Re: Balancing rpc's.
- From: pentagrid@xxxxxxxxx
- Date: Fri, 08 Aug 2008 09:45:57 +0100
On Thu, 7 Aug 2008 08:27:58 -0400, "DrollTroll"
<fitcat@xxxxxxxxxxxxx> wrote:
"Jon Elson" <elson@xxxxxxxxxxxxxxxx> wrote in message
news:hICdndXb2qJJ6AfVnZ2dnUVZ_tjinZ2d@xxxxxxxxxxxxxxx
DrollTroll wrote:
Awl--Balanced current in the MACHINE TOOL motor, or in the RPC? You don't care
What's more important when adjusting capacitor values:
Getting equal voltage between each pair of legs, or balanced current in
each leg? Seems like you can't have both.
about balanced current in the RPC, and it won't be balanced, because each
winding is doing a different job. (The 240 V line windings are running
the RPC, the 3rd leg is providing the shifted phase.)
What you are concerned about is getting roughly equal current in the
machine tool motor. And, equal voltages on the line DO NOT guarantee that
you have the proper phase shift of the generated leg. The voltage from
the 240 V mains ceter tap (neutral) to the generated leg should be at 90
degrees to the mains, and about 207 V. Without an oscilloscope, however,
it is pretty hard to measure phase shifts. If the machine tool motor
draws equal current on all 3 legs, with roughly balanced L-L voltages,
then your phase shift is right.
As long as the current in the generated leg of the RPC motor is not
excessive, I'd ignore it. If it IS much above the nameplate rated amps,
then you need to reduce the caps to bring the current down.
At equalized voltage, the current through the generated leg is throughI'm not too surprised, but what is "through the roof"? twice rated
the roof.
current? More?
At equalized current, if line L1-L2 is 240, then L1-L3 and L2-L3 areWhat are you complaining about? That is quite good for an RPC.
about 230-233, which is not so bad.
Well, I don't know enough to know if I *should* be complaining or not.... so
I just complain reflexively.... :)
I bought a bunch of cheapie analog VOMs (identical), and now have them
permanently wired among the three legs--convenient. I have clamp-on
ammeters now, but I also plan to get 3 cheapie digital clampons (about $20
each, if I'm lucky), and have them installed permantly as well.
A 10 hp baldor Super E will read over 20 A in the 3rd leg, with closely
matched voltages (more capacitance)--idler only.
With matched currents (much less capacitance), it's as above, 240, 230-233.
I will wire up 3 ph resistive loads today (5 A heating elements), in both
delta and wye, and adjust the caps see how the currents/voltages respond *in
the resistive load*. Is this a good test method? Will the delta/wye
config of the load affect things?
Also, what do you think of the comment to add capacitance between L1 and L2,
and that the current through this capacitor "is the only effective
measurement you can get with a clamp-on ammeter in a rpc"?
I did notice that 25 uF between L1 and L2 drops the current in L1 and L2 by
about 2 amps, without affecting the current in L3 (no load).
At the moment, as you have described it, you are running
the idler alone. Matched current setup is right for most
efficient running of an idler alone running light. Matched
current is when the current through the correction capacitance to
the third motor terminal is roughly equal to the average of the
other two motor currents.
This is NOT optimum when it is then connected to one one
or more loaded or unloaded additional motors.
Each added motor even if running light needs the
capacitance to be increased and every increase in mechanical load
on any of the motors requires further increase.
This means that your current balance is best setup with
your load motor(s) connected and preferably working into their
worst case full mechanical load.
Measurements driving a resistive dummy load are of no
help because of the very different characteristics of a motor
load.
While ideally the correction measurements should be made
with the load motors under full load, correction made with partly
or completely unloaded motors is usually acceptable.
This is where the size of your idler becomes important. If
your idler is only the same size as the load motor, addition of
the load motor running light DOUBLES the optimum capacitor size
with a further increase needed for full load operation. If your
idler HP is 5 times the load motor the optimum change drops to
about 20% and this makes the setup adjustment correspondingly
less sensitive.
Your operating setup now consists of two or more motors
all directly connected in parallel.Because of this, as far as
correction is concerned,there's no distinction between idler and
load motor, single motor or multiple motors. It's the total HP in
the system that matters.
With your load motor(s) switched off and the idler running
light the idler will be substantially overcorrected with
excessive phantom phase current. However this is mainly reactive
wattless current with only a small I squared R (i.e. real power
dissipation) component. Provided it remains below idler rated
full load current this is OK. It can be avoided by splitting the
correction capacitance and giving each load motor its own
correction capacitance but it's rarely worth the additional
complication.
Because of the non unity power factor of the motor load
the current drawn from the single phase input is unnecessarily
high and can be reduced by adding power factor correction
capacitance directly across the single phase line input. Although
this connects to L1 & L2 this connection distinction is important
because current balance measurements must only measure the
current into the motor terminals - it must NOT include the power
factor correction current which is drawn by this capacitor.
directly from the supply.
Jim
.
- References:
- Balancing rpc's.
- From: DrollTroll
- Re: Balancing rpc's.
- From: Jon Elson
- Re: Balancing rpc's.
- From: DrollTroll
- Balancing rpc's.
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