Question for motor experts.

I have a single to three phase inverter drive, which I bought in error not realising that the output was only 240V ph-ph. I saw something recently that suggested to me that an ordinary 240V single phase motor could be made to run off this by "throwing away" the capacitor and instead connecting the two windings across two phases of the inverter output, thus giving the advantage of variable speed.
Am I right in thinking this and would it work?

Stuart wrote:
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The answer is a definite maybe. Some motors will work like this with some inverters, other motors you will burn out the winding. It really has to be a PSC type, not the sort with the centrifugal switch that disconnects the start winding once the motor is running.
I'm not sure I understand the problem here though, most 3 phase motors can be wired up as delta for 240V or wye for 480V, and the rare one that comes hardwired wye can usually be opened up and rewired delta.
What size inverter is this and what motor are you trying to run?

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Understood
The motor I bought it for is wired star (wye). However I opened it up and cleaned it (it was on a old second-hand drill press) and decided that messing about trying to find the star point and rewire delta was more than I wanted to be troubled with. I had a newish 1ph motor to hand of similar power so used it instead.
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The inverter is a Eurotherm 601/003/230/F/00/UK.
Input is 220-240 Vac 1ph 50/60Hz
Output is 0..220-240 Vac 3ph 0-240Hz
Input current 5.3A output 2.2A
The question was about general priciplea but the drill press motor might be "favourite". It came of a Nutool (cheap chinese stuff) woodworking lathe. It is rated 370W nominally 1450rpm at 50Hz.
I would obviously need to open it up to confirm it does not have any switching.
The idea was given to by a post on the 7x12 minilathe group refering to the following: http://ww1.microchip.com/downloads/en/AppNotes/00967A.pdf figs 2 & 5

Stuart wrote:
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A motor that small is probably of the PSC type so it will probably work to some extent. I highly recommend modifying the 3 phase motor or acquiring another 3 phase motor. Even if you get the single phase motor working on the inverter, the performance will be far inferior to that of a real 3 phase. The center point in the star is normally right at the surface near where the other leads connect.
Another option you can use is pick up a 480V inverter, they are cheap (I got one for $3.99 on ebay) because few people have any use for one. They all expect 3 phase input, but the first thing they do is rectify and filter it into DC, so in most cases you can feed them single phase and it will work, although you should derate the output ~30% when doing this. To get 480V for supplying the inverter, you can use a boost autotransformer from 240V, or rewire the input rectifier/filter caps as a doubler as is done in older 120V switchmode power supplies.

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That seems to answer my question, thank you, though I suppose we ought to be comparing the performance of the motor running normally on single phase with its capacitor to its performance on the inverter rather than against a 3ph motor.
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That's interesting, I suppose I'd never really thought about it.
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I think I'd go with the transformer idea.

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I have great difficulty fully understanding what you describe. Itg is likely you are in over your head.
Most ac motors do not run well away from synchronous speed. In an induction motor the speed is somewhat lower than synchronous speed by what is called slip.
In principle, a properly designed three phase inverter could make an excellent speed controller. I strongly doubt that your inverter is like that.
A suitable inverter would have variable output frequency with output voltage proportional to the speed up to the rated speed. Such a inverter would provide full current at low speed and develop full torque without overheating the windings.
Bill

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See my reply to James.

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I do not have the patience or inclination to read all the specialized literature that may be out there. I try to think from fundamental understanding.
Bill

Salmon Egg wrote:
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I'm not sure how specialized it is anymore, variable frequency inverter drives are everywhere. Friends of mine own a machine shop and every machine they have uses one, even the drill presses have been refitted with inverters to dispense with the clunky split pulley drives. There are hundreds of VFDs on ebay at any given time and they have become cheap. It's *the* way to go if you want to run a 3 phase motor in a location that only has single phase power, or if you want variable speed in anything above a small fraction of a horsepower.
A few years back 3 phase motors were cheap because nobody wanted them for home shops. Now I've been re-fitting my own single phase machines with 3 phase motors to get the soft starting, reversability, and silky smooth variable speed. You can even brake and reverse on the fly, allowing a basic drill press to do power tapping to rapidly thread holes. Most even allow you to overspeed, many smaller motors can be safely run at 200% or more of rated speed.

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-------- double speed, frequency doubled, core flux halved so core is much more than adequate. Reactance higher but that is not a safety issue. The safety/heating issue is one that occurs at lower frequencies and, in that range, voltage must be proportional to frequency to limit core flux and losses (I.e total magnetizing current) to reasonable (rated) values. At speeds/frequencies higher than rated, the proportional voltage/speed/frequency relationship is not important as machine magnetic design is more than adequate.
Salmon Egg, as usual, is very capable of separating the BS from the facts.

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The problem is not so much heating as it is the forces on the rotating components when operating significantly above rated speed. I've seen at least one motor where the cooling fan shattered violently from centrifugal force. Bearings also wear much more quickly when operating at high speed, at least the plain sleeve bearings in smaller inexpensive motors. Some have balance issues too and will vibrate and resonate at certain speeds.

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That is precisely how these inverters (variable frequency drives) work, it's called VF mode. More fancy units also support sensorless vector mode where they use back EMF to sense the rotor position and are able to provide high torque at very low speed. I've been playing around with some of these lately and you can run a 3 phase induction motor down below 1Hz and it still has enough torque that you can't stop the shaft with your hand. Go slow enough and it will cog much like a stepper motor.