# Thread: Power supply for 3 x (75V, 6A) ac servo motors

1. ## Power supply for 3 x (75V, 6A) ac servo motors

Hello people,

If i want to power 3 servo motors with a continues current of 2A
and a max current of 6A. (motors run on 75V)

Can i just say 3 x 6A = 18A needed?

Or can i use a lower amerage power supply (2/3) x 18 = 12A...?
(that is done with steppers sometimes)

could somebody explain this to me?

Greetz,

Roy

2. Normally your system should be designed and motors sized to operate in the continuous area of operation.
Are you considering a Linear supply or switching type?
Linear are somewhat more tolerant of intermittent current peaks, but with a proper system you normally would not even see the constant current rating x3 at any given instant in normal operation.
Al.

3. Hey Al,

you are right and got me thinking again..
Designed the servo system for continues operation.
Calculated the linear and rotational inertia of moving parts and ballscrews incl, bearing resistance etc. Also made a calculation for the tangential force needed for the milling operation. (max. 6mm endmill diameter)

During maximum material removal and acceleration of table, the needed torque at the servo is at nominal level, so that is ok.

So i only need a FOS to make sure, think 1,5 will do.

What about the linear or switching power supply. for the servo`s i will use a linear i think.

But i also got another question:
For the spindle motor i use a nice small Maxon motor (36V, 3,6A nominal)
Here the drive uses the full Voltage from the power supply and this defines the max. spindle rpm. (speed mode with only hall feedback)
I want the spindle speed at the right rpm, for this case i would like to use a switching power supply and this power supply defines my spindle speed, but now you say that a rapid change in rpm cann cause trouble. How about that?

If i over design the switching power supply (amps), would this give less trouble?

4. Is this a DC brushed Maxon motor or BLDC?
Are you using the hall feedback just for tach purposes?
What controller are you using?
You could use a 40vdc supply and just regulate the maximum rpm, if using a tach, then you should have rpm control.
I find linear supplies are generally more rugged and easier to fix.
Over designing current wise is no problem, just less efficient, power wise.
Al.

5. Read the manual again and dont know for sure how the drive works.

I want to use a max. rpm of 30.000 rpm.
On the drive i can select a rpm range from 1000 to 30.000 rpm by a dipswitch.

The motor is 737 rpm/V, thus:
30.000/737=40,7V

The drive can control the rpm by a analog input from 0.5 to 5 V

If i would use a power supply of 48V (drive max is 50V), would this result in a max speed of 30.000 rpm? Since this is the max with the dipswitch setting for 1 pole pair motor, or will the max rpm be 48x737=35300rpm

Think i can just use a switching 48V supply (10A, drives max) and use the dipswitch till 30.000rpm, then the rpm will never be higher than 30.000rpm?

Page 5 gives this:
Note:
The power supply must be able to buffer the back-fed energy from STOP or di-rection change operations e.g. in a capacitor.
With electronically stabilized power supply units it must be ensured, that the over-current protection responds in no operating condition.

Meaning that a Switching power supply can be used..?

6. As far as I can tell by the PDF's although the drive will operate a 1 pole pair motor up to 60,000rpm, the EC-32 motor itself is rated for <15,000 rpm?
What it is saying is that if you use a switching supply and back EMF exceeds the tolerance of the supply it should be capable of shutting down.
One more reason to use a linear?
Al.

7. The motor is rated to max. 25.000 rpm, atleast, the bearings.
Maxon told me 30.000 rpm would be possible, but bearing life will reduce.

If i use the 30.000 setting instead of 60.000, the drive will stop at 30.000rpm even if the voltage is higher. (hope so)

Dont understand what the EMF is in regards to a switching power supply.

8. Energy reflected back from the motor in the deceleration (generation) mode that may cause the effect stated on the page #5 note you show and mentions the supply must be able to cope with it.
Al.

9. Will look into it a little bit further and contact maxon.
Think then that the linear supply will be the best solution.\\Greetz,

Roy

10. Out of interest what is the spindle going on or being used for?
Al.

11. A nice small benchtop machine (outer dimensions 450x450x450mm)
xyz cube 200x100x90mm, base of epoxy granite (German EG shop will fill the mould with a good EG and vibrate it)
Got almost all the components except the steel parts.

12. Awesome, unique looking build. Are you documenting it anywhere? After a lot reading on cnczone, and I wanted to build something similar - a small, but rigid, machine with an EG base/frame.

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