Page 245 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

Rotary type switches (Fig 28)
Rotary switches are most commonly used in lathes, milling and drilling machines due to their exact visual position
and easiness in operation. These switches are operated by levers or knobs which in turn operate cams inside the
switch to contact various terminals in sequence by the internal contact blocks. These cams and blocks are made
of hard P.V.C. and are designed to withstand many operations. It is possible to get many circuit combinations by
combining various cams and contact blocks. As the contact blocks, terminals and cams are spring-loaded, these
switches should not be opened by inexperienced persons for repairs. Fig 28 shows 250V AC 15 Amps 2-pole
three position flush mounting coin-slot operator.
Function: This switches can do a number of functions, depending upon the cover and contact block combinations.
According they can be used for ON/ OFF switch, manual Forward / Reverse operation, Manual star delta switches,
Pole changing switches, Selection switch for meaning instrument etc..

Fig 27 Fig 28

Motor control (Fig 29)
Introduction
Power distribution systems used in large commercial and industrial applications can be complex. Power may
be distributed through switchgear, switchboards, transformers, and panel boards. Power distributed throughout
a commercial or industrial application is used for a variety of applications such as heating, cooling, lighting, and
motor-driven machinery. Unlike other types of power distribution equipment, which are used with a variety of load
types, motor control centers primarily control the distribution of power to electric motors.

Basic motor control
Wherever motors are used, they must be controlled. In Basics of Control Components, you learned how various
control products are used to control the operation of motors. For example, the most basic type of AC motor
control, involves turning the motor on and off. This is often accomplished using a motor starter made up of a
contactor and an overload relay
The contactor’s contacts are closed to start the motor and opened to stop the motor. This is done electromechanically
and often requires using start and stop pushbuttons and other devices wired to control the contactor.
The overload relay protects the motor by disconnecting power to the motor when an overload condition exists.
Although the overload relay provides protection from overloads, it does not provide short-circuit protection for the
wiring supplying power to the motor. For this reason, a circuit breaker or fuses are also used.

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CITS : Power - Electrician & Wireman - Lesson 93-100