Handbook of Electrical Installation Practice - BeKnowledge

CHAPTER 15
Rotating Machines
D.B. Manning, CEng, MIEE
(Mawdsley’s Ltd)
Reviewed by G. Walton, MIETE
(Internal Sales Engineer, Eurotherm Drives Ltd)
Electricity is a particularly attractive form of energy in that it can easily be produced,
transmitted and converted into some other form. The commonest form of
energy into which electricity is converted is mechanical driving energy and more
than 60% of electrical energy produced is utilised in this way. Conversion of energy
from its electrical to mechanical form is achieved using electric motors. The vast
bulk of industrial electric motors is used to drive pumps, fans and compressors. All
industrial installations, whether manufacturing units or complex process plants,
include electric motors, often of many types, sizes and voltages to suit a wide variety
of applications.
Often much abused, electric motors are expected to perform efficiently and
reliably for many years with the minimum of maintenance. In relation to the cost
of the energy they convert they are extremely inexpensive. Typically, the electrical
energy costs of supplying a motor can equal the prime cost of the motor in only two
months of operation. Installation, cabling and control gear costs will perhaps double
or even treble this period but this is still a small cost in the normal expected machine
life. Many motors supplied over 50 years ago are still performing well in plants
around the world.
Therefore, selection of the right type of machine for the application and duty, and
correct installation and maintenance of the machine, are as important as the
machine design itself.
This chapter concentrates on motors but does make reference where appropriate
to alternators. Similarly, reference is limited to machines up to 11kV and
10MW.
MOTOR TYPES
All motors fall into one of two classes: those suitable for use on a.c. supply and those
suitable for d.c. systems.
Alternating current motors
There is a large range of a.c. motors available, the most widely used being the cage
design.
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