Re: Power supply for Servos

>
> 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

I do not think back emf applies here. The only thing you have to
worry about is regenerative braking and line regulation. From what
Jon Elson posted, he said 2V is about the max you have to worry about
for regenerative braking. Line regulation will probably be under 5v.

Your 100V caps should be ok.

Wally





--- 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:
> >
> > --- 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