Contactor selection

[Asjad]

Dear All,

Being a comptent selector and assembler of electronic circuits, I have recently had the need to buy a contactor for the following parameters:


UK 3 Phase motor (3 pole?, switching voltage at least 415vac?)

I assume that if the coil voltage is say 24ac, this is to control the switching from an external source?

However I wish the contactor to be activated by a switch located nearby, how would I wire the
coil to the switch and what voltage rating coil should I buy?


Is the current rating stated in catalogues the working current, or is it the current at which the cut-out is activated?

If not where is it normally stated the cut-out current?


I hope there is someone who can help me enter the realm of electrics.

Cheers,

 

[RadioRon]

It is common to use coils rated at 12, 24 or 28 volts where I live. I would make the selection based on what power supply I have that will supply the current for the coil. If you don't have an existing 24V supply available, for example, and will use a plug-in AC adapter ("wall wart") to supply the coil current then any voltage is acceptable I would think. You only need to match the coil rating to your power supply rating.

Wiring this is simple. One side of the power supply goes to the switch. The other terminal of the switch goes to the contactor coil, and the other terminal of the contactor coil comes back to the remaining terminal of the power supply.

The current rating in the catalog is usually the typical current drawn when the coil is supplied with its rated voltage. This is the voltage at which pull-in and hold is guaranteed, not the threshold at which the contactor pulls in or cuts out. I don't usually find the cut-out current in the catalog and don't usually care because it is wisest to provide the rated voltage to the coil and let the current be whatever the coil wants (calculated using ohm's law). The reason for this is because the manufacturer of the contactor assumes that you are driving it from a voltage source, not a current source, and that is the way they have designed it and specified it.

 

[stevez]

Another requirement for your situation - the contacts have to be rated for the load - or in excess of that. You might make some selections based on the needs of the load then look at the options you have.

Quite often for motor starters ( another name for the relay) have overloads and other features that might be useful. Again, a thought might be to work this stuff out then see what coil choices exist.

It is not uncommon to use momentary pushbuttons to start or stop a motor. On start the coil is energized and an auxilliary contact provides the power to keep the coil energized until the "stop" switch breaks the circuit momentariliy.

I'd think that if you google on motor starters you ought to find a lot of applications information that would help a great deal.

 

[RODALCO]

As stevez already explains the auxillary relay contact is used to hold the contactor in like a latch, ( the aux. contact is in parallel with the ON switch push button ) the stop button brakes the coil cct so the relay drops off and is ready for the next cycle.

There are different categories to consider regarding duty cycles of the contactors, AC1 is usually resistive, AC 4 is for D.O.L starting of 3Ø motors.
The current rating should be at least 6 times the nominal motor running current to avoid early burning in of the contacts in case of heavy duty application with long start currents on high inertia loads.

If you go to a supplier or google for it and you quote them the HP or kW rating of the motor they usually provide you with the correct contactor.

In case as for a 230 / 400 Volts supply a relay coil of 230 Volts is usually used between one of the phases and the Neutral.