On Sat, 28 Aug 2004 18:47:16 +0800, Old Nick wrote:
So. I have a 20HP motor...3PH. If I ran it up to speed, using a pony,
I can then expect 80 amps, until I get it tuned? I just don't HAVE 80
Amps.
Well, you don't *have* to tune it. Motor loads in general don't care much
if the phase to phase voltage balance is spot on or not. But you *will* want
to use power factor correction to get the reactive currents seen by your
supply wiring and breaker panel down into a more comfortable region.
Once tuned, what current would it use? How much more would it use than
say, a 10HP 3PH as an idler, in and of itself (not under load). If I
have some idea of that, then I can say well, I need so much dfor idle,
and so much for the actual load.
Ok, my tuned 20 HP idler consumes less than 400 watts no load. In other
words, the *real* current drawn by it to keep it running is under 2 amps. With
a fully loaded 7.5 HP load motor being driven by the rotary, total *real* current
draw from the wall is just over 25 amps, so it runs nicely on a 30A motor
rated breaker in the panel.
Now without power factor correction, the imaginary current draw is about
60 amps, so total draw, real and imaginary, with a 7.5 HP load motor is
around 67 amps. (You're dealing with vectors here, so real and imaginary
currents have to vector sum rather than just using simple addition.)
If you don't use power factor correction, you're going to need a breaker,
and associated wiring, heavy enough to handle that 67 amp draw. So use
power factor correction.
The size of the power factor correction capacitor is a function of the
idler motor impedance, which will vary from one idler motor to another
depending on internal motor construction factors not under your control,
how well the phases are balanced, which will vary depending on how
much capacitance you use to balance the phases (good balance here
will reduce power factor, and hence the needed size of the power factor
correction capacitor), and the real load power, which will vary depending
on the size and loading of the load motor(s) being driven (this has only
a small effect when the idler motor is substantially larger than the load
motor, so it can normally be ignored).
In short, you have to experiment to find the right size power factor
correction capacitor for your particular setup. Just keep adding
capacitance across the 1 ph line while monitoring an amp clamp
meter *on the breaker side of the line* until you see a minimum
current value. (Currents on the motor side of the power factor
correction capacitor won't decrease.) For a 20 HP idler, start
with a value of about 200 uF, then adjust from there.
Current will decrease as you increase capacitance until you reach
a minimum current, then it will increase again as the load becomes
capacitive rather than inductive. I like to pick a power factor capacitor
value that stays just a little on the inductive side of minimum. That
avoids large voltage excursions on the load side with changes in load.
(Load voltage regulation becomes touchy right at mimimum.)
You'll need a box full of capacitors to make this adjustment. You
can often get an assortment of used run capacitors from the back
yard of an industrial air conditioning place for the asking, or for a
nominal charge. These capacitors don't normally wear out (oil filled),
unlike start capacitors (which you shouldn't use), so good used ones
are fine. The A/C places generally use new capacitors when they
do a motor replacement, and bill the customer, so they wind up with
a bunch of good used ones laying around. Since disposing of them
requires that they be treated as hazardous waste, they're usually
glad to give them to you.
Gary
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