Re: Power supply for Servos

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "cnc_4_me" <cnc4me@g...>
wrote:

>
> --- 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

I double checked and the transformers are 55v at 800 va. Using 800
watts as the number, I worked it opposite what you did.

dropping 1.2 volts for the bridge gives me 53.8 volts. then times
1.414 for DC filter using a Capacitor I wind up with 76 volts.

800 watts / 76 = 10 amps

But, using your numbers, it looks like I would only need one
transformer of 55 volts and 800 VA for three motors.

If that is correct, is there any special precautions for back EMF on
these ?


I was looking at capacitors from Digi-Key, 10k uF at 100 volts.
running at 76 volts, that would offer about a 30% overhead for back
emf.

Dave