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Single Phase Transformers 101

Yahoo Message Number: 67254
Hi Folks,

Yes I know you are sick of me as well as the subject but a few days ago I was challenged to included a diagram. That is what I am doing.
If someone can find any voltage phase shifts in this diagram I am all ears. Despite the fact that I also have the highest regard for Fred, his picture is an incorrect representation. You can not show the phase relationship between to voltages with an instrument that has only 3 inputs. What you see on the screen is the result a polarity shift caused by the leads. If his picture were true then the sum of the two 120 segments would be 0 volts and clearly that isn't right. Their sum is 240 volts. Check my diagram below. I don't understand why everyone is complicating it so. It is picture of simplicity.
I inserted the diagram. If it doesn't get through I will post it to the pictures.

Don Seager

2004 Allure 31046

Re: Single Phase Transformers 101

Reply #1 –
Yahoo Message Number: 67261
DON I'm pleased that you continued to address the phase shift myth How would the others Describe a Xformer with two taps. The scope photos do show a 180 difference but it is caused by the the scope common used for each measurement. As the waveform rises from the common on one leg it also rises in the direction of the other leg thereby giving the impression of an inverted Waveform because of the common reference The waveforms are caused by the same Xformer Flux movement in the winding the differences are generated by measurement techniques Some Single phase Xformer applications consider one end of the 240 volt winding to be the neutral with A 125 volt tap a scope will show both voltages in phase not 180 different knowing how to use Test equipment is critical to getting correct results. An average meter is not polarity Sensitive and therefore is more suitable for these measurements. THANKS for. Your additional Effort. BOB

Re: Single Phase Transformers 101

Reply #2 –
Yahoo Message Number: 67283
The issues brought up in this thread boils down to how we measure things.
Using a dual trace scope with isolated inputs, (+) non-invrting and (-) inverting, using the center tapped transformer model and connecting one + input to the top of the transformer, - to the center tap, the 2nd + to the center tap and the - to the bottom, the result would be 2 sinusoidal waves in phase. If you vectorially sum them, they would equal 240v.
If you have a scope that has a + input and ground and ground the center tap, ground the scope and connect the +'s to opposite ends of the transformer, the result will be two sinusoidal waves. one displaced by 180 degrees. If you vectorially sum them they will equal 240v.
The issue here is not "not understanding" how to use and oscilloscope but how to interpret the instrumentation that you are using. Most test equipment uses a ground reference, the center tap or neutral wire in a 240v system is grounded so it's convention is to discusses it in that manner. As long as this is understood there should exist not problem.

Joe

98 Intrigue

Re: Single Phase Transformers 101

Reply #3 –
Yahoo Message Number: 67295
Being a retired power systems engineer from Maine I also think it seams simple.
I guess they didn't teach the high powered stuff at UofM

Re: Single Phase Transformers 101

Reply #4 –
Yahoo Message Number: 67296
Being a retired power systems engineer from Maine it looks simple to me.
I guess they didn't teach us the heavy stuff at UofM.

Jw

06 inspire 51544

Re: Single Phase Transformers 101

Reply #5 –
Yahoo Message Number: 67314
Exactly, Jose. Enough time has been wasted on this thread. Let this comment put it to bed.

Al Colby

2000 Intrigue 10979

Re: Single Phase Transformers 101

Reply #6 –
Yahoo Message Number: 67321
I learnd at Cat Power Systems school that a Trensformer seconday wheather stepping up or Down voltage is a Mirror of the primary! I enjoy your thoughts. Think we must move on! Do you know what a revolving Field Alternator is? I dont want to start testing you. At least we are on the same page! Ken 99 Allure

Re: Single Phase Transformers 101

Reply #7 –
Yahoo Message Number: 67323
I agree the subject has been well covered however I would suggest that anyone that has posted to the file or photo section should review their post for additional clarification considering future users may not have the benefit of the extensive comments BOB

Re: Single Phase Transformers 101

Reply #8 –
Yahoo Message Number: 67328
Hi Robert

May I take you up on a very technical point. The original artical was not talking about a phase shift but a phase direction. The phase direction of the two inputs of 120V to an RV are oposite and so I maintain at 180 degrees with respect to one another. During a cycle, there are 60 cycles per second, one leg goes +ve as the other goes -ve. making them 180 different. Always assuming that between the two legs of 120V we can measure 240V.
Bit off topic for the general RV enthusiast. But there are a few of us that like to get into the nity grity of all sorts of things to conjor up a good discussion.
Regards
Peter J

Re: Single Phase Transformers 101

Reply #9 –
Yahoo Message Number: 67332
Peter:

Read with interest your explanation of the 180 degree "phase shift" of the incoming "240V" into our coaches.
But, not being sarcastic in any way, why do we care? Isn't the main purpose of the wiring to (1) allow a less expensive method for the campground to install the electrical service......i.e., higher voltage, lower amps, smaller wire.......and (2) to balance the high 120V loads in the coach that a single circuit couldn't handle on its own?
But back to my "why do we care" question.........are there some ramifications of adding future loads, as, say, using the hot leg from one circuit and the ground or neutral from the other..........or is there some other "gotcha" that we "kindergarten electricians" should be aware of with this imbalanced, phase input?
As a follow up question.........couldn't the coach mfg have taken a circuit from the land line that was phase-to-phase and run it exclusively to the dryer so one could have residential performance?

Thanks for the techy answers.........I like 'em.....

Roger Uhlich

'07 Intrigue 12170
 

Re: Single Phase Transformers 101

Reply #10 –
Yahoo Message Number: 67339
Hi Roger

I dont realy care about the incoming supply but it was a misconseption of the basics that prompted me to comment. 180 degrees out of phase etc.
Other countries do in fact use a higher voltage 240V. In UK and they need no center tapped configuration. Alas in the USA 120V is the norm and we have to get around the high currents the best we can.
However you reconfigure an electrical service to an RV you will never get something for nothing. Wire sizes will take just so much current. Circuit breakers will trip at there alocated load. Reconfiguring different neutrals with different live wires could be quite dangerous. The KISS principle must prevail.
It is not beyond the realms of possibility that a manufacturure could use the two live legs at 240V to feed a dryer. Just needs setting up that way.
Now someone one this forum says they use a 2000kW Generator just for battery top up when boondocking. I have considered this. My Onan LPG gen set is too expensive to run just to top up batteries. Why choose a 2000W unit? I feel that 1000W max would do me. Any comments. Can't afford solar yet.
Regards
Peter J