Running 3 Phase Motor on Single Phase Supply

I have, in the last couple of days, been given an old Meddings floor standing pillar drill. I have a reasonably large Clarke bench mounted one, but the chance of having the Meddings was too good to pass up, especially as it would have been skipped otherwise. It is old, but in great condition.

I have looked at the little Lindsay Publications leaflet 'How to run three phase motors on Single Phase power', and it seems to recommend a running capacitor of '28 mfd/hp'. Now, I take this to be 28 milli farad/hp (would have to be bumped up by 1.2 for 50Hz instead of 60Hz). However, as the biggest oil filled capacitor I can find is 20 micro farad, am I misinterpreting what is written?

Does anyone have any experience in using the Lindsay leaflet? Or any other way of converting three phase to run on single phase CHEAPLY?

This drill was unexpected, and as I don't really need it, I am reluctant to spend much on it - certainly not enough to get a new 1/2hp motor for it. My preferred option would be a 3 phase extension lead from the coachworks next door, but it would be a bit of a cheek! My total investment in it was the effort of getting it up the 5 steps to my workshop (not easy as the pillar is solid, and the head casting is about 1 cwt), and I am allergic to spending more than I need.

Regards,

Richard.

.

I would say you are in the wrong hobby.

Nick

Well Ricard, the Meddings is far and away the better of the two - so, if you do not want to spend any money:

Sell the Clarke and buy a "proper" motor for the Meddings with the proceeds and a tin of paint with what is left. Don't buy half a horsepower either. 1Hp being a starting point.

Personally a three phase motor with inverter is the way forward and you can do this on little money - but you have to be patient and wait for the right bits at the right price.

graham.

Not quite, Nick, but possibly...

I use this hobby to make stuff so I don't spend it elsewhere either. I have a little Clarke 6 x 4 metal bandsaw paid for by what it saved me in making two internal lintles in the house. The cost at the time was £130, I bought a long steel beam, sawed it up into two and had two lintles for £180 total, compared to the £220 each for the same size ones at the builder's merchant. Try to do the same sort of thing with almost everything else too.

Regards,

Richard.

Edited By richardandtracy on 30/03/2015 10:07:15

Graham,

Sometimes I wonder just how stupid I am.

I have a single phase motor. It's attached to the Clarke pillar drill...

Regards,

Richard.

One no-cost option would be to put the Clarke motor on the Meddings until something better come along.

The ultimate option would be to use the 3 phase motor with a VFD. I have bought several secondhand between £15 and £40 and only ever bought one new (the very common Chinese one about £75) because I needed it quickly for a 1.5HP motor.

Be patient...

Ian P

Richard, it's 70 uf (micro farad) per KW so 1/2hp would need 26 uf. I used this formula to run a three phase milling machine on single phase for several years without any problems. I got my capacitor from RS Components at the time for about £12 I believe.

Thanks Vic - sounds as if Lindsay got his micro and milli's muddled. I did wonder, as it seemed so far from what was available.

I think I will get a 25uF running capacitor  **LINK**   and a 56uF starter capacitor  **LINK**   . I'll never use it near full oomph, so there shouldn't be problems.

Regards,

Richard

Edited By richardandtracy on 30/03/2015 11:39:16

Edited By richardandtracy on 30/03/2015 11:39:49

Well, when running a three phase motor off single phase using just capacitors you will never get anything like full oomph anyway.

Your second link is to a totally unsuitable capacitor as its a polarised type.

I suspect by the time you have mounted, enclosed and paid the cost of two motor type capacitors you will be in VFD territory.

Ian P

Sounds good Richard. There was a very good website many years ago that had all the details but it sadly disappeared. I knew the information would be very useful to me so I wrote up my own "idiot guide" and filled it away.

Have you got a link to a £12 VFD?

Richard

I ran a Pollard Corona 100A on single phase for may years using the capacitor trick with no problems. 1/3rd Hp motor on it, memory says 20 µF run capacitor, 100 µF start. Having done the capacitor trick a fair few times I tend to go high on the start capacitor to ensure things work. Ended up devising my own start / run box assembly with automatic start capacitor cut out as being the easiest way to deal with folk asking me to convert their motors! VFD is more cost effective now.

Worst problem with the Pollard was digging into the motor to find the star point to convert the windings from hardwired 440 V wye to delta to run off 240 V. For motor control I usually made a little box with two start push buttons controlling two contactor units. One contactor wired normally via keep alive contacts and a stop button to handle normal running and one with the coil wired directly to the pushbutton so the start cap was only in circuit whilst the button was held down. Usually a couple of seconds was plenty of start time (count "one monkey and two monkeys"). Have used big octal base relays instead of contactors when stocks were low, last well enough for home shop but obviously pukka thing is better.

Mr Pollard used monster "home made" pushbuttons on the A100 which were well up to handling the start cap current so I onkly had to hold the one button down and didn't need to find another contactor.

I do have a circuit to run 440 V motors off 240 single phase using the capacitor trick but it needs more capacitors and, usually, tuning to get the right capacitors for the specific motor.

Ian

Done correctly the UK version of the capacitor trick with a properly specified run capacitor permanently in circuit and separate start capacitor gives close to full power at rated rpm.  At one time this was an official way of exploiting 3 phase motors where only single phase was supplied.  Older books give the technique its proper name and claim up to 95% power with suitable motors.  Major power related drawback is that torque falls off rapidly with motor speed changes so its much easier to stall under load.  On full fat 3 phase torque actually rises if the motor slows a little.  Our American cousins frequently simply add a start capacitor to run-up three phase moters on single phase.  In this case considerable power is lost as only one winding sees full voltage, the other two see half voltage and the phase balance is seriously disrupted.

Clive

Edited By Clive Foster on 30/03/2015 12:14:38

richardandtracy, if you do ever buy and fit that 56uF capacitor, make certain you are wearing your best rubber underpants, full facial mask, rubber apron and ear defenders when you turn it on for the first time. It's an electrolytic capacitor that won't appreciate seeing an AC voltage across it. I'd suggest instead getting a proper motor run cap like the other part.

Funny, we were just exchanging big bang anecdotes the other day on another thread here....

Murray

Clive,

Does it work better with a delta layout? If so, is it possible to show a sketch of the circuit for delta connections as the Lindsay leaflet only covers star connections?

At the moment I don't know whether it's Star or Delta, but would like to have both options covered.

I shall try to up the start capacitor size somewhat. Not really knowing much about capacitors, I'm concerned to hear that the start capacitor I identified was 'not suitable due to being polarised'. I thought capacitor polarisation was automatic & self reversing as an AC voltage goes both ways. I have studied electrics etc for a mechanical engineering degree, but will confess in the intervening 28 years that I have only retained enough to re-wire a house...

Regards,

Richard.

I've sent you a PM Richard.

The '£12 VFD' might exist but I've not got any laying around.

What I was indicating is that running the motor off capacitors is a compromise that allows the machine to work, maybe at close to its original spec. A VFD on the other hand brings a lot of side benefits and enhances the machine significantly. If you have a tapping machine or tapping head or have a fully equipped machine shop than you might not have need of an upgraded pillar drill but I find my Fobco Star indispensable now.

I have the Jim Cox book in which he shows various schemes for running three phase motors. I if you use a start and a run capacitor and fit the appropriate switching and relay, then by the time you have enclosed and mounted it all safely you will have spent quite a bit of time and money, and the result is a machine that will still be less than it was designed to be.

If you have scrapboxes of suitable parts and can put something together that works for you, then go for it. (I'm biased, I just love the flexibility that VFDs bring)

Ian P

Richard

PM me with your E-Mail and I'll send you a scan of the 440 V version. Might be a day or three whilst I find it tho' as the filing gremlins have hidden it! I know I've scanned it but .... Paper copy is somewhere safe, really safe. I'm pretty sure I have used the 440 V version but many years back so I don't recall if it worked any better than the 240 V version.

Ian

According to my 1978 first edition of The Electric Motor Handbook from McGraw Hill edited by E.H.Werninck the official name for the start and run capacitor set-up is Steinmetz connection. There is about half a paragraph on page 228 referring to its use in fan drive applications claiming that the motor can be rated at around 70 to 80% of its nominal power. It is said that single phase starting torque may be as low as 10% and pull out torque could be as low as 60% of three phase values. Low starting torque is of little moment with fans but the reduced pull out torque demands significant de-rating due to the way fan power draw increases rapidly as design speed is approached. I no longer have references for the higher 90 to 95% of rated power mentioned above but older books claim it possible. Given the serious reduction in pull out torque is open to question as to how wise such high rating is in practice as the stall margin becomes very small at full power. Odds are that anything more than a few seconds, perhaps 10 or 20, with power on and motor stalled will be sufficient to let the magic smoke out. The motor will not automatically restart after stalling unless you make a proper job of the control box with voltage sensing control of the start capacitor. That was the way I made my "customer" version but it does add cost and complexity. Never thought the extra complexity really worthwhile for personal machines, especially if its a drill with foot operated emergency stop. At one time you could buy 3 phase motors with capacitor requirements for single phase operation on the data plate. I have seen one such and, as I recall it, power rating was the same for both 3 phase and single phase operation.

I'd certainly agree that a properly done Steinmetz connection box is more expensive than a VFD. Last time I looked at my version, a decade or so back, component costs were approaching £200 for a 1 HP connect & play version.

Clive

My 3 phase 10" Elliott shaper has been running for many years using the capacitor mod. I had to locate the star point to separate and bring out the 3 wires for 240v connection. I already had a suitable capacitor so it cost only my time. The shaper performs well but sometimes needs a very light nudge on the pulley to start easily; this is easy to do so I'm not bothered by that as it's only once to get running then I just use the clutch. I could add a capacitor to help starting. Poor starting torque is always likely to be a disadvantage with this conversion so a machine that has a clutch is the best type to convert. Colin

The Brooke Crompton motor on my Omnimill had instructions inside the wiring box on how to rewire it from Star to Delta. The mill always started fine without an additional start capacitor except once in a while in the winter. When this happened I just used to give it a nudge and let it warm up for a couple of minutes and then it would be fine.

If a motor self starts with only a run capacitor odds are that capacitor is a little oversize. I always used to trim the run capacitors to get the phase voltages as close together as I could. The closer they are the smoother the motor runs and one has to assume that if the voltages (and currents) are nearly equal any stresses will be minimised. As far as I recall once the run capacitor was set as close as I could reasonably get to optimum the motor would not self start on run capacitor alone without a pretty serious spin of the pulley with no belt fitted.

Capacitor optimisation was much more of an issue with larger motors say 1 hp and up than smaller ones.

Clive

Edited By Clive Foster on 31/03/2015 20:14:00