starting a 15hp 3ph motor

I have run a phase converter for years now. It has a 7.5hp idler motor. I use it to run a lathe (3hp), mill (1hp), 2 grinders, bandsaw, press, and
ironworker. My model has always been to use one machine at a time.
A good friend of mine called yesterday. He got (probably free) a 15hp motor. He wants to bring it over and try to start it in my shop. Just the naked motor, only self-loaded. Do I even have a chance of being able to start it? My phase converter is connected through a 40A breaker via a 60A disconnect box which uses Buss fuses. I don't remember what size fuses are in the disconnect box anymore. I have a little rig I made which uses a 30A disconnect and has a cord with large alligator-type clips which I use to test motors, but I've never even tried to start anything bigger than 7.5hp. I figure we'll fire up my phase converter, maybe adding another motor or two as extra idlers, then spin the big motor with a rope, then quickly connecting it to power. I don't much care if it blows the breaker in my fusebox, but it would be a little bit expensive if it blew a bunch of those Buss fuses. Don't remember but it seems they cost at least $5 each.
My alternatives are to tell him no problem, bring it on and we'll try it, but if any fuses blow, he's got to replace them. Second is to tell him to take the motor to a motor shop (there's a good one I've used before) who ought to spin it up for him and even put it on their megger to look for shorts/opens without charging too much.
Thanks, Grant
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Grant writes: ". . . but I've never even tried to start anything bigger than 7.5hp. I figure we'll fire up my phase converter, maybe adding another motor or two as extra idlers, then spin the big motor with a rope, then quickly connecting it to power. I don't much care if it blows the breaker in my fusebox, but it would be a little bit expensive if it blew a bunch of those Buss fuses. . . ."
Grant, I would be tempted to try it via the rope-start method but with no other motors running. The reason is that starting surge occurs primarily in the single phase leg. Other motors, even though running are still loads across the single phase. IMO, your best bet would be to test start the 15 HP machine on single-phase only. I recommend a few practice spins of the large motor before a "hot" test. The "pre-spun" rotor will effectively reduce starting surge current to a value that should not blow your fuse(s). Do this with the disconnect "single-phased" only to the 15 HP machine, with no other loads. In other words, test with the 15 HP motor connected single-phase.
Bob Swinney

motor.
it?
box
disconnect
has
remember but it seems

but
shorts/opens
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You have a 50% chance that the direction won't be correct. Pre-spinning the motor 'might' increase the starting current as the motor whips around in the opposite direction.
Earle Rich Mont Vernon, NH
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On Fri, 04 Jun 2004 05:58:15 -0700, Grant Erwin

Ive started a 15hp motor on my 5hp rotary..though I did give it a spin with a rope before hitting the power.
Gunner
"A vote for Kerry is a de facto vote for bin Laden." Strider
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Do you have a smaller motor you can use to bring it up to speed? I can't start my 15hp 2-pole motor with the rope method only, I can't get anywhere near enough RPM from the rope to get it in "the zone". I meant to use my 15hp motor as a rotary converter, so I built a capacitor starter. The capacitors bring the motor up to full speed very quickly, but you need enough relays and other parts that this method isn't suitable for a quick bench test. My motor takes 800uF of caps for it to start on it's own from 240V 1-phase without any false starts or prespinning required. If I give the shaft a little spin with a rope, it can get going with as little as 400uF and also pick the direction.
Here is what I ended up building as a converter (see the bottom of the page):
http://www.airraidsirens.com/pistonhorn/fullsize/phase480schematic.gif
Notice that the main start capacitor in the diagram is marked "200uF". This is because the schematic shown is for 480V operation. When I use this converter at 240V, I rearrange the caps for an 800uF bank and change the wiring on the 9-lead motor to its low-voltage configuration. Likewise, in 240V mode the motor-run caps would also be 4 times the capacitance shown.
-Adam
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My phase converter didn't even burp. The 15hp motor started right up and ran smooth as glass. Nice motor. Real quiet.
We first had it wired opposite the way we spun it. When I hit the juice, it slowed and was going to plug reverse smoothly but I got paranoid and killed the power. I bet it would have worked, but we didn't test it and now it's back on it's way to another city.
Thanks, all.
Grant Erwin

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Grant, Do you mean it would start on your rotary without pre-spinning? Bob Swinney

it
killed
motor.
it?
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That's exactly what I meant. After spinning it up and starting it, then we shut it down, stopped the rotor, then from a dead stop just turned on the power as output by my phase converter. Spun up smoothly. Of course, my phase converter has a potential relay and start caps, so if the potential in the 3rd leg dropped too low the start caps would have kicked in. But yes, it started fine.
Grant Erwin
Robert Swinney wrote:

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On Fri, 04 Jun 2004 12:39:43 -0700, Grant Erwin

My 7.5 hp Clausing lathe tosses the 30 amp breakers on my 5hp rotary converter when flipping into reverse. It would probably have smoked your fuses.
Gunner

"A vote for Kerry is a de facto vote for bin Laden." Strider
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it
killed
Hmm....
But once you get a sufficiently bigger one started, if you plug it into reverse the smaller idler is the one that actually ends up reversing, no ???
--

SVL



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Hey Grant,
You may or may not recall my recent "research" in to using a 20HP wound rotor motor as the idler. I had the single phase 220 line (237 measured) to a fused disconnect with 2 fuses of 45 Amp type "D" (dual element slow-blow), and it feeds a 50 amp three phase manual circuit breaker switch. I was able to rope wrap the shaft and "pull-start" it enough to throw the switch. The motor was labeled as drawing 67 Amps at FL, but in fact my Fluke amprobe said it draws over 80Amp at idle. It would run OK for about 2 or 3 minutes, then pop a fuse.
I gave up on it due to the perceived cost to operate, and got a 7-1/2HP instead. Not "In Service" quite yet, but seems to draw about 20 Amps.
I think you should be able to disconnect your idler, and then hook-up and manually rotate the 15HPi and "single phase" start the 15HP for a test on your wiring set-up with-out popping any fuses.
Take care. Let us know what happens.
Brian Lawson, Bothwell, Ontario. XXXXXXXXXXXXXXXXXXXXXXXXXXX
-On Fri, 04 Jun 2004 05:58:15 -0700, Grant Erwin

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    You are measuring the reactive current there -- but that is sufficient to blow your fuses.
    I suspect that if you had put some power-factor compensation capacitors across the line, it would have drawn a lot less current.

    Yep -- one very good reason for power-factor compensation (normally tuned after tuning the phase balance of the generated phase.
    Of course, both require oil-filled AC rated capacitors, *not* motor-starting capacitors, which will pop rather soon after you get started. :-)

    Again -- try tuning the reactive load by adding power-factor compensation capacitors until you get a minimum (with no load on the idler), and then put a maximum load on it, and re-tune. Probably want to aim for a value between the two, unless it spends most of its time idling, or most of its time at maximum load.
    Enjoy,         DoN.
--
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Hey DoN,
OK. Thanks for the tip(s). Yard work around the just-less-than-a-year-old-TO-US -house has taken precedent over the shop for the past month or so. Actually, work on the abode in general has been on-going for the whole year really!!, but I can see a 3-phase light at the end of the shop pretty soon!
I was able to run the mill and lathes in the short spin time of the 20HP idler, and the 7-1/2 will run the lathe all day although the idler is getting a bit warmer than i would have thought. Still "touchable", but warmer than I expected. The motor is from some sort of machine that had it flange mounted vertically, and I will attempt to mount it that way when I'm ready, but there is just a bit of a screw around because the motor leads come out "un-protected" (no connection box) immediately adjacent to the flange. I have to do something about that.
I have been accumulating a small bunch of oil-impregnated caps for a "testing phase", and when I find the most desire able I will get a single correct size for the job. Interesting reading of one of the fellas that did some research, in that he sees very little difference in currents and voltages if the caps are placed across just legs 1-3, instead of 1-3 and 2-3. I'll diddle with that a bit too.
Take care.
Brian Lawson, Bothwell, Ontario. XXXXXXXXXXXXXXXXXXX
On 4 Jun 2004 17:57:20 -0400, snipped-for-privacy@d-and-d.com (DoN. Nichols) wrote:

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Sure it will start. I have no idea about what will happen to your fuses, however.
My 7.5HP phase converter started a 20HP motor with no problems.
Steve Smith
Grant Erwin wrote:

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Motor started fine, fusa intacta. - GWE
Steve Smith wrote:

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Grant My home made phase converter sports a 15hp idler, and is fed from a 40 amp breaker. Go for it.
John Normile
On Fri, 04 Jun 2004 05:58:15 -0700, Grant Erwin

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