Background

Electrical relays and contactors are indispensable items when considering of using control circuits in electrical installation work.

Relays are used for low current consuming control apparatus and the usage of contactors may vary from a few Amperes to thousands of Amperes in practical usage.

Contactors are mainly used to control machinery which uses electric motors.It consists of a coil which connects to a voltage source. Very often for Single phase Motors , 230V coils are used and for Three phase motors, 415V coils are used . The contactor has three main NO contacts and lesser power rated contacts named as Auxiliary Contacts [NO and NC]used for the control circuit. A contact is a conducting metal part which completes or interrupt an electrical circuit.

Consideration is given here on few basic usages of contactors, which can be made used in many similar electrical power control applications.

NO-normally open

NC-normally closed

Installation work described here is according to British Standards. [IEE Regulations and Practice ].50Hz, 230V [1P]/400V [3P] AC Power Supply.

Material Requirement

1.0 mm2 [called as 1.0 mm squared] Cross sectioned double insulated Cu [Copper] Cables [Cu/PVC/PVC] are used for the Phase [Live ] and the Neutral cables for field wiring

[ Single insulated Cu cables can be used when wiring the contactors or relays inside an enclosure ]

Accessory Requirement

10A Rated 3 nos. Normally Open Contactor Capacity, 1 nos. Normally Open Auxiliary contacts ,230 Volt rated Coil [ 415 V Coil for 3 Ph. Motors]

Thermal Overload unit

DIN rail to mount the above [ normally the relay base or the contactor can be mounted on a din rail]

On and Off Push Button Switches- 2 nos

Preparation

Contactor Layout and Terminals for Field Wiring

PDF File

Contactor Layout and Terminals -Photo Identification

PDF File

Thermal Overload Layout and Terminals -Photo Identification

PDF File

Single Phase DOL (Direct On Line) Starting

PDF File

Three Phase DOL (Direct On Line) Starting

PDF File

Method of Wiring - for Three Phase DOL Motor

The Main contactor has three NO contacts and one NO and NC Auxiliary contacts. A thermal overload relay is shown in the example but one can use only a contactor without the overload unit. If a thermal overload unit is used, it's rating should correspond to the motor running amperage.

Install the 415 V coil contactor in a suitable enclosure and fix the On/Off Push Button switches at the required operating point.

Wire from a 3 Phase power supply of a 3 Pole MCB or a Main switch of the Power Distribution to the L1,L2 and L3 terminals of the contactor. If a Thermal overload is connected to the contactor's T1,T2 and T3 terminals, then connect the load wires to the motor to the T1,T2 and T3 terminals of the thermal overload.

Now for the control wiring.

Connect a wire to L1 terminal and connect the other end to a terminal of the On Push Button.

Connect the other terminal of the On Push Butto to the terminal of the Off Push button.

The remaining therminal of the OFF Push button should be wired to one of the Thermal Overload's NC contact terminal.

The other terminal of the Thermal Overload's NC contact should be wired to the coil terminal A1 of the contactor.

[ If a Thermal overload is not used, then the remaining therminal of the OFF Push button should be wired to A1 terminal of the contactor.]

Connect the A2 terminal to L2 terminal of the contactor.

Wire from NO Auxiliary contact terminals of the contactor to On Push Button's Terminals.

Now energize the 3 Ph. supply to the contactor. Pressing the On button momentaraly will energize and hold the contactor until pressing Off Push button . Any overload will be detected by the Thermal overload and will open up the NC contact. Until manually reset it, the circuit will not be energized even when the On Push button is pressed.

[ Wiring - for Single Phase DOL Motor starting is almost similar but the coil voltage is 230 V. Please see Single Phase DOL (Direct On Line) Starting PDF File.