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Trying to understand rotary phase converter output...

  1. Oct 8, 2015 #1
    I am looking to build my own rotary phase converter for utilizing 3ph devices however I am having a difficult time wrapping my head around the aspect of producing 120* out of 180* power. PracticalMachinist forums have a plethora of threads on this topic but after reading many of them I find that a third of the people are left clueless, another third make convincing arguments but lack any visual demonstration (waveforms) and the final third simply resolve that it just works and because it just works it must be producing clean, 120* separated, AC signals.

    The windings are physically separated by 120* within the 3ph motor, so I completely understand how clean 3ph power would be generated by externally driving the motor. However, in the RPC circuit, the two single phase wires are [directly] two legs of your final output but are also used to drive the motor via two windings and generate voltage on the third leg in the motor.

    I fail to see how at least two phases of your final output would not have equal but opposite voltage at the same exact point in time.

    Here is an image that I thought represents the difference between RPC 3ph and true 3ph.... is it correct?

    14902d1251037838-origins-rotary-converters-rpcvoltages.jpg
     
  2. jcsd
  3. Oct 8, 2015 #2

    Averagesupernova

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    Rotary machines work, however their output is generally not ideal 3 phase power with perfectly symmetrical sine waves. It is very hard to compete with the quality of power we get from the power company concerning this. Phase is always relative. In the case of single phase power we can think of each leg being 180 degrees out of phase with one another when the common of our oscilloscope is the neutral center tap of the transformer. I have been in discussions on this forum over whether or not this is true but in my opinion it is a matter of semantics. With 3 phase power having 3 legs 120 degrees out of phase the common connection (as far as 120 degree measurement goes) is no longer a center tap on one of the transformer windings. 3 phase delta connected transformers in an industrial building will often have one winding center tapped and this tap is connected to ground. This transformer feeds the building in the same way a single phase service would and you will find checking around with the oscilloscope with the common connection connected to the center tap that this transformer has its legs 180 degrees out of phase with one another when the scope is connected this way. Confused yet? :)
     
  4. Oct 8, 2015 #3
    Yes, very confused... I don't understand why this is such a debatable topic, there has to be a clear answer.

    Is the image of the waveforms I posted accurate? It demonstrates that the generated leg (2) will never reach the same maximum potential in reference to 1 or 3 as 1 and 3 would reach relative to each other. So say 120v, 1 to 3 can achieve 240 between each other but 2 will never reach 240 relative to any reference...
     
  5. Oct 8, 2015 #4

    Averagesupernova

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    Your question is more about why 2 legs of the convertor are the same legs as your single phase supply right? Your diagram looks like typical 3 phase.
     
  6. Oct 8, 2015 #5
    There are two graphs in the image, one claiming to be a RPC 3ph output (top) and the other to be representative of PowerCo, true 3ph.

    I guess my questions are somewhat hinged on the accuracy of that top graph, it is somewhat more clear to me if that is an accurate representation of RPC output. In which case I would think something such as a 3ph motor being driven by a RPC would exhibit rougher operation than compared to running it on PowerCo 3ph.
     
  7. Oct 8, 2015 #6
    WIkipedia has a good description - under How RPC work - basically it is a 3 phase motor powered by a single phase and this excites/ energizes the third leg.
     
  8. Oct 8, 2015 #7

    Averagesupernova

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    Again, what is the common point on the scope that displays these waveforms? A true legitimate display of the 3 phases needs the common of the scope to be connected in a symmetrical manner. Wye connected transformers have this as the center node but delta connected transformers need something more creative. I would say that your attachment is a bit misleading. The top one would probably be close if the ground of the scope is connected to the neutral of the single phase supply. However, to have an apples to apples comparison you would need the same connection on one of the center taps of the transformers in the lower display of your attachment. Clearly this is not the case. I have built and used a rotary phase convertor.
     
  9. Oct 8, 2015 #8

    Baluncore

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  10. Oct 13, 2015 #9
    The understanding is more important than the application, in this case, for me. Why not look at this in terms of current in each of the legs in relation to time? In true 3ph, current through each leg to the load should oscillate symmetrically in relation to each of the other legs; ∆I = 0 (or Imax) at evenly spaced time intervals, indefinitely, for current in either direction. This is represented by the "true 3ph" illustration above, which I think all of us can agree is ideal 3ph power. So if the other illustration is misleading or inaccurate (because of where the illustrator chose common); my question is what would period of ∆I = 0 (or Imax) look like with an RPC output?

    I don't see how Imax (independent flow directions) could have a consistent period. Perhaps this can be best represented after performing Fourier analysis?
     
  11. Oct 13, 2015 #10

    Averagesupernova

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    I have never looked at mine on a scope. The voltages and currents are balanced fairly well. When I bought the machine I power my convertor with it came with a three phase motor that was manually spun up to speed before power was connected to it. Various schemes have been used over the years to start the convertor. I ended up using a starting capacitors with an adjustable potential relay to switch out the starting capacitors. The main difference in the quality of a rotary phase convertor versus commercial 3 phase power is the ability to supply the required current in the manufactured leg. It takes a huge convertor to have good voltage regulation on the manufactured leg over a wide range of loads.
     
  12. Oct 13, 2015 #11
    I can visualize the ability if a typical induction motor - properly generating the 3rd leg such that there is proper phase displacement. The Sketch you show from Practical Machinist is in a thread that has a debate. After spending too much time searching I have found this Commercial Statement - claiming that their technology provides true 3 phase and that lesser machines do not. In most commercially available designs there is a run time capacitance between one of the "input legs" and the created leg. Since it would be pretty easy to just hook up a motor and many posts are saying this - it is possible that the sketch you are providing is a home made unit that was not fully engineered. IT actually looks a lot like a scott connected transformer ( one for the few cases I would call true phase system) -- the waveforms see to be the same - and some may be calling it a phase converter and it is not really an RPC.
    As is pointed out above - the WAY the phases are measured when taking the scope measurement is also important - however I would not call this a common point on the scope - three phase power measurements can not use the scope common point(generates smoke) - and need isolation to be connected to a true scope. If the sketch artist IS using a scope common (on the Neutral of the single phase system) than he could have an RPC - and I do think that is the waveform he would get.
    You need ether a true 3 phase neutral -- or three separate phase to phase reading (A-B, B-C, C-A).
     
  13. Oct 13, 2015 #12

    Baluncore

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    The period and frequency will be set by the primary supply. The 120° phase separation of the voltages in the “three phases” generated will be what determines the quality of output balance. That does not require Fourier analysis for a single frequency. It only requires computation of the phasor angles.

    If there is no neutral return, then the sum of currents in the three phases must be zero.

    What gives? Ideally you generate three phase voltages at 120°. You then load the converter. The voltage phases should remain stable and balanced in phase, independent of load. The load current deviations will then be due to unbalanced load.

    If you have an input neutral and an output neutral then you have a terminology problem. You can synthesise a neutral output wire on a 3PH delta connected converter by using a star, = Y, connected three phase transformer wound on a single three legged core. I keep a few 3PH transformers from old equipment for that purpose. Unfortunately you then need an input isolation transformer if for safety and protection the output neutral is going to be connected to the input neutral and ground. It is usually better to wire your rotary converter as a star.

    A four channel digital scope will give you a view of three phases relative to neutral=earth by subtracting the neutral channel from the phase channels. A three channel digital scope can compute the average of the three inputs and subtract that from the 3 phases for display. But that requires you select a common reference, ground or neutral for all phase inputs.

    You can synthesise an output reference voltage by using three load resistors in a star. But then you need an analogue three phase scope with a floating neutral input that can be subtracted from all phase inputs.

    Have you seen the Scott–T transformer? https://en.wikipedia.org/wiki/Scott-T_transformer
     
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