Motor Transformer Sizing

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tblough
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Motor Transformer Sizing

Post by tblough » Sun Feb 11, 2018 10:07 pm

With my lathe finished, I'm considering a mill conversion. I have have two 29oz-in motors for Z and X and a 40in-lb for Y. I also have a 16in-oz motor for a rotary table if I can ever find time to work on it. All of these are Centroid 180V motors.

Since I need 230V for my spindle motor, it looks like I'll need a 230/110 step down transformer for the drives. Will a 2KVA transformer be large enough? I think the high power transformer from Centroid is 2.3KVA and it says it's good for four 40oz-in motors. With my mix of motors it seems like a slight smaller 2KVA transformer should work.
Cheers,

Tom
Confidence is the feeling you have before you fully understand the situation.

bfreestone
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Re: Motor Transformer Sizing

Post by bfreestone » Sun Feb 11, 2018 10:44 pm

Tom,

If all your motors are rated for 180VDC you don't necessarily need a transformer if you power the full wave bridge and smoothing capacitor with a single 120V Leg. You should end up in the 160-170VDC range with that setup. It's been a while but If I remember correctly, 120VAC RMS x 1.414 = 169.68VAC Peak then - 1.4V Drop from the full wave bridge Diodes would leave you with 168.28VDC Peak before you run it into the smoothing capacitor. If you want to have 220VAC Input to the DC supply to balance the load or isolation you can go with the transformer. We've been using 180VDC glentek servos with the allin1dc and no transformer for several years now.

tblough
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Re: Motor Transformer Sizing

Post by tblough » Mon Feb 12, 2018 12:08 am

I had originally thought about running 4-wire service to the machine, but running all the drives on a single leg is probably not a good idea.
Cheers,

Tom
Confidence is the feeling you have before you fully understand the situation.

diycncscott
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Re: Motor Transformer Sizing

Post by diycncscott » Mon Feb 12, 2018 8:34 am

Tom,

120VAC @ 20A is 2.4KVA. You'll probably be ok on 120VAC but you definitely want to make sure you have an inrush limiter. The Centroid "Cap & Bridge" do have an integrated inrush limiter installed. http://www.centroidcnc.com/centroid_diy ... nents.html PN12726

tblough
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Re: Motor Transformer Sizing

Post by tblough » Mon Feb 12, 2018 12:24 pm

Scott,

I plan on using the Centroid cap and bridge, but my question is if 16A is enough to run the 4 motors I have. Big difference in price between 2KVA and 3KVA transformers on eBay.

I curious where the 20A number comes from? If I add up the amperage based on the values from the recommend current switch settings for the Allin1DC, I come up with 48A for my motors. If I take four 40oz-in motors and the recommended current switch settings it's 60A. Is there a "rule of thumb" that says to size the transformer at 1/3 the maximum?
Cheers,

Tom
Confidence is the feeling you have before you fully understand the situation.

eng199
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Re: Motor Transformer Sizing

Post by eng199 » Mon Feb 12, 2018 1:53 pm

I think diycncscott was referring to the 20A that is commonly available from 110/120 outlets. Centroid 40inlb motors are rated about 8A continuous, so in theory, a 4 axis could require 32A. In reality, the current requirement could vary greatly depending on the user. Most users will not run try to run the machine for all its worth. Also, chances are that only X and Y will move with a heavy load most of the time while Z and W hold. The other common situation is that Z is loaded heavily while all other axes are near idle.

To summarize, normally two or less axes are running at 8A or less for any significant amount of time. Add a few amps for the remaining axes to hold and you have enough. If your motors are oversized or you're not in a hurry (lower feeds, lighter cuts) 16A will be fine. If the motors are undersized or you're machining as fast as possible, 16A may not be enough.

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