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Running a 5 HP motor off a 3 HP rotary?

Caspian

Hot Rolled
Joined
Nov 2, 2007
Location
ohio
Found a drill press I really want. Problem is, it is 5 HP and my rotary is only 3 HP.

I would never put a strain on the motor since 5 HP is a much larger motor than anything I'll ever drill.

Would a 3 HP convertor work and just provide less "juice" thus making the drill press perform a little weak? That would be fine.

or

Would it burn out the motor? I suspect this is probably the case, but i figured I would ask. If there is any way I could get this to work, i'd be thrilled.

Thanks
Caspian
 
Nope, won't work.

Ideally, you should have a bigger idler motor than the machine's motor. There's NO WAY you should have a bigger motor on the MACHINE! MINIMUM you'd need a 5hp motor on the rotary. It's not so much the problem of the load you'd put on the motor, but starting it. Too big of a mismatch, and you'll stall BOTH motors out, even if it only drew what the RPC was capable while running. Time to upgrade the RPC, or better yet, stick a VFD on it. :cheers:
 
That's what i was afraid of. I'm not a big fan of VFDs after having both RPCs and VFDs. I'll probably buy the drill press and begin the lookout for a 5 HP 3 phase motor to make a RPC.

Thanks,
Caspian
 
Hi Caspian,

Im not sure where you are in Ohio but you should check out HGR in Cleveland. They have a
great number of motors at any given time and the prices are reasonable. Talk to Paul Cossick and tell him that Ernie sent you. The number is 216-486-4567. I have no connection with the company other than the fact that they are nice to deal with and I buy from them occasionally.

ErnieD

Edit was spelling.
 
5 hp drill press, that's pretty large. Do you really need 5 horse?

Thing is, you could put the current 3 hp idler on the drill press, and
use the exising drill press motor, the 5 hp one, for the new idler....?

Jim
 
I was thinking that, but it might be a PITA.

5 hp drill press, that's pretty large. Do you really need 5 horse?

Thing is, you could put the current 3 hp idler on the drill press, and
use the exising drill press motor, the 5 hp one, for the new idler....?

Jim


I was thinking about swapping the motors around, but I've seen a few drill presses where that wouldn't be possible, or a ROYAL PITA! Not from an electrical standpoint, but mechanical. It MAY be possible, and if it's EASY, it's a good idea. :cheers:
 
I wouldn't worry about it

For most 3 phase motors, all you need is to get them started, and they can run fine on just one phase. As you said, you won't be loading it heavily.

Also, a drill press does not have a heavy starting load, compared with, say, a lathe. The drill press spindle has much less inertia than a lathe spindle might have.

As I understand it, you can increase the effective HP rating of the rotary phase converter simply by plugging in another 3 phase motor, and letting that one idle too. Then, the extra idler motor will be working in tandem with the original idler motor, providing more possible electrical power to the generated legs. For example, if you can find an extra old 3hp 3 phase motor, you can plug that one into the existing 3hp RPC circuit, let it idle, and now you have a 6hp rotary phase converter.

Getting an surplus 3 phase motor and adding it might be the easiest and cheapest way to be sure.

But if it was me, I would first try running it with what you already have, and see if it seems to work OK.

-Jon
 
5 hp drill press, that's pretty large. Do you really need 5 horse?

Jim

Heck no I don't need 5 HP, but it is a helluva press for a helluva price. I'll get it for less than the "industrial" drill press from Harbor Freight.

Thanks for the ideas, guys. I'll first try hooking it up and seeing if it will work. Odds are, it won't be enough to jump start it. If that's the case, then I'll look for a suitable 3 HP motor to mount. If that fails, then I'll try the tandem idler motor idea. That would be the most economic/quietest operation/simplist hook up progression.

Caspian
 
I have a 5hp rotary phase converter that I will be selling within the next couple of weeks. I have purchased some new machines and now need to upgrade my rpc. The one I have has worked well for me for the past 9 years. I have run a mill, an engine lathe, and a surface grinder with it. I am in the Elyria area. Let me know if you are interested.
Joe
 
definitely try it before you buy anything else. As long as you really are drawing less than 3 HP it will work.

The load any motor presents when first switched on greatly exceeds the rating, but only for the few milliseconds it takes to start rotating. (This is the basis for how Sears claims their shop vacs "develop" 6 hp but operate just fine on a 15 amp circuit - throw the switch and there's a momentary ~60 amp load, which almost immediately drops to something like 2 or 3 amps)

As soon as the motor starts rotating, the load starts falling off until it's up to speed. So, as long as the 3 HP RPC can handle the starting current load of the 5 HP motor you should have joy (until the DP presents a steady load of more than 3 HP) Better idea might be to swap motors (if practical) as was mentioned above.
 
definitely try it before you buy anything else. As long as you really are drawing less than 3 HP it will work.

Good news. Got the DP hooked up and the 3 HP RPC will start the 5 HP motor w/o any problem. I will wait to change anything (swap motor, bigger RPC) if i ever bog down the 3 HP worth of juice running it. I doubt that will happen, so all should be well.

Thanks for the input.

Caspian
 
interesting experiment . . .

Now that you have it connected, want to have to "fun" ?

This happened to me a couple of decades ago. I built a RPC from a relatively "new" 2 hp mtr. My lathe at the time was a 13 x 5 SB and it had an old heavy mtr, forget the HP. I would habitually plug the machine to stop it while working to get the most out of the day. (yeah, it had a screw spindle and I never had it come loose, maybe lucky, eh?)

Anyway, . . . I had this _heavy_ roller to rebuild the bearing ends and surface. I mounted it on the lathe between ctrs and completed the setup. Now to start working. :rolleyes5: Made a cut, plugged the mtr, and it didn't stop. But the RPC made a nose that I had not heard from it. A kinda' decreasing pitch groan and then it took off again. I muttered a "what the hey" and noticed the work was still spinning in the same direction, but the switch was in reverse.

yeah, the bloody RPC reversed. :eek:

You might be able to do the same with the different mass of the mtrs you have. Well, I assume that given the HP, age, and my assumption that the RPC mtr you are is a recent manufacture with lower mass armature than the old mtrs.

Or if that does not sound like "fun", just be aware of the possibility. :D

- Reed
Raleigh, NC
 
You could install a second idler, possibly a 5HP. Your 3HP will easily start the second idler. The second idler would only have to be powered up if you run the drill press. The combined idler 3+5=8 HP would be more than enough to handle the drill.
 
I always wondered if you could push the RPC idler to reverse.

I was wondering if you overloaded an RPC if you could push it to spin backwards or if it would stall if it had too big of a load doing the instant reverse thing. I guess it all depends, buecause I've heard of them stalling too.

The second idler is a good idea. It would ASSURE the drill press wouldn't stall the idlers, or keep spinning the same direction when doing instant reverse.

That gives me another idea, would adding a flywheel for extra inertia help with an RPC that needed a little more oomph for starting a heavy load?? Sure seems it would! It would keep it from stalling, and allow it to generate a little extra juice on the third leg. :cheers:
 
The flywheel would add more rotating mass like one of the older motors but the windings and magnetic components may still fail or be overloaded. The incoming power supply to the RPC may trip the breakers also. A second idler has it's own power supply and breakers.

A standard motor starter for the second idler can be wired so it won't start or will drop out if the third phase is not present.
 








 
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