Page 251 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

Electrical specification of contactor
Electrically, contactors consist of two main parts, the operating coil and the switching contacts. A contactor will
have a number of contacts (or poles), usually three normally open contacts for power switching and a set of
auxiliary contacts for use at lower current in the control circuit.
Their basic electrical specifications are mainly concerned with:
• the voltage required to operate the coil;

• whether the coil needs AC or DC;
• the current-carrying capacity of the contacts;
• the maximum voltage the contacts can switch.
The type of operation they will be used for further complicates the specification - for example, how often they will
make and break in an hour and whether the load is inductive (an electric motor) or resistive (a heater element).
The choice of contactor depends upon:
• the type of voltage and mains supply;
• the load power;
• the load characteristics;

• the duty requirements.
These are combined into several categories. Briefly they are as follows:
For AC loads:
AC1 - resistive load switching. Least severe conditions.

AC2 - slip ring motor control switching.
AC3 - squirrel cage motor starting and breaking during normal running.
AC4 - as for AC3 but with higher operating frequency and also where the contactor may be required to break the
motor starting current. Most severe conditions.

For DC loads:
DC1 - mainly resistive loads.Least severe conditions.
DC2 - starting and stopping shunt motors.
DC3 - as DC2 but allowing inching and plugging control.
DC4 - starting and stopping series motors.

DC5 - as DC4 but allowing inching and plugging control functions. Most severe conditions.
The use of a contactor - or relay - that is not up to the conditions in the circuit will rapidly fail in service.
The contacts may weld or stick together causing power to be applied to a circuit after the contactor has been
switched off.
Too much current can cause the contact to melt and disintegrate like a fuse.
Relay operating coil symbols
The symbol for a relay is in two parts: the operating coil and the contact set.
• General symbol

• Slow-to-release relay.
• Slow-to-operate relay.
• Polarised relay.
• Mechanically latched relay

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