Wiring in air compressor

[ericmark]

To draw 40A at 230v 3 phase is really some massive motor.

With single phase the motor has to have a helping hand. Be it a capacitor, centrifugal switch, timer, together with start and run windings one way or another we have to get it turning with most squirrel cage motors.

With three phase we have the reverse problem we need in some way to reduce the current used at start be it an electronic soft start or resistor, auto transformer or star delta.

The inverter allows us to use three phase motors on a single phase supply and control the ramp up and down to within limits never before through possible with a squirrel cage motor.

However with a compressor it all to do with the matching of de-load with the motor start. Three phase is easy as the motor goes into delta so we drop the de-load. However with other types of starting the matching can be a problem.

I remember one compressor which would regularly blow fuses having us all scratching our heads as to why as there seemed to be no electrical fault. In fact in the end it proved to be a mechanical fault the outlet valve was leaking just slightly and pressurising the cylinder once this had happened there was not enough pressure to lift the inlet valve to de-load. As a result of trying to start under load the fuse would blow.

It was cured by instead of lifting valve in star it was dropped in delta so when not running inlet valve always open so pressure could not build up.

Faults like this are hard to find and it needs someone with both mechanical and electrical knowledge to cure.

I will guess it's the start amps rather than run which is causing the problem here. And rather than a larger supply what is required is some other way to start it.

With the fault I related it would start and run OK with empty tank it was only as the pressure increased and a re-start with pressure already in the tank was required that the problem arose.

Now it may be for things like paint spraying it is just a case of setting the pressure to a lower value. But since we don't know what the compressor is like it's all guess work.

But I can see no reason for the way the query was answered. The idea is to say how he can get around a problem not just say you can't do it that way.

 

[SimonH2]

a motor drawing 40 amps must be a 240 volt 3 phase, a lot of people think if its 240v its single phase.

Except that it isn't a motor drawing 40A - the OP said it needs a minimum of a 40A supply which is a different matter.

My (admittedly limited) experience is that some manufacturers quote excessive supply requirements as that allows them to avoid a lot of hassle. Eg, it may well be perfectly suitable for a 32A supply if the right type of breaker is used, but then they'd get people complaining it trips a curve B breaker when the manufacturer specified a curve C or D. Ie, it's not the running current, but the starting current as someone else has already picked up on.
So I suspect they just say "minimum <some silly large value> supply" just so that they have an easy get out and don't have to think further than the characteristics of a B curve MCB.

It would help if the OP posted details - or just a clear photo of the rating plate. I wouldn't be surprised to find that it's only 4hp or so.
When a friend replaced his compressor some years ago, he put a 13A plug on the new one (4hp/3kW) and found that it blew fuses quite regularly - but didn't blow fuses when he plugged it into an extension lead (1.5mm^2 arctic flex) because that limited the inrush current. He had melted the switches in several of his sockets before asking me to put a supply in for it.

Another mate has a 2x 3hp compressor running fine off a 30A fuse. Had some troubles when it was new and "tight", but once run in it's been fine except in very cold weather when the second unit can be a bit slow starting - the trick being to recognise it's not starting and switch off for a minute. Once it's run and warmed up it's fine all day.

Yes, some of my mates are "not very technical" when it comes to 'lectrics

EDIT: In my day job I've come across this with UPS manufacturers. They seem to like to quote minimum supplies of at least twice the rating of the load - and at least one has told me they will invalidate the guarantee if connected to a lesser supply. Ordinarily this might not be an issue, but when your load is half the rating of the main fuse and you're planning for some increase, then that becomes and issue. And I've not found one manufacturer with any ability to restrict battery recharging rate to reduce the headroom required - I think they live on a planet where unlimited supplies are the norm

 

[ban-all-sheds]

The idea is to say how he can get around a problem not just say you can't do it that way.

If you want the compressor in teh garage then you are going to have to run some cable around.

The CEEFORM connectors are available in various sizes
eg 16A, 32A, 40A. 63A. 80A. 100A. 125A.

But you are going to need to add a new circuit and you wont be able to do it on that old fuseboard.

So a separate 40amp switch fuse will be needed plus some hefty cable.
This is notifiable work and you'll need a registered electrician to do the stuff for you.

 

[SimonH2]

If you want the compressor in teh garage then you are going to have to run some cable around.

To be fair to the OP, leaving aside his gender issues*, he did already have a plan to put a longer lead on the compressor and have a socket next to the fuseboard that he can use when required (his fuseboard would need capacity, and he could use an RCD socket to avoid the need for RCD in the board.) It would work, but from experience I think he'd soon tire of running the flex out every time he wants to use the compressor (which tends to get used much more if it's "instantly available"), and will probably find the WAF** quite low at having to leave the door open between garage and house. The latter might be eased if the house layout would allow the circuit to be taken out the back of the CU and into the garage - but I suspect not from the way the original question was worded.

* He confuses plug and socket, but otherwise his plan seems reasonable.

** Wife Acceptance Factor