/s
Ν revolutions per minute rev/min, or r.p.m.
Acceleration, linear f foot per second per second ft/s2
Acceleration, angular α radian per second per rad/s2
second
Acceleration, g foot per second per second ft/s2
gravitational
Coefficient of friction Μ — —
Stress, tensile and f pound-force per square Ibf/in2
compressive inch
SSttrreessss,, sshheeaarr q pound-force per square llbbff//iinn22
inch
Strain, tensile and e — —
compressive
Strain, shear Φ — —
Modulus of Elasticity Ε pound-force per square lbf/in*
inch
Coefficient of linear α — —
expansion
HHeeaatt,, qquuaannttiittyy ooff Q jjoouullee J
British thermal unit Btu
xiv
SYMBOLS, UNITS AND ABBREVIATIONS XV
Quantity Symbol Units Abbreviations
Heat, quantity of Centigrade heat unit Chu
gram-calorie cal
kilocalorie kcal
Specific heat c — —
Heat, total H British thermal unit per Btu/lb
S
pound
Heat, latent L British thermal unit per Btu/lb
pound
Heat, sensible h British thermal unit per Btu/lb
pound
Capacitance C farad, microfarad F, /iF
Charge (electric) Q coulomb, microcoulomb C, MC
Current I ampere, milliampere A, mA
Electric flux ψ coulomb C
Electric flux density D coulomb per square metre C/m2
Electric force or Ε volt per metre V/m
potential gradient
Electromotive force Ε volt, millivolt, kilovolt V, mV, kV
Electrical energy W joule (or volt-coulomb) J (or V-C)
watt-second, watt-hour, Ws, Wh,
kilowatt-hour kWh
Frequency f hertz Hz
Impedance Ζ ohm Ω
Inductance (self) L henry, millihenry H, mH
Inductance (mutual) M henry, millihenry H,mH
Magnetic flux Φ weber, milliweber Wb, mWb
Magnetic flux density Β weber per square metre Wb/m2
Magnetising force Η ampere-turn per metre AT/m
Magnetomotive force F ampere-turn AT
Permeability of free Mo henry per metre H/m
space
Permeability (relative) Mr — —
Permittivity of free ε farad per metre F/m
0
space
Permittivity (relative) — —
Potential difference V volt, millivolt, kilovolt V, mV, kV
Power (electric) Ρ joule per second (or watt) J/s (or W)
kilowatt, megawatt kW, MW
Power (apparent) — volt-ampere, VA,
kilovolt-ampere kVA
Power factor — — —
Reactance (capacitive) Xc ohm Ω
Reactance (inductive) XL ohm Ω
Reluctance S ampere-turn per weber AT/Wb
Resistance R ohm, microhm, megohm Ω, μΩ, ΜΩ
Resistivity Ρ ohm-inch, ohm-metre, Ω-in, Ω-m,
ohm-centimetre Ω-cm
CHAPTER 1
Forces and Equilibrium
1.1. Types of force
When we think of forces we usually consider the action of lifting
a weight, bending a piece of wire or setting a vehicle in motion.
Whenever we move, or move other objects around us, then forces
are brought into play. In fact, approximately three centuries ago
Sir Isaac Newton gave us a definition of a force which includes all
the forces that we can imagine. "Force changes or tends to change
the state of a body's rest or uniform motion in a straight line."
One of the most important forces we meet is the one which causes
objects to fall downwards, the Force of Gravity.
It has been found that between all bodies there is a force of
attraction defined as the gravitational force. The larger the bodies
the greater the attraction between them, but as the distance
between the bodies increases the attraction decreases. Normally
bodies on the surface of the earth do not show this attraction
between themselves because the force is very small, but if we
consider two masses, one of which is the earth, then the force of
attraction between these masses can be considerable. The earth
being the larger body will not move perceptibly and therefore the
smaller body, if free to do so, will move towards the centre of the
earth. This force of gravity acting on the body is called its Weight.
It should be understood that not all the forces we meet are
gravitational forces. Friction forces are introduced when two
surfaces slide over each other; inertia forces occur when a mass
has to be accelerated; tensile, compressive and shear forces are
considered in dealing with the strength of materials (Fig. 1.1). The
1
2 ENGINEERING PRINCIPLES FOR ELECTRICAL TECHNICIANS
idea of the gravitational force can be used to give us a unit of
measurement of a force.
Accelerating
Gravitational force causes Acceleration
force causes an increase Mass
a body to fall in velocity
Earth
GRAVITATIONAL FORCE ACCELERATING FORCE
Motion
Friction force
Applied^ resisting
force_ — motion
Rough surface
FRICTION FORCE SHEAR FORCE
Compressive
force in leg
TENSILE FORCE COMPRESSIVE FORCE
FIG. 1.1. Different applications of forces. The gravitational force
causes all bodies to fall towards the centre of the earth. An accelerat-
ing force causes a body to change its speed or velocity. When two
surfaces slide over each other the resistance to motion is called the
friction force. A pair of scissors cuts paper by a shearing force.
A tensile force is introduced into a rope when pulling a box along
the ground. The stool legs are subject to a compressive force when
anyone sits on the stool.
1.2. Unit of force
if we wish to compare forces, then we must have a standard of
reference. In England a cylinder of platinum of a certain size is
FORCES AND EQUILIBRIUM 3
kept at the Board of Trade in London representing a mass of one
pound. The gravitational force exerted on this mass at mean sea
level is called the pound-force. Other weights are then made to
U.K. Primary standard
Authorized
copies
Ε —I
lib nib I lib I lib nib nib Secondary
Tertiary
Local
1 lb 1 lb 11b Working
r ~ —ι
TuTI î~nb~| |iib| |iib| I lib I | lib I Tradesmen
FIG. 1.2. The U.K. Standard One Pound Mass. A cylinder
of platinum kept at the Board of Trade represents the
United Kingdom Primary Standard one pound mass. Five
copies of this are distributed to various other offices in
London. Secondary standards are made to compare with the
authorised copies and tertiary standards are compared to
the secondary standards. In each town and city a set of local
standards are kept and periodically checked with the
tertiary standards. The Weights and Measures Officer uses
a set of working standards for checking tradesmen's weights
which he maintains by checking with the local standards.
This flow enables a continual check to be made on the
accuracy of all weights used in the country.
compare with the standard and distributed to Weights and
Measures offices throughout the country from which the weights
you see in shops or factories are checked. See Fig. 1.2.
Another standard of mass used on the Continent is the kilo-
gramme, represented by a cylinder of platinum-iridium kept in
4 ENGINEERING PRINCIPLES FOR ELECTRICAL TECHNICIANS
France. The gravitational force exerted on this mass is called the
kilogramme-force.
The relationship between the pound-force and the kilogramme-
force is
1 pound-force (lbf) = 0-4536 kilogramme-force (kgf)
or 1 kilogramme-force (kgf) = 2-205 pound-force (lbf).
We can use multiples of these forces or weights.
The pound-force is related to the ton-force (tonf) such that
2240 lbf = 1 tonf.
Similarly 1 gramme force (βΟ = ττ^ kgf.
1.3. Effects of forces
If we consider a mass of 5 lb hanging from a rope (Fig. 1.3)
we should say that the force in the rope is 5 lbf. If we now sup-
port the rope over a pulley and hang a 5 lb mass on each end
(Fig. 1.4) would the force on the rope now be 5 lbf? If you are in
doubt, cut the string and insert a spring balance in the system
(Fig. 1.5) and take a reading from the balance. This should be
5 lbf. Now ask yourself why this is only 5 lbf and not 10 lbf.
The answer was given again by Sir Isaac Newton, who stated:
"To every action there is an equal and opposite reaction."
In Fig. 1.3 the support is providing an upward force of 5 lbf to
balance the weight hanging downwards, therefore in Fig. 1.4. to
keep the system balanced, a mass of 5 lb must be attached at each
end of the string. If one mass were taken away then the other
would fall to the ground. This situation occurs when anything
rests on the ground. If you weigh 145 lbf and you are standing on
the ground, then the earth is pressing upwards with an equal force
of 145 lbf to keep you at rest. When you stand on the bathroom
scales, the scales are sandwiched between your feet and the floor
so that pressure is exerted on the top and bottom of the scales to
enable a reading of your weight to be taken. If you attempted to
jump off a high diving board with the scales strapped to your
