Single Phase 230V supply and 3ph Compressor install.

[Simon47]

I'd have thought that it woukd run fine in delta. L & N to two terminals, cap between one of those and the other corner to assist the third phase. That is the basis of a rotary converter.
You'll probably want extra cap during start.

Having played with making a rotary converter in the past, look forward to a few blown fuses/tripped breaker while you work out the amount of cap needed.

It will help a lot if the compressor can be unloaded during start.

 

[ElectroChem]

Thanks for the reply.
I'd be happy to find a 2nd hand Rotary converter or VFD, but all about timing that appears.
It does look like a VFD is 6/4 Fav though. They are expensive, and must decide. I have looked at a few, avoiding e-bay and Amazon, just blatantly gear from China. Was tempted all but 30seconds. Invertek make them, I have an one on pool system, good gear but pricey.
Is there a make you might be able to suggest or know of that is decent. This compressor wont be used much.
Thanks again?

I'd have thought that it woukd run fine in delta. L & N to two terminals, cap between one of those and the other corner to assist the third phase. That is the basis of a rotary converter. You'll probably want extra cap during start. Having played with making a rotary converter in the past, look forward to a few blown fuses/tripped breaker while you work out the amount of cap needed. It will help a lot if the compressor can be unloaded during start.

Using caps to make a pseudo-three phase supply is always a bodge. True single phase motors have their run winding designed to carry the full power of the motor, three phase motors are intended to carry a balanced load across the three windings.

I would also not use a cheap rotary converter, as these supply three lines at 0/90/180 not 0/120/240 phase angle so you still need to de-rate your motor. A compressor is exactly the wrong load for any kind of kludged up pretend three phase supply.

Use the right tool for the job, in this case that is a true 400V supply or rewire the motor to delta and use a 230V 1-to-3 phase VSD. Otherwise your next thread will be entitled "I blew up my compressor motor"

 

[itstony]

Using caps to make a pseudo-three phase supply is always a bodge. True single phase motors have their run winding designed to carry the full power of the motor, three phase motors are intended to carry a balanced load across the three windings.

I would also not use a cheap rotary converter, as these supply three lines at 0/90/180 not 0/120/240 phase angle so you still need to de-rate your motor. A compressor is exactly the wrong load for any kind of kludged up pretend three phase supply.

Use the right tool for the job, in this case that is a true 400V supply or rewire the motor to delta and use a 230V 1-to-3 phase VSD. Otherwise your next thread will be entitled "I blew up my compressor motor"

Thanks for the reply.
Yes Sir, I have seen a capacitor setup, but it was as an experiment and deemed not a good method.
Not clear on your last reply after your first an earlier one you confirmed you would be happy on this setup to go with a VFD.
Have you has second thought, or have I missed something there?
I had accepted using an Invertek, would be over £2-250. If this is not the tool for the job, I definitely would not spend that.

 

[James]

Use the right tool for the job, in this case that is a true 400V supply or rewire the motor to delta and use a 230V 1-to-3 phase VSD. Otherwise your next thread will be entitled "I blew up my compressor motor"

FVD & FSD are the same thing, variable frequency drive or variable speed drive.
that is the route i would be going down, wire the motor in delta and use a VSD.

 

[davesparks]

Use the right tool for the job, in this case that is a true 400V supply or rewire the motor to delta and use a 230V 1-to-3 phase VSD. Otherwise your next thread will be entitled "I blew up my compressor motor"

A true 400V supply would be the wrong thing for this motor, as stated in the OP it is rated for 220V delta 380V star

 

[Simon47]

I would also not use a cheap rotary converter, as these supply three lines at 0/90/180 not 0/120/240 phase angle so you still need to de-rate your motor

Wouldn't 0/90/180 be harder to make than (at least an approximation of) 0/120/240 ? Reason I say that is that for a rotary machine, regular 3 phase stuff if common and cheap, to get 0/90/180 would need a custom wound machine. A static converter (i.e. a box with a few caps in it) will give you a 90° third phase - but only under no load - I have a Transwave example of that in the garage, and under load you can get quite a reasonable balance acroos the lines.
As I mentioned, a whike ago I was experimenting because we wanted a 3P supply for my mates home garage, partly to drive the 4 post lift we'd got hold of. Dad just happened to have a 5hp motor lying around which was going to be the basis of it - plus a few caps and an old rescued manual star-delta starter - the biggest probkem we had was getting it going ! I never finished it because my mate got divorced and the house got sold - so we no longer had a need for the 3P.
Sadly, that motor and the step up transformer were among the 400kg of motors I had to weigh in when dad died. If I'd had the room I'd have kept a lot of them.

 

[ElectroChem]

Wouldn't 0/90/180 be harder to make than (at least an approximation of) 0/120/240 ? Reason I say that is that for a rotary machine, regular 3 phase stuff if common and cheap, to get 0/90/180 would need a custom wound machine. A static converter (i.e. a box with a few caps in it) will give you a 90° third phase - but only under no load - I have a Transwave example of that in the garage, and under load you can get quite a reasonable balance acroos the lines.
As I mentioned, a whike ago I was experimenting because we wanted a 3P supply for my mates home garage, partly to drive the 4 post lift we'd got hold of. Dad just happened to have a 5hp motor lying around which was going to be the basis of it - plus a few caps and an old rescued manual star-delta starter - the biggest probkem we had was getting it going ! I never finished it because my mate got divorced and the house got sold - so we no longer had a need for the 3P.
Sadly, that motor and the step up transformer were among the 400kg of motors I had to weigh in when dad died. If I'd had the room I'd have kept a lot of them.

Yes and no. A full rotary converter that uses a single phase motor to drive a three phase generator will produce all the right angles and voltages. The cheap rotary converters use single phase with a cap to spin up a three phase motor, then tap the unused terminal for the third line. From my understanding, the magnetic interactions inside the motor produce a voltage at this terminal that is midway between the other two, which from a single phase source will give 0/90/180. The third line also has no guts and will sag under load, meaning this style of rotary converter can't drive heavy loads.

 

[davesparks]

A full rotary converter that uses a single phase motor to drive a three phase generator will produce all the right angles and voltages.

That would be a motor-generator set, not normally described as a rotary converter.

 

[Simon47]

The cheap rotary converters use single phase with a cap to spin up a three phase motor, then tap the unused terminal for the third line. From my understanding, the magnetic interactions inside the motor produce a voltage at this terminal that is midway between the other two, which from a single phase source will give 0/90/180. The third line also has no guts and will sag under load, meaning this style of rotary converter can't drive heavy loads.

No, I think you are misinformed there.
If you connect 1P across 2 phases of a 3P motor, give it a spin, and then apply power, you will get something resembling 3P out of the 3 terminals. It may not be too well balanced, but it will be something resembling 3P. It will do this without any caps, but adding caps will a) add a rotating element so you can actually start the converter, and b) assist in balancing the phases. As you say, without the caps, you have problems starting large loads, but once running these loads will add their own conversion.

One of the tales my late father used to tell was of a callout to a joiner's workshop - this was maaaaany years ago when he was an on the tools electrician. They had a problem with one machine - but only one machine. All the small machines were working fine, just this one big one wouldn't start. Turned out they'd lost a phase at the incoming mains, the machines running created enough of the missing phase for the smaller machines to start, but not enough to start the biggest machine. I guess they must have had something already running when the phase was lost, and that was able to start other machines.

 

[ElectroChem]

I'll freely admit that you all have more experience with rotary converters than myself, which would be none. One of the benefits of living in Australia is a very unified electrical system, and the regulatory backing to tell customers that no, we can't always 'just make it work'.
That said, I'd want to see a scope trace of a single phase driven rotary converter giving anything resembling a balanced phase angle output. When you're starting with two lines that are by definition opposite/180 apart, there's only so much you can do.

 

[itstony]

I'll freely admit that you all have more experience with rotary converters than myself, which would be none. One of the benefits of living in Australia is a very unified electrical system, and the regulatory backing to tell customers that no, we can't always 'just make it work'.
That said, I'd want to see a scope trace of a single phase driven rotary converter giving anything resembling a balanced phase angle output. When you're starting with two lines that are by definition opposite/180 apart, there's only so much you can do.

Well, I do feel a little better this was not something easy or common.
But then not received anything broadly recommended.
I will speak to Invertek tech guy and request what view he has on this situation a little more clearer if it's the only alternative.
Speed control is not a requirement.
However load might be an issue with a compressor when pressure drops and resumes under load.

 

[Simon47]

That said, I'd want to see a scope trace of a single phase driven rotary converter giving anything resembling a balanced phase angle output. When you're starting with two lines that are by definition opposite/180 apart, there's only so much you can do.

That's mostly the point of using a rotating machine as part of the converter - you have something capable of generating out of phase components relative to the input. I'm sure my late father could have explained it clearly - he used to design motors etc. for Brush back in the 60s - but my machines theory is rather scant.
I would imagine that if you take a standard 3P squirrel cage induction motor and apply 1P across two connections (with some current limiting !), then as you infer, without the rotor spinning then you'd simply get some variation on a single phase transformer. But once you spin up the rotor, with only one field present, the rotor is going to be going in and out of that field - rather than moving between fields as the fields rotate. That's going to create currents in the rotor cage, and the resulting field is bound to have some effect back into the field windings. As I say, my machine theory was never strong, and it's a lot of years since we did it at uni, but going on the principle of the current in coils trying to counteract change, I'd expect the currents in the rotor cage to act so as to try and reduce the variation in magnetic field as the rotor rotates. That's then going to induce currents in the stator windings with a similar relationship (magnitude and phase angle) to those you'd find if it was fed with "real" 3 phase.
So some of the 1 phase input current goes into creating rotor currents, and those rotor currents go into making 3 phase stators currents. Just how strong this is I don't know !

However load might be an issue with a compressor when pressure drops and resumes under load.

Compressors don't normally start under load. Small ones typically incorporate an unloading valve in the pressure switch that controls the motor - when the switch clicks over, as well as opening the electrical contacts, it opens a dump valve that unloads the pressure in the delivery pipe between compressor and reciever (there's a non-return valve that keeps the air in the receiver). Beyond that, there are various schemes for allowing the compressor to run up before getting onto full load - soft-start valves vent the air from the delivery pipe until it reaches a certain pressure, a dump valve may just allow the air out until some condition (may be just a simple timer) is hit, and there's probably other schemes I've never come across.

 

[itstony]

Appreciate all replies and contributions to an interesting topic. For me anyway. Contacted Invertek Tech man who was very patient and helpful. It does appear the VFD would be the best option for 230~1 to 230~3 and have a stable control. IP20 unit will be +/-£ 220 Which is a costly option. My moto is “Never buy Cheap, only good Value”. If anyone does know of a trusted lower cost option VFD that would run this compressor only on infrequent times on short time, please let me know. Thanks

 

[James]

I doubt you can do it properly for any less than that price.

 

[Lucien Nunes]

Not specifically useful to the OP but just to clarify...

A motor generator is exactly what the name says: A motor driving a generator.

(+) Any input to any output, DC-DC, DC-AC both ways, AC-AC of different freq or phases
(+) Many control options including full variable voltage control
(+) Can often be assembled from standard machines
(-) Large, inefficient; all power has to be converted to mechanical and back to electrical.
* Popular 1900- 1980 for DC machine drives e.g. Ward Leonard, gearless express lifts.

There are two kinds of rotary converter.
A 'proper' rotary converter is a single machine that has a wound armature rotating in a field system. The armature can be wound for multiple voltages and have both commutator(s) and sliprings.
(+) Smaller, more efficient than corresponding motor generator because some power flows straight through. The remainder is converted to mechanical in part of the winding and back to electrical in another part.
(-) Limited permutations of input and output; DC-DC, DC-AC, not usually AC-AC.
(-) Limited freedom to control output, input and output voltage, frequency and pf are interdependent.
(-) Usually needs to be purpose-designed, AC machines often need specialised transformers.
* Popular 1890 - 1960 for very large constant voltage traction supplies e.g. NYC subway, and small DC-DC units for radio HT.

Then there is the rotary phase converter which is what we are talking about here. The popular units are basically induction motors rated to regenerate the missing phase. Simon47 generally nails the whole story above.
(+) Compact, low-cost way to make polyphase AC from single.
(-) Symmetry and load regulation poor especially with difficult loads
(-) Basically a whole bunch of compromises that sits in the corner making a whirring noise, but gets the job done.
* The only kind that is still in widespread use