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  #11  Post / In Thread 
Old 27th May 2016, 10:57 AM
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naomicornish naomicornish is offline
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Thank you, all! This is very helpful.

Story with this lathe, is, that where it came from, the guy fitted a single phase motor to it..... but it is a 1/2 or 3/4 HP.... something like that..... greatly under powered..... (and for some reason it is running in reverse...... but that is meant to be getting sorted out before it is delivered to me).... It still has the original 3 phase motor and panel, too, so I could easily get that refitted, and buy an inverter/converter....

Hmm, talking of which..... Some people use inverters... others converters..... I know what the visual differences are, and know someone who uses a transwave converter, which is pretty good..... but are inverters the better option? They certainly seem a lot cheaper.... I am happy to do that option if a suitable single phase motor doesn't appear....

Your advise would be very much appreciated
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Old 27th May 2016, 11:47 AM
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Standard AC motors are 415V 3 phase, however most modern motors have six terminal so can be wired for 240V AC 3 phase.
To use an inverter which is single phase 240V AC in gives out 240V AC 3 phase, so for best results you need a motor that can be converted.
I believe the transwave converter capacitors and inductors to give out 415V AC 3 phase.
I don't want to diss somebodies produced, but believe me the inverter is the best way of doing it.
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Old 27th May 2016, 11:58 AM
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You can make a 3 phase motor to run on single phase, the trick is knowing the capacitor value

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Old 27th May 2016, 12:18 PM
Phil Procter Phil Procter is offline
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You will find that the original motor is a two speed one and that will possibly cause problems when trying to run from an inverter.
Also you would need to make sure that any motor you choose can be connected in Delta, ie the windings form a triangle and each phase of the inverter is connected to a corner of the triangle.
Some motors are only connected in Star, ie the windings have one common centre connection, this is OK when used on a normal 3 phase supply at 415V, but for inverter use it needs to be Delta when driven from a household 240V supply.

Don't be put off by all this, most motors can be used but it is just something to be aware of
The inverter link I sent on ebay was just a random example, I agree with the comments about buying from a reputable source.

A converter will give you an artificial 3 phase supply, but they will not give any speed control so have only a limited use and tend to be fairly expensive.
An inverter gives an artificial 3 phase supply, but it's frequency can be adjusted to give a good range of variable speed.
At work we tend to work on approx half or double the rated speed of the motor, if you go too slow the motor fan will not push enough cooling air through, too fast and you can exceed the speed that the bearings are happy at

I only have one machine in my workshop (pillar drill) that is not on an inverter, but I have the parts ready to convert it very soon.

My dad used to run his three phase machines using the capacitor method, but this was long before inverters became affordable.

Phil
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Old 27th May 2016, 12:48 PM
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I got an inverter and would never want anything else.

I used these guys:-

https://www.inverterdrive.com/
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Old 27th May 2016, 01:50 PM
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Hmm, yes this inverter option is looking more and more appealing...... will continue to research and will ask an electrician friend for some advise, too.....

Julia / David - which inverter did you get? (same question to anyone else who has an inverter, too)
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Old 27th May 2016, 02:11 PM
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Ring them up, or email them tell them what you are trying to do, and let them specify it,
that way if there are any problems you can go back to them.
David
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Old 27th May 2016, 07:15 PM
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This is just some observations.

I haven’t used an inverter but I used to have a 415V phase converter, if I needed one I would have an inverter.

I used to work in the Machine tool industry and the Motors that we used on our CNC machines were much bigger HP than those on conventional machines, this is because the power advantage of a mechanical speed reduction is lost.

For example a 1HP motor with a 4 to 1 mechanical speed reduction would give 4HP at the output. (losses not considered)

With electronic speed control a 1HP motor running at ¼ speed would have an output of less than 1HP taking internal losses into consideration.


See Phil Procter’s post above regarding motor speeds and cooling.

You would need to avoid running the motor too slow, a Pulley change might have to be considered to keep the speed up and the motor cool.

Someone with a two speed motor would be able to run their motor on the low speed at double the rated speed, it is better running fast than slow.

The motors that we used on our CNC machines had an independent motor driven cooling Fan allowing the main motor to have a wider speed range.

I would be interested to hear if anyone using inverters has overheating or lack of power at low motor speeds?
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Old 27th May 2016, 08:23 PM
Phil Procter Phil Procter is offline
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I run a little Boley & Leinen (BCA) jig borer with a 0.55 kW motor on a Jaguar inverter. Parameters are normally set to run at an absolute minimum of 15Hz and a maximum of 70Hz

As an experiment I set it to run at 5Hz to see if it warmed up, it ran like that for over an hour with no issues at all, and it is almost impossible to stall it either.
I think the trick is to have a motor that is more than man enough for the job then amount of metal in the casing can keep it cool even without the effects of its fan.

I ran the Harrison lathe for a good few hours at 10Hz when turning my big flywheel, again no problems were apparent.

Phil
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Old 28th May 2016, 06:05 AM
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naomicornish naomicornish is offline
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Quote:
Originally Posted by davidnclare View Post
Ring them up, or email them tell them what you are trying to do, and let them specify it,
that way if there are any problems you can go back to them.
David
Yes, that's a good call.....

Am also interested to know about brands.... Some names I've heard of, others not.... Be interesting to know if people are going for a specific brand - why - any known issues, etc etc.... What's hot? What's not?
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