Power a PLC using a 3-Phase supply

Hi,

So, i am new to PLC's so if i make a foolish comment please point me in the right direction.

So we are trying to find a PLC to which we can input a 3 phase power supply and specify that we want a third of the I/O pins to be powered through each phase. If that is possible please point me to an existing product.
If not, are there any alternative approaches that i can look into to obtain similar results.

Thanks

waleedqk said:

Hi,

So, i am new to PLC's so if i make a foolish comment please point me in the right direction.

So we are trying to find a PLC to which we can input a 3 phase power supply and specify that we want a third of the I/O pins to be powered through each phase. If that is possible please point me to an existing product.
If not, are there any alternative approaches that i can look into to obtain similar results.

Thanks

Click to expand...

Well to answer your first question you wouldn't generally power your PLC with a 3 phase supply. You would use a control transformer to generate 110V to power the PLC or a Power Supply to generate 24V to power the PLC. The PLC is a logic controller, it controls the logix it doesn't directly drive anything with big power that would generally require 3 phase power.

I would add more but first of all it would be helpful to know just what you are planning on controlling with the PLC? Motors, solenoids, lights, etc... Also so information on your background will help. Are you an Electrician, an Engineer, are you familiar with Electrical controls?

The more information that you give us in regards to what you are trying to accomplish the more information and help we can give back. You will find a wealth of information here and people are generally willing to help but you will only get as much back as you put in.

Welcome to the forum.

So we are trying to find a PLC to which we can input a 3 phase power supply and specify that we want a third of the I/O pins to be powered through each phase.

Click to expand...

I think what you mean is NOT "powering the PLC", but rather 1/3 of the Inputs and Outputs powered by each phase of a 3-phase power supply. The PLC itself (the CPU module and the backplane) should be powered by a single phase or by a DC power supply, depending on the model of the PLC.

Almost any PLC could meet those specifications, as the Inputs and Outputs can always be independently powered by whatever source desired, as long as it is within the voltage limits of the PLC specifications.

Where different phases are used for the Inputs and Outputs, warning labels should be applied to the cabinets so that electricans and maintenance technicians will be aware of the 3-phase voltage.

Something smells fishy. There are a variety of ways that this could be done, but I'm having trouble thinking of a good reason you would want to do it this way. It's not very helpful if we can tell you how to do something that ends up being the wrong thing to do.

In general, a PLC will be powered either by a 24VDC power supply or its own 120VAC 1ph power supply, and regardless of what power sources you have available, you can buy a power supply or transformer do condition your power to match the requirements of your PLC.

Furthermore, usually the onboard I/O on a PLC are designed for low power applications, and most are for low current signals under 120 VAC. If you're using three phase power, chances are you aren't just sending low power signals but actually driving motors or something, in which case you don't want to run that directly through a PLC. Instead, just use a contactor which switches on and off the high power signal from each of the phases.

It's common for PLCs to allow you to land your own VDC/VAC and Common signals to your I/O. For example, you might run a 24VDC signal to the VDC and then all the outputs on that bank are now 24VDC when on and 0VDC when off. If you instead ran 12VDC they would now switch between 12VDC and 0VDC.

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One sort of valid reason would be if a large plant has a 10 or 20 3-phase panels, 208/120 volts, for their PLC loads, then they might want to equally load all phases with PLC inputs and output circuits. I don't see any problem with doing that with just about all brands and models of PLCs, although it would work best with the Remote I/O types, where the Remote PLC units could be powered from the nearest 3-phase 208/120 volt panel.

Lancie1 said:

One sort of valid reason would be if a large plant has a 10 or 20 3-phase panels, 208/120 volts, for their PLC loads, then they might want to equally load all phases with PLC inputs and output circuits. I don't see any problem with doing that with just about all brands and models of PLCs, although it would work best with the Remote I/O types, where the Remote PLC units could be powered from the nearest 3-phase 208/120 volt panel.

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This is why I asked for clarification. The OP said he was new to PLC's and like you said there may be a sort of valid reason for this I am inclined to believe that he is just starting out and doesn't exactly know what he needs.

All of the 22 PLC's in my plant are connected to a 3 phase power source, there just happens to be something in between the 3 phase and the PLC in any installation I have ever seen...

The power supply of a PLC can be three phase (have several here) giving 24 volt output for a PLC, they even work fine when a phase is broken so no problem here.
The phase inputs is never a problem as they are almost always isolated from the rest of the plc. they will need a Null (not ground).
outputs are when relays are used always isolated from the PLC however sometimes the Commons of some of them are attached to each other.
I use WAGO(Beckhoff) and each unit is separate so i can do almost anything.
I do use powercontacts for the coils and solenoids they have separate PSU.

...so, could you replace one of these 3-phase power supplies with an ordinary single phase supply. I have a bad one that is 380v 3-phase in and 10A 24v DC out. Is there anything "special" about the output of these 3-phase PS that I don't know about...maybe specific to the PLC it is powering? It's a Siemens PLC and Siemens 3-phase PS.

bluenoser337 said:

...so, could you replace one of these 3-phase power supplies with an ordinary single phase supply. I have a bad one that is 380v 3-phase in and 10A 24v DC out. Is there anything "special" about the output of these 3-phase PS that I don't know about...maybe specific to the PLC it is powering? It's a Siemens PLC and Siemens 3-phase PS.

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Yes you can just make sure you use a good regulated PS. The only things to watch out for are the Voltage ratings of the input to the new power supply and the output Ampacity. Also note whether or not the common out of the DC side is tied to ground or not. I have had to replace a few Siemens 3 phase power supplies over the years.

Thanks allscott !!

i do have one of these in spare.
They are built rather simple. with some rectifiers and a DC/DC regulator