contactor question

Is the A1 and A2 on a contactor the coil to trigger the contactor from the output of a plc?

Yes, make sure the voltage is compatible and the inductive load of the contactor is within the PLC card rating.

or use an interposing relay, this is a good practice IMO

Instead of an interposing relay, you can get special low power consumption 24VDC versions that are specifically designed to connect to a PLC's solid state output. If you don't get one of those, then use an interposing relay with a solid state output. You can also use a PLC relay output with an inline fuse rated at 1A. The fuse is to keep the amperage below the contact ratings.

Newbie,

A1 and A2 are usually the direct ends of the coil wires. If it is a motor starter contactor, you normally would have an overload relay on one end of the coil. Your PLC output should be wired to A1 and the other side of the overload contact, so that if the overload trips, the motor will not keep running. A less-desirable alternative is to have the overload wired back to the PLC as a fault input, so that the PLC then turns off the contactor.

Lancie1 said:

A less-desirable alternative is to have the overload wired back to the PLC as a fault input, so that the PLC then turns off the contactor.

Click to expand...

You should always have a signal from the overload to the PLC input, regardless on whether you also use a contact in series with the output.

Lancie1 said:

A less-desirable alternative is to have the overload wired back to the PLC as a fault input, so that the PLC then turns off the contactor.

Click to expand...

Lancie
I am curious why you say it is less desirable to have motor overload as an input to PLC.
GRANTED if they are in series with coil the coil drops
the aux contact opens (and if wired to PLC) would tell PLC motor tripped
BUT PLC would have no idea why.

IF overloads were wired into PLC then it would have idea and could put motor trip warning on HMI etc etc.
It would take another input (or threee ??) of course which is the downside of doing it this way.

Like to hear your thoughts

Dan Bentler

leitmotif said:

Lancie
I am curious why you say it is less desirable to have motor overload as an input to PLC.
GRANTED if they are in series with coil the coil drops
the aux contact opens (and if wired to PLC) would tell PLC motor tripped
BUT PLC would have no idea why.

IF overloads were wired into PLC then it would have idea and could put motor trip warning on HMI etc etc.
It would take another input (or threee ??) of course which is the downside of doing it this way.

Like to hear your thoughts

Dan Bentler

Click to expand...

Think what he meant was that the O/L should be hardwired to the Coil. Dont want the PLC only to turn of the contactor. Hardwire the O/L and also wire input back into PLC

Usually overloads have 2 contacts - use N/C in control ciruit and N/O to PLC - or at least the DIN ones do - do not know about NEMA as the huge great things are not used in Ozz. Then you could use both contact sets.

I do not use overloads anymore at all but use motor circuit breakers instead with plug in auxiliary contacts. Wish they were electronic though - take a lot of heat out of the control panel.

Dan, I was talking about the case where you have only one motor overload contact as in the traditional NEMA starter. If you use that for the PLC input, then there is no hard-wired overload (not desirable). If you have an two OL contacts, then by all means use one for each function.

BobB said:

I do not use overloads anymore at all but use motor circuit breakers instead with plug in auxiliary contacts.

Click to expand...

BobB, tell me you don't mean this as it reads! If you do, you need to go back and take another look.

Overloads are intended to monitor for sustained overload situations such as those occur as a result of loose connections or insulation breaking down over time. Such overloads tend to cause heat build-up but may not cause an overcurrent situation.

Circuit breakers are intended to monitor for instantaneous overcurrent situations such as short circuits. These tend to be very sudden such that overload detection devices cannot respond quickly enough to prevent significant damage.

Clearly, a correctly designed motor control circuit needs to have both overcurrent and overload protection and, unless you have a device that is designed to do both, they are two separate items. If you are attempting to use a circuit breaker in place of an overload detection device, you have created an unsafe condition.

Steve

You should always have a signal from the overload to the PLC input, regardless on whether you also use a contact in series with the output.

Click to expand...

I am not talking about a contact in series with the PLC output, but an overload contact at the motor starter that shuts off the contactor upon motor overload. I am sorry to disagree, but if I only have one overload contact, it always goes to provide safety of the equipment and personnel. If the PLC doesn't know the motor has tripped, too bad. I can live with that, but not with burning down the plant. Running the overload function to the PLC FIRST (and considering the PLC as the overload controller) is not good design. You are adding another failure mode to a vital safety device.

Lancie1 said:

I was talking about the case where you have only one motor overload contact.

Click to expand...

Hi Lance,

When I come across this, I replace them and also pull an input back to the PLC, I know that you can not allways do this but I try...

I like having the interlock and having the input coming off the auxiliary.

Steve Etter said:

BobB, tell me you don't mean this as it reads! If you do, you need to go back and take another look.

Overloads are intended to monitor for sustained overload situations such as those occur as a result of loose connections or insulation breaking down over time. Such overloads tend to cause heat build-up but may not cause an overcurrent situation.

Circuit breakers are intended to monitor for instantaneous overcurrent situations such as short circuits. These tend to be very sudden such that overload detection devices cannot respond quickly enough to prevent significant damage.

Clearly, a correctly designed motor control circuit needs to have both overcurrent and overload protection and, unless you have a device that is designed to do both, they are two separate items. If you are attempting to use a circuit breaker in place of an overload detection device, you have created an unsafe condition.

Steve

Click to expand...

I feel he maybe talking about what we call motor starters, which combine a couple of protection methods in one unit.

Steve
Motor circuit breakers are designed to take the place of overloads - they are specifically designed for motor protection. They have an adjustment the same as overloads and have a higher magnetic component than normal circuit breakers. Suggest you have a look at Telemecanique type GV2 (up to 15kW), GV3 (up to 37kW)or GV7R (up to 110kW) that are the types I use.
They offer automatic type 2 co-ordination as well and the GV2 type have a "connecting" bar set so that you can just hang a contactor off the bottom. The whole lot in a nutshell.Also, they have busbars rated at 63 amps for the GV2 that can be connected into sevaral motor circuit breakers saving heaps of wiring. They are only available in DIN type to the best of my knowledge.
A variety of auxiliary contacts are available also to indicate tripped, open etc.
The GV7R type can also have a Merlin Gerin Vigi RCD block connected directly to them - the same Vigi block that connects directly to a Merlin Gerin NS250 circuit breaker. Normal trip coils can also be fitted to these.
The motor circuit breakers are part of a starter but are only the protection side - you still need a contactor. They break all 3 phases at whole current. No need to wire them into the contactor circuit as if they trip there ain't no phases connected to the contactor.
They also have a very high kA rating.