I finally got a 10 hp air compressor running. I have a 10 hp rated
contactor for it. But my Idler motor for my phase converter is 15 hp.
I have used a momentary switch in the past to start it but I was
reading that a contactor is preferred. Since I am starting the 15 hp
converter first and it is only idling do I need a 15 hp rated
contactor? Or since it will eventially see the additional 10hp
compressor (once the converter is running) do I need a 25 hp rated
contactor?

I want my converter to cycle on and off outside my compressor range so
I was looking at some type of relay and protecting the motors.

On 2 Aug 2005 05:20:06 -0700, "gtslabs" wrote:

I finally got a 10 hp air compressor running. I have a 10 hp rated
contactor for it. But my Idler motor for my phase converter is 15 hp.
I have used a momentary switch in the past to start it but I was
reading that a contactor is preferred. Since I am starting the 15 hp
converter first and it is only idling do I need a 15 hp rated
contactor? Or since it will eventially see the additional 10hp
compressor (once the converter is running) do I need a 25 hp rated
contactor?

I want my converter to cycle on and off outside my compressor range so
I was looking at some type of relay and protecting the motors.
The thing about induction motors that makes them need the proper
contactor is the starting current, not the running current. This
current is at the maximum when the contacts first close. It can three
times the rated current of the motor. This is why big contactors are
used. If you spin up the motor with a smaller pony motor first then
the starting current will be lower.
ERS

I think the answer depends on how you are starting the 15HP idler. The
issue isn't the fact that the motor says "15HP" on it, the issue is how
much current does it draw when starting. If you're using capacitor
start, you get a pretty large surge current, related to capacitor size,
not HP itself. I know the caps are sized based on HP, but I'm trying to
explain what the real issue is.

In your situation I'd have the 10HP contactor between the 15HP idler and
the compressor. This way you start the idler and let it run. The
compressor would start via its contactor exactly as if it were on power
company 3 phase (assuming your third leg doesn't sag...).

Steve

gtslabs wrote:

I finally got a 10 hp air compressor running. I have a 10 hp rated
contactor for it. But my Idler motor for my phase converter is 15 hp.
I have used a momentary switch in the past to start it but I was
reading that a contactor is preferred. Since I am starting the 15 hp
converter first and it is only idling do I need a 15 hp rated
contactor? Or since it will eventially see the additional 10hp
compressor (once the converter is running) do I need a 25 hp rated
contactor?

I want my converter to cycle on and off outside my compressor range so
I was looking at some type of relay and protecting the motors.

Ignoramus6455 wrote:

Personally, I am considering making a two stage phase converter.

First it would start a 10 HP idler, and then it once the idler is up
to speed, it would start another 7.5 HP idler. Much easier on the
electrical circuit. The reason for this is that I have a 7.5 HP motor
that I do not know what to do with, caps, contactors etc. Any cost to
this project, at this point, will be my time.

i



This is how I had my converter set up (sold when I moved). I had a 3HP
capacitor start converter. For my one larger machine, I then would
switch in a 7.5HP idler with some additional run caps. It started just
fine off the 3HP idler, since it was starting no-load. Two stage
starting, but way easier than what I had before--pull start a 7.5HP with
a rope. I got pretty tired of that.

Steve

This is what I am doing now. However I was wondering if I should use a
contactor just to start the 15hp converter.
I am using 7 108-130 uF capacitors to start it up in about 2 seconds
with the momentary button.

Ignoramus6455 wrote:
On 2 Aug 2005 05:20:06 -0700, gtslabs wrote:

I finally got a 10 hp air compressor running. I have a 10 hp rated
contactor for it. But my Idler motor for my phase converter is 15 hp.
I have used a momentary switch in the past to start it but I was
reading that a contactor is preferred. Since I am starting the 15 hp
converter first and it is only idling do I need a 15 hp rated
contactor? Or since it will eventially see the additional 10hp
compressor (once the converter is running) do I need a 25 hp rated
contactor?


Your maximum load is about 10 HP, and your maximum starting load is 15
HP. That makes me believe that 15 HP rated contactor would be
appropriate. I am not, by any means, an exp[ert on this.

You can start a motor, especially with no load, on a much smaller
starter than is recommended- they're meant to deal with a unit putting
out the full rated power. In truth, I think the hardest part of a
starter's job is shutting down under load, you'll get arcing then.

John

Ignoramus6455 wrote:
On Tue, 02 Aug 2005 20:02:41 -0400, Steve Smith wrote:


Ignoramus6455 wrote:


Personally, I am considering making a two stage phase converter.

First it would start a 10 HP idler, and then it once the idler is up
to speed, it would start another 7.5 HP idler. Much easier on the
electrical circuit. The reason for this is that I have a 7.5 HP motor
that I do not know what to do with, caps, contactors etc. Any cost to
this project, at this point, will be my time.

i




This is how I had my converter set up (sold when I moved). I had a 3HP
capacitor start converter. For my one larger machine, I then would
switch in a 7.5HP idler with some additional run caps. It started just
fine off the 3HP idler, since it was starting no-load. Two stage
starting, but way easier than what I had before--pull start a 7.5HP with
a rope. I got pretty tired of that.


That's very encouraging. Thank you Steve. I have 3 more 92 mF run
capacitors that I can use as run caps, and plenty of contactors.

i

Caps aren't too pricy, although you got a good deal on the ones you
have. Surplus Center has good prices on them, and if you need a bunch of
capacitance for a big unit you can do the trick with DC caps hooked back
to back- IIRC, they are wired + to +, but look it up to be sure.

John