Interlocking Plc with motor control cct

[mikeybale]

Can you interlock your plc with n/c contact of your motor overload relay?

 

[CaseyK]

I'm not sure if interlock is the word you want to use here.

Have the NC contact tell the plc when everything is okay? Sure

Have the NC contact tell the plc that a problem exists, and to shut of motor? Maybe, I think this is kind of a grey area. I have seen it in one older plc book that I got at the library. I can't think of any code problems off the top of my head.

If it was neccesary to shut down another part of a conveyor line or something, it would make programming things easier.

If there were any obvious safety concerns, then Definitely Not.

I am pretty sure that I have done this in the past, and would consider it in the future.

Perhaps someone else has some insight on something that I havent considered.

regards.....casey

 

[Eric Nelson]

The Telemecanique overloads I use have two sets of contacts. One normally-open, and one normally-closed. I'm pretty sure the majority of other brands are the same way.

Common practice is to hardwire the power to the motor contactor coil through the N.C. contact, so the contactor opens upon overload. The overload should be setup for manual reset. I don't know why they even OFFER automatic reset on overloads...

I use the N.O. contact to let the PLC know that the overload has tripped, so it can take the appropriate action (i.e. shut down the rest of the machine and inform the operator about the overload). If that motor contactor is controlled by the PLC, I'll ADDITIONALLY turn off the output to the contactor as a backup, but the hardwire circuit should have already turned off the contactor anyway.

beerchug

-Eric

 

[Bruce99]

Motor OL

One common practice is to wire the OL to an input, yet still keeping
the control integrety intact.

Wire the starter control circuit from the control fuse to a door switch (if you use them) then through the OL block and to the stop button or other interlock devices. Then using an isolated output module wire the supply for the coil from the load side of the stop button. This maintains hardwire control, OL trips, supply for output is lost and coil shuts off. Your logic will back this up, loss of the OL input turns the output off. Operator gets a message saying motor is tripped due to overload.
This is for thermal overloads. Electronic are different. Motor portection relays are different again.

No one really trusts electronic OL, we have motors that have two levels of electronic protection, but scabbed onto the side of the starter tucked away neatly there is an AB OL block. My favorite is the large frequency drives with the trusty AB thermal OL block sitting on the bottom "just in case".

Hope this helps

Bruce.

 

[robert mooli]

Hi mike , interlocking n/c contact of o/l relay with your plc is simple , hard wire that i/p to yu plc i/p card give it an adress, use logic , rem plc dont mind whether youare scaning an ip as nc or no its about your logic

 

[elevmike]

Please dont trust your PLC to shutdown your motor in the event of an overload trip. Use the OL to directly shut down the motor, and an aux contact to indicate the fault to your PLC.

Mike.

 

[rdrast]

The preferred solution here is to use Motor Starter Protectors instead of plain old overload relays. They give the best of three worlds (or 4).
First, an aux contact from the MSP can go to the PLC to indicate an overload trip for one, but the MSP itself trips and disconnects the motor.
Second, the MSP's we use have nice switches on them and even little lockout tabs, so a motor can be manually shut down and locked out.
Third, related to second, we use an aux contact off the MSP (not the overload trip contact) to indicate back to the control system that the starter is manually commanded off. That's useful to bypass alarms, or do some interlocking based on a hardware off command.
Fourth, the MSP's are rated as full circuit protection devices, eliminating the need for external breakers or fusing, and they bolt directly to magnetic starters to save wiring.

Fifth, I guess, is that where we don't need magnetic (controlled) starters, the MSP's are just fine by themselves as manual starters.

Oh, the original topic? I generally don't like using a PLC to interlock a motor-starter overload, but rather just to indicate that one has occurred. It's too easy for someone in a hurry (and anonymously) to force the input on, or hard-wire it on to prevent 'nuisance trips', which can lead to fire or other severe damage.

 

[electrified3]

One quick question on wiring O/L directly to plc input, is it your neutral going through your N/C contact? That is common practice in alot of motor starter circuits that I have seen, if it is, will tying it to your plc actually give you an input? Also what type of inputs are you using? 24VDC or 120VAC and what is the voltage of your motor starter?
If any of the above situations exist then you might want to look at other options, is it possible to add an aux contact or a control relay to the motor starter for your input?

 

[Tom Jenkins]

I agree with electrified. Most motor starters have the overload contact hardwired in series between the starter coil and neutral. You don't want to be in series with that, since you won't have 120 VAC to either device. You don't want to be in parallel, since you will "backfeed" 120 to both sides of the coil.

The best solution is to add an auxiliary contact to the motor overload. For most starters that is a simple add on contact block that can be field retrofitted to the side of the overload, and will provide an isolated contact that you can wire to your PLC input.

 

[DonsDaMan]

If you are connected on the neutral side, then you'll have a high input when there is an overload and you'll have a low when you do not.

Just use negative logic in your code.

 

[elevmike]

Can you interlock your plc with n/c contact of your motor overload relay?

Short answer to short and vague question: NOT RECCOMENEDED!!!

Wire your contactor like so..

   OL            Starter Coil         Neutral

---|/|-----------(        )-------------*

If you want a seperate signal to the PLC then follow Tom's advice above, and install an aux contact on the overload.

Additionally, (please dont take this too hard) make sure you have somebody in the know check your work!

 

[rdrast]

If you are connected on the neutral side, then you'll have a high input when there is an overload and you'll have a low when you do not.

Just use negative logic in your code.

Sorry, but I'd disagree there, strongly, for a couple reasons.

One, is that if the OL contact opens, breaking the coil circuit, the collapse of the starter armature could generate a huge spike voltage, potentially damaging the input module. Most people do apply snubber networks to output drivers, but I don't know of many that put them across inputs!

The second, is that I absolutely HATE tapping off of a node in a control point, especially a safety point. It is, indeed, highly unlikely that a fault in a PLC input module could act as a short to neutral (in this case), but it is possible. If such a fault did occur (say the third time the OL Tripped, blowing out the input circuit in the module), then the physical OL contact could open, but the coil would still have a neutral path, and could remain energized.

Tom and Electrified are correct. Use an independent auxiliary contact for status indication. 5 Bucks for an extra contact, is a whole lot cheaper than if there is a failure down the road.

 

[Sliver]

We use a relay, called a 'P' relay in every starter.
the coil of the relay is powered through the control transformer, control fuse, overload N/C contact to common. The motor coil is powered the same way but also includes the computer output contact.
N/O contacts from the 'P' relay are connected to an input (DCS or PLC)
Control can tell when the starter looses main power because the control transformer looses power, or overloads are tripped both drop 'P'. Both require a visit to either engage disconnect or reset overloads. We also use an aux starter contact N/O to drive an 'M' input to DCS/PLC. This tells us if the starter picks up properly or drops out unexpectedly. Main 575V 3phase, starter control 120VAC, DCS/PLC control -125VDC.

 

[Bruce99]

To Clarify

There is no rule that states the OL contacts have to be in the neutral wire. To wire to an input it has to be on the otherside of the coil. This is used when you have many interlocks in the motor control cct. Oil flow, presure, vibration, brg temp or cooling water etc etc... The more info you give the operator the better. I hate driving 6Km to a crusher only to find the lube oil tank low caused the motor to shut down due to low oil presure. Got to go find a millwright, where do they hide that oil anyway?

There are issues also with opening the neutral. This wire is, by code, not to be opened. But this is a different subject..