3 phase - aka headache

[Bill Edwards]

Well I have got myself a lovely conpressor (I think).
But how to use it?

Am I right in thinking that as it can be configured to run on 240v that the best approach is an inverter? They're not cheap!

Is this what I will need?
http://www.ebay.co.uk/itm/1-5kw-2-H...752566?hash=item4ac0a70a76:g:nQUAAOSwLVZVkoWg

 

[AndyStobbs]

At 1.5KW the cheapest approach is a single phase motor.

 

[piman]

Hello Bill,

what do you call 'not cheap', some less than £100 on E-Bay, cheaperthan a new single phase motor.

Alec

 

[brightspark]

u can pick a single 3hp one up for less than 100 quid

 

[Bill Edwards]

But are the sub £100 ones able to supply 1.5kw? The ones I saw with that capacity started at £160.

 

[piman]

Hello Bill,

yes if you look hard enough, e.g. :- http://www.ebay.co.uk/sch/i.html?_f....H1.X2+H.P.+vfd.TRS0&_nkw=2+H.P.+vfd&_sacat=0

Alec

 

[Wendelspanswick]

This is the one I use:
https://www.ebay.co.uk/ulk/itm/261939824052
A 3KW (4HP) Huanyang inverter, it runs the majority of my 240V 3 phase kit, £105.

 

[HughF]

Bill, a 3kw vfd is around 105 gbp on eBay. The best way to use it on a compressor is to use the pressure switch to switch the control wiring on the vfd, the start/stop terminals. I have had success doing this in the past.

You would re-configure the motor for 220v, connect it directly to the vfd then run control wiring to the pressure switch from the vfd.

 

[Bill Edwards]

Ahh, I didn't use the best search term when on eBay...

So, before spending £105 I'd like to check - can I use smaller? The motor says 1.5kw, and such an inverter is £83 which is more attractive. Or, should I, for not much extra, buy a bigger unit to give a better margin for optimistic seller claims...? Am I trying to pinch the pennies too far?

 

[dobbslc]

If you're going to buy an inverter get a bigger one that way you can use it to run a 3 phase pillar drill, lathe, mill, grinder.....

 

[HughF]

Insert joke about yorkshireman and parting with money here....

In all seriousness, buy the 3kw one.

 

[Bill Edwards]

What sort of size would be needed for a typical lathe, mill, shaper etc? Is 3kw enough for in case such things appear?

 

[HughF]

The biggest Chinese vfd's with single phase input are the 3kw ones. You can get bigger, but they are 380/415 in and the same out.

They are cheap enough to buy 2nd hand that it makes sense to have one one each machine you buy. A small 3 phase shaper might have a 1.5kw motor for example and a vfd of that size can be picked up used for 35 quid if you wait and watch eBay.

 

[Wendelspanswick]

My mill is 3kw, Pollard Corona drill 2Kw, Colchester Student 2Kw, anything bigger than 3Kw only tends to be found on large woodworking machinery as it takes alot of HP to remove large volumes of wood whereas with metal you tend to remove small amounts slowly.

 

[MattF]

Ahh, I didn't use the best search term when on eBay...

So, before spending £105 I'd like to check - can I use smaller? The motor says 1.5kw, and such an inverter is £83 which is more attractive. Or, should I, for not much extra, buy a bigger unit to give a better margin for optimistic seller claims...? Am I trying to pinch the pennies too far?

I can understand your reluctance to part with extra unless necessary, but with stuff like this, it's worth paying the extra £20 or so and having that extra capability. It's always there if you need it. Spend once, spend right.

 

[rikrobson]

I'd go for the 2.2 one at least. There is a power factor of. 7 on your motor . Take that into consideration and the 2.2 is just enough.

 

[Pete.]

For a compressor I'd go for a single phase motor. It doesn't require any of the features provided by a VFD.

 

[HughF]

For a compressor I'd go for a single phase motor. It doesn't require any of the features provided by a VFD.

VFD is easier to retrofit than a new motor, no need to **** about with mounts that don't line up or pulleys that have the wrong bore... Just bolt it to the wall near the compressor and change the wiring... I'd go VFD personally.

 

[Bill Edwards]

This is the beast:

I don't really fancy swapping moyors tbh, I would need to go out and buy one of the right size and couple it up and mount it (not as easy as a belt drive would be).

 

[dannyp]

personaly my preferd option would be to resell it and buy the right machene.

my 2nd option would be to fit a 3 hp 2.2 Kw singel phase motor ( sigel phase motors allways seem to need to be bigger than theer 3 phase mates when it comes to starting up as they struggel with torque compared to a 3 phase )

the absolute last option would be a VFD, the functionality just isn't needed, it's something extra to go wrong, ( tho this motor can not all motors can be configured to 220v 3 phase ) VFD's and electricity meters i've had it where i'm not sure what was going on but the meter was clocking up a fair rate faster than it should have for some reason. also the vfd will need to be bigger than you think for, 1.5 KW motor on a 1.5KW vfd dosent allways work going to need more like a 2.2 kw and even still may run into starting problems.

motors pull a hell of current during startup that a vfd can't supply so they tend to have soft start, but a compressor needs to get to working speed quite quickly, enertia is needed in teh flywheel to somooth over the compressions else tehy can stall when pumping at pressure, compressosrs have a small resovor ( usally the pipework between the NRV on the tank and the compressor itself also used for cooling the air, this pipework is drained down when the compressor shuts off, this volume gives means teh pump is only outputing at low pressure to start with and gives teh motor chance to get upto speed before the pump starts presenting much resistance