Re: Transformer sizing

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Jon Elson <elson@p...> wrote:

> Hi Jon

Thanks for the input. Yes, the drives are Rutex. I will do the
mechanical calculation as you sugested and I will install a 2KVA
transformer.

Craig

>
> bolttjza wrote:
>
> >Hi All,
> >
> >I need some help in sizing a power supply for a Bridgeport series
> >1/EMC conversion. I will be driving each axis with 140V SEM PMDC
> >servo motor of which the stall current is 26A. The drive powering
> >these motors are capable of supplying 100V @ 20A. I will drive a

5mm

> >pitch ball screw thourgh 2:1 reduction ratio. What VA transformer
> >should I purchase? Am I ever likley to use 100V @ 20A? Should the
> >reduction ratio be bigger ie 3:1 or 4:1?
> >
> >
> First, you need to get the torque/Amp constant (Kt) for your motor.
> Figuring the desired linear force on the table back through the

ballscrew

> and belt drive, you can figure out the torque you need, and then the
> drive ratio that is most appropriate. Then, check the top rapid

feed rate

> you want, and make sure the motor speed at 100 V will reach that.
>
> Sounds like you'll be using Rutex drives?
>
> Note that these motors are really strong, if you apply 26 A it may

break

> belts even 3/4" wide! So, check the specs on the belts you plan to

use

> to make sure you stay within their ratings. Keep in mind that these
> belts need to be kept REAL tight to prevent creep, and add the

tension

> to the motor force.
>
> As for the transformer, it is very unlikely you will ever run 100 V

and

> 20 A through the motor at the same time, except for a very short

instant

> when accelerating. But, when all 3 motors are accelerating at the

same

> time, the power draw will be substatial. Note that PWM servo drives
> are switching power supplies, and so power in roughly equals power

out.

> So, if the drive is delivering 20 A at 5 V out, it will only need

100 W

> at the input (plus losses) and so will only draw 1 A from the power

supply!

>
> A 1 KVA transformer soulds like a good minimum. You could
> probably go to 1.5 or even 2 KVA. There is no reason to go above
> that, however, for a milling machine. Even though the theoretical
> power draw could be 6 KVA (100V x 20A x 3 axes) you just never
> do that except for instantaneous surges . Make sure there is plenty
> of filter capacitor for energy storage, and that the rectifiers are
> sufficient. I might go with 50 A diodes for this.
>
> Jon