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what is the basic differences between contactor and relay and why can't we use the contactor instead of relay and vice versa?

Simply, a relay is cheaper than a contactor - generally. A contactor is used for a higher load. For example, we have pump control applications where the electronics control a relay and the relay controls the contactor which starts the pump.

Also - the relay load to the electronics is lighter where the contactor may draw heavy amps and produce a bigger arc potential when it fires.

Thanks,
Bob Hogg

Responding to Anonymous' Feb 5, queries... the following discussion applies to only electrically operated devices:

IEEE Definition
Std 100-1992 "Standard Dictionary of Electrical and Electronic Terms" (Std 100-1992) has many definitions for 'Relay'! Conversely, the Standard's only 'Contactor' definition calls it a 'Load-Switch'! The point is that there is no simple definition.

Historical Perspective
In 1905 'relay' and 'contactor' were defined by the AIEE (predecessor of today's IEEE) as follows:

Relay
A device... by which contacts in one circuit are operated by a change in conditions in the same circuit or in one or more associated circuits.

Contactor
A device... for repeatedly establishing and interrupting an electric power circuit.

Now, in response to your "difference" question... the terms are delineated by the power they must carry. Here are definitions presented in Struthers-Dunn "Relay Engineering" manual (ca 1945):

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.

Regards,
Phil Corso, PE {Boca Raton, FL, USA}
[tal-2@webtv.net] (Cepsicon@aol.com)

Size. Actually, "contactor" is just a trade name for a type of relay used for power switching, and is sometimes referred to as such. Like any other relay, it should be used within its limits with due consideration given to the application specifics such as switching frequency.
--
Steve Myres, PE
Automation Solutions
(480) 813-1145

relay is for control logic. Contactor is for outputs. However, relays can also be used for low-load outputs.

EG

Hai,

A contactor is a contactor a relay is a relay.

Ya You are very right ...Relay is relay and contactor is contactor... I fully agree with you...

Ans:- there is no such a difference between Relya and contactor as I know. So as a conclusion Relay is relay and contactor is contactor. And relay can be contactor and contactor can be relay.

Ans. contactor is a heavy duty switch whereas the relay is light duty switch

contactor has a hold on contact whereas relay has no such provision

How about the speed of operation? Any difference between the operating time of relay & contactor relay?

As a rule, contactors are a little slower than relays because the armature is more massive. But there will be overlap as there are slow relays and fast contactors. If you don't mind supplying the current, contactors can be made very fast. I've seen some where the actuation was so fast and positive it sounded like someone smacked the panel with a hammer.

And there are even ballistic contactors made to actuate so quickly that they make contact before the HV they switch has a chance to ionize the air and arc. So it's hard to generalize. Smaller will almost always be faster. We used some tiny reed relays on test equipment that were almost as fast as garden variety transistors.

Regards
cww

I think you are all mistaken, We have a contactor relay which is used in switchgears, a Relay used in control systems.