3 phase - aka headache

[daleyd]

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

Sorry mate but not much of that is true. Inverters usually have a ramp function programmed in because it puts everything under less stress, but if you wanted to you could have an almost instant start up, but most applications don't require this.

Lots also have a torque boost feature for starting, providing much more torque than you get out of a normal motor start, much more controllable too.

 

[Pete.]

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.

It's a fair argument, especiallysince we now know it's a direct drive unit.

 

[dannyp]

Sorry mate but not much of that is true. Inverters usually have a ramp function programmed in because it puts everything under less stress, but if you wanted to you could have an almost instant start up, but most applications don't require this.

Lots also have a torque boost feature for starting, providing much more torque than you get out of a normal motor start, much more controllable too.

inverters suck on comrpessors. when it comes to RAW power there is only so much they can handel power that goes in is powr that goes in. a lot of motors will draw or would like to draw meany times there rates run current on start up. if you had a 5KW inverter it wouldn't be a problem, but if the inverter can say only handel 1.5 KW theer is only 1.5KW avalible to accselarate the machene upto working speed and on a lot of machenes a gentel ramp up isn't a problem. on ( relativly ) cheep compressors it is. unless that motor is running at a decent speed before the pump sees much pressure it may stall. that is the problem i was trying t make the op aware of then you end up haveing to fit unloader valves that just vent all the pumped air untill the pump has got upto speed.

i may not be 100% correct on exactly why the problem occurs, but it's a problem i've encouterd a few times but i should know better by now than tp try and give a relativly detailed explanation as to why things i've tryed havent worked, i sall simply use the YES NO aproach.

now daleyd if your going to call me a liar i find that insulting and would like to call you a few things in person !

 

[daleyd]

I can only speak from my experience of designing and installing inverters and motor drives for the last 20odd years on anything from conveyors, fans, pumps, and I've not come across many situations where an online start is the preferred method other than simplicity and cost.

An inverter of a rated capacity is usually able to output a fair bit more for short periods during start up, typically anything up to 200% rated current. Any motor starting dol has a huge inrush current, it's a characteristic of that type of starting.

 

[dannyp]

i guess your used to working with decent quality inverters and not £100 chinese specials tho. high end invertes are good bits of kit and very capable but i dought man in shed bodgeing a 3 phase machene to run singel phase is going to use one of them. i priced one up recently for a mandrel pipe bender after ringing round and speacking to people the inverter that would do the job was more like £1500 not the £150 the seller claimed.

im my experiance playing about with compressors, and admitely ive got it horrobly wrong a few times. there are so meany factors that come into play and a lot of compressors have some nasty desing features.

i agree that in most situateons a steady wind up is preferd, but compressors without unloader valves is proably not one of them. i've retrofitted unloaders before ( justa gas selonoid and a timer ) so the output of the pump is just vented untill the pump has had time to get upto full speed. as what ive found with some compressors is once the pipe from the pump to the tank has come upto pressure unless the pump is upto speed so the flywheel has enough inertia to carry the pump over the compressions it may stall. then the VFD trips out to protect itslef and the motor, but as the pressure switch hasent been actuated it dosent drain the pipework from the pump to the nrv on the tank down it trys to start again and struggels like hell.

it may work but i think it's a big mistake to think of an inverter to be the same a true 3 phase supply

 

[Bill Edwards]

Well, it's looking to me like the £105 inverter is the best way to get it running hassle free. More than I want to spend, but choices are limited.

I'm expecting the thing to work, and it seems to be a well made piece of kit. I hope it's worth the outlay! It's an Atlas Copco LE6.

 

[dannyp]

well if it plays silly buggers you may have to fit an unloader

 

[HughF]

Or just make the pipe between the cylinder heads and the non return valve on the tank nice and long. Personally I doubt you will have a problem. Nice compressor by the way Bill...

 

[Bill Edwards]

Ta, I'm hoping it's a good 'un! I'm not sure on the output mind, but hopefully it'll be reasonable.

I can easily sort out pressure relief - I will probably need to get a typical pressure switch combined with unloader valve anyway, and a long pipe/tiny tank wouldn't be hard to give it chance to spin up to speed. The wiring for the inverter control may or may not be simple, if the instructions are in Chinglish I'll end up asking on here!

 

[Bill Edwards]

Ok, time to get this running.

I've gone and bought an inverter, then shortly afterwards discovered I have one I wasn't aware of.

I'll try to use the one I already have and keep the new one new (might end up for sale on here if it's surplus).

But I really don't know what to connect to what?

Can anyone tell me where to connect my 240v input, which connections go to the L1, L2, L3 etc on the motor and if there's any control circuitry to join up, perhaps for the pressure switch when I get that far?

 

[HughF]

U,v,w are the motor terminals. N and l1 are the mains inputs. 9&8 are probably an 'enable' jumper. You need to get the user manual for the inverter so I can advise how to connect the pressure switch to the control wiring, and how to set the inverter to the correct start/stop ramp timer etc.

 

[HughF]

Manual is here:

http://download.lenze.com/TD/8201-8204__Inverter__v02-08__EN.pdf

 

[HughF]

Pressure switch should make a connection between 20 and 28. You need to set the rotation direction by leaving e4 disconnected for Cw or connect it to 20 for ccw.

Set the acceleration and deceleration times in the menu.

 

[Robotstar5]

Lenze are a good bit of kit, used a few to run welding positioners.

 

[hareng]

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.

Been using a 7.5KW 3ph 400+V for the last 5 years from single phase 240V input.

 

[daleyd]

Lenze is really good kit, but you will need a keypad to programme it - they are usually included with the drive but sometimes get removed and hence lost(Or software, gdc lite version is a free download, but youll need a serial cable and adaptor.)

 

[daleyd]

If you have to buy a keypad make sure you get one for the right model, I think yours is 8200 vector series but the keypad from 9300 servo models fits - but doesn't work!!

 

[CornishPete]

in the pictures in the first post don't the data plates imply you can use either 240 or 415v by altering the copper bridging things? I'm assuming I'm missing something as everyone is saying to use an inverter!
how does an old oxford welder differ/work that runs from 240 or 415v? I've never known anyone need an inverter for one of those.

 

[HughF]

Umm, no it implies you can retire the motor for star or delta, therefore needing 249v phase-phase or 380/415 phase-phase...

Either way you still need 3 phases

 

[CornishPete]

cheers for that, I thought I must be missing something! Does the same apply to an old Oxford oil welder too? And is the 240v setting specifically for use from single phase converted to three phase, or are some 3 phase supply's only 240v to start with?
apologies for lots of questions if they seem daft but I've never had any experience with 3 phase.