I have 2 phases (400v), can I utilise this for running my 3 phase motor? - Page 2

# Thread: I have 2 phases (400v), can I utilise this for running my 3 phase motor?

1. Plastic
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Is this what you mean?, what size transformers do I need & likely place to find some? thanks

2. Originally Posted by Mark Rand
If you've got two phases plus neutral, then the Scott T connection is what is needed.
A Scott T converts 120 degree three phase to 90 degree two phase in either direction. He has 120 degree two phase, not 90.

In a while I will post a diagram.

Bill

3. Here is is with the transformer windings shown as a vector diagram. All windings are 240 volt, simple line transformers. Naturally, you need to get the polarity right, for example when phase A is on a positive half cycle, the bottom end of the phase A secondary needs to be on a negative half cycle.

No doubt some of the other transformer guys will check my logic. Fine, if i have overlooked something, I want to know it.

Bill

4. Plastic
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Loooking at your supply intake point there is a fair chance it was upgraded from the old DC mains which was converted to AC in the 50's you have 2 phases of a 3 phase supply but there will not be the 4th core to give you the last phase even in the road Parts of Swindon are like this from the old railway works supplies.
Does anyone know if a vfd fed at 400v single phase would output 3 phase 400v?

5. ## 1Φ Wye Service to 3Φ Wye Service, Open Wye- Open Delta Bank

Nice job Bill, you got their attention better than I did. How about some phasing dots for our Euro friends, that never heard of using a Yankee open delta connection, center tapped delta, open Wye or Scott T connections.

In fact, many Yankee's, might make use of the drawing where they have a light commercial space that is served by a 1Φ Y-208V network service.

A little formula added, for sizing the bank KVA, would complete the thing and make it useful to many more folks, worldwide. Adding some info on providing over-current protection to the bank would also be beneficial. A note stating neutral current draw will be the same or higher, as the phase current, might prevent some from under-sizing the neutral as is sometimes common.

Nice description as well...

If that is the case, the third phase can be produced with two single phase transformers, each connected from a phase to neutral and their secondaries in series with the polarities reversed and connected to the neutral. They need not be very big since each will only supply 1/6 of the total power, although they should have some margin for starting. This would give stable three phase without the noise and instability of a rotary converter.

Bill
I would add a note that this connection is not suitable for a 1Φ, split phase service, due to the wrong phase angles.

It should prove useful to many smallholder shops out there.

SAF Ω

6. Originally Posted by Oilyneil
Loooking at your supply intake point there is a fair chance it was upgraded from the old DC mains which was converted to AC in the 50's you have 2 phases of a 3 phase supply but there will not be the 4th core to give you the last phase even in the road Parts of Swindon are like this from the old railway works supplies.
Does anyone know if a vfd fed at 400v single phase would output 3 phase 400v?
There have been some strange power supplies around here, too. The Central Corridor of St. Louis still had DC in the 50s. I don't think they ever upgraded it, just tore the buildings down to make Kiener Plaza, a park that runs west from a few blocks from the arch. In the mid 70s I worked in a plant that still had 25 cycle AC. A young US Army engineer named Robert E. Lee noticed that some rapids near Keokuk Iowa would be a good place to collect hydro power and later they put in 25 cycle generators and sent it all the way down to St. Louis. It must have been a real loser to maintain all those wires for the few customers left. Too bad that Lee couldn't stick to being a river engineer, which he apparently did well.

I don't know a specific VFD to suggest but they certainly can use single phase input and output three phase. They convert the incoming power to DC and then can do anything they want with it.

Bill

7. Originally Posted by SAF
A little formula added, for sizing the bank KVA, would complete the thing and make it useful to many more folks, worldwide. Adding some info on providing over-current protection to the bank would also be beneficial. A note stating neutral current draw will be the same or higher, as the phase current, might prevent some from under-sizing the neutral as is sometimes common.

.

SAF Ω
Thanks for the appreciation. Re sizing, I misspoke a bit. While it is true that the transformers are each only supplying 1/6 of the power, after more contemplation, they are working harder than that, sort of the same way that bad power factor heats a generator, A and B phase will have to supply 10 A directly to the load and another 10 A to the C phase. Therefore, the transformers will have to have current ratings equal to the load on each phase. Most of that will not show on an electric meter unless you have one that measures power factor as well as KVA. 10 amps will run a 9.6 horsepower perfect motor, so you probably figure on running 7 or 8 Hp in real life. A 2.4 KVA line transformer is not very big and the size scales, i.e. 15 Hp would need 5 KVA transformers plus some headroom for starting. You could tolerate some voltage drop in the C phase since the A & B are supplied directly.

Re single phase lines, You need some phase shifted power somewhere in there. Once you have an adequate amount, it doesn't have to be the right voltage or phase angle- you can gerrymander it around to almost anything. The problem is that the two main sources of phase change, capacitors and inductors, change with load. I have been playing with a system that introduces the original phase shift and then mushes the phases together to more or less average them out. I made the transformers and found that I had resonances with the capacitor that did all sorts of strange things. That is where it stands.

Bill
Last edited by 9100; 12-16-2019 at 04:04 AM. Reason: Correct dumb mistake in arithmetic.

8. Plastic
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Thanks for the input, but Im out of my depth with this....

9. Stainless
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Originally Posted by pxr5
Thanks for the input, but Im out of my depth with this....
all you need is two 240:240v single phase transformers and you've got 3 phase when hooked up correctly.

10. Plastic
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yeah I get the idea, but I can not see from the schematics how to wire up the transformers, & what exact transformers I need.... Do I get them from arc welders? are they 2 wires in, 2 wires out items? etc...

11. Here are some examples of transformers from EBAY. I am not recommending them, just for illustration. Shipping would probably be more than the purchase price.

Wall-Mount 240/480VAC Transformer, 2kVA, 120/240VAC Output Volts (DR) 190735257593 | eBay

Dongan 5 KVA 240/400x120/240 Volt 1 Phase Transformer- T670 | eBay

To an electrician this is simple. It sounds like you need one. These are heavy so the shipping to England would be prohibitive.

To the transformer gurus here, I wonder if you could connect the two phases to the outer legs of a three phase core and get the combination voltage off the center leg. I have never heard of anything like that being done. I have some three phase transformers and a big three phase Variac so I could bring it up slowly and watch for smoke.

Bill

12. Plastic
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I can not find anyone to help on this, so Ill have to use a RPC, I assume given I have 2 live legs allready It wont need a lot of caps to smooth the generated 3P power?

13. Stainless
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Originally Posted by pxr5
I can not find anyone to help on this, so Ill have to use a RPC, I assume given I have 2 live legs allready It wont need a lot of caps to smooth the generated 3P power?
find the neutral of your motor and connect it to ground, two legs go to your two existing phases.

the motor will start on its own, and run. you can run machines this way and you can get about half the motor's rated shaft hp from them safely.

the third leg will generate your third phase. to boost the voltage you can connect a 240v rated motor run capacitor from the third leg to ground. or you can connect >415 volt rated capacitors from the third generated leg to both the other lines.

if you do not connect the motor's neutral to the ground then the motor will not self start and you will need a rpc start box, start and run caps, potential relay, overloads, etc. and the motor won't produce as stiff of an output, the voltage will drop under load more.

14. Diamond
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Delete ground and insert Neutral.

15. Plastic
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Can some one confirm, whats the correct connections? thanks

16. Aluminum
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Hi pxr5, I have all ready told you in my earlier post not to use the neutral.
I really need to see a photo of the manufacturers name plate of Idler motor before I can tell you for sure. What some people think is you have a Star connected motor, I think highly unlikely.
In Australia (and your Supply Installation Requirement are probably the same) 3 phase motors under 4 HP with windings connected Star will operate on 415v. Any motor higher in HP, the motor would have to be in Delta for the continuous running duty.
Please put a photo up and I can give you the answers.
Do not connect anything to an earthing conductor unless it requires earthing like your Main Swithboard Main Earth Neutral connection or for reference testing.

Jim

17. IF you had a star connected motor, AND you could find the star point, THEN, theoretically, you could run a motor like that, at about 58% power. It would probably run a bit "lumpy".

Motors are essentially never run like that (for a variety of reasons). I'd not recommend it.

I would treat it as a standard RPC input supply, and set up an RPC using the two you have as the inputs to an " idler motor" which makes the third phase. That is the standard normal way to do it, and the most straight-forward.

While there is a valid argument that you COULD run the idler off the neutral and two lines, that ends up with "differences and complications", meaning that probably you would never find anyone else running theirs that way, and anyone else working with it (to solve problems, etc) might be confused by it. Confusion about electrical wiring is hardly ever good.

Also, it would NOT work as-is with any other type of input. So unless that is common in your area (which it could be) it is not recommended. Whenever you have a choice with stuff like that, do it the way most people do, that way there is less to explain and everything is the way others expect it to be.

.

18. Plastic
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Ive yet to find a motor big enough for the RFC, Ill post the details as of when, thanks for your help!

19. Plastic
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Sorry the pics are really poor the name plate states,
ouput 30KW
RPM1460.
415 DELTA
A? 56.0
Hz 50
RATEING MO??
INSL CLASS B

20. Aluminum
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Hi pxr5. Thats a big idler motor,at 56 Amps 415v minimum cable size 10²mm maybe 16²mm copper. Is your home shop supply up to the job? Most probably pony start motor for starting this 30kW Idler. I thought you would have gone for a 10 HP or 15 HP Idler. Have you bought this motor? Did you buy the Unique 3 phase book? if so what do you think and do the DVD's still come with it. Have you tried yet to start the 30kW motor with a rope around the shaft.

Jim