The 3 phase question

I have a Bantham running off a Transwave static converter for the last 20 years. I have just bought my first mill A Tom Senior Model E also with a 3 phase motor. Mooching around on the web to check out what to do with my domestic supply, I find I havn't been doing my lathe any favours . The converter does not produce 3 phase at all - no wonder the speeds always felt odd. As an amateur without electrical engineering knowledge how do I choose a suitable product to get my machines running properly. Do I have to spend £500+ as requested by some of the commercial companies I have contacted - for a piece of kit I don't understand. To complicate matters The Bantham has an AEI twin voltage & twin speed motor. The TS has a Brook Crompton single voltage single speed motor that can be wound in star configuration only. My reading of Wickipedia gives me the impression this might make a difference asto what can be done. Any advice would be welcome- Thanks MLM

Hi There

Does the Tom Senior say 220/440 volts or similar on the motor?

If do it is a three phase motor I would think.

Often, the 3 phase motor comes with some links that can be changed from Star to Delta.

I have a Tom Senior light vertical which I think has the same head as a Model E but can't remeber if it is wired star or delta.

It runs of a Transwave invertor and I have had no problems with it.

As the Bantam has a two speed motor I would think it might be a rotary converter rather than a static one.

regards David

Maurice,

I have a Myford S7 and a Harrison M300 both running on VFDs (both from Newton Tesla). In both cases, absolutely splendid, and I would never run a lathe on aything other than 3 phase/VFD again. However, it is fair to point out that both have motors which can be configured in delta and can run on 230V output. There are numerous instructions out there on how you can convert star-only motors to delta, but if not, I would say it was worth replacing the motor with one which can be so configured, the benefits are substantial.

David

Edited By David Littlewood on 13/07/2012 22:18:06

I've got a lathe, a mill and a shaper all running off inverters from the household single phase supply. The lathe and the mill had their motors replaced with 230/440v of equivalent HP. The shaper had it's original 440v Hoover wired in Delta mode but that took a lot of work and help from people on forums.

I'd buy an inverter and replacement motor and enjoy hassle free VFD - it makes life so easy

Can you add photos to an album of the motor rating plates and also of your converter. That will help identify exactly what you have and enable folk to best advise a way forward.

If your Crompton can only be wired in star then then that presents an additional difficulty. As has been said, some motors can be converted to Delta wiring but it requires digging out the star point from the windings and this can be tricky on some motors, a good rewind company should be able to do this for you..

I run both the spindle and feed gearbox motors on my mill on Teco VFD's, neither of which cost anywhere near the £500 you are quoting. It all depends on HP capacity and specification/features as to the general cost

CB

A Star wound motor has to run on 440 (standard 3 phase) because the electricity has to go down one winding and back up one of the others - twice as far as a Delta wound motor which only needs 240V.

Modern inverters come in two versions - one pnly produced 240 @ 3 phase and the other can also produce 440V. Sometimes the latter are called 'double conversion'. However the 440 ones are twice as expensive so you have to balance the extra cost there to the cost of perhaps getting a new Delta style motor, or on a small machine just a single phase motor and no inverter.

A 2 speed motor will be more fussy about what it gets so worth talking to the inverter supplier to confirm compatibility.

Some people say a 3 phase motor is smoother - bit like comparing a 2 cylinder bike engine to a V6 but both get you there.

You won't have done you lathe motor any harm and I don't see how your converter has altered the lathe speeds either, as the motor speed is dependant on the mains frequency. Its not easy to reconfigure switching arrangement on a two speed motor, so leave well alone, your doing alright as you are. Yes converters don't produce a true three phase, but does it really matter in your situation? What you might consider obtain a cheap large say 3 or 4 hp motor 3 phase 415 volt, running it a as a pilot motor idling a way (doing no work) ,acting a a rotary converter, sound wise, insulated from the floor, tucked away some where close by but out of the way. Then have one socket outlet from the converter and a plug for each machine.Then use either lathe or mill one at a time,negligable cost and safe.

So how exactly does a three-phase supply on its own manage to give you soft starts, variable speed running, controlled braking and easy reversing? And to a first approximation, 300A would let you run a total of 100hp worth of motors, all at once. Perhaps just a tad excessive for a home workshop?

It's fine to have the three-phase in there, but I'd still recommend using inverters with it. And often, three-phase inverters (ie 3 to 3) are cheaper to purchase, at least second-hand.

You should be able to buy a 1/2 hp inverter for under £100 and a new motor for about £50. If someone is asking £500 it must be something very special or a rip-off.

I would however recommend going up to 2/3 hp (0.55 kW) because, as you reduce the speed the torque remains more or less constant so the power output is reduced.

Have a look here

Russell

It's excessive, simply because that's way more power available than you could sensibly use, even though it's 'standard'. Most home workshops are just that - home workshops, running at most two machines at a time. Even with two big machines, that's only 6-7hp, tops, plus the lights. Soft starts on smaller mains distribution systems cut out the surges that a lot of users have trouble with. Admittedly the better arguments though are the ones you carefully didn't mention... Firstly, controlled braking - your 3-phase motor running directly from the supply will only have that if you purchase one of those very expensive DC injection boxes for it. On a lathe without a clutch, controlled braking saves you a lot of time every time you stop and restart, especially if it's a larger lathe. Once you've experienced this, you won't want to go back. And having a variable speed option gives you all sorts of opportunities you didn't have before, especially at lower speeds with some screw-cutting jobs.

I take your point Steve, mine too is a home work shop.

I would be interested in a 3-3 inverter with variable speed etc. Would you have a link so I could look at one?

I am at present motorising a milling head and have bought an IMO invertor for £69 and a 1/2 hp 3 phase motor (dual voltage) for £50 (both new) and have bench wired it for testing puposes and must admit it works great!

Output frequency can be varied from 0hz to 200hz giving up to 4 times normal rpm - I would not be using this 200hz option but it shows its versitility)

Using a toothed drive belt it is allowing me to use stepless speed changes and the assembly is far more compact for it

Alan

Perhaps the relative cost and convenience of 3 phase v. converted single phase is more important than other factors to most M.E.'s . If it cost say £500 to supply and wire the workshop with a 3 phase supply then that would be around the cost of a suitable single phase alternative. The actual cost of having a 3 phase feed, meter change and new 3 phase wiring into the workshop ( to current regs) is likely to be above the cost of single phase conversion.

The three phase supply is simple and 100% reliable, you can run more than one machine at a time and the tools are designed to work on a 100% duty cycle- suits someone working from home.

The single phase solution suits the hobby workshop-one machine at a time- by swapping connectors.

100A supply is the norm for single and three phase supply, has been or a long time.

Billy.

I should perhaps explain first - the reason that they are generally cheaper is that they are rated to operate at 415 volts, rather than 240. Other than that, calling them 'three phase input' is ever so slightly misleading, because essentially they're identical to the single phase input ones. All that happens in any of them is that all of the incoming supply, regardless of the number of phases, is converted to one lot of DC, and it's this DC that is recreated into three new phases. Since this is under microprocessor control, everything' s available to be controlled.

 I found a cheap one quite easily on ebay - http://www.ebay.co.uk/itm/SCHNEIDER-ELECTRIC-1-5KW-3-PHASE-INVERTER-VFD-VSD-IMO-JAGUAR-ABB-/120934869127?pt=UK_BOI_Industrial_Automation_Control_ET&hash=item1c2847a887

(Now all I have to do is to find out how to make links work the way they did last time I did this - they've changed it!)

Edited By Steve Garnett on 14/07/2012 22:19:34

Perhaps I was just lucky, but the inverter I found, from 'PowTran' (via Ebay) provided 415 out, and it was at the bottom end of the price range. That avoided the star-delta change although my motor was dual voltage.

One problem i didnt expect converting a 3phase mill to run off a 3 phase VFD is that the built in light expects to run off 415 volts, and that is only available when the motor is running, (not a big deal, need to install a new transformer and globe, but still, its another hours work), and the original NVR switch, which i had planned to use as an emergency stop, with the original interlocks on covers etc, won't hold in with just the load of the inverter.

The wiring job therefore turned out bigger than i had expected.

-regards, russell

Maybe worth investigating Steves link but check the KW/HP of the Bantam motor.

To give you some idea a 3hp/2.2KW motor may want to draw as much as 9hp on startup, thats what knackered my £650 4KW inverter! Now repaired £236 not bad for 1 1/2hrs use from new. Add to that spindle is easy to get to slow down with minor cuts, not so with the 7.5KW inverter.

Whoever said soft start is quicker, think again - you have to wait for it to build up speed. Not much point in DC braking unless the controls are wired in. Plus cant run coolant at a constant level.

Of course you can slow down the spindle speed via the inverter, you just have to program the ramp down time. What is the point of applying a minor cut to slow down the spindle when you have finished that particular part of the job? You must consider the whole machine and its function when you apply an inverter, this does mean that you must consider how other parts of the machine function, light, stop controls etc. As Steve Garnett comments, there are many advantages but you cannot just jump in and expect it all to work without a little thought.

Absolutely - it's a system change you are implementing here, and you have to plan it carefully.

I don't recall anybody saying that a soft start would be quicker - that's plain illogical! With an electronic inverter, you pretty much have to use the device controls to gain any real benefit from it - so controlled braking is going to be there.

The coolant thing is certainly going to be an issue. Since there appears to be little benefit to having the coolant flow a little more even by powering it with a three-phase motor (don't discuss, that is a joke...), you might as well retrofit a single-phase one to it, and have done with it. Same goes for modifying the lighting arrangements.

Having converted several machines now, I still think that the benefits outweigh the conversion problems.