Relay V Contactor

What is the difference between a relay and a contactor, and in what situation would you use either?

Below is a definition of both, these are not mine but what I found on the net, are they correct.

Relay
An electrically controlled device that opens and closes electrical contacts to effect the operation of other devices in the same or another circuit.

Contactor
A magnetically-operated device, for repeatedly establishing and interrupting an electrical power circuit. It is usually applied to devices controlling power above 5kW, whereas the term 'relay' is ordinarily employed below 5kW. The terms are often used interchangeably.

Jim

Contactors can be lower than 5kW, if you look at the construction difference you can see that relays usually have open contacts where you can see them but contactors do not. This is generally because contactors are designed to control the arc upon opening and an arc will result from higher current levels and different loads. In my experience the description is true regarding relays for 'control circuits' and contactors for 'power circuits' although like everything there will be exceptions such as a control circuit for loads like very large contactor coils.

Eddy is spot on with his reply. Contactors are generally used in conjunction with relays and other equipment like thermal overload switches. BOB's normally have a small low voltage relay built in for switching your spindle or other equipment on or off from your software. These low voltage relays can then be used as a control switch to switch a higher mains voltage relays.
Mike :)

Also relays generally rely on the moving armature to shift the moving contact and the 'springiness' of the contact material to maintain contact closure, whereas contactors often have mechanical arrangements and heavy-duty springs to minimise the transit time of the generally much heavier contacts. Relays 'click', contactors 'ker-chunk' :)

Jim, just incase you're interested, I used this contactor for switching the main power in my circuit : NC1-2501Z - Detailed item view - Chalon Components (datasheet here : http://www.ingramproducts.com/Contactors/NC1-N.pdf ). I think Eddy and Wobbly used the same model too.

Si.

I always thought a contactor was a device that would self power on when power is applied such that a power cut will release the contactor so it will not come on when power is resumed.
You can do the same with a relay but it will need extra contacts to do it.

Peter

From a textbook point, a contactor is just a relay, and the terms are interchangeable.
However, it's generally accepted practice that a contactor is a specific type of relay, which utilises a bar (I'm sure it has a proper name, but my mind's gone blank!) to bridge two individual contacts. Their main advantage is they're far less likely to weld shut, and due to their inherent design, are cheaper to make larger to handle larger currents.
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What's normally called a relay usually only has a single swinging contact, which if it welds shut (should only ever happen if overloaded) will remain shut. The swinging contact is more reliable and far cheaper/easier to produce in small sizes, however it's not something that scales to large sizes easily.
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There are off course variations of each to increase safety i.e. force guided for standard relays that provides a monitor for welded contacts, or travel return contacts that monitor a contactor to check the contacts have fully disengaged. And then you can get them in a mind boggling array of contact options.

Thanks for all your responses. So it seems to come down to current handling abilities.

Quote:

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Originally Posted by ptjw7uk

I always thought a contactor was a device that would self power on when power is applied such that a power cut will release the contactor so it will not come on when power is resumed.
You can do the same with a relay but it will need extra contacts to do it.

Peter

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No, this contactor definition is incorrect.

The sort of switch that stays off if the power fails is a No-Voltage-Reset or NVR. The On button puts power to the relay coil and a relay contact holds it. When the power to this hold circuit goes, the relay drops out and is only latched back on by the ON button. Having a push-to-break switch in the hold circuit gives you an OFF switch.
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You can make an NVR switch with a relay or contactor with extra (to the main switching requirement) contacts. Some contactors have auxiliary contacts to do this or other things.
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My whole workshop is Contactor NVR controlled, so if I am back in the house I can push the remote off button when I hear the compressor start up, without having to go back there.
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Cheers,
Rob