Plc controlled dol

[Lucky_Bugger]

Hi,

I'm a bit new to all this, however...

I'm trying to control a DOL motor using a PLC output.

At the moment i have a PLC output directly driving a contactor supplying power to the motor. The rating of the PLC output is high enough to drive the contactor coils.

On other circuits ive seen, people sometimes use an intermediate relay on the plc output to drive the contactor coil. This relay would be supplied from the PLC output in series with the auxilliary contacts on the motor overload, breaker and isolator. I.e. its hard wired.

What i'm intending to do is to feedback the overload auxilliary, the isolator auxilliary and the breaker auxilliary to the PLC and get the program to decide when to drop out the contactor.

The question is, is this the wrong way of doing it?

Ive attached a PDF of the circuit.

I should have added, if it is the wrong way of doing it, why is it?

 

[Gomez_]

I don't see anything wrong, it all depends on your application. I will say that where I work they prefer a hardware stop rather than relying on the PLC. For example the output from the PLC you are using to engage the contactor would go through the auxiliary contacts on the breaker and isolator, so if either of those trip the contactor drops out.

 

[geniusintraining]

I will say that where I work they prefer a hardware stop rather than relying on the PLC.

I agree... Have the CPU get an input saying the overload/heater/breaker has tripped but drop out the relay with the interlock, this will prevent someone in the future from forcing the PLC to run the motor if something is wrong with the circuit/motor control, its just safer

I would just have the PLC light a error light and stop the process but leave the control to the hard wire

 

[Lucky_Bugger]

On thing I have just realised, is that if I use the auxiliary contact of the isolator in series with the PLC output then I definitely know the incoming side of the isolator is dead when the isolator is switched off irrespective of the state of the PLC output.

Thanks both for taking the time to reply.

 

[blackandredwarrior]

A project I did I just monitored the aux contacts for alarming and "locking out" the motor starter. This "lockout" was just so the PLC wouldn't try to start it. It was not any other kind of Lockout/tagout system. These were on A/B 140M MPCB/109C starters. If the MPCB was off or tripped I knew and I also made sure that the aux on the starter contactor closed. Your coil should always be running through the aux of whatever protection you have (overload relays, breakers, etc.) If they trip it's going to drop the contactor and doesn't matter if you are trying to hold the contactor in.

If there is a separate power source for the starter then you need to take that into account. I had the PLCs on a UPS and had an input coming into the PLC so I could monitor power. If that input dropped we lost 120V control power and I wouldn't even set any alarms for any open/tripped MPCBs because I wouldn't have power anyways.