1. ## Power supply question

Hi,

I'm about to finish my first cnc machine (moving gantry type). I am running the x-axis with two lead screws/two motors, y-axis and z-axis with one motor. I also plan to make/buy a rotary table. However, the motors I'm using is three Nema34 in bipolar (p) 55V/4A/Phase and two Nema24 in bipolar (p) 55V/2.4A/Phase and need a little help on the power supply...here is my calculations:

(3*4A+2*2.4A)*2/3 = 11.2A

11.2A*55V = 612VA

I've already got one EI power transformer with the following spec: 220V Primary and 0-24-27-30 secundary 400VA.

My qustion is, how to use this transformer in combination with another transformer. Does size, type or volts and amps matters? Can I connect a 225VA 2x30V in parallel to obtain the VA's and missing volts?

Thanks,

Thomas

2. You can parallel two transformer secondaries, but ideally they should be identical transformers, also you have to make sure you phase them correctly, there is a lot of past post on how to do this.
Al.

3. Thanks!

yes, I've done it before with two identical transformers. Please verify my setup (see attachment).

Best regards, thanks

Thomas

4. Yes, you show one with parallel windings in series with the second transformer, obviously the current rating will be the lowest of the two.
So the VA rating will be the lowest current rating x total voltage.
Al.

• hmm..I was hoping to get 400VA + 225VA..guess I'll have to buy another transformer..anyway, thanks for your help Al

Thomas

• HI Guys, Is there a table that gives the nema ratings for amps and voltage for stepper motors?

• Dear Thomas - You are going in circles here!!! When you connect transformer secondaries together you need to know the voltage of the winding. You can connect only the exact winding in parallel with another one. When you do that, the voltage is the same as each separate winding, but the current doubles (if the current is the same in each winding) and your VA would also double. If you connect two same windings in series, your voltage will double but your VA will remain the same. You can NOT add up random VA ratings to get a VA rating that you are looking for. You need to know what voltage you need and at what current you need, and work toward that. I couldn't read any voltages or current ratings on the two transformers in the series/parallel drawing. Transformers are AC devices. Are you connecting to an AC motor? DC is another animal. You said "bipolar". Is this a stepper motor? Stepper motors are DC. Maybe I am missing the whole point here, but what you gave doesn't make sense. Tell me more.

That was also my findings! ..sorry for the missing/wrong info.

I've got 5 stepper motors and want to supply 3x56VDC/4amp and 2x56VDC/2.4A. That gives me a total of 56VDC and (3*4A+2*2.4A)*2/3 = 11.2A

56VDC = 40VAC

And..11.2A*40VAC = 448VA

I've got two transformers, one rated 400VA and 30V and another rated 330VA and 20V. But as you said, I cannot use them in series or in parallel.

So I guess I'll have to buy a new transformer rated 500VA and 40V, right?

Best regards

Thomas

• Originally Posted by Thomasdj
I've got two transformers, one rated 400VA and 30V and another rated 330VA and 20V. But as you said, I cannot use them in series or in parallel.

So I guess I'll have to buy a new transformer rated 500VA and 40V, right?

Best regards

Thomas
Your two transformers in series will equal 30v + 20v = 50v the VA will be 50v at the lower current rating of 13a, therefore your new TOTAL VA rating will be 50v * 13a = 650VA.
Al.

• Hello Thomas

I wrote something wrong in my previous post - if you connect two same windings in series, the voltage will double, but the CURRENT will remain the same, but the VA rating will double. What AL wrote is correct. I got similar numbers - 20 volts (at 16.5 amps) + 30 volts (at 13.3 amps) = 50 volts. Using the lower current of 13.3 amps, you will have 50 volts at 13.3 amps which is 665 VA. But I do not follow your idea about 56 volts DC being equal to 40 volts AC. 50 volts AC should be around 35v DC, after rectification. How will you rectify? Full wave bridge?

• Originally Posted by Roger Z
Hello Thomas

I wrote something wrong in my previous post - if you connect two same windings in series, the voltage will double, but the CURRENT will remain the same, but the VA rating will double. What AL wrote is correct. I got similar numbers - 20 volts (at 16.5 amps) + 30 volts (at 13.3 amps) = 50 volts. Using the lower current of 13.3 amps, you will have 50 volts at 13.3 amps which is 665 VA. But I do not follow your idea about 56 volts DC being equal to 40 volts AC. 50 volts AC should be around 35v DC, after rectification. How will you rectify? Full wave bridge?
Using a full wave bridge the formula is secondary AC voltage * 1.4 = (rectified and filtered) DC voltage. So 40 * 1.4 = 56.

Alan

• Originally Posted by Roger Z
But I do not follow your idea about 56 volts DC being equal to 40 volts AC. 50 volts AC should be around 35v DC, after rectification. How will you rectify? Full wave bridge?
I think you have it backwards, 50vac would = 50 x 1.414 = 70.7dc after capacitively removing ripple.
OR using the figure of 56vdc, the AC would = 56 x .707 = 39.5vac.
Al.