Considering a 3 phase CNC small lathe - - - have 240V single phase

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Aug 14, 2020
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In Canada having 3 phase wiring is strictly a commercial or industrial option.
While the 2 phase wiring is standard for residential. This allows 110 volt single phase and 220 volt two phase appliances.
Having used single phase, two phase and three phase wiring there are advantages to each.
Three phase being what it is is strictly a management decision.
Really doesn't need to be that way but getting the management to understand the advantages of being 'like the rest of the world' is a very very difficult position!!

I'd like to hear what the advantages of each as you see them.

(I'm seeing not a lot of advantages to our present system except that its legacy and that makes it 'cheap' but its sure not helping our competitiveness!!!)



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Oct 18, 2013
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Keswick, Ontario
(I'm seeing not a lot of advantages to our present system except that its legacy and that makes it 'cheap' but its sure not helping our competitiveness!!!)
The only thing that makes it 'cheap' is that it is legacy, 110v uses more copper which is quickly becoming a problem but removing and replacing with smaller wire for 220/240v is more expensive than it's worth and just like Imperial size tooling, it's harder and more expensive to buy internationally.


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May 21, 2016
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Hmmmmmmmmm - - - - I've seen VFDs even past 2k HP - - - - - but I would bet that they're NOT cheap.
The fact that one needs to oversize the VFD (the rating that I've found was to use 58% when using the vfd to produce the third leg) that gets to be the expensive part!
I'm thinking that when you calculate the costs of up-rating 3 or 4 larger VFDs it might be cheaper to get an electronic inverter and then size the vfds for just rated but I haven't calculated that as of yet!
My largest VFD is rated to 3.8kW (5HP). It is designed for 230VAC single phase input. VFD derating is only required if a 3 phase unit is driven from a single phase supply.

Old tech, or low quality modern VFDs produced switching voltage spikes that required motors with better insulation. Not so much of a problem with modern high quality VFDs.

The biggest rotary converters (motor-generators) I have worked with were rated to 1MW power. The motors were supplied with 11kV. The generators produced 6.6kV. Those were expensive. Cost is relative.

I would not touch cheap no-name Chinese VFDs. I have 3x VFDs. All are recognised industrial brands. They cost more but are better value for money.


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Dec 13, 2019
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Culberson, NC
well, I can give you my thoughts on this since I ran a motor, generator, alternator shop and built several Phase converters.

If you use a phase converter, you have to wire in 3 phase power, that means additional wiring and all that expense. If the whole shop will be wired and all stuff is 3 phase, great. If not, add money for the wiring and box and breakers for 3 phase and they ain't the cheapest things you can buy.

3 phase motors are cheap, single phase are not cheap. If the windings in 3 phase are bad, new motor. Bearings are cheap. Single phase motors have capacitors and centrifugal switches and as a rule, the caps go bad first, then the cent. switch which then burns the motor up. I have repaired a few centrifugal switches but it was lucky that the motor used a switch I had. Usually single phase motors are made somewhere on earth and nothing is standard.

And remember, with a VFD and 3 phase motor, you get variable speed, simple switch reversing, and, If you order now, motor deceleration which means an electrical brake to slow the motor quickly instead of waiting for it to run down.

3 phase motors are easy to reverse while each time you start and stop a single phase motor, the caps. and the centrifugal switch cycle, and they only have so many cycles in their life. Wiring a single phase, reversable motor with a switch or relays is a pain. VFD reverses for you. I don't like lathes with single phase motors that cycle a lot because of the failure of both the caps. and the centrifugal switch causes motor changes more often than I like. All the lathes I worked on were either 3 phase or converted to 3 phase with a VFD. My current lathe is single phase and the 3 phase motor and VFD are on the shelve ready to change out but then, I'm only a hobby machinist and don't use it like we did in the shop. When I got my lathe, I spent 9 hours fixing the control circuits and wiring so it worked right.

On the negative side, you don't fix VFD's unless you got the skills and equipment. I tried several times and found it easier to just get a new one. If you pay someone, do a cost benefit for sure. Also, wiring one up for your application takes a bit of electrical skill which you may or may not have. Your decision.

Boils down to what you have. If you have a spare 3 phase motor around, use it to make a phase converter. They are not complicated in their primitive state and the wiring diagrams are out there on the net. I have done most conversions using 220 single phase to a VFD. Rarely does it pay, if you already are wired for single phase, to run a rotary phase converter for 1 or 2 machines unless the wiring between them is close. I used rotary phase converters where I had really big motors or lots of them and then the phase converter usually is cheaper than a VFD, especially to run a 30-50 horse motor or to power a whole machine shop.

Ken I

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Oct 31, 2010
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Cape Town, South Africa
Just my 10c worth.....
Most VFD's are intolerant of out of balance phase loads - like a single phase coolant pump on a 3 phase lathe - so check out the ratings of any VFD for phase imbalance limits.
I would think a single phase butt welder (blade welder) on a 3 phase bandsaw would not work.
Regards, Ken