PLC switching

[netblindpaul]

Not very wide spread since manufacturers started value engineering!

 

[joel89]

Warning... dont switch contactor coils, big relay coils threw the plc go threw a seperate transformer or power supply, and switch the "no volt contacts" as sparking can blow plc cards..

 

[netblindpaul]

joel,
Your post is somewhat confusing.
I have to assume you mean through, rather than threw?
Why should there be sparking at a PLC IO card when switching a contactor coil?
A large transformer will have more inrush current and back EMF than a contactor coil.
The back EMF from a contactor is easily suppressible if you are using good equipment the contactor manufacturer will have a stock snubber of flywheel diode etc. that you can just fit to eliminate the back EMF through the PLC.

 

[WaterSparky]

Bit late but oh well, what usually is the case is that a 24V DC interposing relay be used to switch the 110V wiring to initiate the DOL starter. This also helps with isolation to the PLC so that in the event of a fault you are less likely to get 415V etc flowing up into the PLC!! This all depends on the size of system really, larger systems will be transistorised outputs @ 24V DC,(IMO) small systems like the TSX micros (now Schneiders Twidos) will like has been said relay outputs.

 

[Darkwood]

Most plc do all output options....you choose before you buy and unsure why you get 415 flowing into the plc unless you coincidently had a lost supply neutral With a selv short to lv... your outputs normally if relay are galvanically isolated ...volts free

 

[WaterSparky]

Im talking about modular systems in which the I/O can be added and changed with different (cards) rather the smaller unitary type. Also me talking about a fault, I mean you may get 415V/230V/110V flowing via a cable becoming severed and joining with a control cable all kinds of faults the possibilities are endless. I agree with the volt free contacts on relay output cards being isolated, but i was just saying if a large system leading to more often than not being transitorised outputs isolation would be good practise via interposing relay or opto-isolator (more commonly Analogue I/O)

 

[joel89]

joel,
Your post is somewhat confusing.
I have to assume you mean through, rather than threw?
Why should there be sparking at a PLC IO card when switching a contactor coil?
A large transformer will have more inrush current and back EMF than a contactor coil.
The back EMF from a contactor is easily suppressible if you are using good equipment the contactor manufacturer will have a stock snubber of flywheel diode etc. that you can just fit to eliminate the back EMF through the PLC.

well i was taught to not switch big coils i.e contactors, solenoid's etc directly off the plc due to inrush and arcing, so the panels i've built i have switched the "big coils" from small relays or a seperate power supply.... i do love this forum though all people do is moan other peoples posts

 

[netblindpaul]

joel,
Don't be a prat.
I genuinely could not understand your reasoning.
Your response is one of the reasons I post less and less on here.

I won't bother trying to help any more as you obviously aren't interested.

 

[Darkwood]

I must agree with netblindpaul here... I already expressed options of suppressed low inrush contactors etc.. one of the beauties of using a PLC is it can help reduce the number of timers, relays other control devices but starts to defeats the object if you end up operating all your contactors through a slave relay when its just a matter of picking the correct gear in the first place or fitting suppressors.

I probably would throw caution to frequent switching or pulsing of solenoids and other inductive loads direct from plc as this is best done through a slave relay so as to make a programmed maintenance replacement of the slave relay simple and easy and you don't sacrifice a whole plc just because you have a damaged output that should be for-seen.

@ joel -your post was strongly worded that its more or less a no no to run contactors direct of plc outputs but this just isn't the case as ive been doing it many yrs without issue, ts just a matter of knowing what the inrush will be and what you plc is capable of.

 

[i4Automation]

I always think its better to use a relay between the PLC and the load. It's easier to change a relay if anything goes wrong. The exception to this is if I'm building several units and need to keep the price down. Then you need to look at all the details outlined here and make a case by case decision.