Contactors & Relays

[john37]

I know this must be a dumb question but coming from a guy that is just starting out in the Electrical field please bare with me....
Can someone please explain what contactors do and what does relays do?

 

[infinity]

They are electromagnetic switches. The coil on the device allows them to open and close. This gives you the advantage of using a small relay control circuit to switch larger circuits. For example a large pump motor may require #3/0 conductors but it needs to be operated by a pressure switch. Running the #3/0 conductors through the switch wouldn't work so a contactor or starter is used to allow for a small control circuit to operate the contactor to start and stop the motor.

 

[celtic]

Contactors and relays are essentially the same thing: a switch.

Typically, a relay is small - sometimes called an "ice cube":

Maybe about 1" or 2"


A contactor, is usually large:

 

[john37]

Thank you very much Ininity and Celtic. That was very helpfull.

 

[celtic]

To expand on Inifity's example....store lighting.

If you wanted all the lighting to come up at the same time, that would a humungous switch bank...and who is going to flip all those switches before the store opens?

A contactor.

You could run 110v to a timeclock, then to a contactor which could turn an entire lighting panel (277/480)on.

 

[infinity]

Here is an example of a 12 pole contactor. This could control 12 lighting circuits with just one switch.

 

[growler]

I saw a strange use of a contactor not long ago. At a certain hotel there is a 150 Amp. lighting panel being controlled by a contactor ( one big sucker ). It's wired to a time clock and scares the maintenance men when it energizes.

It seemed stupid because if this one contactor goes bad they lose all exterior lighting. There is no flexability of lighting control.

Has anyone else seen anything like this?

 

[celtic]

Has anyone else seen anything like this?

Yup...all the time

It seemed stupid because if this one contactor goes bad they lose all exterior lighting. There is no flexability of lighting control.

The same might be said about transformers going bad.

 

[infinity]

I saw a strange use of a contactor not long ago. At a certain hotel there is a 150 Amp. lighting panel being controlled by a contactor ( one big sucker ). It's wired to a time clock and scares the maintenance men when it energizes.

It seemed stupid because if this one contactor goes bad they lose all exterior lighting. There is no flexability of lighting control.

Has anyone else seen anything like this?

These go up to 225 amps to control entire panels. If I remember correctly they have a manual switch so that you can manually change the switch position.

 

[iwire]

It seemed stupid because if this one contactor goes bad they lose all exterior lighting. There is no flexability of lighting control.

Has anyone else seen anything like this?

Yes very common in the buildings we work in.

As far as one part going bad keeping things in darkness, yes it does happen. It can be a real surprise to a mall or a "Super Store" when none of the outdoor lighting comes on.

Some of the good contactors can be manually operated but not by a store manger, custodian etc. they need an electrician to do it.

For large stores they really don't need 'flexibility' as this location is just a cookie cutter image of the last 400 they built.

 

[iwire]

By the way Trevor seems to favor Asco, I don't blame him.

Both of the contactors he posted are rock solid, long lasting and if need be easily repairable contactors.

Both are latching or mechanically held contactors so you use a 3 wire control circuit which allows unlimited points of control.

With two wire control if you turn it on in one place it's difficult to turn off from another place.

 

[LarryFine]

Has anyone else seen anything like this?

Sure again. It can be the best and most economical way to de-energize electrical equipment under a commercial kitchen exhaust hood for fire-suppression control.

 

[infinity]

You're right Bob we do favor them for their reliability and repair-ability. Many of the parts can be replaced. They are expensive however. We usually use them with a momentary time clock and momentary center off switches as overrides. This allows for an infinite number of locations where you can switch them on and off. We recently did a high bay lighting job that had about ten lighting circuits. The customer wanted to have all of the light switches next to the front door. This would have required thousands of feet of wire to bring the feeds (panel at the opposite end of the building) and switch legs over to that location. Instead we installed a 12 pole Asco contactor with one three wire cable to a three way switch at the door. This saved a considerable amount of pipe, wire and labor and of course money.

 

[iwire]

They are expensive however.

No doubt.

But have you seen some of the real junk some of the big names have been releasing in the past couple of years.

There was a very large multi floor Furniture store, the lighting was mostly fancy track lights.

At close the tracks would shut down and work / clean up lights would come on.......or where supposed to come on.

Many times the tracks shut down but the work lights stayed off as well. No one in the store could get them on so they would send the night crew home.

The problem was the same each time, junk contactors made primely of cheap plastic, the entire unit is 'snap together' but it falls apart.

 

[growler]

Sure again. It can be the best and most economical way to de-energize electrical equipment under a commercial kitchen exhaust hood for fire-suppression control.

For fire supression it sounds fine. You want everthing to go off at once.

I didn't think the idea stupid from an electrical stand point but from a design stand point. For the electrician it looks faster and cheaper.

I don't think they consulted management before installation in that particular hotel. Picky people. They didn't even like the noise it made when it energized. I explained that it's just a big contactor in a metal cabinate it going to sound loud when you are close and the cover is open.

I removed the old mechanical time clock and installed a digital with battery backup. That would a least keep them from losing time every time the power was off . ( solved one problem ). Flexibility would require more work and money than they wanted to spend.

I do tend to over analyze. Looking at it from a different perspective it's safe and works well it's just not what the customer is happy with . Who's fault is that ?

 

[john37]

Thanks everyone for the insight. It was very helpful. I have a related question that I would appreciate some insight.
There is a currently a new Elevator going in on a project and the elevator contractor is providing a battery recall system ( I believe that is the correct name for it) Anyway, the bldg doesn't have an emergency backup generator and it's only 3 stories. He is requesting that a relay or normally open contactor be put in that is tied back into the fire alarm system. What would the purpose of the relay or normally open contactor?
In case you are wondering....I won't be doing this myself. I just come across things in the field and want to learn how they are done so in case I do have to do a similar thing in the future I'll know how to do it. Thanks.

 

[Smart $]

What would the purpose of the relay or normally open contactor?

Purpose: under a fire alarm condition, elevators cease normal function and are automatically returned to the main floor of egress. In order to do this, the fire detection system must communicate with the elevator control system. This is accomplished by the fire detection system providing an electrically controlled switch, otherwise known as a contact. The contact can either be within the fire alarm panel or remote to it in the elevator machine room. In either case, the contact is provided through the use of a relay and its contact. This is done to keep the respective power supplies isolated.

Relay contacts, of which there may be one to several sets, are basically of either normally open or normally closed designation. A relay may provide both with a common terminal:
View attachment 236
The normally open and normally closed designations refer to the state of the contacts electrical path when the relay's coil is de-energized.

 

[john37]

Thanks Smart $. I'm still a little confused by when you would use a contactor and when you would use a relay and how you would know when to use a normally open or normally close contactor. I would appreciate if someone could explain. Thanks.

 

[Smart $]

Thanks Smart $. I'm still a little confused by when you would use a contactor and when you would use a relay and how you would know when to use a normally open or normally close contactor. I would appreciate if someone could explain. Thanks.

While a contactor is a relay, and vice-versa I suppose, you'll find that manufacturers generally refer to the ones which are open-framed as contactors and those which have only the terminals exposed as relays ...but exceptions to this premise exist. I believe the added cost of enclosing larger versions is the reason you will generally find smaller versions called relays and larger versions dubbed contactors. The size is basically determined by and directly proportional to the voltage, amperage ratings, and number of contact(s) plus the life expectancy, i.e. number of make/break operations at rated current. Additionally there are similar considerations for the coil: voltage mostly, but current, and duty cycle also come to mind. To assist in making a choice easier, manufacturers also classify their product using generic terms, such as general purpose or heavy duty.

As for when to use a normally open or normally closed contactor is as you guessed: a design consideration. In the high current arena, contactors are typically normally open, as the majority of applications call for supplying a lower voltage and current to the coil to "switch on" a higher voltage, high amperage circuit connected to the contacts. Just in case you haven't figured it out, a "normally open" contact closes when the coil is energized, while a "normally closed" contact opens. In the control arena, relays come in a variety of configurations. Typically, IMO, general purpose relays such as those dubbed as "ice cube", are furnished with two sets of C-NO-NC contacts, so you can connect it either way, or both, twice over.

 

[iwire]

I'm still a little confused by when you would use a contactor and when you would use a relay

In general relays are physically small low, capacity devices often (but not always) limited to about 15 amps of current.

The operating coil draws little current and the coil voltage may be 5 volts to 600 volts AC or DC

A contactor on the other hand is large, robust, and can be purchased with very high current ratings. The largest I work with are in the 600 Volt 400 amp range.

The coil current on contactors is much higher and as the size increases so does the operating voltage of the coil.

You will not likely find a 225 amp lighting contactor available with a 24 VAC coil., the coil voltage will be 120 VAC or higher.

By the way the 'coil' is what you energize to operate the relay or coil.

and how you would know when to use a normally open or normally close contactor.

That is a large question.

Here are a quick two examples.

1) You want to shut down a bunch of equipment when the fire alarm operates. This would be a good application for normally open contactors as when they fail they will fail open meaning the load will shut down.

2) You want to be able to turn on a load under emergency conditions, say lighting in a auditorium. This would be a good application of normally closed contactors. If normally closed contactors will fail closed or ON.

Many contactors now come with reversible contacts so if you look at this ASCO 12 pole contactor

you see 12 single pole contacts, I can make some normally open and some normally closed.

It is all about flexibility in design.

Keep asking whatever questions you want, I happen to work with contactors all the time so it's not that I am particularly bright, just experienced.