Difference between relay & contactor ?

[danishdeshmuk]

Difference between relay & contactor ?

When we should use relay ?
In what circumstance we should prefer contactor over relay ?

 

[goldsmith]

Hello my friend
At the basics of operation , in fact nothing especial .they are pretty alike with together . But we have some differences : 1- as i know , you can not find them ( contactors) with DC coils ( such as relays) . 2- the power of contactors is very higher than relays . 3- the contactors , usually , have more contacts instead of relays.
Best Wishes
Goldsmith

 

[alexxx]

If I may add something on this... I've failed to notice any normally close contacts on a contactor until now. If someone has, please let know.

 

[betwixt]

Contactors are usually latched on by the power they are switching and as such need to be turned on initially by some other method, normally a 'start' switch. The idea is that if power fails, they turn themselves off and remain off until started again.

Relays are similar but they turn on whenever power is applied to their coil and they turn off when the power is removed. There are some relays which have a latching action but these do not turn off by themselves like a contactor would.

A relay, wired with it's coil downside of its own contacts would work as as a contactor. Given a method of initially turning on, the power to it's own coil would come through it's own contacts and it would hold itself 'on'. If the power failed, the coil would de-energize, open the contacts then even if power was restored it would not switch on again.

Brian.

 

[FvM]

N.C. is quite common for auxilary circuits of contactors.

Contactors with DC coils are available from all major manufacturers.

P.S.: betwixt, I won't follow your characterization of contactors. I think, it only applies to simple hardwired contactor circuits. Contactors in complex automation systems, e.g. controlled by a PLD are nothing but "power relays".

 

[betwixt]

Thanks FvM, I'm sure you are correct. I think it's another one of those cases where the name and the function have become blurred over the years.

Brian.

 

[tpetar]

Contactors mechanics are bigger then relays, bigger contacts with special material platine, that allow contacting higher amperage. Some special contactors have special chamber filled with gas to prevent sparking udner very high and strong amperage, to eliminate Volta flux!!!. Relays are smaller and should not be used for higher amperage, but if used lifetime is shorten and contacts may stick together under high amperage!!!!.

All combinations NO,NC,.... exist in markets, almost all serious manufacturer have that. Also DC coil for contactors exist usually at 24V, but I see and for 12V.

Also exist impulse contactors and relays. They change state on some impuls, with that they consupt energy only for short impulse for changing state of contacts, that allows saving power. Used especially in situations where there is less energy such as solar systems.

 

[danishdeshmuk]

so what are the difference then between the two ?
What's the criteria of selecting any of the two ? only the amperage

 

[tpetar]

so what are the difference then between the two ?
What's the criteria of selecting any of the two ? only the amperage

Yes voltage and amperage. If you see relay, see relay contacts, they are close open, for higher amperage and voltage that can be dangerous. Also see relay area of contacts. I have relays for 75A but for 12V or 24V.
For higher loads, especially on 220V, for heaters, ovens, three phase motors,..... always use contactors, small relay from control circuit just turn on/off contactor coil.
Contactor have more space between contacts, and bigger area of contacts.

When You whant to be safe on high amperage and voltage always use contactors!!!.

Relay not designed for higher amperage, relay should be used for smaller power and signaling as they are designed.

 

[danishdeshmuk]

For low voltages & amperages & for light loads we should use relays & for heavier loads & for higher voltages & amperages we should use contactors whether voltages are AC or DC doesn't matter ???? Am i right ?

 

[tpetar]

For low voltages & amperages & for light loads we should use relays & for heavier loads & for higher voltages & amperages we should use contactors whether voltages are AC or DC doesn't matter ???? Am i right ?

Manufacturer give data about relay and contactors, maximum amperage and voltage DC or AC.

Example some relay:
Chansin

NO contacts 240V 7A 12A 125V
NC contacts 240V 7A 12A 125V
28V DC max

 

[phatcreators]

tpetar i think danishdeshmuk is right in the post # 10

but we always use Relays with plcs ..... whether for plcs inputs or their outputs

 

[tpetar]

tpetar i think danishdeshmuk is right in the post # 10

but we always use Relays with plcs ..... whether for plcs inputs or their outputs

Yes I agree with that.



Additional:

Some relays have their bases soldered on PCB. When relay need to be changded just pull relay from base and put new.
But that bases dont permit higher amperage for relays, this is just to make changing easy, and for some relay to make better wire contact with screw.




Also both relay and contactors make noise when contacting, because that (one of reason) people use solid state relay for switching.

 

[nmbg011]

Use relay just for signal and for lower current, contactors for higher current.

 

[danishdeshmuk]

If an interviewer ask that question then what should be the answer to that question ?

 

[tpetar]

If an interviewer ask that question then what should be the answer to that question ?

Relays for signaling, and for smaller voltages and amperages, can be often in variant for 220V @10A.

Contactors for higher power, voltage (220V,380V...., amperage (16A,25A,40A,63A,150A,....), three phase,.... much stronger mechanics, coil can be on lower voltage, and on higher (220V/110V)

 

[sam_sdpl]

Re: Difference between relay & contactor ?

If i use solid state relays for switching 100A @240 V how can i manage power dissipation problem

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If i use solid state relays for switching 100A @240 V how can i manage power dissipation problem

 

[FvM]

If i use solid state relays for switching 100A @240 V how can i manage power dissipation problem

Generally not a good idea. You'll do it only in special cases (very frequent switching) and have to accept power dissipation and respective heatsinks.

 

[chuckey]

It has not been stated (yet), for the uninitiated, a contactor, has two fixed contacts with a moveable set of contacts, that are pulled in when energised, to bridge the fixed contacts. The main contacts are therefore always normally open. The contact sets are often replaceable. Relays always rely on flexible flat springs that carry the contacts, which are pushed or pulled when energised to make or break the contacts on the end of the flat springs. As the flat springs carry the current to the contacts, there is a limit on how much current they can carry, before they get too big and stiff to be able to be bent by the power available in the coil. This is when you use a contactor.
Frank