[CR250Greg]

I am *attempting* to convert a mill with a 3 axis Grex G100/G340 Servo setup. I have a single 72V 8.5A Continous/38A peak and two 60V 3.5A Continous/20A peak servos.

Once I have the 3 axis setup running, I will be moving towards a 4th axis which utilizes another 60V/3.5 Continous/20A peak servo. I need to make sure that the power supply can supply enough amperage for all four servos while still being safe to use with three for the time being.

The Gecko drives will handle 20A/80V peak, voltage is not so much a concern as I do not have a need for maximum speed.

Would a power supply of 60V between 18-20A work in this situation? I will also need 5V and 12V regulated outputs.

Any input appreciated.
-Greg

[NC Cams]

The only time you would/should ever reach peak current (78 amps) would be if ALL the servos were locked rotor/stalled SIMULTANEOSLY. The chances of that are two, slim and none.

The next problem you have is that the motors have different voltage ratings. if you run at 60 volts, the rated speed of the higher voltage motor will not be reached. If you run at the higher voltage, the lower voltage motors could be oversped or perhaps burned out due to overcurrent from the higher than rated voltage. Running separate P/S's would fix that problem and solve some of your current concerns.

In your combination, if any two servos would be running, you'd see between 7 and 12 amps depending on which two were running. Load would spike this up but not anything of much conseqence.

The only issue of not enough amperage will be at high current demand. Either the motors will not pull rated current and thus rated torque or the voltage of the P/S will dip as you load the supply and it can not supply current in the amount asked for.

As an approximation, a peak current capability of 60%-70% of your absolute max current draw potential should be adequate for most of your needs. More is better of course but it all depends on how much you want to spend. When all things are considered, you will not be disappointed if you build a bigger as opposed to a p/s that is too small.

[CR250Greg]

Ok I understand what you are saying about peak theoretical maximum current draw. I will play it safe and stick within the guidelines put forth in the gecko manual. I realize there is a fine line when dealing with motor drives which I must abide by; the last thing I am intrested in is killing a servo drive. I am less concerned for the servos than I am the drives.

Can you explain the necessary steps to ensure over current protection to the drives?

[Al_The_Man]

If you read up on the engineering section on the A-M-C site it will explain about PS voltage etc, wiring practices etc.
They suggest power supply voltage of 110% to 150% over the rated motor voltage with PWM drives.
The max motor current is usually set to a current limit value in the drive to protect both motor and drive.
Al.

[yantra3d]

And yet another very valuable link! I'm just starting on a power supply for a servo system and this info is super helpful. Thanks again Al...

[CR250Greg]

I tried to digest some of the A-M-C tech literature. I will have to read it over again when I am less tired to gain a better understanding. Now at least I have the general concept of power supply requirements. Electrical parts may not be my forte however I will limp though this if I have to.

I did some further reading on drive protection. From what I have read, I should include slow blow fuses inline with the servo drives power. Should I also look into a soft start circuit for the power supply?


Thanks

[yantra3d]

You might be interested in checking out this guy's power supply setup:

http://www.truetex.com/bpcnc.htm

He is using a torroidal transformer w/ a random ratio (to provide line isolation) then correcting the actual output voltage/current w/ a variable autotransformer (variac) in order to acheive the correct ratio for his system.

0-140V@10AMP variacs are easily available for low prices on the block. I'm seriously considering giving this a try since I can't seem to find the ratio I need for my system.

Does anyone have any experience with this method or any advice for or against it?

[Al_The_Man]

IMO it is far easier to modify a toriodal than go to that bother.
Get one that has a slightly higher secondary and take turns off.
also the beauty of a toroidal is it is easy to add low voltage auxiliary winding like a 5v or even 24v for instance.
There is a seller of Toroidals on ebay that has all kinds of ratings.
Al.

[David_Web]

Just don't runt a steel bolt through a toroidal.
I have heard of people do that and while they figure out why it draws so much power the case will get burning hot.

[Al_The_Man]

A steel bolt through the centre has no effect, in fact it is included in most manufacturers hardware, it is when you complete the path arount the outside that the shorted turn comes into play, as in a U shaped cover/clamp etc.
Al.

[David_Web]

I was reffering to one that goes all the way through the case.
Of cource it has to compleat the circuit.