Handbook of Electrical Installation Practice - BeKnowledge

CHAPTER 17
Motor Control Gear
T. Fairhall, IEng, MIIE
Senior Engineer, Allenwest Electrical Ltd
Motor control gear and associated circuitry are fundamental parts of electrical distribution
and utilisation systems. Motor control gear in its simplest form may be a
direct on-line starter consisting of a switching device (contactor) and tripping device
(overload relay) mounted within an appropriate enclosure.
Motor control gear has two main functions. It must make and break the connections
between the motor and the supply and it must also automatically disconnect
the motor (a) in the event of excessive current being passed which would overheat
and ultimately destroy the windings, (b) if the supply fails, or (c) in combination
with its external short-circuit protective devices (fuses or circuit-breakers), in the
event of an earth fault.
In addition to the main functions described above, motor control gear may
also be required to: limit the starting current to a value consistent with the requirements
of the drive; apply braking when rapid stopping is required, reverse rotation
or vary speed as necessary; and carry out automatic operating cycles or
sequences.
It follows that the control gear must be appropriately selected to suit the characteristics
of the motor and the drive.
Control devices for manual operation are normally pushbuttons or selector
switches, alternatively automatic control of a starter may be achieved by use of a
float switch, pressure switch, thermostat or other similar device; these are usually
covered by the generic term ‘pilot switch’ on starter diagrams.
Actuation of the contactor (switching device) may be by a number of different
methods, the most usual being electromagnetic, where the energisation of a coil and
magnetic system provides the force to close the spring-loaded contacts which switch
the motor current, although some hand-operated starters are available where finger
pressure on the start pushbutton directly closes the switching device and others were
available where the changeover from ‘start’ to ‘run’ was carried out manually. It
should be remembered that these starters could not be remotely controlled,
although in some instances the option of a remote stop was offered; also, they may
not provide no-volt protection so were unsuitable for machine-tool or machine
control applications.
In order to provide a ‘no-volt’ release function and be able to provide remote
control, an electromagnetic contactor is used in the majority of industrial
applications.
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