Need help w/ gecko drivers and power supply

I'd like to use geckos in a new controller for cnc mill. My question is what are the determining factors in deciding on what my voltage and aperage should be on my power supply. I'll be using gecko 201's and the motors are rated at 6 amps. I read so many peoples suggestions and now i have another guy telling me i need a power supply that's 58volts and 20 amps. Is he off track? I was always under the assumption that steppers only recieved 2/3 of their rated amperage.

Please help all help appreciated.
Thanks
Mark

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Originally Posted by m8kingit

I'd like to use geckos in a new controller for cnc mill. My question is what are the determining factors in deciding on what my voltage and aperage should be on my power supply. I'll be using gecko 201's and the motors are rated at 6 amps. I read so many peoples suggestions and now i have another guy telling me i need a power supply that's 58volts and 20 amps. Is he off track? I was always under the assumption that steppers only recieved 2/3 of their rated amperage.

Please help all help appreciated.
Thanks
Mark

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Hi Mark,

I'm still new at this, but I think I learned the following. At least I hope so. :)

You are almost right. You need enough amperage to run all 3 or 4 stepper motors at the max amperage simultaneously that they can use. Most likely, this may never happen or only happen for a split second or two. More voltage means more power for the amperage to do it's work. When doing any heavy cutting, the amperage needs to be there to move the stepper/servo motor to its new position.

I realize that this isn't a very detailed account, but it should get the thought across.

Your driver determines what amperage to feed your motor at any given moment. The max amperage to your motor is determined by the Limit resistor size, mounted on the driver, depending on your motor/driver amperage limits. There are formulas on the Zone that will let you determine what resistance and wattage to use for this.

As far as voltage goes, two considerations are taken into account.
1) Motor voltage times 20 (max)
2) Max voltage the driver can handle including any back emf for when the motor stops.

Not all drives are equal in the emf department and should be checked out for what they can handle.

Either way, the lesser of the two criteria is your maximum voltage.

Others, who are more experienced will, I hope, chime in and fix my errors or confirm them. We have some real smart guys on here. Between them almost any correct answer can be gotten.

Al

P.S. The Gecko web site has some very useful information on what you are asking.

Allow 4 amps per motor, and for the voltage, use 32 times the square root of the motors inductance in mH.

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Originally Posted by ger21

Allow 4 amps per motor, and for the voltage, use 32 times the square root of the motors inductance in mH.

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That's right. Total motor rated amps times .67 = total PSU amps. And you might want to allow enough amps for a fourth axis, even if you don't have one now.

Try to get PSU voltage as close to max voltage as possible. It's okay if you are a volt or two over. Most power supplies lose a few volts when under load.

CR.

Quote:

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Originally Posted by ger21

Allow 4 amps per motor, and for the voltage, use 32 times the square root of the motors inductance in mH.

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Gerri, i'm confused, you say allow 4 amps per motor. I was under the impression, that say for instance, these motors are rated at 5 amps, then I will use 10 amps x 3(motors), (being they are 2 phase) then take .67 (percent of that to arrive at the amperage i need. Is this correct?

Also, are you referring to the combined inductance of all three motors?

Please correct me where ever i'm wrong.
All help appreciated!

Thanks
mark

Quote:

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Originally Posted by m8kingit

Gerri, i'm confused, you say allow 4 amps per motor. I was under the impression, that say for instance, these motors are rated at 5 amps, then I will use 10 amps x 3(motors), (being they are 2 phase) then take .67 (percent of that to arrive at the amperage i need. Is this correct?

Also, are you referring to the combined inductance of all three motors?

Please correct me where ever i'm wrong.
All help appreciated!

Thanks
mark

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If the motor current rating is unipolar current, then

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The easiest factor in choosing a power supply is its current rating. The current rating of the supply
is based on your motor choice. The drive will always draw less than 2/3 of the motor’s rated
current when it is parallel (or half-winding) connected and 1/3 of the motor’s rated current when it
is series (or full-winding) connected. That is to say, a 6 Amp / phase motor will require a 4 Amp
rated supply when parallel connected and a 2 Amp rated supply when series connected. If
multiple motors and drives are used, add the current requirements of each to arrive at the total
power supply current rating.

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I think this document was written by Marris Freimanis of Geckodrive but I am not sure.

Alan

No, not the combined inductance. If the motors are all the same, then multiplying 32 times the square root of ONE motor's inductance will determine the best/max voltage.

If the motors are different, then you can only go as high as the lowest voltage motor. The best all around motor wiring method is Bipolar Parallel.

CR.

For an interesting experiment hookup 3 or 4 motors on a table under normal cutting conditions and insert a DC ammeter in the Supply line and measure RMS (average) current. You will then start to wonder where all of the numbers and rule of thumb come from. Since amps don't do work (only Horsepower, Watt hours, BTU's and similar values do) you have to know the applied voltage and the duty cycle. You also have to have all of the motors at full stall before you see anything near their rated amps.

The inductance times some number to get volts may define the magnetic force in the motor structure BUT that voltage may cause excessive motor heating from IR losses...in short the motor wiring may not be able to handle the loses and magnetic core saturation can be a factor of too many volts. If you saturate a magnetic core (transformer, motor, etc) it looks like a short circuit to the drive force.

Remember that big power transfomers and caps have a flywheel effect so can supply peak currents of higher values providing the duty cycle is not 100%.

TOM CAUDLE
www.CandCNC.com

Quote:

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Originally Posted by m8kingit

Gerri, i'm confused, you say allow 4 amps per motor. I was under the impression, that say for instance, these motors are rated at 5 amps, then I will use 10 amps x 3(motors), (being they are 2 phase) then take .67 (percent of that to arrive at the amperage i need. Is this correct?

Also, are you referring to the combined inductance of all three motors?

Please correct me where ever i'm wrong.
All help appreciated!

Thanks
mark

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The 4 amps per motor was for your 6 amp motors. Take 2/3 of each motors current and add them up, so, for three 6 amp motors, you need 12 amps. This is assuming either 4 wire motors ar 8 wire motors wired bipolar parallel. If using them bipolar series, you only need 1/3.

And as was mentioned, use the inductance of one motor when calculating the voltage.