What do you know about Transformer sizing?

Timeismoney08 · Apr 8, 2020

I have an application that has 3 24VDC motors.

Each motor pulls around 10 amps, but none of them will run at the same time.

If I fuse them accordingly, can I use a transformer that is, say a 15A? Or do I need a min of 30A?

Thank you!

rupej · Apr 8, 2020

Confused- if these are DC motors, what is your transformer doing?

Timeismoney08 · Apr 8, 2020

When I said xformer, I was referring to a DC power supply.

It'll be 480V-24VDC Power Supply.

The PS is providing the power to the motors.

rupej · Apr 8, 2020

Nothing wrong with using the 15A in the manner you're describing, IMO.

lfe · Apr 8, 2020

With a switching PS you do not need a transformer, less occupied space and less heat inside he electrical cabinet, for example one like these:

https://www.tracopower.com/series/tsp-3p

Timeismoney08 · Apr 8, 2020

rupej said:
Nothing wrong with using the 15A in the manner you're describing, IMO.

Thank you for answering.

Is there any standards describing what I'm trying to do?

Not sure if it is allowed or not.

Timeismoney08 · Apr 8, 2020

lfe said:
With a switching PS you do not need a transformer, less occupied space and less heat inside he electrical cabinet, for example one like these:

https://www.tracopower.com/series/tsp-3p

Thank you, That is good to know. I still need to know if it is allowed to size the xformer based off of one running at a time or if I need to size it based off of max capacity.

lfe · Apr 8, 2020

Timeismoney08 said:
Thank you, That is good to know. I still need to know if it is allowed to size the xformer based off of one running at a time or if I need to size it based off of max capacity.

If only one is going to work at the same time, you should size it for one, but with some margin, better to choose a minimum 15A power supply.

Timeismoney08 · Apr 8, 2020

lfe said:
If only one is going to work at the same time, you should size it for one, but with some margin, better to choose a minimum 15A power supply.

Thanks for the info.

padees · Apr 8, 2020

No one knows the motor specs and if there is a controlling drive involved.

I have used a number of 24VDC SMC actuators with drives. Even tho these are rather low power consumption, you still have to size the DC power supply quite a bit higher so it doesn't crowbar the power supply just starting the motor.

No clue as to what you really have, but also don't think anyone could answer your original question without specifics.

jraef · Apr 8, 2020

Most modern 24VDC switch mode power supplies generally have what's referred to as a "crowbar circuit" that will turn off the output if the current draw is higher than the PS is capable of delivering, and/or the rate of increase is too rapid for it. So not knowing the nature of or how you will be controlling / driving these DC motors, it's a safer assumption to over size the power supply. Depending on these unknown factors, the starting current could easily be 200-300% of rated current. It could also be 100%, we just don't know from what you have provided.

And stop calling it a transformer, you will get yourself screwed up. Transformers are for AC, it's a Power Supply...

padees · Apr 8, 2020

jraef said:
Most modern 24VDC switch mode power supplies generally have what's referred to as a "crowbar circuit" that will turn off the output if the current draw is higher than the PS is capable of delivering, and/or the rate of increase is too rapid for it. So not knowing the nature of how you will be controlling / driving these DC motors, it's a safer assumption to over size the power supply.

Yes...

padees · Apr 8, 2020

jraef said:
And stop calling it a transformer, you will get yourself screwed up. Transformers are for AC, it's a Power Supply...

Double Yes...

Gene Bond · Apr 8, 2020

If you are planning to full-voltage start a DC motor, you better plan on a current surge of 10x for a few mS... If you can get the specs, or the winding resistance, it's easily sized, but most don't give you enough data.

Just like circuit breakers, not all power supplies are created equally. Motor starting is a better application for a 'brute-force' power supply, and now your primary concern *IS* the transformer (and the rectifier). Not much need for a capacitor if a motor is the only load.

With a simple transformer/rectifier circuit driving the motor, the transformer can withstand a little overload (150-200%) to start the motor, so now you can size it for 5x instead of 10x (or so).

Just a thought.

Timeismoney08 · Apr 8, 2020

I can post the motor data tomorrow, but....

The question was if I need to size the power supply to handle all Motors at the same time even if it will never be turned on at the same time.

It doesn't even have to be a motor...Let's pretend it's a LED drawing 500ma...If I had 10 of them, but only 1 ever turns on at a time...Do I need to size the Power supply for worse case scenario @ 5 amps, even if it's fused properly and my program prevents it? Or can I limit the power supply to handle 2 or 3 LEDs since I know I will never turn them on in the program?

What do you know about Transformer sizing?

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