Downsizing air compressor motor and powering with rotary converter

Here's the situation, a friend just bought a really nice compressor and I'm to help him fiugure out how to run it:

-Quincy 370 compressor, currently 15HP 3 phase, just over 1000 RPM.
-Spec sheet for compressor says it can run as low as 400 RPM and would need a 7.5HP motor to do so.
-The 370 at low RPM will satisfy his air requirements.
-the Quincy starts unloaded until oil pressure builds, it's not a terribly hard starting compressor.
-No desire to run it at full speed and power or purchase the electrical components necessary to feed a true 15HP 3 phase load.
-Shop it's going in doesn't have 3 phase or a converter.
-Looking for a satisfactory and reasonably cheap solution,(likely excludes the Phase perfect and even a VFD)

Here's what I'm thinking,
Option 1. Remove 15HP motor from compressor, belt it with a pony motor to make simple rotary converter. Repower compressor with 10HP 3 phase motor and pulley size to suit the 400 RPM minimum.

Option 2 find 15 or 20HP idler motor for making phase converter. Keep existing 15HP motor on compressor but reduce pulley size to suit the minimum 400RPM.

Option 3 ??? and why is it a good one?


I like option 1 because wiring and components to be found are smaller and cheaper, downside is a little more work and uncertain if 15HP converter will run a 10 HP motor and pull a possible 7.5HP load.

I like option 2 because only modification to compressor is a smaller pulley and shorter belts, and motor is well oversized for it's new load. Downside is finding the larger idler motor for converter, and perhaps somewhat larger wire and breaker necessary to setup everything.

Any input on what size motors are really necessary in each case?

I'm not an electrician but do have a fair understanding of the different methods of and how to make a phase converter. Not real concerned about finding/purchasing an idler motor, in my experience they seem to turn up free or real cheap if you just let folks know you're looking for a big old 3 phase motor.

Thanks for any suggestions,
Grigg

Option 3 - find someone who wants to pay $ to upgrade their 7.5 hp Quincy to 15 hp? Use the money to buy a RPC?

Option #3 buy a 7.5HP single phase motor and be done with it. Keep the bigger motor in case your buddy wants to sell the quincy at a later date, or needs additional air in the future.

7.5HP is about the biggest you can run on single phase. I've got two kaeser rotary screws and even though I had three phase I got single phase motors on the compressors because my three phase box was maxed out.

That's certainly a good simple option except for the price of a single phase 7.5 HP motor. And they're hard to find used, but worth looking.

Any other ideas?

I like using the 15 hp motor for the RPC. A friend runs his 10 hp compressor om a 15 hp RPC. He has found that the compressor starts easier when he is running another load motor with the RPC.

Option 3.5

Look on Ebay until you find a BIG VFD ;-)
Should be less than cost of a new motor and a RPC.

run the present motor at half speed (30 hz)

Quincy QR-25 Compressor Pump Data

Buy a 5hp single phase compressor duty motor and be done with it.

A vfd to go from sf to 15 hp for a compressor would cost ?

Heavey Metal said:

A vfd to go from sf to 15 hp for a compressor would cost ?

Click to expand...

Isn't 15 HP if run slower........ still could be expensive, but maybe less than a SF motor. Then you talk about derating for SF input, and cost can go back up.

SF motor isn't too bad, if you can find one. Lower power means lower speed.

Too much slower and oiling may not be adequate, though, is it splash lubricated? You want to go fast enough that the "splash" is going to actually get oil where it needs to go.

A 5HP motor? Maybe..... but if it has to run really slow, then you may have oiling problems. Easy to find, though, even in compressor duty. Every big box standup tank unit seems to be 5HP.

Heavey Metal said:

Quincy QR-25 Compressor Pump Data

Buy a 5hp single phase compressor duty motor and be done with it.

A vfd to go from sf to 15 hp for a compressor would cost ?

Click to expand...

That info does show 5HP, but the genuine Quincy flier shows 7.5HP needed for a model 370 run at 400 RPM, better safe than sorry and why I'd like to plan on 7.5HP.

As mentioned earlier the literature says the minimum operating speed is 400, and it does have an oil pump and filter. If they say it'll run fine at 400 I'm not worried about it, it should actually last considerably longer than if run at the max speed of 1,070.

So far most talked about option is to find a single phase motor, but have you seen a 7.5HP single phase motor used laying around for similar money to any of the other possible solutions, and or if t'll cost more is it that much better an option?

How about the VFD, I haven't ever used one but what would one cost capable of single phase in, to run a 15HP 3ph motor at normal speed with a 7.5HP load? This assuming it'd still be smart to downsize the motor pulley instead of running the motor at 30Hz.

I guess I'm still leaning toward finding a 10HP motor for the compressor, smaller pulley, and make a simple converter with the old 15HP. Should be under $200 for everything, assuming a freeish 10HP motor can be found, which I expect one can be.
Is there a cheaper, or even if more expensive a noticeably better way to do it?

Thanks,
Grigg

Grigg said:

. . . Is there a cheaper, or even if more expensive a noticeably better way to do it?

Thanks,
Grigg

Click to expand...

You can sell a used 15hp Quincy for $1500-2000. You can buy a decent used 7.5hp Quincy for $500-750; possibly single phase. You don't have to help your friend figure this out. Instead of hunting for a cheap 10hp motor and building a $200 RPC (gotta find those capacitors, too?) that may or may not be convenient to use, your friend can spend $0 (if he buys a 3 phase unit and a commercial RPC) to $750 (if he sells the 10hp for market value and happens to find a 7.5hp single phase one).

And then you're not your friends' RPC and compressor service tech for the next 10 years.

The situation is a bit like "My friend has got a used F350 and wants to convert it to a more fuel efficient F150. What's the best way for me to quickly and cheaply do the conversion?" Could be to sell the F350 -- for which there is market demand and $$$ prices-- and buy a significantly cheaper F150 -- which are even more frequently available.

I don't mind at all helping him, we've actually been looking for a decent Quincy compressor for his shop for over a year now. We're quite fortunate that this one turned up close by and was a really good deal to boot, even considering the work necessary to run it. The pony start converter won't need any capacitors unless desired for balancing.

I hear your argument but not sure it's true in this case. The time and effort in finding as nice a 7.5HP compressor as this one is (new in 2005 and used as backup) combined with the shipping and or hauling and selling this one would be a step backwards. In the time we've been looking we couldn't turn up a good 7.5HP Quincy within reasonable driving distance even at a fair price, so continuing the search is not appealing. Bird in hand is worth two in the bush sort of thing, and it's a very nice bird!

Thanks,
Grigg

The 7.5 is for 200psi 400 rpm

At 175 psi it is 5 hp 400rpm.

I know where at least 20 of them are used for backups in one shop alone been there sence vietnam most with the original motors (5 hp 3p 220)

Exactly, and anyone that thinks they need 175PSI for general (machine) shop use is out of their mind anyway. 110-120 PSI is a good range and the 5HP should have no problem at that level.

A 370 is a pretty good size hunk of air compressor iron.....

Heavey Metal said:

The 7.5 is for 200psi 400 rpm
At 175 psi it is 5 hp 400rpm.

Click to expand...

Excellent! Being able to run on 5 HP HP makes a big difference. I have a 5HP single phase already, that'll be a great solution. He'll probably run it about 140-150 PSI, agree that 175 is more than necessary.

Thanks for the info, suggestions, and pointing out that 7.5 HP was for 200 PSI.

Grigg