On 2009-01-11, wrote:
On 11 Jan 2009 07:03:37 GMT, "DoN. Nichols"
wrote:

On 2009-01-11, willray wrote:

[ ... ]

For current monitoring, I've stuffed 3x 100:5 current
transformers into the 3P subpanel, and brought them out
to current-sensing resistors.

Good -- that gives you isolation.

For voltage monitoring I need voltage in the range -1V -- 1V
to match my scope inputs, so I need to build a voltage-divider.
This would be no big deal, except I just tossed myself a puzzler
when getting ready to solder the bits together.

[ ... ]

I hadn't originally thought of using the "Wye connected" meter
setup, but I can't currently think of a downside. ?I'm
wondering if anyone who does this stuff more regularly might
know some reason not to do this, that I haven't thought of.

Yes -- one very good reason -- have you tried measuring the
voltage from ground to the center point? I'll bet that it is a nasty
range above ground, since you have no isolation between the incoming
power line and the rotary converter.

Get three identical filament transformers with input voltage
appropriate to the leg-to-leg voltage. You can then tie one side of
each secondary to ground and safely measure voltages (taking into
account the ratio of the transformer, of course.


Hmm - while I'm at it, another question -- Is there any reason
that I can't tie together one leg of each current-sensing
resistor for the current monitor part, and use this as a
"common", for metering purposes?

Yes -- since the current transformers give you isolation.

I'm suspicious that my
scope inputs don't have isolated grounds.

Most don't. The probe ground is connected to the safety ground
pin on the power cord.

You want the isolation that the current transformers give you,
and similar isolation on the voltage side with the filament
transformers. (Those used to be cheap enough and common enough so you
should be able to pick up three identical ones.

[ ... ]

I see you're aiming for the luxury of both current and
voltage measurement.

When there is significant unbalance, the apparent 3 phase
balance of the current set and the voltage set can be very
different.

With both sets of figures available you may be tempted to
aim for a rough average betweeen voltage and current balance.

Because motors are essentially current driven devices
this is not the way. Balanced current should be your aim even if
this results in larger voltage unbalance.

That depends. Yes, if a motor is the only load. But if it is
also powering something like a Bridgeport BOSS-3 through BOSS-6, a
voltage imbalance will pop stepper driver power transistors like
popocorn on the opening night of a big movie. :-)

The transistors used in these systems were just barely high
enough voltage rating to handle what was being applied. (Of course,
really high voltage power transistors were very rare and very expensive
when the system was designed. I think that the BOSS-3 hit the streets
about 1977. :-)

If you're using such a machine, it might be good to use two
rotary converters -- one tuned for current balance for the spindle
motor, and one tuned for voltage balance for the electronics boxes.

Enjoy,
DoN.

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