Re: variable transformer for dc power supply

> Mike,
>
> I designed industrial machines with DC. Like I said we did it both

ways. I

> liked it more when we didn't let the DC float. On the larger

voltage

> systems we ALWAYS grounded the DC bus from a center tap on the

transformer.

> Those were 600 volt DC systems so each side was either +300 VDC or -

300 VDC.

> If we didn't ground the center tap and let it float through

isolation, then

> whenever a DC wire touched the machine frame the other side was at

a full

> 600 volts relative to ground. If we did ground the center tap, then
> whenever a DC wire touched the machine frame we would blow a

breaker and

> everything stopped as it should.

Hi Greg,
In a perfect world that is the case. But things fail, grounds come
loose through, use, stress, vibration, etc.

And now theirs an added factor, this is a group of do it yourselfers.
In a manufactured machine, power supply... the user isn't building,
testing, troubleshooting it himself, many times not even installing
or wiring it himself. The user/builders here usually have less than
complete knowledge of electrical wiring and safety precautions. The
lack of and distribution of knowledge is why this group thrives. The
chassis, wiring, equipment... may not be properly grounded to begin
with. Lets face it, a 3 phase motor runs just fine with only 3 hot
wires, no ground is needed for it to run, and the user may not
realize the potential danger. I've even heard people say "its sitting
on the ground (concrete) so it must be grounded" or just assume its
grounded.

A typical homebrew CNC setup can have a power supply in one box, the
controller circuitry in another, and the machine sitting next to
them. It also generally has a seperate computer connected to the
controller box. Now we have power and grounds comming from 3 sources.
Also the connections most likely are via plugs, small plugs, possibly
with no locking mechamisms, that the user/builder solders. The solder
joints of the inexperienced usually aren't very mechanically sound,
through flexing, use, stress, vibration.... they can come loose.
Small plugs may be salvage from other equipment and not make good
connections... The equipment that you design may have heavy
grounding straps connecting everything togeather, chassis to chassis.
Theres a good chace that a homebrew CNC setup does not.

Then theres the building and troubleshooting phase, an open
powersupply not hooked to the rest of the equipment, in a metal
enclosure...
Is the chassis strapped to ground? (A home brewer probably didn't
think or know to)
Was an extension cord, possibly faulty or without ground used?
Are all connections tight?
Is a wire pinched to the chassis?
Did a wire come loose an lay against the chassis unseen?
Is it even wired correctly?
Are all the parts in a new or known good condition?


There is a big difference in being an experienced equipment designer
in an industrial enviroment using all new, or known good parts and
building your first CNC setup, or even a simple power supply at home
or in your home shop using scrounged or bought used/surplus parts to
save a few $$$.

> To my way of thinking, isolation does not make the system safer.

I hope i've helped you see things from a different perspective as
applies to this group.


In

> particular, don't think that by isolating the system you can be

less careful

> with the DC wiring.

Of course not. A lot of the people here are machinists, and
building/wiring their CNC setup should take just as much care as they
take to turn, mill, grind... their work to precision. Do both
striving for Quality and Safety.


Wires should never get loose in the system and any

> loose wire that contacts the machine should shut the machine down,

not

> electrify it.

A sad fact of life, especially for the inexperienced homebrewer, is
that things are not always as they should be.

> For anyone that is really concerned with safety, just get a $6 GFI

and wire

> it into the main supply of the machine. This is a standard

industrial

> practice on 115 vac industrial circuits and UL encourages it.

GFIs are great, but everyone doesn't have or use them. Also the
Computer, powersupply... anything connected to AC that you can touch
need to have a GFIs their outlets or it defeats their purpose.


The bottom line here is that whats posted has to be for everyones
safety.

mike