PLC switching

[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,

 

[telectrix]

darkwood's your man for this, mate. what i know about plc's can be written on a beermat with a 3" paintbrush.

 

[silva.foxx]

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

 

[Richard3009]

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?

 

[LeeH]

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....

 

[silva.foxx]

... 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.

 

[Richard3009]

yep good point, if the distance it too great (volt drop), I suppose an option would be a 110v card then or remote I/O

 

[Darkwood]

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.

 

[Knobhead]

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.

 

[Knobhead]

How wide spread is this system where VD has to be taken in to account for OP’s?