Running a pump from twin motors?

[gasket999]

Hi folks.

I've been getting advice on my compressor in another thread - given this is an odd request I thought I'd start a new thread (old one can be viewed here: http://www.mig-welding.co.uk/forum/threads/how-does-a-petrol-compressor-work.51906/)

Is it possible to use two electric motors to power a single compressor pulley? I can't imagine the arrangement.

I have a 3-phase compressor like this:

It owes me nothing and kicks out about 23CFM, which is plenty for me. But the motor is 5.5HP three phase and I'd like to run it from 240v. I'm on a budget so a digital convertor is out but I do have access to two 3hp 240v motors for the price of a crate of beer.

We're currently in the middle of rearing the house and the garage (10m from supply) is being rewired to a high spec at the same time. Garage has/will have a 40 amp socket fed from a garage consumer unit which in turn comes via a 10mm-square cable straight from the meter.

Having said that I'd imagine the motors would need to start in sequence so I can't picture the pulley arrangement to let me do that.

I need 15CFM FAD for the little blaster and spray gun and if I pull it off I'll have 23CFM for next to nothing (alternative is a Clarke one at £969+VAT)

Any ideas?

 

[gasket999]

Effectively I'm thinking about something like this ARB off road tyre inflator, but on a much bigger scale:

Oh! and the compressor flywheel already has two grooves - there's twin belts connecting it to the (current) single 5.5hp 3-phase pulley.

 

[eil]

Your best bet would to fit a 4hp motor on it and a smaller pulley, or make the compressor cut out at a lower pressure, you cant fit two electric motors to run the compressor, because the rpm of the motors wont be exactly the same which would mean one would be doing all the work and the second motor would then be driven, making it more like a generator.

 

[gasket999]

Ah… that makes sense, thank you for the pointer. That being the case I'll go back to the conversion to petrol/diesel. At 4hp I'd be as well selling it complete and buying a 4hp one straight from Machine Mart.

 

[piman]

Hello Eil,

I don't think that will be the case, the motor speed is dependant on load and even if they are slightly different loads on each motor they will both drive. (All motors are generators when they are running, singly or in a multiple)

I can't see anything to lose by trying it.

Alec

 

[Anguz]

it will work, but for protection of the motors you will need a proper overload device for each motor

 

[Fazerruss]

Your best bet would to fit a 4hp motor on it and a smaller pulley, or make the compressor cut out at a lower pressure, you cant fit two electric motors to run the compressor, because the rpm of the motors wont be exactly the same which would mean one would be doing all the work and the second motor would then be driven, making it more like a generator.

As far as I'm aware induction motor speed is governed by the frequency of the mains supply. So it should work. Just make sure you have a hefty supply to deal with the start up current.

 

[rtbcomp]

The no-load speed is governed by the mains frequency, the running speed will be load dependent. It will work in theory, they will sort the torque & speed out between themselves.

You'll need a pretty robust mains supply though.

That ARB unit looks like two pumps rather than one pump and 2 motors.

 

[normspanners]

I had thought about doing this for a comp, I did give it some thought but not too deeply, as I went down the three ph and inverter road.
My idea was not to start the motors in sequence (because you would be adding extra load on the first starting motor), but to start the motors in series abit like a star delta starter, this system would be working in time with an unloader solenoid so that once running, and as the motors where switched to DOL the solenoid allowed the compressor to load up.
by putting them in series it would lesson the start up load and inrush current.
The pulley arrangement is quite simple you need an extension to the motor mounting plate then mount the second motor one space further away from the pump. obviously you would have to re-site the pressure switch.
The original placed motor would have a three groove pulley and the extra motor one groove, both motor pulleys would be the same size.
The original belts would be used on two of the three grooves and a new short belt used between the two motors with a separate tensioner.
I would place the extra belt on the inside of the the original two belts to lessen the load on the motor brgs.

 

[rtbcomp]

Check that both motors turn in the same direction

 

[normspanners]

Check that both motors turn in the same direction

That sounds a good plan.
Counter rotating:---- Now that would reduce the torque somewhat

 

[lshigham]

Yes no reason who it won't work
As said above though they will both need separate overload protection

 

[eil]

The only compressor I have worked on with what was near to having two motors is one that was part load which had a 3hp winding and a 2hp winding and the idea behind it was to lessen the start up current, but that was on 3ph though, it also had cylinder unloaders fitted. I as yet haven't seen two single phase motors running one compressor pump but would be very interested to see that kind of setup, just wondering why its not common to see, instead of putting two motors and two pumps to get more capacity.

 

[the snooper]

Hi folks.

I've been getting advice on my compressor in another thread - given this is an odd request I thought I'd start a new thread (old one can be viewed here: http://www.mig-welding.co.uk/forum/threads/how-does-a-petrol-compressor-work.51906/)

Is it possible to use two electric motors to power a single compressor pulley? I can't imagine the arrangement.

I have a 3-phase compressor like this:

It owes me nothing and kicks out about 23CFM, which is plenty for me. But the motor is 5.5HP three phase and I'd like to run it from 240v. I'm on a budget so a digital convertor is out but I do have access to two 3hp 240v motors for the price of a crate of beer.

We're currently in the middle of rearing the house and the garage (10m from supply) is being rewired to a high spec at the same time. Garage has/will have a 40 amp socket fed from a garage consumer unit which in turn comes via a 10mm-square cable straight from the meter.

Having said that I'd imagine the motors would need to start in sequence so I can't picture the pulley arrangement to let me do that.

I need 15CFM FAD for the little blaster and spray gun and if I pull it off I'll have 23CFM for next to nothing (alternative is a Clarke one at £969+VAT)

Any ideas?

does the 3phase motor have 2 belts if so it will be simple to connect 2 motors and as it will already have 3 phase switch gear there will be no problem starting both motors at the same time

 

[rtbcomp]

.......... just wondering why its not common to see, instead of putting two motors and two pumps to get more capacity.

I suspect the demand for 6hp single phase compressors isn't that great so it's more economical to use 2 existing 3hp units. Also it's easier to manage the startup surge with 2 independent units.

 

[eil]

Granted its more economical to run one of two when you don't need it Rog, but if you want to run a spray gun and mask then 3hp is not enough or if you want to do a bit of blasting like your good self you need more than 3hp really. Just haven't seen two single phase motors being used to run just one pump as of yet, Regarding the start up I would imagine a 3hp motor would be able to start his compressor if it had a big enough solenoid valve to unload the discharge till it got up to speed.

 

[rtbcomp]

I meant more economical to manufacture using existing components.

 

[Maker]

The only compressor I have worked on with what was near to having two motors is one that was part load which had a 3hp winding and a 2hp winding and the idea behind it was to lessen the start up current, but that was on 3ph though, it also had cylinder unloaders fitted. I as yet haven't seen two single phase motors running one compressor pump but would be very interested to see that kind of setup, just wondering why its not common to see, instead of putting two motors and two pumps to get more capacity.

I've heard of old wood lathes to have a motor wound to run with 3 or 6 hp, I'm suprised they would bother to make one with such a small difference between the two powers.

 

[gasket999]

Wow - just logged back in and seen the replies above - guess the idea is not dead yet!

There were a couple of queries above, so to answer these first:

1) needing a 'beefy' mains supply. One that score I am in a great position. We're in the middle of a complete property rewire and the garage is being set up to be the very best it can be on a domestic supply. I'm not doing the wiring but can change the specs if anyone suggests improvements: There is a single 10mm-square cable that runs from the meter to a dedicated consumer unit in the garage. I've asked for two 40amp plugs/breakers/etc to be fitted in the garage as I've been thinking ahead with regard to compressors and my welding sets, etc.

2) Does the compressor have twin belts? Yes it does - see attached picture

 

[the snooper]

Wow - just logged back in and seen the replies above - guess the idea is not dead yet!

There were a couple of queries above, so to answer these first:


2) Does the compressor have twin belts? Yes it does - see attached picture

then simply mount one motor above the other to one would need to be a little further towards the pump if the same size belts were to be used, job done