Compressor motor confusion

[Paul99]

I got my hands on a 2nd big compressor a few weeks ago that is in need of a new motor. It has a 200l tank and I presume pump will be 14fcm - it possibly could be even bigger but there is no plate or markings on the pump itself. Now reading about it seems that the usual answer is that for a 14cfm pump you'd need a 3hp (2.2kw) motor as that's about the biggest that will run on single phase. I'd need to source a drive pulley to match motor to pump rpm this is where I'm getting a bit confused....

It seems that there are two speeds of 3hp single phase motors 1440rpm and 2880rpm if my pump is the same as many I've looked at they can have a operating range of anything between 700 and 1400rpm so in theory I could use either speed motor with a 7" pulley and still be within the correct operating speed for the pump - that seems almost implausible given it would, give or take a bit, double the output of the pump yet still be drawing a maximum load of 13amps - or am I missing something.

Now I don't intend running the pump at 1440rpm as that is likely the very maximum the pump would take when new (which it's most definitely not) but on the other hand there's no point in having the pump running for longer longer periods at half capacity either.

Any thoughts on this - I think I maybe over thinking this way too much and confusing myself...

 

[rtbcomp]

Go for the slower motor and source a pulley to suit.

The faster motor will require a smaller pulley and there's a chance that the belt may not grip properly. I found that out the hard way!

Driving the pump too fast could actually reduce its output - the piston could be on its way back down before the all air has been able to get out of the cylinder.

 

[AndyStobbs]

The 2 pole motor only has half the natural torque of the 4 pole. You might find that without appropriate gearing, the 2 pole motor wouldn't have the torque to drive the pump as the tank pressure climbs.

So, you use a smaller motor pulley to compensate, reducing pump speed, but giving the 2 pole motor the torque advantage it needed. Potentially running into slippage as mentioned.

 

[piman]

Hello Paul,
that seems almost implausible given it would, give or take a bit, double the output of the pump yet still be drawing a maximum load of 13amps - or am I missing something."
"
Because a motor name plate gives a current rating, this is at full load but the actual current drawn depends on the load on the motor, if you try and drive the compressor too fast it could overload the motor and it will (Should) trip or blow a fuse.

Alec

 

[dobbslc]

I think if you have a pump that wants to run at 1400 rpm you will need a 4 pole motor. 2 pole = 2400 rpm, 4 pole = 1700 rpm roughly. Less speed = more torque.
You could then make a pully that will run the pump at your chosen speed say 1000 rpm.
What say everyone?

 

[rtbcomp]

Actually the speed/torque of the motor doesn't matter, it's the speed/torque at the pump that gives you the air output - you match the two with the pulley ratio.

Power = Speed x Torque = 3hp

As I said earlier. try to go for a large pulley on the motor to get good belt grip, this would mean using the slower (1400 rpm) motor.

 

[Paul99]

Thank you all for the replies - the slower 4 pole motor it is then. Probably going to target 1000rpm and go from there. For the time being I've rigged up a spare 1.1kw with a small pulley which is working albeit slow (500rpm or so) hopefully I'll get a vat free voucher from machine mart at some point as that seems to be about the cheapest option I've found so far.

 

[addjunkie]

Post a picture of said compressor, some one may have one the same.......

 

[Paul99]

Post a picture of said compressor, some one may have one the same.......

I had planned to do that but been a bit busy. Here it is minus the belt guards whilst it's being worked on:

and the motor that was on it 1400rpm 1hp:

I didn't have a mug to hand so a piece of 40 x 80 box for scale... It's old, huge and heavy - it must weigh 30kg+ it worked to charge the tank half a dozen times... great I thought - then out of the blue it just won't start on it's own - a helping hand with no load and away it goes. There's - no start capacitor on this old thing so presume its a repulsion start and again presume there is a fault in the winding somewhere as the brushes are good and no obvious signs of problems on the armature. I'd really like to get this one running again TBH but it's going to have to be a diy repair job so need to do some research first - the closest motor rewinding place wanted £500+vat to re-wind it having not even looked at it - they just wanted to sell me a very overpriced equivalent given time I was going to start a separate thread asking more specifically about fixing old motors like this.

Clearly that's not the original motor or drive pulley - I'm amazed that this ancient 1hp motor was driving the pump at 700rpm with no light flickering - no blown fuses - which makes me wonder if the pump originally ran a bit faster with a larger motor...

 

[rtbcomp]

If it's a repulsion motor you will see a commutator and brushes. It most likely has a start winding and centrifugal switch.

 

[Paul99]

If it's a repulsion motor you will see a commutator and brushes. It most likely has a start winding and centrifugal switch.

Yep it has brushes and a commutator (which I've always called the armature - a bit of reading cleared that up). After all the years of messing with things I've done very little with AC motors and consequently never done much reading about them I'll try and get the housing off the other end this evening and check that the centrifugal switch is working correctly (if it has one) - fingers crossed it's just stuck...

 

[Paul99]

I had an unexpected early finish at work today so took that old motor apart - no centrifugal switch and the brushes are in permanent contact with the commutator so I assume that the start winding adds more torque once it is up to speed. I couldn't spot anything obviously wrong so cleaned loads of dried grease (from the bearings) and dust off everything, put it back together and hoped for the best but no improvement it just sits there humming until you give it a hand and away it goes - so it looks like my earlier assumption of a short in the starter winding may be correct - the insulation was starting to flake off where it emerges from each winding and attaches to the commutator pads so it's not looking good. I should have taken the time to check for shorts or breaks with the multimeter while I was at it and taken some photos too but I plan to have another go at it in the future and will do all that then.

 

[addjunkie]

Think if just pull the pulley off the origional motor and "make" it fit a new slightly larger motor say 1.5 hp. Running at 1400