Help me figure my transformer size

[scheek]

I have 3 stepper motors. (2) 640 oz @ 2.3 volts/5.5 amps and (1) 270oz
@3.75 volt/2.8 amps.

I think I bought to big. I bought a 800V 60V + 60 V toroidal.

Any help would be appreicated.

My capacitor is a 37,000 uf

[ViperTX]

You can never be too big.....we've done these calculations at least a thousand times on this web site....do some searching.....we can then clarify any confusion....

[scheek]

I did some searching and not sure I got anything out of it. Found one person using same set up (Gecko 202 and 640 oz motors) but was using a 48VAC transformer. Another guy had a 100VAC transformer.

I'm stupid but learning this Greek. All I want to know is if I hook this 60 V + 60 V/ 800VA up and use a 25 amp rectifier and have a capacitor (37,000 uf)with a 75V surge rating will it be enough and not kill my motors and blow up my single capacitor.

I guess in the long run the motors will only draw what they want if the supply is there. Right?

[Al_The_Man]

If you want to reduce the secondary voltage, there are plenty of posts here describing it, it is relatively easy with a toroidal transformer.
Al.

[scheek]

Thanks, but I don't know if I'm too big or just right? One response was that I couldn't get too big. Is that right? If so, maybe I'm okay.

Okay...look at this. say my 60VAC is what I need or about and I need to convert to DC. I suppose with the right rectifier I will come out with 60VDC or about.

That rectifier, should it be a minimum 60V or can it be a 50V to do the job. I don't see a 60V anywhere is why I ask. 25 amps is what the desired amperage should be. So when ordering what should I look for. I know now I probably ordered the wrong one.

Thanks.

[sdantonio]

The rule of thumb I was given was that voltage should be 4 to 20 times the rated voltage of the motor. I forget the rule of thumb gfor amperage.

[scheek]

I think I'm going to send my transformer back and get a smaller one. I just read where one guy needed about 57VDC and bought a 55VAC. When the he checked on the back side of his capacitor he was over 84 VDC, which was too much for a Gecko Drive.

I've got a 60VAC secondary so there is no question what I'll have too. More than him likely.

This has been a horrible experience. It's so clear to someone who is in the electrical circles but I'm not and having to dig for info is tough!

All I want to know is what I have work. I don't think so.

[fyffe555]

60vac after a rectifier will be ~85vdc ( Vdc=1.414*Vac). That's too high and why Al the Man was suggesting the secondary vac needs to be reduced.

I wouldn't run the 2.3 motors at more than ~48-55vdc, (20-25x plate voltage), 3.75 motor could be 75-90vdc but then drivers get expensive. For convenience you could run all three on the same voltage, the voltage is also governed by the capacity of your drivers..

48vdc needs ~34Vac into the rectifier. 55vdc needs ~39vac etc..

I'd suggest Amps would need to be ~11amps. For a transformer with ~11amps at ~39vac (55vdc) i'd get a transformer with secondaries of ~39vdc and at least 450va rating.

Capacitor needs to be rated at over the VDC voltage, uf rating depends upon the ripple you'll accept but the usuall rule is uF = (80000 * I) / V, so for 11a and 48vdc you get 18333uf at greater than 48vdc rating

[scheek]

Thank you, Thank you. finally an answer I can understand.

My 60V is packed and ready to go back. All I'be out is the freight.

So rectifier wise?? maybe a 50V /25 amp or something like that.

[Al_The_Man]

You do not need to size the rectifier close because there is very little difference in price between sizes, go with a 25amp 300~400 PIV.
50v is too low.
Al.

[scheek]

Got cha. Just ordered it from Parts Plus.

Thanks again. You've answered two weeks of questions in about 1 hour.

Gee! Great help.

[sdantonio]

Remember, it you have to much voltage coming through you can always cut it down with a zenner diode or two.

Steven