Contactors | Electrical starters | For equipment control

● Contactor – explained simply

A contactor is an electrically controlled switching device that allows a low-power control circuit to switch a considerably higher-power electrical load on or off.

Contactors are commonly used to control electric motors, pumps, fans, compressors, heating equipment, lighting groups and automation systems.

● How does a contactor work?

A control voltage is applied to the contactor coil terminals A1–A2. The coil creates a magnetic field which mechanically operates the main and auxiliary contacts.

This allows a push button, relay, PLC or another automation device to control an electrical load of considerably higher power.

● Contactor schematic designations

These designations are used in electrical schematics and help identify the function of contactor terminals regardless of the language used in the documentation.

● What do NO and NC mean?

NO (Normally Open) – in the normal de-energised state, the contact is open. When the contactor operates, it closes.

NC (Normally Closed) – in the normal de-energised state, the contact is closed. When the contactor operates, it opens.

● How to choose a contactor?

A contactor should not be selected only by the highest current value shown on its housing.

• Load type – motor, heater, lighting or another load.
• Motor power – check the manufacturer's specified kW rating at the relevant supply voltage.
• AC-3 current – one of the most important parameters when controlling asynchronous electric motors.
• AC-1 current – mainly applies to resistive or slightly inductive loads.
• Coil voltage – must correspond to the control circuit voltage.
• Auxiliary contacts – NO and NC contacts are used for self-holding, interlocking, signalling and automation.
• Number of poles – 3-pole contactors are commonly used for three-phase motors.

● Warning – coil voltage is not the main power circuit voltage

If the contactor coil is marked 24 V AC, 42 V AC, 110 V AC or 230 V AC, this value refers to the A1–A2 control coil voltage. It must not be confused with the main power circuit voltage. Applying an incorrect voltage to the coil may damage the contactor.

● What do AC-1 and AC-3 mean?

AC-1 – utilisation category for resistive or slightly inductive loads, for example electrical heating.

AC-3 – one of the most important utilisation categories when controlling three-phase asynchronous motors. Therefore, a motor contactor should not be selected solely by its higher AC-1 rating – always check the AC-3 current and motor power.

● Where are contactors used?

A contactor is generally used when an electrical load must be switched frequently, automatically or remotely.

● Key difference: contactor or relay?

A contactor is generally designed for larger loads and frequent switching. Relays are more commonly used in signalling, control and automation circuits and for smaller loads. This is a useful rule of thumb – the actual capabilities of a device must always be checked against the manufacturer's technical data.

● Common selection mistake

A higher rated contactor current does not automatically mean that it is suitable for a particular motor. Motor starting current can be considerably higher than normal operating current, therefore the AC-3 category, motor power, supply voltage and manufacturer's recommendations must be considered.

● Quick terminology guide

● LOTO – Lockout / Tagout

LOTO is a safety procedure for isolating, locking and identifying energy sources in order to prevent accidental or unauthorised energisation of equipment while work is being carried out.

Where applicable, the energy source must be physically isolated, locked and clearly tagged. Re-energisation must only take place in accordance with the established safety procedure and by authorised personnel.

Selection, connection, installation, replacement, adjustment and commissioning of electrical equipment must only be carried out by a specialist who has the qualifications, competence and authorisation required for the specific work.

Contact with live parts, working on an improperly isolated circuit or incorrect connection of equipment is dangerous. Such errors may cause fatal electric shock, arc flash, severe burns, fire, explosion or equipment damage.

Before starting work:

• isolate all relevant energy sources;
• protect the equipment against accidental re-energisation;
• where applicable, use the LOTO lockout/tagout procedure;
• verify absence of voltage using suitable test equipment;
• use personal protective equipment appropriate for the work;
• follow the manufacturer's documentation and applicable electrical safety and occupational safety requirements.

If there is any doubt whether the work can be carried out safely, do not start the work and consult a qualified electrical professional.