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Old 13th Nov 2010, 10:26 pm   #1
Al (astral highway)
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Default what happens if you only run 2 phases on 3 phase equipment

....I am new to 3 phase stuff and am getting my head round this. will my small, ex-avionics fan run if I supply just two phases at 400 Hz ? What would happen? Amd what about with just one phase?

Also, there are 3 wires epoxied into the assembly, presumably ground was the common ground with the rest of the kit in the radar it came from? So I can measure dc resistance between any phase and the casing off load?

You can give patronising answers, I won't be offended!

Thanks,

Al
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Old 13th Nov 2010, 10:58 pm   #2
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Default Re: what happens if you only run 2 phases on 3 phase equipment

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So I can measure dc resistance between any phase and the casing off load?
Is that a question or a statement?
If you lose a phase on a running delta connected (ie no neutral)3 phase motor, it will generally continue running on just one winding which can cause that winding to burn out. It will not restart however as there is no rotating field.
I should imagine your fan is delta connected, originally fed from a 2 phase plus neutral supply. I seem to recall that is how the fan is connected in an ARC-44 helicopter set, fed from windings on the dynamotor.
I'm sure I posted a link in another of your posts for a company called Transwave, I'm pretty sure there's detail there on how to run a 3 phase motor from a single phase supply, although it's esentially a case of connecting a capacitor between Phase 1 and Phase 2, the supply going to Phase 1 and Phase 3 (or vice-versa depending on direction of rotation).
Rob.
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Old 14th Nov 2010, 1:09 am   #3
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Default Re: what happens if you only run 2 phases on 3 phase equipment

Hello,

A three phase motor will not start on only two phases. If it is running and only lightly loaded it will continue to run on two phases. If heavily loaded it will stall or overheat.

The simple way to run a three phase motor off single phase is to connect the single phase supply to two of the phase connections (it may be necessary to use a transformer to get the right voltage) and then use a capacitor connected from either (but not both) of the two phase connections to the third phase connection.

The problem is to get the right size of capacitor to balance the three phases. The ideal size of this capacitor depends on the size of the motor, the inductance of the windings, the AC frequency, and also the motor load but with a fan this load should be more or less constant, so this is probably not a problem.

The other problem is that because the starting current is much higher than the running current you may need a much larger capacitor to start the motor so you usually have two capacitors, a starting and running capacitor, the starting capacitor being switched out when the motor has started. Here again you might be lucky with something like a fan which has a low starting torque that it might start with just the run capacitor.

Yours, Richard
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Old 14th Nov 2010, 7:57 am   #4
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Default Re: what happens if you only run 2 phases on 3 phase equipment

Thanks Richard, brilliant and thorough answer!!!
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Old 14th Nov 2010, 11:21 am   #5
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Default Re: what happens if you only run 2 phases on 3 phase equipment

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The other problem is that because the starting current is much higher than the running current you may need a much larger capacitor to start the motor so you usually have two capacitors, a starting and running capacitor, the starting capacitor being switched out when the motor has started.
My 3hp Colchester Chipmaster requires 600uF total to get it rolling but only 100 to keep it going.
You probably won't need quite that much
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Old 14th Nov 2010, 2:43 pm   #6
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Default Re: what happens if you only run 2 phases on 3 phase equipment

Hello,

I have a three phase bench grinder with a 1/3 hp 415v motor which runs on single phase via a 240V to 415V transformer and a 7uF running capacitor and I hand switch in and out an additional 14uF for starting.

You can use a speed sensitive switch, current sensing relay or a timer instead of hand switching the start capacitor.

If yours is 400Hz then one would expect that the capacitor would need to be about 1/8 the size of one for 50Hz. If your voltage is 110V then it would take a higher current so the capacitor might have to be about 4 times the size of one for 415v which would make overall about half the size for the capacitor.

Your motor may well be smaller than 1/3hp so the capacitor size would have to be adjusted accordingly.

I just offer these ideas to give you some idea of what you might want but there are many variables.

If you can get the motor to run then you can adjust the capacitor to give equal voltage between phases. Don't run it for long if it doesn't start or until you have got the voltages somewhere near right.

Yours, Richard
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Old 14th Nov 2010, 3:34 pm   #7
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Arrow Re: what happens if you only run 2 phases on 3 phase equipment

2-phase / 3-phase conversion.
Since no one else has mentioned this, I will: Scott transformer connection: http://en.wikipedia.org/wiki/Scott-T_transformer.

Al. / Skywave.
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Old 14th Nov 2010, 5:30 pm   #8
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Default Re: what happens if you only run 2 phases on 3 phase equipment

Hi Gents, the problem with the Scott connection is that it gives 2 phases 90deg apart from 3 phases 120deg apart, not what is really needed. Camden Minature Steam Services do an excellent book (satisfied customer only) on 1 to 3 phase concersion and running 3 phase motors on single phase.
Note that 3 phase motors are normally wound for 415V, single phase is 240V. This will lead to reduced power. First thing to do is to get an auto transformer to boost to 415v (cheaper than double wound). Then suitable capacitors to give the phase shift for the 3rd phase. Note that there is no neutral used or required on this motor.
If you can get a further tap on the autotransformer at about 600V, then the capacitor sizes are smaller (but rated volts higher). This technique was used on at least 1 commercial switched cap converter.

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Old 14th Nov 2010, 5:47 pm   #9
AlanBeckett
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Default Re: what happens if you only run 2 phases on 3 phase equipment

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Originally Posted by Ed_Dinning View Post
Camden Minature Steam Services do an excellent book (satisfied customer only) on 1 to 3 phase concersion and running 3 phase motors on single phase.
Indeed so

Quote:
Originally Posted by Ed_Dinning View Post
Note that 3 phase motors are normally wound for 415V, single phase is 240V.
Ed
Ish.
415V is normally Star. If all the windings are available (6 ends) then it can be reconnected Delta for 240V.
Unfortunately I think that the OP said that he only had three connections.

Alan
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Old 14th Nov 2010, 5:52 pm   #10
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Default Re: what happens if you only run 2 phases on 3 phase equipment

Running big 3 phase motors in say machine tools from a single phase supply is fine, but I doubt the wisdom of doing it for a small avionics fan. It would probably be easier to get a new fan.
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Old 14th Nov 2010, 6:02 pm   #11
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Arrow Re: what happens if you only run 2 phases on 3 phase equipment

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Originally Posted by Ed_Dinning View Post
Hi Gents, the problem with the Scott connection is that it gives 2 phases 90deg apart from 3 phases 120deg apart, not what is really needed.
Ed
Yes, I realise that Ed: "Not what is really needed" - but close . . . .

The line I am thinking along is that since Astral has a 400 Hz supply to start with (as implied in his OP.), it is a relatively simple matter to produce an approx. 90° phase shift from this, thus producing the 2 phases. These two phases can then be fed to a Scott-connected transformer to produce the 3 phases, each 120° apart - which is what I understand is required.

Or perhaps I've misunderstood something in the above.

Al.
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Old 14th Nov 2010, 8:06 pm   #12
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Default Re: what happens if you only run 2 phases on 3 phase equipment

There are a couple of similar threads in another section. The snag with the single to three phase converter for mains motors is running them on the 240V delta connection, you get some horrendous harmonics, circulating currents and everything else. It is much better to run them from the star winding, do need a 240-400V transformer though. Otherwise they work a treat. Bob
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Old 14th Nov 2010, 10:20 pm   #13
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Default Re: what happens if you only run 2 phases on 3 phase equipment

Hi Al, I don't think a Scott connection will work well with a 120deg supply; however if there are only 2 wires available then it is a single phase supply, no matter what it is derived from.
If the neutral is available then things change a lot and it is possible to generate 3 phases, but you will need to wind some special transformers, and start dawing vector diagrams.
The only Scott transformers I've seen recently were for instrument testing use and were a horrenduse price for 500VA transformers. Still at 400Hz the total turns come down a good bit so not too hard to hand wind.

Ed
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