3 phase motor on single phase supply.

[igotalake]

hi all.

i've read several things saying this (title subject) can be done with capacitors but with a certain loss of power. having said that, the information mostly seems to be american and there are often references to '2 phase power'. at first i assumed it was a mistake on their part, confusing 2 wires with 2 phases but looking further it seems that they possibly may have 2 phase supplies.
anyway, what i would like to know from you electrical engineers is whether it's possible on a simple 1ph + N supply.
i know i could get a phase convertor and i may well do in the end but i would like to try this solution first always having the other option if this is a 'non-starter' (pun intended).

thanks,

 

[Jackrae]

Do you mean you want to build your own or simply can it be done. The answer to the second question is yes - but it all depends on the size and type of motor you have. There are basically three ways of doing it
a) use a static inverter to produce an "electronic" 3-phase supply (can be either 230 or 415 but the latter is more expensive)
b) use capacitors and inductors to produce a pseudo 3-phase which is enough to get the motor rotating, after which it generates it's own 3 phase. Again the 415 version is more expensive
c) use a pony motor (single phase) to start the 3-phase motor then a series of switches/capacitors to excite the other windings.

As a complete novice, your chances of a self-build are slim.

What size is your motor and does it have 3 or 6 wire ends within the terminal box. If only 3 then it's fixed at 415 volts so you will need either a 230-415 transformer to get a suitably high single phase voltage to start your build. If 6 terminals then you can reconfigure the winding such that the motor will operate with 230 3-phase.

Cheap (or not so cheap) type (b) units are available from the likes of machine mart.

I'd suggest you go for the type (a) unit, an inverter. These also give you the ability to vary the motor speed, which you may find useful.

If you really want a try-it-and-see then I'd suggest you'll get promising results if you have a play with the type (c) system

But beware, you are dealing with lethal voltages !!

 

[igotalake]

hi and thanks for the reply.

yes it is with a view to actually doing it.

it's a 1.8kw and it's got all six ends of the stator coils available.

i don't really want the added hassle with a pony motor or the expense of buying an invertor so i am going with the capacitors/inductors route although all the things i've read have not mentioned inductors but it makes perfect sense to use both. i have read that the capacitor will give you enough phase shift to 'create' a second phase and that the motor will run ok, albeit underpowered, without any third phase or a third phase which is shifted less than the second by a smaller cap. i can see that an inductor would give a shift in the other direction so would make more sense than two capactors of different values.
do you have any rule of thumb that i can use to estimate the capacitance and inductance required for the motor in question?

 

[bernardgreen]

There is no "rule of thumb" that will create a safe and effective pseudo 3 phase supply at that power level ( 1.8 kW ) The loading on the capacitors and other components has to be correctly calcuated using the impedance and inductance of the motor windings. Both at still stand and normal running speed / load. The capacitors need to be able to survive the very high voltage spikes that will be created when the supply switch is opened.

You would be better using two phase regulating transformers to create a pseudo phase 120deg leading and a pseudo phase 120deg lagging.

https://www.pti-us.com/pti/company/...se_shifting_transformer_modeling_in_PSSE.html

The most sensible and ( almost certainly ) lower cost method is to use a single phase to three phase invertor.

 

[SimonH2]

I have a commercial converter in the garage - I was given it when it was ripped out* where I used to work. It is basically an autotransformer to give 415V, and a pile of caps. It has a switch that varies the amount of cap so as to get the best running (probably the easiest way to work out how much you need !).
Also, it has a voltage sensitive relay that (IIRC) adds in more cap if the third phase is overloaded - during starting I'd guess.

On the lathe we have an inverter drive - couple of hundred quid some years ago, I think they are under hundred these days. We do use the variable speed on that, though there's nothing to stop you running at a fixed speed. Unless you spend a lot of money then they only do 240V on the 3P output - that's because they don't have the headroom on the DC bus do the same voltage out as in, and remembering that the input voltage disappears 100 times a second.

Which to choose also depends on the nature of the load. The soft-start (ramp up) feature on the inverter drives can be quite useful for some loads.


* It was in a factory that we acquired and which was later closed down. The daft thing is, it was only about 10 feet away from a 3P dist board, but their on-site maintenance guy decided that this was easier If I hadn't "bagsed it" then it would have gone in the skip.

 

[igotalake]

i'd love a variable speed drive but the application doesn't really call for it and i suppose what it boils down to is that i can replace the 3phase motor with a 1phase for about 120 quid but if i could buy a couple of capacitors for 10 quid each and run it without problems i would be happier and probably (for a week or so) carry with me the odour of some better quality wines than the ones i currently drink. so it is a worthy cause.

 

[SimonH2]

Indeed, the only problem is that you need to work out how much capacitance you need - and I suspect the best way is by experimentation. That requires you to have a selection of values. I can do that as we have a few in a box (more of those useful bits that mum calls "rubbish") so can often find enough to work out what's needed and then buy some.

 

[igotalake]

i envy your box of caps. i collect as much 'scrap' as i can and i've got a fair bit, although i refer to it as 'useful salvage items' to other people but i don;t have anything in the way of caps sadly. i could nick the caps off various motor driven things like my compressor but i would be worried about blowing them and then having to replace them and still be no further forward than when i started.
do you think a single phase motor might be the easiest way?

thanks,

john.

 

[bernardgreen]

As the motor is more than 1 kW you need to make some arrangement for either a soft start to prevent the start up load ( considerable greater than 1.8 kW ) from dipping the supply voltage to your neighbours or notifying your DNO and asking their advice about using the motor on the existing supply.

Many DNOs require notification of the use of motors greater than one 1 kW so they can identify sources of volt drop in their network. The requirement is normally in the contract the builder had with the DNO to install the supply.

 

[SimonH2]

A single phase motor would be by far the easiest way to do it - just select a motor of the same capacity, frame and mounting, speed, etc and it should work. It won't be as satisfying as doing it yourself.

As to notifying the DNO. It may be in a contract somewhere, but does anyone ever do it ? Interestingly, if it was the builder that agreed to it, then that contract is not binding on the house purchaser unless the builder passes on that contractual bit via the sales contract or the DNO enters into a new contract with the purchaser (and ditto on each transfer of ownership).
It is notable that on the two purchases I've done (and a few service takeovers when tenants have moved out), on no occasion was anything said/mentioned about that.
That could be an interesting one to discuss. From the end user's perspective, I agree a supply contract with <some lecky provider> - I have no contract with the DNO. So the lecky provider has a contract with the DNO and meter operator to make the physical supply - but the terms of that contract don't apply to me as I am not a party to it.

Says the guy in a household with a 3hp compressor, 3 welders, ...

 

[Jackrae]

Excellent 2-part you-tube video on the subject available here :
http://www.youtube.com/watch?v=6UcDM3hm0XM (part 1)
http://www.youtube.com/watch?v=43eAEeyTbs8 (part 2)

The man's crazy but it's still a good tutorial

If you want a self starting circuit, for your 1.8KW you're probably looking at 50mF for run and around 100mF to start. Drop me a private message if you need an auto start circuit.

 

[igotalake]

i've got a 3hp compressor as well and i've never known of my neighbours having voltage issues. then again my neighbours have got 3 phase supplies. and if we're showing off i've got 2 welders and a plasma cutter!

those videos were very useful and have definitely shown it can be done quite easily. i never thought to look on youtube. my particular motor is for a hydraulic pump on a 4 post ramp so it will start under load so i guess i will need a start cap circuit.

 

[igotalake]

p.s. Jackrae, i don't know how to p.m. but i would like an auto start circuit.

 

[SimonH2]

my particular motor is for a hydraulic pump on a 4 post ramp so it will start under load so i guess i will need a start cap circuit.

Funnily enough, a few years ago I was looking to do just such a setup - but before we could finish installing the lift, my mate got divorced and had to sell the house, then we had no-where to put it It was a 3.5t, 16' lift as well, great when you've got Land Rovers.
A load like that is quite difficult to start. It may be worth looking at whether you can unload it for starting - a dump valve to prevent pressure build up, and a non-return valve to prevent the dump valve lowering the lift.

What we were looking at was to provide a 3P supply to the workshop. To that end we'd got as far as mounting an old car flywheel on a largish 3P motor (probably 3hp I think), using an old manual star-delta starter to get it going, and an old transformer to get the voltage. I said we never throw stuff away - this was almost all stuff my dad has collected over the decades. The plan was that once the motor & flywheel were running, that would give us a good enough supply to DOL start the lift motor. I think I tested it by just capping off the outlet and letting it pump against it's pressure relief valve, and I vaguely recall that it worked.

 

[bernardgreen]

i've got a 3hp compressor as well and i've never known of my neighbours having voltage issues.

One of my past neighbours used a large electric saw in his garage for few days and that dipped our lights and the other houses on the same phase. I knew it was only a temporary matter and only in day time so only mentioned it to him. One of the neighbours who could not hear the saw reported the dips and the DNO came out, heard the saw and asked him to stop using it.