10hp air compressor....how to power it.

LD1 · Mar 29, 2018

Ok guys, I have mentioned in a few other threads that I acquired a locked up old screw compressor. A joy twistair TA010....40cfm compressor.

I didnt want to invest too heavily in this project because I honestly didnt know weather or not I could save it.

Well, so far with minimal investment in new bearings and an oil filter.....the thing lives and breaths very well. It will go from 0-100 psi in 3min flat and thats with an 80 gallon tank:thumbsup:

And pressure on off being 100psi and 120psi......that recovery time (if no air is being used).....a mere 30 seconds:thumbsup: So I think its safe to say this thing makes way more air than I will ever use.

Being it is a screw compressor with vent/unload/load valves....its not a hard starting machine at all. For test purposes I have been running it off my 15hp American rotary phase converter. But I am not liking the amperage imbalance I am seeing.

At the machine.....I am pulling 38-40 amps on L1 and L2....and 22a on the generated L3. All the while the nameplate amperage for 230v is only 25.6. So this is not a good way to run, and would assume the converter is simply too small??? But certainly worked for my test stages to make sure this compressor was even worth investing more money into.

And while running on the RPC......my single phase supply supplying everything.....right around 55a. So that certainly doesnt sound efficient at all since a with only ~40a single phase I could be running a single phase motor.

And options get alot slimmer for VFD's to power 10hp motors that I am starting to shy away from that as an option.

I can get 10hp/3600rpm single phase motors for ~$750-$800. Which is certainly worth it for this beast of a compressor considering that even spending that for a motor, I'd still have under $1000 total in a 40cfm screw machine.

Another option I am considering....is drop to a 1800rpm motor. Currently the pulley sizing has the airend running 4000rpm. Cutting motor RPM in half.....and upsizing the pulley a bit but not quite doubling it....get the pump to maybe 3000rpm is what I am thinking. 40cfm is way more than I need, slower will pull less amps, and make things last longer....and 1800rpm motors in the 10hp range are alot easier to come by and a few hundred bucks cheaper.

Whats everyones thoughts? Dont really think there is a wrong way to do it. Just trying to decide what is most practical/economical.

Also came across some old references on practicalmachinist about Ronk add-a-phase static converters. That use an autotransformer or something. Supposed to be able to use a 3ph motor up to rated HP with their static device...and have good amp balance too.

Bingo9991 · Mar 30, 2018

Get a Wisconsin thd/tjd with 100lbs compression and run it at idle, not exactly efficient but bulletproof

LD1 · Mar 30, 2018

Bingo9991 said:
Get a Wisconsin thd/tjd with 100lbs compression and run it at idle, not exactly efficient but bulletproof

Don't think I can make that run on electric

Mendonsy · Mar 30, 2018

The current readings that you are seeing are not particularly surprising for an RPC running a loaded motor. The phase angle of the output from an RPC isn't the same as true three phase power so the two phases connected to line draw much more current than the generated phase.
About the only option with the existing motor would be a phase converter VFD. That would allow you to adjust the motor RPM which should improve the situation, but I'm not sure how rpm effects a screw type compressor. Slower speed might reduce both CFM and pressure?? The other option would be to go with one of your single phase motor choices.

oldnslo · Mar 30, 2018

seems like the 10 HP 230 VAC single phase would be the simplest solution. Only failure I ever had on larger single phase motors was capacitors in cold weather. This was in central MN at -15 to -25 degrees. Doubt you will see that to often in central Ohio.

LD1 · Mar 30, 2018

Mendonsy said:
The current readings that you are seeing are not particularly surprising for an RPC running a loaded motor. The phase angle of the output from an RPC isn't the same as true three phase power so the two phases connected to line draw much more current than the generated phase.
About the only option with the existing motor would be a phase converter VFD. That would allow you to adjust the motor RPM which should improve the situation, but I'm not sure how rpm effects a screw type compressor. Slower speed might reduce both CFM and pressure?? The other option would be to go with one of your single phase motor choices.

RPM wont effect the pressure....within reason.

As I said, this compressor airend is the exact same one used on 7.5hp/29cfm models......the 10hp/40cfm model that I have, and 15hp/60cfm models. they just did the speed change with pulley sizing. So yes it is feasible to run a drive and reduce speed.

But from an economics standpoint.....I cannot buy a drive big enough for any less money than a single phase motor.

LD1 · Mar 30, 2018

oldnslo said:
seems like the 10 HP 230 VAC single phase would be the simplest solution. Only failure I ever had on larger single phase motors was capacitors in cold weather. This was in central MN at -15 to -25 degrees. Doubt you will see that to often in central Ohio.

No cold issues in a heated shop

Still trying to decide if I want to spend ~$800 for a 10hp and have a 40cfm compressor......or spend $400 for a 7.5hp and ~$25 for a new pulley and have a 29cfm machine?

Decisions decisions.

bigdeano · Mar 30, 2018

I put a 10hp 3ph motor to use on single phase by using capacitors and a starting relay. I used 220ufd run capacitor and 900ufd start capacitor. I had the relay, the caps cost a little over 100 dollars total. It works real well, runs just like on 3ph, but I suppose it wouldn't develop full power. I'm probably using about 7 or 8 hp. I bought the run caps at surplus center, the start caps on Ebay.

LD1 · Mar 31, 2018

bigdeano said:
I put a 10hp 3ph motor to use on single phase by using capacitors and a starting relay. I used 220ufd run capacitor and 900ufd start capacitor. I had the relay, the caps cost a little over 100 dollars total. It works real well, runs just like on 3ph, but I suppose it wouldn't develop full power. I'm probably using about 7 or 8 hp. I bought the run caps at surplus center, the start caps on Ebay.

Yea, you basically made a static converter. And no they dont run full power. You are probably closer to 6-7hp rather than 7-8

Erik M · Mar 31, 2018

If you want simplicity and have the current to do it, just get the single phase motor and the starter as it will be cheaper than any three phase solution. It will need plenty of power when you start it though so I wouldn’t do it if you have only enough power to run the motor. The inrush current will be very large as a compressor is a hard start. If power is limited a three phase solution with a VFD will use less current on start.

LD1 · Mar 31, 2018

Erik M said:
If you want simplicity and have the current to do it, just get the single phase motor and the starter as it will be cheaper than any three phase solution. It will need plenty of power when you start it though so I wouldn’t do it if you have only enough power to run the motor. The inrush current will be very large as a compressor is a hard start. If power is limited a three phase solution with a VFD will use less current on start.

200a service at shop. Yea I have enough power.

Everyone keeps saying compressor is a hard start load....

No, not a screw compressor. Not any harder than starting a fan, or even a clutched machine like a lathe. It doesnt actually "load" the compressor til things are at speed

Erik M · Mar 31, 2018

Then you should definitely stick with the single phase solution. Easiest by far.

LD1 · Mar 31, 2018

Erik M said:
Then you should definitely stick with the single phase solution. Easiest by far.

Thats the conclusion I have come to.

But I am far from knowing everything. Thus the question as to a more economical solution if one so happened to exist before I pony up $800 for a motor

Bingo9991 · Mar 31, 2018

I thought you meant engine, when you were talking about hp and rpms, guess I was just being a tard

John Fitzgerald · Mar 31, 2018

When I was advising consumers at the electric coop, this came up fairly often. I would advise to get a single phase motor. Many cooperatives in rural areas limit single phase motors to 10 hp and smaller.

Industrial Toys · Mar 31, 2018

I was reading an article this morning about oversizing conductors to save money in the long run. Had no idea a 100HP motor (costing 10,000) running 24/7 will use a hundred grand of power in a year!

Most time, one doesn't need much air. I would run a thing like that for sandblasting using a petrol motor and have a smaller compressor for filling up the tires.

I built a 10hp RPC years ago for a 3ph lincoln mig welder that I bought by accident. I think the power poles in the neighborhood jumped out of the ground every time I start that thing! I used to turn off all my sensitive electronics and appliances!

bigdeano · Mar 31, 2018

LD1 said:
Yea, you basically made a static converter. And no they dont run full power. You are probably closer to 6-7hp rather than 7-8

It's a capacitor start, capacitor run motor. All three windings are energized while it's running, unlike the static converters which don't use run capacitors from what I understand. They're commonly used in submersible well pumps. The OP could change the pulley to lighten the load on the motor a little. It's the cheapest way to get the compressor running and should still put out a lot of air.

the old grind · Mar 31, 2018

LD1 said:
.... this compressor airend is the exact same one used on 7.5hp/29cfm models......the 10hp/40cfm model that I have, and 15hp/60cfm models. they just did the speed change with pulley sizing.

....I cannot buy a drive big enough for any less money than a single phase motor.

.. also drives are fragile, as smoking when caps fail .. that said, you might could rebalance the RPC's legs, and that's often needed for largely disparate loads. Idling another connected motor might (a long shot, but sometimes) help balance, YMMV depending on what they are and what you have now (balance caps)

LD1 said:
... Still trying to decide if I want to spend ~$800 for a 10hp and have a 40cfm compressor......or spend $400 for a 7.5hp and ~$25 for a new pulley and have a 29cfm machine? ...

Based on quote #1, gosh yeah. I suggest also that you set a much wider spread for your switch settings. Two stage piston jobs may set from 125 to 175 psi. I might use, or recommend on at 110 and off at 150. It's kinda like having a bigger well tank but using pressure vs volume to store-up because outlets are regulated, eg 90 psi for most small air tools 120 psi or so for tire changer or other high pressure OR volume tools. Fewer cycles is part of o'all efficiency, and I'd guess it's unlikely you use for long tools that need 20 cfm or more.

LD1 said:
Thats the conclusion (confusion?) I have come to. ...Thus the question as to a more economical solution if one so happened to exist before I pony up $800 for a motor

I suggest you'd rarely need all of 5 of 7.5 hp you had on tap, and IMO you best explained why that would be a good choice. (you're good at that

... thanks, btw)

LD1 · Mar 31, 2018

Was hoping you'd chime in old grind....rather than try to parse out quotes to reply to....I'll just do it all at once.

Tried balancing with another idler. 10hp. no load as it is a 10hp off another converter. That brought L1 and L2 down from 40ish amps to about 37...and added a few on L3 where it was at 20 and now 22. But didnt do me any favors for efficiency as single phase amps....feeding the main converter box...went from ~57 up to the lower 70's.

Regarding pressure....cant really up it any. Tank and compressor....according to Joy MFG....is only rated to 125psi max. And the L1 and L2 loads....the ones I am concerned with about the high amps....pull about 38A @ 100psi and is up to ~41A @ 120psi right before it kicks off. But those two legs only pull ~24A @ 50psi.....and a tad less under that.
Watching the Amp meter clamped on the leads....watching the amps climb just like watching the pressure gauge climb.

Now regarding my air usage......current compressor is a 11cfm 30 gal tank. Single biggest thing it cannot keep up with is my can crusher. 4" bore....6" stroke air cylinder @ 80psi. When I have a barrel full of cans at the house and want to crush....cannot crush more than 30 at a time without waiting on compressor. And the barrel holds ~150.

But I also do some sandblasting and painting, and running a die grinder which is an air hog for an 11cfm compressor.

Based on your reply....I am unsure if you are suggesting that I drop it to 7.5hp and adjust airend RPM's accordingly.....or stick with 10HP?

Do I think 29cfm/7.5 HP is plenty for now....sure. But dont want to regret spending ~$450 setting it up as such....and wish I had gone bigger if my air needs change.

Boy would I like to stumble on a use working 10 horse single phase motor