Hello all, could do with some advice about my compressor please....I have a nice Alup one that has been trouble free since I bought it 7 or 8 years ago but its 3 phase and I'm moving to a different workshop that only has single phase. I'd rather keep it if possible so (bear in mind I don't really know what I'm talking about here!) I guess my options are either a phase converter or replace the motor with a single phase one? Which would be better....both in terms of cost and reliability....and any problems I'm likely to have with either option? Thanks for any help......
- Thread starter rja52
- Start date
brightspark
Member
- Messages
- 40,165
- Location
- yarm stockton on tees
as long as the rpm and hp are the same just a bit of wiring and starter switch to alter and re drill the base that motor sits on as the single phase one might be slightly larger . you may need a longer belt as well
Do you not usually have to fit a larger HP single phase motor due to it having less torque than a 3 phase?
The figure 1/3 larger comes to mind. So to replace a 3 hp 3 phase you use a 4 hp single phase.
Either that or gear it down by changing the pulley ratios, just takes longer to fill.
Thanks for that....I'm definitely preferring the motor change option....though didn't realise I'd need more hp
Its a piston compressor....a V twin
Will the existing pressure switch work/be modifiable....or simpler to get a new one
No concerns about actually fitting the motor.....making brackets etc is no problem ( I build m'cycle frames) just know nothing about compressor electrics.....last post is totally over my head I'm afraid!!
Its a piston compressor....a V twin
Will the existing pressure switch work/be modifiable....or simpler to get a new one
No concerns about actually fitting the motor.....making brackets etc is no problem ( I build m'cycle frames) just know nothing about compressor electrics.....last post is totally over my head I'm afraid!!
mtt.tr
Member
- Messages
- 8,431
- Location
- Essex/Suffolk border
You could use the pressure switch to engage the vfd will be the easiest option
timgunn1962
Member
- Messages
- 315
- Location
- Lancashire UK
I'm not sure it's necessarily quite that simple. The pressure switch has a little unloader valve on the bottom which depressurises the line between the compressor and the Non-Return Valve where air enters the receiver. The idea is that the first rotation of the pump (usually 2 to 3 rotations of the motor with a belt-drive machine) is against minimal pressure. If you use a VFD and ramp up the speed gently, you may find the current spikes too soon/too high (though I may be overthinking things).
What is the motor? Most belt-driven 3HP compressors use a 2-pole (about 3000 RPM), 90-frame foot-mount motor, whether single- or 3-phase. I've changed 3 HP, 2.2 kW, compressors between single- and 3-phase on many occasions. It usually means changing the motor, cable and sometimes the pressure switch (always if going from single- to 3-phase. Sometimes when going from 3-phase to single-phase, depending on the pressure switch). The single-phase motor does need to be a capacitor-start type intended for high starting torque.
It looks like Alup may use a 4-pole motor (about 1500 RPM) on some of theirs. If so, it's likely to be a larger, 100-frame.
A pic of the motor plate and another of the pressure switch would be helpful, along with a wider-angle pic showing how it all fits together.
mtt.tr
Member
- Messages
- 8,431
- Location
- Essex/Suffolk border
Can't see it being an issue just set the ramp up to an appropriate time and the ramp down to coast
timgunn1962
Member
- Messages
- 315
- Location
- Lancashire UK
It is a while since I looked at it (running a 3HP, 3-phase compressor at home), but it just seemed the ramp-up would need to be really short, effectively duplicating a DOL start, and that would probably take the current higher than most of the VFDs I'd used would allow. DOL start is usually 6-7 times FLC IIRC. I think most of the VFDs I've used have instantaneous trips at 200% or 250% of rated current.
It was easier to go single-phase at the time, so I never got to find out whether it was a problem, though I had got the bits together for an unloader solenoid valve, to be closed by the "at speed" output from the VFD.
It was easier to go single-phase at the time, so I never got to find out whether it was a problem, though I had got the bits together for an unloader solenoid valve, to be closed by the "at speed" output from the VFD.
