On 2008-10-06, DrollTroll wrote:

GeoLane at PTD dot NET wrote in message
...
I'll be picking up my Bridgeport 2 HP 2J in a couple of weeks, so
it's time to start working on a rotary phase converter. I know that
my ideal size is 3 HP for the converter. There's a possibility of
getting two 2 HP motors incredibly cheap. Can you gang them together,
or is there a disadvantage to doing that?

I"m early in my search, so I don't know if there are any reasonably
priced 3 HP motors out there in our rural area at the present time.

As Ig stated, there are numerous advantages, which hopefully he'll elaborate
on.

I will list them, and I know that you mentioned quite a few.

1. Being able to use what you have
2. Lower startup current
3. Ability to add extra capacitors at the same time as switching in
the other motor, for perfectly balanced third phase

My phase converter is one of relatively few homemade things that I am
actually very happy with and that does not give me any trouble.

i

I see the advantages as lower start up cost, lower running cost, greater
flexibility (run as many as you need), easier start up (won't brown out the
neighborhood), AND, I suspect, overall better power regulation, and, all
things being equal, quieter operation. Also less heat buildup.

I have an arsenal of about 6 idlers.

In fact, I see no advantages to one big idler over numerous small ones,
except one: simpler wiring.
Others may disagree, however.

Best to separate start up caps from run caps, and even better to have a bank
of switchable caps for best load/voltage matching.

With a bank of switchable caps (to both L1-L3 and L2-L3, (L1-L2=line)), you
can use the caps you need to start up, then switch those out, switch in the
run caps predetermined for the idlers chosen, *and* the anticipated load.
Stage the idlers in, one by one, as needed. Only need start up caps for the
first idler started.

Get 3 cheap digital multimeters from HF (also 3 of their dert cheap clamp-on
multimeters) to let you observe voltage and current in all three legs
simultaneously. Really helps in the tuning process.

Another poss. superior alternative for your BP is a VFD, esp. if the head is
belt, and not variable speed. This way, you won't need an rpc, at least not
for this machine.


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