contactor or relay?

E
A relay is use to switch on/off low power devices. A contactor use to switch on/off high power devices.
 
The differences:

A Relay:

1. Goes Click when energized.
2. Switches loads of lower wattage
3. Applications include control circuitry
4. Generally switch currents less than 10-15 Amperes.
5. Generally switch voltages less than 240 VAC
6. Switch AC or DC
7. Not normally rated in horsepower.
8. Double throw is typical (normally open and normally closed contacts in one unit).
9. Relay coils are AC or DC, but tend to be 5-24VDC with back emf power diodes to protect relay control electronics.
10. The buck just gets passed here. Relays tend to be "pilot" control devices, controlling more powerful contactors.

A Contactor:

1. Goes Clunk when energized.
2. Switches loads of higher wattage
3. Applications include power circuits (from electric motors to a country's main power grids).
4. Generally switch currents above 10-15 Amperes.
5. Generally switch voltages 120 VAC or greater.
6. Switch AC or DC, resistive or inductive loads, but generally switch AC Loads.
7. Usually includes a horsepower rating.
8. Single throw is the norm (either normally open or normally closed contacts, rarely both).
9. Usually A.C. powered coils (24 VAC for safety- interlocked with EPO or EMO, and Panel open interlock switches; 120-240VAC for simplified operations are typical applications)
10. The buck stops here. Contactors are the real deal. They can be controlled by smaller, less powerful relays.
 
P

Petar Timotijevic

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

Basic and main difference is that, contactor have special designed chamber vacuumed or filled with special inert gas, which allow HV to switching but not start ARC!!! Relay don't have this, and when use relay on HV with high current arc flux will occur and this situation will be dangerous. Plus relay is smaller contacts are closer and contact platine are smaller may glue together. Relay are often for current to 10A, contactors may go 25A 63A 150A 300A ...
 
W
"Basic and main difference is that, contactor have special designed chamber vacuumed or filled with special inert gas, which allow HV to switching but not start ARC!!! "

I have to disagree with the above statement. While a high voltage contactor may have those features, low voltage (<600V) contactors do not typically have those features. Also, there are high voltage, air-break contactors available. Low voltage contactors commonly have arch extinguishing capabilities via arch chutes (air). The basic difference between a relay and a contactors is that contactors typically handle higher currents and in some cases higher voltages, are of sturdy construction, and have application specific features like high interrupting capacity, high withstand ratings, motor service, cyclic service, heavy-duty service, etc.


William (Bill) L. Mostia, Jr. PE
Sr. Consultant
SIS-TECH Solutions, LP

Any information is provided on Caveat Emptor basis.
 
The 15A limit for a relay is not always appropriate as can be seen on this site concerning a relay which it calls:

"FINDER - 66.82.8.230.0000 - RELAY, PANEL, DPDT, 30A, 230VAC "

see http://uk.farnell.com/finder/66-82-230-0000/relay-panel-dpdt-30a-230vac/dp/1329690

The current rating is also not a simple specification because you need to take into account the steady state value as well as the current breaking capability. The graphs for the "Finder" relay in their technical data sheet in the link above cover these considerations very well. I have just installed this relay to control a 3KW hot water cylinder at 220 Volts. It has to switch 13.6 Amperes. I have a timer which turns on this relay. I selected this relay rather than a three phase contactor because it is about a quarter of the price a similar contactor.
 
G

Gireesh Gokuldas

Thanks Mr.Bobb,

its just now my superior asked me this simple question. actually i was not aware of this thing. for me it was both same. so now you helped me to answer his question in a proper way. once again thanks and do upload things like this.

Regards,
Gireesh Gokuldas
 
D

Darryl Dearing

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

It is my opinion that a contactor is a device that has a 'floating' contact bar that is drawn either away from or towards contacts wheras a relay always has one end of its switch contact mechanism attached to the connection pin/tag. It is nothing to do with current rating as they can be interchanged to a large extent.
 
R
The plug-in relay often does and all form "C" contacts have a common contact end - however, most industrial rated "relays" are similar to contactors with a floating contact bar (see the AB 700 series - they have both types listed under "relay").

There are many sub groups under the title relay - safety relays, definite purpose, magnetic or "reed" relays, and of course latching relays. You cna make some generalizations about typical relays but so many styles have been in use over the years that there is always an exception or two.

Russ
 
hey man i build and test air circuit breakers which have heavy duty springs which close and open the circuits to bus-bars and they're fn loud too especially 4000a and up
 
The distinction of a relay as switching AC (almost always only single phase) or DC pilot/control signals vs a contactor actually switching the full line voltage and current (often three phase) to lights, motors, etc. seems like a highly useful distinction. Contactors often have auxillary contact sets suitable for much lower currents built right into the operating mechanism of the contactor in addition to the main contacts. I don't think I've ever seen that in a relay, just add other relays in parallel/series to perform the equivalent function.
 
C
I like this answer a lot. It's one of the few that actually specifies when you would actually use a contactor over a relay. I also like history lessons. Thanks!

I do have a question on application. What if I were trying to switch the power off to a 1kW power supply? 220VAC in. 15VDC out. I only want to turn it off about once or twice every day or two. It's less than 5kW but still switching off power.

> 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 &#8216;Contactor' definition calls it a 'Load-Switch'! The point is that there is no simple definition.

&#8212;&#8212;snip &#8212;&#8212;

> 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&#8221; 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.
 
C
My favorite answer. Made me giggle.

> 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.

> ;-)
 
> The distinction of ...
> Contactors often have auxiliary contact sets suitable for much lower currents
> built right into the operating mechanism...

Right,
in Germany we call the contacts for lower current "Hilfskontakte"
and those for higher current "Schaltkontakte"
http://de.wikipedia.org/wiki/Schütz_(Schalter)

With the "helper-"contacts you can implement your PLC (SPS) logic by wiring the logic with copper,
where the actuators are the "Schaltkontakte".

have 3 -dependent switch contacts for three-phase socket .
 
The "real world" difference is generally one of power. A contactor is usually used for switching higher ampacity loads. For example, adding an overload relay module to a contactor creates a motor starter. From a design perspective, use a relay when switching control signals and an appropriately rated contactor for switching power.
Contactors are more flexible to use in that some allow the arrangement of N.O./N.C. contacts to be changed as needed. Most contactor manufacturers also sell auxiliary contact blocks so that more contacts can be added without the need for another contactor.

Please note that this information is provided by an electrician and not an automation technician.
 
in all practical terms a relay and contactor perform the same basic role - switching current to a load however a "relay" nowadays something associated with CONTROL circuitry and a CONTACTOR is something associated with POWER circuitry.

for example a relay could be that small ice cube looking device that is energised by a microprocessor/ computer/ brain to switch power to a contactor to bring on heating elements. the relay is part of the control circuit controlled by a "brain" to switch power to a heating element carrying lots of current.

relays and contactors are often different in size due to their role, contactors carrying lots of current are bigger than relays carrying smalls amount of current.
 
F

Frederick Mwambafula

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

<i>Contactor and Relay have same mechanism in operation.
There is a coil to be energized and the contacts to be attracted upon magnetization of coil.</i>

THE MAIN DIFFERENCE:
1. Voltage to energize coil of relay is very low compared to coils of contactors.

2. Contactors are used in High power application eg. reconfiguration of Induction motors for forward-reverse operations, or Star-delta starting. WHILE Relays are used in low power applications eg. in Control, Protection and HV measurements.
 
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