Two Speed Three Phase Motor From Single Phase

Hi, I'm contemplating a mill with a 3ph 2 speed motor and don't know if it is possible to run (in both speeds) from a 1ph supply and if so what is needed, VFD or Phase Converter? I guess I could replace the motor with a dual wound one with a VFD but it them gets to be a fairly expensive machine. Any help/advice would be much appreciated.

 

Regards

Keith

If it's anything like most 2-speed motors, it will have two sets of windings, and be star connected, I suspect. Which means an expensive inverter taking single-phase 340v in and giving you 415v out per phase. The other thing you will have a bit of a problem with is that there's no way you can switch speeds with the inverter actually running the motor. In at least one switching direction (if not both), the reverse emfs generated in the windings will blow it apart, unless it has a staggeringly high protection rating. And generally they don't. So you can use it as a dual speed motor, but you have to bring it to a standstill before switching speeds.

 

I have a Bridgeport head to rebuild which has exactly the same dual-speed motor issues, and I'm faced with exactly the same potential solutions. One of the things that I'm thinking is that since most inverters provide pretty good torque compensation at slower speeds these days, it might be possible to leave it set to the higher speed and just use the variable speed control on the inverter. But I haven't got anywhere near trying this yet, so I don't know how practical an idea this really is.

Steve, thanks for that it was as I feared. Having looked at the machine today I have decided not to go ahead with it so back to the search. I have been looking for a replacement for my Bridgeport for a few years now and still haven't found the right machine. I looked at a bunch of new mills without finding an excuse to break out the wallet. I seem to be in the gap between the hobby and small industrial markets, in one corner the VMC/RF45 clones and the other the 836 type. Price difference is significant but I must say the apparent quality of the imports was fairly impressive over what I was looking at 10 years ago. The 836 with DRO, power feed and variable speed was very tempting until we got to the nearly £6K price tag.

 

Anyway, thanks again for your help.

 

Best regards

Keith

I fear that you're correct about the affordable gap in the market. There seem to be plenty of imported mills that fit into that small industrial category, but I haven't seen a cheapish one either. Oh, and I'm sorry about the typo in the first post - obviously it should be 240v not 340! The inverters that run 240v to 415v 3-phase also appear to have a pretty hefty price tag on them - I wonder who buys them? The people who purchase the £6k mills, perhaps? For a high power one, that's at least another 10% on top of the cost of the mill.

 

The really daft thing is that the one type of mill that generally falls into the just about affordable category, and seems to be what a lot of people go for - is second-hand Bridgeports, just like what you're trying to replace...

I am curious about the nearly 6K price tag.

 

One of my customers has recently bought the largest mill that Warco sell, the WM40.

This comes with inbuilt rise and fall motor, power feed to X, DRO and hardened ways on X and Y.

 

I was very impressed with the fit and finish of it and the price tag was 6K including VAT, 5K without.

I thought this was a steal for what you get for the money and streets in front of a Bridgeport [ Hawk - Spit - Ding ]

 

Not associated with Warco and currently don't own any Warco machines.

 

John S.

Steve, John Hi,

 

Good guess Steve, the Chester 836 was one of the machines I looked at yesterday. Very well specified (Variable speed, DRO, Power feed and full Coolant) and (the one I looked at) very impressive build. My only drawback was the price at a little below £6K. I also looked at a more basic version of the 836 from another supplier that would have been about £4.5K. However, as I want a DRO, and to run it from single phase I guess I would need to put another £750 into it.

 

John is absolutely correct in that there are some very good deals out there for what look like excellent machines. Unfortunately, Warco do not currently have any WM20 (their 836 varient) machines and I understand the lead time is likely to be long. There is another excellent offer from Warco at the moment that might temp me for the interim.

 

Steve is also correct in the point that the Bridgeport is as good a machine as most model engineers will ever need (Sorry John), but my issue is that for medical reasons it and more importantly the tooling is too heavy for me at the moment. That might also be an issue for me with the 836 type if the future is less than kind to me. I'm 90% decided to go with the Warco offer as if things improve then I can always trade up again. Small mills seem to sell reasonably well so I might not loose too much and in fact SWMBO has just said "what's the matter with having two mills". I guess I'm lucky in some ways.

 

Best regards

Keith

Lets think laterally - How much would it cost to have a 3 phase supply to the workshop, or see if your neighbour(s) would let you pinch a bit of his electricity - it may be on a different phase(s).

You'd almost certainly need the co-operation of the neighbours on both sides to get all three phases into your property that way - and you'd definitely need all three. The cost of adding a three-phase supply varies somewhat according to who supplies your power. Chances are though that it will cost you more than an inverter will - and it won't give you variable speed options, controlled braking and ramp-up, or anything else like that. What it amounts to is that even if you have a three-phase supply, using an inverter is still a good idea. And as long as your single phase supply to your workshop is up to it, you can run anything up to a 4hp motor under load without popping the breakers. But, you wouldn't be wanting to start one of those without a controlled ramp-up...

 

As for the mill compromise - well, when I get it finished, I'll end up with a SeniorPort (with a J head). That's a couple or three jobs down the line though, and I'm not getting anything like as much time on this as I'd like.

There is a solution to running multi-speed motors from single phase without great cost, but you need to understand and be comfortable with some electrics to do it. I use this technique on my Holbrook with a 3 speed motor and it has worked without fuss for some years.

 

Buy a 3 phase input inverter (pretty cheap), at least 50% bigger capacity than the motor you intend to run (I'm using an old AB rated at 2.2KW to drive a 2HP motor). Open it up and you should find two rows of large capacitors in the rectification stage (inverters turn the incoming supply into DC and then chop it up to provide the output). The rows may not be obvious, but they've been there in most inverters I've seen, especially big old ones. You are looking to make a connection between the two rows (the rectifier and the output to the next stages will be across the outside of the two rows since these capacitors are in series pairs in parallel with other series pairs, usually 3 sets). Cable this connection out to a suitable point where you can connect it to the incoming neutral. The incoming line connection will go to one (or more) of the normal connection points. This method produces more or less double the usual incoming voltage at the DC stage of the inverter, so a bit more than rectifying 415V, but there's usually enough tolerance.

 

So that's the supply, now the control. This will depend greatly on the machine, but on the Holbrook the lever that changes the speed using a large complex switch had a switch that cut the mains during the changeover. I replaced this with a modern micro switch wired into the inverter enable line (so 24VDC, low current). This means that the switching is not done with the output of the inverter live, nor the switch contacts, nor the motor.

 

Job done!

 

As I said you need to be comfortable that you know what you are doing, but it can be done with a bit of time and care.

 

Nick

Hi guys

 

Thanks for the ideas but our next door neighbours would be convinced that I was doing something illegal

 

Nick, thanks for your advice but I'm sorry to say that I fail completely on the "being comfortable with some electrics" part. I don't really understand what you are doing but will read it again (a few times) and see if I can grasp enough to at least not put the lights out.

 

At the moment I have taken the easy route and ordered one of the Warco special offers as an interim solution. In the longer term if all goes well I will look to convert that one to CNC and look for an 836 to keep my hand in as they say.

 

Thanks again for all your advice

 

Best regards

Keith

Hi Keith,

 

Don't worry it's not an intuitive solution; you need to understand some electronics , though not much. The AB inverter I first did this on (I've since also done it on a 5.5KW Lenze - much harder) came from a skip at the place I was working at then. I tried running it from 240v into the ordinary terminals, but it came up with a message which meant 'under voltage in DC section'. This and a little reading made me realise that inverters don't use AC directly at all, but convert it all to DC. The output IGBT (BIG and fancy transistors) are then controlled so as to simulate the output frequency required (generally they don't change the voltage in sine wave fashion but simply change the 'mark/space ratio' to imitate changing voltage over time). I realised that the key was not input phases but simply getting enough volts to the DC stage; More reading and I found the 'voltage doubling' technique I tried to describe earlier. So I tried it (standing well clear!); worked fine. This AB inverter was first used on a big pillar drill from the same source, but re-cycled to the Holbrook when I acquired it and decided that attempting to change the windings to delta was a risk too far! Changing most old motors to delta by digging out he star point and wiring back to a new connection block is usually fiddly, but not difficult; multi-speed would be much harder, if possible at all.

 

Whichever way you go I wish you luck,

 

Nick

Posted by KMP on 30/07/2011 20:55:41:

I can probably offer you a little help... What Nick's done is effectively a slightly cruder version of what happens in the expensive converter anyway - essentially he's built a voltage doubler input for the inverter.The basic principle of operation of the device goes like this:

 

For any inverter to work, the available DC created from the input has to be able to provide the peak voltage of the resulting three-phase output, otherwise the motor won't run anywhere near its rated capacity. What the drive actually does is to chop up the DC voltage in varying widths, and it's the rate of doing this that gives the variable speed. The time-sliced output's voltage doesn't look too good; it's essentially a chopped up square wave with different sized bits, but when it's fed to the motor, current is drawn. But unlike the voltage waveform, the averaged-out current waveform looks like a good sine wave. Do this three times, with each output 120 degrees shifted, and you have yourself a three-phase inverter.

 

With a star-connected motor, you need the maximum voltage available to be not quite double the voltage required for a delta connected motor, which is why Nick is saying that technically, the voltage doubling arrangement produces too much. What's happening is that with a star-connected 415v motor, whatever is connected to each terminal has to flow in part through all of the windings in order to have a return path, but with a delta connected motor running at 240v, there's only one winding (albeit with the other two connected across it in series) between the terminals. Each individual winding is the same for both type of connection, which is why you can rewire some motors to be either star or delta connected. Of course, the exception to this is the dual speed motors we started with - they can only realistically be connected in star, because of the different way the windings are arranged. Clearly there are some niceties to this that are much easier to explain with diagrams, but you should at least get the gist of what's happening.

 

Another way you can run star three-phase motors from a single-phase supply is to use a 415v inverter with a large step-up transformer at the input. My bro-in-law has been doing experiments along these lines, but I haven't heard definitively about the outcome yet.