Can you safely under run a piston compressor?

[Reman]

Pretty much as the title says.
I finally got to grab my "Proper" compressor out of my sisters garage (It's been blocked in with household crap after one of her friends downsized and piled the rest of the garage full of the junk they didn't have room for.......... FOR 8 YEARS!!!!! ). It's a reasonably chunky industrial Clarke Air unit with a 160L tank.
When I bought it second hand it had a 4hp 3 phase motor on it. Didn't have 3 phase so I put a 4hp single phase on it, But that drew so much current that it ended up needing it's own power line running to a new RCD in the fuse box. The garage I have now isn't attached to the house and it would be a huge b*** ache getting another dedicated spur out from the fuse box, So I'd rather not have to go down that route.

I seem to remember that this thing could pressurise the tank way faster that any of the tools I used could empty it, So I'm now toying with the idea of replacing the motor with a 2hp unit so I can power it off one of the garages existing 3 pin sockets, But running a smaller pulley to under speed the compressor a bit to give the motor a bit more leverage over the pump when it's getting up to pressure.

In my head it all seems like a perfectly reasonable workaround to the problem, But I'm not too headstrong to assume I might have missed something (Almost too headstrong, But not quite. ).

So, Is there something I'm not seeing? Could running the pump slower damage it?

If it's all gravy, I'm wondering, As I'll have to get a different pulley anyway, Would a 2800RPM motor and an even smaller pulley be better than a 1400RPM unit? I'm thinking the faster motor speed is going to be carrying more momentum and that could help as the tank fills up and the pump get's harder to turn, But would a 1400RPM unit have considerably more torque?

Alternatively, I've still got the 4hp 3 phase motor kicking about, If I ran that off an inverter (A technology I'm shockingly unknowledgeable about) could it be made to "Soft start" to lower the start up power spike enough to be ran off a mains socket? Would that be a cheaper option? If that's possible, How would I wire an inverter to the pressure cut off switch?

 

[Hopefuldave]

You can definitely put an undersize pulley on the motor - 1/2 the power at the same speed is half the torque, so a half-size pulley shoulld do it, though the tank will take twice as long to fill, you'd get half the CFM... similarly, a 2800 rpm motor is going to have half the torque from the same power in, you'd be in tiny pulley territory with a loss of grip on the belt.

An inverter would work, BUT - if the motor's taking 4HP when getting close to pressure it needs 3kw+ into the VFD, allowing for losses you're going to need a minimum of a 16A supply. Could be worth a try, take the compressor's cutoff switch to the VFD low-voltage control inputs - the control voltage (e.g. 24v) out of the VFD through the switch (opening when at pressure) to the forward input.

Dave H. (the other one)

 

[dobbslc]

I've got a SIP 150 litre compressor with a 3hp motor on it that runs off a 13A plug, I reckon that'll be better than using a 2hp motor.
Is yours a single or twin cylinder?

 

[daleyd]

I've got a SIP 150 litre compressor with a 3hp motor on it that runs off a 13A plug, I reckon that'll be better than using a 2hp motor.
Is yours a single or twin cylinder?

I’ve got a FIAC 150 l and 3 Hp motor - the thing to check would be the pump as tank size isn’t really relevant to what power motor is required, though obviously the bigger tanks generally have bigger pumps on them. If you could get some pump details it may be worth a gander to see if it could run off a 3 Hp motor to save messing with pulleys?

 

[Reman]

Hopefuldave - Fair enough. So an inverter is a possible solution, But the motor swap is a definite.
Pulley size shouldn't really be a problem as the original one on the 1400RPM 3 phase was about 4.5". It looked odd being that big, But it's got factory paint all over it, So it much be the original. So that means If I find a 2 to 3" pulley I should be in the rough ball park.

Dobbslc - On startup the average 3HP motor must be pushing it reasonably close to the allowable limit for a UK mains socket wiring. If it's on a ring main it's probabbly OK, But I guess SIP must have speced the motor very carefully to prevent it popping the RCD every time the pump kicks in on a half pressurised tank. Maybe it's got some kind of soft start to flatten the power spike when it first restarts? Or possibly they use another trick like a bleed valve that only seals after the motor and pump are up to speed?

 

[daleyd]

A 3 Hp motor will pull more than 13 amps on start up but should only do so very briefly. A compressor should also start “off load”, a valve should release the pressure coming from the pump to the tank when the pressure switch stops the motor. If it’s not and it’s trying to start with pressure in the line then it will struggle.

A c type circuit breaker may help if you get tripping but I’ve never had mine trip the breaker on start up - I’d have to double check but I’m pretty sure I only have standard b type breaker fitted on a 16 amp circuits

 

[Reman]

I've just been doing some digging. It's a single piston pump, But it doesn't have any markings on it. The tank has a plate with the model number shown as Clarke air se15c160, So if I remember how Clarke label there compressors rightly, That means it's a 13 to 15cfm pump sitting on a 160L tank. I can't find specs for this exact one (Though I did find a post about someone from this very forum (Gasket999) who was doing a pump rebuild), But the comparable current models DO seem to come with 3hp 1ph motors, But most also say they need to be on a 30 Amp minimum power supply.

The 3hp motors on the "DIY grade" compressors are rated to 2200w at 230v, So if I've worked it out correctly, That's a shade under 10A....... That doesn't sound quite right to me?!?!

 

[brightspark]

I've just been doing some digging. It's a single piston pump, But it doesn't have any markings on it. The tank has a plate with the model number shown as Clarke air se15c160, So if I remember how Clarke label there compressors rightly, That means it's a 13 to 15cfm pump sitting on a 160L tank. I can't find specs for this exact one (Though I did find a post about someone from this very forum (Gasket999) who was doing a pump rebuild), But the comparable current models DO seem to come with 3hp 1ph motors, But most also say they need to be on a 30 Amp minimum power supply.

The 3hp motors on the "DIY grade" compressors are rated to 2200w at 230v, So if I've worked it out correctly, That's a shade under 10A....... That doesn't sound quite right to me?!?!

3hp generaly run about 12 full load amps starting will be about 50

 

[daleyd]

3 Hp is 3 Hp (2.2kw) - diy or pro. They will say they need a 30 amp supply to cover their ‘arris - they will almost certainly run off a 13 amp plug, providing things like the unloader valve is working ok. It may be worthwhile getting your wiring checked to see if it’s suitable to fit a c type breaker in case it may be needed, though as I said I’m sure my breaker is a b type and I’ve had no problems.

 

[Seadog]

Maybe it's got some kind of soft start to flatten the power spike when it first restarts? Or possibly they use another trick like a bleed valve that only seals after the motor and pump are up to speed?

There should be an unloader valve that releases pressure on the pump side of the non-return valve once the motor stops.

 

[brightspark]

There should be an unloader valve that releases pressure on the pump side of the non-return valve once the motor stops.

and if it hasn't fit one because the motor wont like starting under load if its on the limit

 

[Reman]

Yeah, Mine's got an unloader fitted. Haven't seen a compressor without this feature for quite a few years.

I was more thinking about what tricks manufacturers might use to make a 3hp motor happy on a domestic mains plug, And not to current spike so badly on startup that it's going to trip the average consumer RCD whenever it starts up. The unloader air circuits only bleeds the pressure from the short link pipe between the compressor and the one way valve on the tank, So on any reasonable displacement pump it's only going to take one full rotation before the pressure in that pipe is enough for the pumps torque requirements to start climbing. I was just pondering if some manufacturers might have a bigger bleed valve in there that waits for the pump to be up to full speed (And the startup current spike to be over) before closing?
Alternatively, I could imagine some manufacturers using a oversized/extra long link pipe so the initial pressure buildup was slightly less aggressive? That way they might be able to under spec their motors a little to save money and still get the CFM ratings they want.

 

[brightspark]

it would want to be plugged in to sockets on the ring main that has a 32 amp mcb . an rcd is only an earth leakage breaker and has nothing to do with current start up or load

 

[hermetic]

I have a very similar, maybe identical compressor, and 3hp is right on the limit of what a 13A fuse will carry. A 13a fuse does not actually blow at 13a, because there is a "fusing factor" of about 1.5, which means a 13a fuse will actually blow at about 19.5 amps. When I first got my Clarke, I occasionally turned it off at the socket instead of allowing the pressure switch turn it off (and open the pressure relief valve) the result is that when you turn it on again, fuse blows instantly. What size is the supply to your garage?
Phil

 

[qwakers]

I doubt you'll get a 3hp motor to run on a 13a supply. My 2hp motor blows a fuse on every 3rd or so startup... I've had to stick it on a 16a commando plug.

 

[dobbslc]

Mines on a normal 13a fuse, it did blow the odd mcb in my old house but seems ok here......
The lights do dim when I fire it up though!
My workshop is temporarily run off the end of the upstairs radial circuit (don't ask) on a 16A B type mcb and that hasn't tripped.
I did go through a few fuses before I found a "good one" though.

 

[Reman]

The garage here is on a 32a spur directly from the houses fuse box. It then goes into a small fuse box in the garage where it has a couple of (IIRC) 16a ring mains and the lighting circuit. Not sure WHY a one car garage would need 2x ring mains (but I did hear a rumour that the previous tenant might have had a waste cooking oil processing setup in there to make bio diesel).

I'll have to double check the numbers when I get chance.

 

[minimutly]

You should be ok, but beware, just slowing this down will not reduce the torque needed to turn the compressor over tdc. It will improve you torque availabilty though. If the peak current fluctuates too much you will bet flickering on your lights, and your neighbours might too. Been there, I put a 2 hp on a big single cylinder broom wade, the whole streets lights flickered...

 

[Reman]

I'm having a little prod about at motor specs at the minute. I know from building quadcopters (Yes, I'm one of THOSE people. ) that the difference in Amp draw Vs power output can be quite noticeable with different quality's of motor. Cheap motors tend to be less efficient, and the manufacturers have a habit of rounding output figures up (Sometimes by quite a scandalous margin). I'm trying to compare 2 and 3hp motors from different manufacturers to see if any are worth a closer look to work out if some really are more efficient, Or if it's just bull **** marketing.

Unfortunately, The more I dig, The more it looks like there's only really a handful of motor manufacturers left now, And everyone else is just re-branding their products.

 

[daleyd]

Your overthinking it - unless you have a long run to your consumer unit or you have particularly small cross section cable then you should be able to run a 3 Hp motor ok from a 13 a plug supply. It’s at the limit of what you can run but lots of people can and do run all sorts of machines with this size motors from 3 pin plugs (compressors, woodworking machinery, lathes etc)

If your voltage at the garage is low, or your wiring is a bit borderline then it’s possible you may have problems - if you know your cabling is good then I see no reason why it would cause problems.