Alternating Real vs PLC

[ControlConcepts]

I'm in the process of coming up with a 3 motor alternating scheme with a trip indicator. I am wondering if it is possible to do it with relays or if it would be easier to use a PLC.

 

[Macbeth]

You can do this with relays but a PLC would be easier. Also with PLC any changes are faster by reprogramming than rewiring. PLC add more advantages in the short and long run. By the time you buy all your relays switches and lights. You could have bought a micro PLC. Add some more money get a small display, now you have an enunciator with time stamp. Also a runtime counter, changeable seq. & ect..

Although relays wont go into fault after a thunderstorm.

 

[vanskat]

Agreed.

I assume that coordinating these relays will become tedious depending on how many are used. Futhermore, the hardwiring bit becomes a headache when minor changes have to be made very frequently..

Condideration: optoisolators for PLC protection

 

[abeltio]

I've seen a combination of both, the PLC will:
1. drive control relays (due to the contact rating of the plc's built in relays) and,
2. perform all the logic, annunciation functions

Make sure the logic is failsafe (e.g. if it is driving lube oil pumps: the pumps run when the system is tripped (control relay is DOTR - dropout to run), the only way to stop the pumps will be to open the MCC breaker, if it is driving fuel-oil pumps the control relay MUST be PUTR (pick-up to run) otherwise you'll be pumping fuel into you-do-not know-what...

The thunderstorm problems I had were always troubleshot to poorly installed shielding and grounding system.

HTH
Saludos.
a.