Re: Power supply for Servos

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "turbulatordude"
<dave_mucha@y...> wrote:

>
> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "cnc_4_me" <cnc4me@g...>
> wrote:
> >
> > These servo suppplys are a little hard to nail down. A couple of
> > weeks ago i tried to get a formula for making one. Both Mariss

and

> > Jon Elson gave me advice for building a supply. If i understood
> > them. The straight forward approach means you need the wattage
> > rating of the servo. Second best is the constant current spec

and

> > maybe Ke constant. Third worst is the voltage and current rating
> like
> > you gave.
> >
> > Do you have any other specs on your motors...
>
>
> PACIFIC SCIENTIFIC, Permanent magnet DC motor.
> 1 1/2 hp
> 110 VDC
> 11.6 Amps
> 3,200 RPM
> Continous Duty
>
>
> If I look at the wattage of the servo, 110VDC @ 11.6 amps, that is
> 1,276 watts.
>

This is the same mistake I was making. As Jon pointed out, if 1276
watts is being turned into heat in the motor then no power is coming
out the shaft. Or in other words, this is supposed to be a motor,
not a toaster oven.

The reason the straight calc of E x I = P does not work here is
because of back EMF. This is also what makes it so difficult to
calculate exactly what power supply is needed. That is why I asked
for the servo wattage rating. That rating for a motor this size is
going to be 125 to 250 watts. I think it comes from the armature
resistance and current through the motor. We will have to make up
some numbers because we do not have them. Let's say the armature
resistance is between 1 and 2 ohm with 12 amps.

P = I squared x R
12 squared x 1ohm = 144 watts
12 squared x 2ohm = 288 watts

So you are probably somewhere between these numbers. You would them
multiply times each axis you have. Lets use 200 watts

200W x 3axis = 600 watts

Now we have to size transformer to deliver 600W. I = P/E I=
600w/80v = 7.5 amps

Now remember when we use a bridge rectifier and capacitor to make DC
the actual current drawn from transformer is 1.8 x desired current.
So 1.8 x 7.5A = 13.5A
Now we need voltage. Transformer voltage = .707 x desired DC
voltage
.707 x 80 = 56.6 Vac. So transformer power in watts = 13.5A x
56.6V = 764 watts

Since we guessed at some of these values and since 1kw is a standard
value I would bump it up to that value.

For capacitor size I would go with the Gecko stepper power supply
guide. And do not forget to watch your capacitor ripple current
rating per my stepper power supply write up.

This is the best guess I can make for a servo power supply design.
If anyone else thinks I made mistakes or has other ideas please
respond.

Wally

> But, if I look at it with the max voltage from my power supply, it

is

> 76 volts time 11.6 amps = 881 watts.
>
> For 881, I could use 2 transformers to get 2,000 watts, that is

light

> for one calculation, and heavy for the other.
>
> I had thought there was some number one could use for duty cycle as
> the motors are not driven DC, but pulsed.....
>
> I'll go back and re-read Jon's post. IIRC, it was more guidelinesh
> than calculations.
>
> Dave