Supplying a Power Supply


Thread Starter


I am powering a 24 volt PLC which controls 2 240 volt three phase motors. Starter coils are 24 volt. Do i need a control transformer to power my power supply which requires 115v.input? Also, is it a good rule of thumb to keep the high voltage (>120 volt) outside of the control panel?

Bob Peterson

There is no absolute requirement to have a Transformer at all in a control panel. Use a Transformer if you have input voltage that needs to be changed to a different voltage. If you can use hey supplied voltage directly to your power supply there is no need for a Transformer.

We don't normally consider hundred twenty volts to be an especially high voltage. Generally it is considered good practice to separate 120 volt DC from 24 volts DC but usually it is not actually required by any commonly recognized standard. Please note the words generally and usually in the previous sentence.
A couple weeks ago my escort electrician in the steel mill had to put on all his arc flash protection garb to open the panel door to a 24x20" panel with a single 115Vac line powered strip chart recorder so that I could count the I/O and get a model number from it.

I don't know whether keeping a transformer out of a 24V panel would avoid the requirement for arc flash protection or not.
What is the hold up time of the PLC power supply?

Do the motor controls have any auto restart or re-acceleration logic?

The issue with a PLC controlling a MCC is that relays can drop out when there is a power blip and re-energise immediately with power but a PLC may reset itself. Therefore being less reliable; plus the issue of the reboot or restart time. Relays come back on line much quicker. So you might want to look for a more reliable PS feed; UPS or dual power supplies fed from different sources.
Somebody didn't know what they were doing then.

Your Arc Flash PPE (Personal Protective Equipment) requirement is based on the amount of potential energy in the enclosure in terms of cal/cm2. The lowest dangerous level is considered 1.2 cal/cm2 because it can result in a 2nd degree burn, like a sunburn. But I have NEVER seen a 120V AC circuit that calculates out to more than 1.2 cal/cm2. At that level all you need is basic work cloting, called PPE Level 0 in the past, although now they just say "no PPE required".

You have to get to 4 cal/cm2 before you get to needing even PPE level 1, and that's just a long sleeve shirt and pants made of fire resistant material such as cotton. You don't need the full blown Arc Suit until you get to 40 cal/cm2. there is no way that box could have had 40 cal/cm2 in it if the only thing in there was 120V...

What more likely happened is that the plant manager was being LAZY and didn't bother having someone calculate and LABEL the enclosure with the proper PPE requirement, a 5 minute process, so the boor electrician was forced to ASSUME the worst because he "LEGALLY" did not know otherwise.
<i>> Do i need a control transformer to power my power supply which requires 115v.input?</i>

You do if you do not have 120V 60Hz available.

What do you have available?

I would assume you have 240V 3Ã because that is what powers the motors.

If 240/120V 3Ã 4W, no CT needed. You can simply use Line A or C to Neutral to get 120VAC 60Hz.

If 240V 3Ã 3W, yes CT needed. You cannot get 120V 60hz out of a turnip.
<i>> Also, is it a good rule of thumb to keep the high voltage (>120 volt) outside of the control panel?</i>

It is generally possible to get the arc flash PPE level down to 0 or 1. It is not just about the voltage.