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Richard3009

If you wanted to use an output of a PLC to start a DOL motor, would the PLC output power be used to energise the coil on the contactor at 110v,
 
darkwood's your man for this, mate. what i know about plc's can be written on a beermat with a 3" paintbrush.
 
Richard3009;9 If you wanted to use an output of a PLC to start a DOL motor said:
You can use a PLC to switch the DOL but depends on a couple of things. Firstly does your PLC output 110v and secondly could it handle the current switching.

Is your PLC a micro logic block, brick or modular?

Is the output a relay, transisor or triac?

You could also use an interposing relay which is the PLC output switching a relay to switch in the DOL contactor.

A word of warning; follow all PLC manufacturer's instructions for switching inductive loads else you could end up replacing components or the PLC itself... back up the PLC program too!

regards s.f
 
Its more of a general question as in what's the most common approach, I suppose it depends upon application equipment etc. Lets say we have motor and it's load current is too large for the plc contacts, would normal practice be to energise the contactor coil in the starter panel local to the motor via the PLC?
 
If you wanted to use an output of a PLC to start a DOL motor, would the PLC output power be used to energise the coil on the contactor at 110v,

No, use a 24VDC relay to bring the 110v coil in. Or change the coil in the contractor to 24VDC if the on the PLC in man enough.


I doubt the card will be 110v....
 
... I doubt the card will be 110v....


If the output card has volt free relay contacts you can switch most voltages within spec.

I wouldn't power a motor directly through the PLC contacts without exploring relay control thoroughly.
 
Continuing from Silva.foxx excellent advice -You can get low power consumption contactors that are PLC friendly driving off a 24dc output of your PLC, they have suppression fitted and should do what you require... although this is an option many things have to be considered as you could also go for a volts free relay output PLC and follow the manufacturers guidelines as to inductive loading of the relays..

Contactor example would be a Schneider CA4KN22BW3 which have a suppressed inrush of 1.8w.

You need to factor many things ..your PLC is usually one of your last things to pick after you know how your going to build you control panel, what voltage and AC or DC ... 24v dc is usually a favoured Voltage as many accessories have a range in this voltage but again depends on many things.
 
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The best I’ve used were Modicon (Telemecanique) TSX micro. 5A relay units aren’t cheap, but 16 x 5A relays in one unit are a godsend. All OP’s totally separated.

If you want more or cheaper then you may as well start out with some form of interface switching from the start.
 
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..
 
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.
 
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.
 
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
 
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)
 
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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
 
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.
 
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.
 
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.
 

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