contactor or relay?


Thread Starter


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.

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:

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.

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):

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

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.

Phil Corso, PE {Boca Raton, FL, USA}
[[email protected]] ([email protected])
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.

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

amit shekhar sharma

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.

I think you are all mistaken, We have a contactor relay which is used in switchgears, a Relay used in control systems.
>contactor has a hold on contact whereas relay has no such provision <

Hold on contact is something that can be applied to either component. If anything, it would more apply to the relay than the contactor because the hold is a smaller voltage to keep the coil engaged than what a contactor passes through. Once the coil is closed, one of the normally open circuits will close as well passing a constant power to the coil keeping it energized.
>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 <

Sorry Sir, but it's not convincing answer. There are several relay which have 4 contacts, 2NO and 2 NC, example Omron MY4N. If we are using only one NO contact then, we can use the other one as latching/holding contact.
Contactors have non-hinged armatures, typically double-make or double-break device. They are typically in higher load applications and they typically have less contact bounce and more powerful closing forces than standard relays.
Ans:- there is a difference between relay and contactor. relay is used in control circuit only but contactor is used in both control and power circuit.

a contactor can be relay but a relay cannot be a contactor.
I prefer a more pragmatic definition of Relay vs. Contactor. If, when you energize the device, it goes "click", then it's a relay. If it goes "clunk" then it's a contactor.

means "click" little amount of current to energize the coil

means "clunk" a large amount of current drawn by to start the motor..

relay is mostly integrated in PLC controller for controlling a close loop process..
while MC or TC is typically used for a large motor to start with a large amount of current...

Wm Seán Glen

Lots of ways of looking at this. Yes, in general, the relay is lighter duty. A light duty relay might be used for current isolation or providing a signal to some kind of monitoring software whilst the contactor would be heavier duty and provide power to a device with more current capability.

The contactor is usually built to be repairable. You can change out the coil for different voltages or flip the contacts to be NO or NC and have at least four sets of contacts (three phases and holding contacts) with the option of adding more to the same armature. If there were an overload added to interrupt current to the coil, it would be referred to as a "motor starter."
Although a Relay is generally non-repairable, you can, however, have different coil voltages and multiple contact options