Discuss Contactors : and how they work in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net

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Hi Luke,
the only winding a contactor has is the coil ,this is fed with a set voltage via the control circuit,most coils are marked with the opperating voltage and can be ac or dc,24volt ,110 volt ,240 volt ,or 415 volt, upon completion of the control circuit the coil is energised and operates as a solinoid and attracts an armature which makes the conection between the contacts, most contactors have three main contacts for the phases and various auxillary contacts for the control circuit ,these can be normally open or normally closed ,for motor control the contactor is often "held in"by taking the coil feed via a n/o contact, hope this helps a bit ,its difficult to explain in words and i cant upload a picture at present
atvbitwww
 
i tried to figure out the difference between contactors and relays the only real answer i came up with is size and purpose, nothing definitive
both are used so a smaller circuit can energise a larger circuit
 
whats the main reason for using a contactor, rather than connecting direct to the board? large currents?

when I helped wire one up, the contactor were feeding outside lights, and had 2 cables coming into the contactor from the board, one to close (i'd say trigger) the contactor, and one fed the lights. would it have been possible to just have one cable incoming from the board to the contactor, and using that to close the contactor and feed to the lights, rather than having 2 cables, get me??? :S cheers
 
i presume the cable to "trigger", as you say ,i would say" bring in" ,the contactor went to the coil and was switched in some way
 
Hi,
We've fitted one at a hotel to monitor freezers that are in an remote building.
If the supply to the freezers failed then an external light would come on to alert the staff.

Best regards,
Lofty
 
is there some sort of light on the contactor? or are there external lights wired to the contactor, which become lit when power to the freezers is lost?

luke, contactors are absolutely vital for industrial circuits.

lets say you have a circuit carrying, say, 1000A at 1000V, by using a contactor you could control that circuit using 12V dc at 100mA. From miles away if you wish.

imagine a 3 phase motor. If you strapped it accross the supply, it would just run continously. By using a contactor (or a configuration of them) you can start it and stop it at will, and again, remotely.

Regarding the light, a contactor will frequently have 'normally open' (N/O) contacts and 'normally closed' (N/C) contacts. 'Normally' relates to its de-energised state.

So take the circuit mentioned above, by using a set of N/C contacts, when the contactor de-energises (loss of power to the freezer) the N/C contacts will close and complete a circuit to the warning light. The N/C contacts could even be in the 'pull in' circuit for a seperate contactor which will feed the lights.

and so on, and so on

You can use the same principle to bring in a stand-by generator, for, say, a hospital, when the mains fails.

And imagine connecting a circuit to a generator that was off. the genny would be at zero hertz and volts. By the time it got to 50Hz and 230/400V the circuits connected to it would be fried. So we put in a contactor which wont connect the output until its at correct V and Hz. And the same contactor will 'drop out' if it drops to say, 45Hz (when the diesel runs out and its stopping), for example

PLC's are just banks of 'electronic contactors', and without them, most industrial machinery would be buggered.

Many many uses.

hope this helps:)
 
Ive installed them in schools for lighting controls where lights in a room had 2 circuits, so i ran it through a contactor to switch them on and off at the same time through a PIR, also ive used them for switching a large load through a time clock.
 
Luke

Not quite sure what you mean?

you could use a switch to operate the contactor, and the contactor could control the lights

do you have specific circuit in mind?
 
am unsure how contactors work, and hardly installed them. but basically with what you've said I sort of understand. so I could put a switch on the contactor, which would trigger the coil for the contactor to control (allow power to) the lights???
 
Yup, thats it

the contactor is just a coil, with an armature connected to it. When you energise the coil, the magnetic field around it attracts the armature, this pulls in the sets of contacts

it is normal practice to have a N/O on (or start) switch in series with a N/C off (or stop switch), in series with the coil

you then wire a set of N/O auxilliary contacts on the contactor in parralel with the on switch, which act as the hold on.

So when you press on, the coil energises and closes the contactor, the N/O contacts close and short out the on push, keeping the contactor energised. The contacts can be used for any circuit, within their current rating

This system is used because if you were to lose power, the contactor would drop out, and would not re-energise until the 'on' push is pressed again.

If you were to use a straightforward toggle switch, for example, the contactor would re-energise when power was restored. And the person working on the motor (for example) which would suddenly restart would NOT be impressed!!!!
 
righty, could we possibly explain how to wire this up, bare with me please cus I don't do this everyday, lets imagine that I'm doing one next week (most unlikely) anyways

this is the contactor http://www.mkelectric.co.uk/images/products/6420S.JPG

lets pretend we've got 2 circuits, one for the contactor and one for alot of high powered lights

am guessing A1 and A2 energise the coil ? so.... line to a1, neutral to a2 ?

whats next, and wiring the lights to - from and via the contactor :)

regards
 
oh f**k here we go
two circuits one to energise the coil one to power the lights
1st circuit ,wire from your mcb(control circuit ) to your light switch (normal everyday thing)then back to your a1 terminal .neutral of this circuit is taken straight to the a2 terminal
2nd circuit (power circuit ) is taken through the terminals of the contactor (l1,l2,l3...t1,t2,t3)and back out to your lights .neutral is taken straight to the lights
dont forget the relevant earths and make sure the contactor is correctly sized ,also check the control voltage info
this is the most simple way of doing things it gets fun when you add pir sensors ,timers photocells................
have i missed anything?
 
a contactor is a self contained item , if you are talking about the enclosure then lets continue
you have two circuits ,that is two feeds two neutrals two earths
if you are junctioning the neutral through the enclosure via a teminal block or crimp,for the power circuit either is fine i prefer terminal block
 

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