Re: How many kva for a control transformer...?

Lance,

I think you may be mixing metaphors.

I can see seperating 60 hz A.C. from D.C. I can see isolating
noisemakers (like vfd's) from things that can be affected. It is
vital that proper attention be shown to shielding, transient
suppression and proper grounding techniques. The is no reason that I
can think of to isolate single phase AC from 3 phase AC. Assuming
your 3 phase feed comes from the same panel as your single phase, Jon
is right, why not use a 110v circuit from the same panel (can be
pulled in same conduit as three phase) instead of adding the
complexity, heat losses, expense and "real estate" of an additional
transformer?

I would put a DECENT surge suppressor/EMI filter on where the 110
feed comes in (I use Corcom filters, then a Tripplite Isobar on
mine). This also helps cut down on "backfed" crap from the VFD
(loads of filtering and transient suppression on that as well).

Just because you CAN put a transformer in, doesn't mean you have to
<G>

Jeff



--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Lance Hopper"
<snaggletto@c...> wrote:

>
> Hey,
> Yes I know, but this machine will actually be in an industrial
> environment. My goal is to have only the 220 3ph coming into the
> machine as power. Therefore, all lower voltages will be derived

from

> the 220 3ph input power.
>
>
> I've got 2x 1 kva transformers with 120 secondaries, and then

I've

> got a single 2 kva with 120 secondary.
>
> I've got 2 separate NEMA 3R enclosures. My plan is to mount all
> the 3ph in one and 1ph in the other. These will be physically
> separated on the machine, about a 5 ft separation.
>
> 1)... Being that the VFD is the big noise maker, should I mount

the

> control transformer(s) in the 3ph box with the VFD and run the 120
> 1ph across to the 1ph enclosure?. Or should I run the 3ph across

to

> the 1ph enclosure and mount the control transformer(s) in the 1ph

box?

>
>
> Also note, there will be 120 back to the 3ph box for the contactor
> coils.
>
> 2)... Then there is the Digispeed, PWM to analog converter, which

is

> used to control the VFD. Somehow these low voltage control wires
> will have to cross the ~5ft span to the 3ph box. Which would be

more

> noise tolerant, the PWM side or the analog output side? (IE should
> the Digispeed be mounted in the 1ph box with the breakout boards or
> in the 3ph box with the VFD it will control?)
>
>
> Thanks for any info.
>
> > 36 V 10 A is only 360 VA. You can get 1700 VA out of a 15 A 120 V
> > outlet.
> >
> > > Wouldn't it be beneficial, electrical isolation-wise, to have
> > >separate transformers supplying the servo drives and for the PC
> and
> > >control components? Or, should I just go with one big

transformer

> to
> > >power all the 120VAC components?
> > >
> > >
> > >
> > Possibly, but most computers, etc. are pretty tolerant of noisy
> power.
> > Actually, the VFD is the real culprit, the servo drives are going
> to be
> > much more benign.
> >
> > Jon