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Metalworking (rec.crafts.metalworking) Discuss various aspects of working with metal, such as machining, welding, metal joining, screwing, casting, hardening/tempering, blacksmithing/forging, spinning and hammer work, sheet metal work. |
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Re. Rotary phase converters - magic or myths
In a recent long and wandering thread, (phase converter questions 8/24) it
was written: "A pair of good posts. It really is refreshing to see some solid input on 3 phase phase conversions which is both soundly based and backed up by direct measurement. I hope it will dispel some of the myths on minimum sizes of idlers and the practical usefulness of precision "tuning"." This, of course, refers mainly to the excellent report from Jerry Martes and tests made with his dynamometer. Perhaps, I know a bit more than others about Jerry and his dynamometer. I have knowledge of some of the design particulars of Jerry's dyno and I know that it is a first class machine both with regards to the way it loads and the way it measures HP. Therefore, I would be the first to say Jerry's test were soundly based and backed up by direct measurement. Jerry's tests proved that well designed 3-phase motors have a built in power reserve. The amount of "reserve" varies among manufacturers, and probably the quality of their products; it is known as "service factor" and would account for a 3-phase motor running on single-phase being able to deliver full rated 3-phase power out for a limited time. Jerry explained it very well. To those which may be "new" to rotary phase converters, I would caution not to be mislead by erroneous conclusions that may be drawn from the RCM thread, above. Jerry reported the load vs. current characteristics of 3-phase motors quite accurately and showed they can be driven beyond full rated output for short periods, whether running on single phase or 3-phase. Note that Jerry did not recommend this practice - he only reported on it. Having a bit of experience with rotary phase converters, frankly I cannot say where the "1.5 X" minimum size of idler came from. Perhaps Fitch threw it out several years ago when researching the subject, I just don't know. I do know that to successfully start a 3-phase load from an idler motor, whether "balanced" or not the idler must have a certain minimum size in order to take the load. How large, I'm not sure and have never experimented in this area. Perhaps, for starting duty only, the 1.5 figure is a bit high in some cases. I do know, though, that 1.5 is a good "platform" to work from toward voltage balancing a rotary and load system. The case for balancing can be made from a starting current standpoint. As various respondents have reported, no ordinary residential circuit breaker could stand the surge of starting a 15 or 20 HP non-balanced motor on single phase current. Precision "tuning", like magic, lies in the mind of the beholder. How much precision lies somewhere between none and much too much. Myths? I think not. No, a proper RPC system is merely an example of a serious metalworking hobbyist adhering to good engineering procedure. Bob Swinney |
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In article , Robert Swinney says...
Having a bit of experience with rotary phase converters, frankly I cannot say where the "1.5 X" minimum size of idler came from. Perhaps Fitch threw it out several years ago when researching the subject, I just don't know. I may be guilty here - if not of originating, then at least propogating the factor. Seems like anyone who ever tried to build a converter with a motor that's 1X never gets it to work, over the years of observing. They need to go bigger - the 1.5 factor works well if you do the 'capacitor thing.' I probably first saw the number from Fitch Williams. Likewise the old saw that the 3600 rpm motors don't make good converters. Jim -- ================================================== please reply to: JRR(zero) at pkmfgvm4 (dot) vnet (dot) ibm (dot) com ================================================== |
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"jim rozen" wrote in message ... In article , Robert Swinney says... Having a bit of experience with rotary phase converters, frankly I cannot say where the "1.5 X" minimum size of idler came from. Perhaps Fitch threw it out several years ago when researching the subject, I just don't know. I may be guilty here - if not of originating, then at least propogating the factor. Seems like anyone who ever tried to build a converter with a motor that's 1X never gets it to work, over the years of observing. They need to go bigger - the 1.5 factor works well if you do the 'capacitor thing.' I probably first saw the number from Fitch Williams. Likewise the old saw that the 3600 rpm motors don't make good converters. Jim Jim I suspect there is alot of good guidance to be obtained from most "old wives tales". Anything written by Fitch Williams or Don Foreman is valuable. I havent been able to determine why the 3600 RPM idler would be undesireable. I sure dont have data 'either way', 1800 or 3600 being better for idlers. The 1 1/2 time tool motor seems like a good starting place for identifying an idler size. I was actually surprized to learn that idlers as small as 1/10th the tool motor's HP could spin up the 3 phase tool motor. The "size factor" is very dependent on the amount of load on the tool while 'spinning-up'. I have noticed that a Big idler and Tuning (ala Fitch) really helps get a heavily loaded 3 phase tool spin-up. The Big idler and the *proper* tuning* both really help smooth out the 3 phase tool motor's pulsating when the tool is heavily loaded. What really surprizes me is that the 3 phase tool motor's performance doesnt seem to care if it has an idler or not whenever the tool is loaded to less than about 1/2 its name plate max rating. I may have missed something in my testing and thinking. But, I do have confidance in the findings since Don Foreman has been kind and patient enough to guide me thru all the aspects of this project I couldnt understand without his help. I suspect he would have picked up on any serious errors I made and corrected my approach and faulty conclusions. It sure pleases me to read that Pentigrid and Bob Swinney appreciate the efforts Don and I have made to get some data on how RPCs perform for RCM type considerations. Jerry |
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Yeah, Jim. It's funny how things get lifted from context only to be shoved
back down as gospel. Thanx for the confirmation. Bob Swinney "jim rozen" wrote in message ... In article , Robert Swinney says... Having a bit of experience with rotary phase converters, frankly I cannot say where the "1.5 X" minimum size of idler came from. Perhaps Fitch threw it out several years ago when researching the subject, I just don't know. I may be guilty here - if not of originating, then at least propogating the factor. Seems like anyone who ever tried to build a converter with a motor that's 1X never gets it to work, over the years of observing. They need to go bigger - the 1.5 factor works well if you do the 'capacitor thing.' I probably first saw the number from Fitch Williams. Likewise the old saw that the 3600 rpm motors don't make good converters. Jim -- ================================================== please reply to: JRR(zero) at pkmfgvm4 (dot) vnet (dot) ibm (dot) com ================================================== |
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I probably first saw the number from Fitch Williams. Likewise
the old saw that the 3600 rpm motors don't make good converters. I looked at some of Fitch's old posts. He contents that any good motor will make a good RPC. The original theory was that a 3600 RPM motor will have more mass and hold more energy for that peak. Fitch debunks that theory with his own theory. His theory is that the idler motor is a rotary transformer not a flywheel. If the motor is big enough and has enough copper and iron to make the transformation the motor RPM doesn't matter. I have been following this thread and experimenting a bit on my own. I now believe that a RPC isn't doing much until the load motor has a significant load. Both motors are pretty much running on single phase. At some point, the load motor starts to slow down(ie slip) and the RPC motor is still running at full speed and starts suppling some power to the load motor on the third leg. Maybe my theory is all wet, but I have tried connecting up three different motors without balancing caps and I see almost no current in the third leg when the motors are unloaded. chuck |
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On 30 Aug 2004 08:26:51 -0700, jim rozen
wrote: I probably first saw the number from Fitch Williams. Likewise the old saw that the 3600 rpm motors don't make good converters. Perhaps a 3600 RPM motor is less desirable because it's harder to start, having to build much more angular momentum to reach operating speed. |
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On Mon, 30 Aug 2004 13:48:01 -0500, Don Foreman
wrote: On 30 Aug 2004 08:26:51 -0700, jim rozen wrote: I probably first saw the number from Fitch Williams. Likewise the old saw that the 3600 rpm motors don't make good converters. Perhaps a 3600 RPM motor is less desirable because it's harder to start, having to build much more angular momentum to reach operating speed. I've been looking and can't find it but I read the math which shows that all things being equal the 1800 rpm motor is better. I was surprised when I read it a few years ago and I'm no motor guy so I could be totally wrong. All that being said, I've got three store bought RPCs. Two spin at 1725 and the other at 3450. The fast one came without enough capacitance to balance the load correctly when the load was close to the maximum rated load. I added capacitors using info gained from several posters to this group. The other two are really well balanced machines and the voltage on all legs on the largest is always within 6% when the load is between 1 and 33 hp. I have not measured the current to see how well it is balanced at all possible loads. The manuals for my machines all only talk about the voltage in the supply. ERS |
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On 30 Aug 2004 08:26:51 -0700, jim rozen
wrote: I probably first saw the number from Fitch Williams. Likewise the old saw that the 3600 rpm motors don't make good converters. Perhaps a 3600 RPM motor is less desirable because it's harder to start, having to build much more angular momentum to reach operating speed. I've been looking and can't find it but I read the math which shows that all things being equal the 1800 rpm motor is better. I was surprised when I read it a few years ago and I'm no motor guy so I could be totally wrong. All that being said, I've got three store bought RPCs. Two spin at 1725 and the other at 3450. The fast one came without enough capacitance to balance the load correctly when the load was close to the maximum rated load. I added capacitors using info gained from several posters to this group. The other two are really well balanced machines and the voltage on all legs on the largest is always within 6% when the load is between 1 and 33 hp. I have not measured the current to see how well it is balanced at all possible loads. The manuals for my machines all only talk about the voltage in the supply. ERS FWIW years ago (before they made RPC's too) Phase-A-Matic had instructions on how you could make a rotary phase converter starting with one of their static units and buying a used 3 phase motor. Quoting them: " The idler motor should be at least 50% larger than the largest motor that you want to run to accomodate the higher starting current. A good quality 3600 R.P.M. Y wound 3 phase 220-V motor is the best choice. 1800 R.P.M. motors can be used on applications not heavily loaded." David Lindquist |
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In article , Don Foreman says...
With some trepidation, I have to say that I don't think it's "just a rotary transformer". Energy must be stored and released somewhere to create a third phase, Sure it does - it takes energy to provide the rotating B field that spins with the rotor's squirrel cage. The energy is in the kinetic energy of the rotor. That seems to be pretty consistent with the 'rotary transformer' approach to converter operation. Jim -- ================================================== please reply to: JRR(zero) at pkmfgvm4 (dot) vnet (dot) ibm (dot) com ================================================== |
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Don sez: "Perhaps a 3600 RPM motor is less desirable because it's harder
to start, having to build much more angular momentum to reach operating speed." That would have to be true. In my limited experience, low-power installations such as we encounter in home-shop applications, 3600 RPM idler motors seem to work as well as the more-common 1800 RPM types. I have seen some 7-1/2 HP, 3600 RPM idlers that appear "normal". They are sorta like ugly babies - if they are otherwise able to kick and scream, their mothers don't notice the difference. As an aside to all the great points set forth in recent threads, I want to acknowledge a key point that Don Foreman set me straight on. I sent Don a copy of a small, unpublished "paper" I had written on RPC's. In there, were comments on the obvious (obvious to me anyway) point that an idler and its load motor are in parallel. BAM! Don hit me right between the eyes with "dead squirrel" reasoning which proved they were *not* in parallel. Don's reasoning was eloquent in its simplicity and enhanced my understanding of RPC current flow by a quantum leap! Previous to Don's proofing of my little paper, I could not reconcile 3rd leg current measurements against the notion that the 3rd legs were in parallel. Duh! Of course, they can't be in parallel. Thanks Don. Current flow in a RPC is quite complex and does not easily yield itself to calculation. The stated goal of "balancing" the 3 legs of a RPC is to achieve reasonably similar *voltages* across all 3 legs, or phases. Interestingly, from measurements of loaded and unloaded voltage balanced RPC systems, it can be seen that the *current* through the 3rd leg is always maximum; or of an amount sufficient to create the desired 3rd leg voltage. That is to say if the 3rd leg voltage is made to equal the voltages in the other 2 legs, then the 3rd leg current has to be manipulated in such a way as to make that happen. Third leg *current* flows in a tortuous path indeed. It might be stated that in a voltage balanced RPC, 3rd leg current is made to flow in a manner that emulates a parallel connection of idler and load. Measurements taken at various nodes show current tends to flow in opposite directions through the 3rd leg - but, of course, the resulting aggregate current is what establishes 3rd leg voltage. In other words, idler and load are made to *behave* as if they are in parallel in a voltage balanced RPC. Thanks again to Don Foreman for showing that the idler and load in a RPC are not in parallel. Bob Swinney "Don Foreman" wrote in message ... On 30 Aug 2004 08:26:51 -0700, jim rozen wrote: I probably first saw the number from Fitch Williams. Likewise the old saw that the 3600 rpm motors don't make good converters. |
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In article , Robert Swinney says...
load motor are in parallel. BAM! Don hit me right between the eyes with "dead squirrel" reasoning which proved they were *not* in parallel. Um, maybe in some non-intuitive theoretical treatment - but from a topological sense the motor windings *are* in a parallel circuit, at least for delta wired motors. Granted the incoming line is across two of the paralleled windings of course. Where's the dead squirrel? Jim -- ================================================== please reply to: JRR(zero) at pkmfgvm4 (dot) vnet (dot) ibm (dot) com ================================================== |
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In article , Gary Coffman says...
In summary, during starting, the load motor back EMF is small, so significant current flows on the wild leg, properly starting the motor. While running at speed with no load, wild leg current will be small, Actually, for the driven motor running at speed with no load, *all* the currents will be nearly zero. Granted an amp-clamp will show some larger value, but once again that is reactive current. Measuring the in-phase currents on all three legs of the driven motor, with no mechanical load, will probably yeild nearly equal values. SWAG. Jim -- ================================================== please reply to: JRR(zero) at pkmfgvm4 (dot) vnet (dot) ibm (dot) com ================================================== |
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I like your explanation of induced EMF in the load's wild leg
varying with load motor slip speed, Gary. That clearly and concisely shows how the load motor draws additional third-phase power under load. I'm sure not inclined to dispute Fitch's observations from data. . I will, however, try to understand what that data tells us. He may have found that *additional* flywheel makes no observable difference. I'd bet that he did his experiment with an idler at least 1.5 times as large (HP) as the load motor. I rather doubt that he built an induction motor with a rotor of negligable mass. The mere fact that an induction motor draws many times rated load current to reach speed in a period long compared to 1 cycle suggests that the kinetic energy in the rotor is considerably greater than the integral of rated line power over one cycle or fraction of a cycle. Variation in rotor speed (kinetic energy) over 1/60th of a cycle would therefore already be difficult to observe. The effect of adding more mass would then be even more difficult to observe. One would need a high resolution (fraction of RPM) speed sensor with a very high sample rate to observe the ebb and flow of kinetic energy over each cycle. Some clarification of the term "rotary transformer" might be helpful. Simply stating that it is a rotary transformer doesn't clear anything up for me! What exactly is a rotary transformer? If the rotating mass is irrelevant, then how is rotation relevant? If the wild leg is generating power during periods when little or no power is available from the mains each cycle, where does that power come from if not from kinetic energy stored in the idler rotor? If that *is* where it comes from, then I contend that idler motor rotor mass is indeed relevant, though I could sure see how adding more mass may not make any noticable difference. On Wed, 01 Sep 2004 01:41:14 -0400, Gary Coffman wrote: On 30 Aug 2004 18:34:58 GMT, (Charles A. Sherwood) wrote: I probably first saw the number from Fitch Williams. Likewise the old saw that the 3600 rpm motors don't make good converters. I looked at some of Fitch's old posts. He contents that any good motor will make a good RPC. The original theory was that a 3600 RPM motor will have more mass and hold more energy for that peak. Fitch debunks that theory with his own theory. His theory is that the idler motor is a rotary transformer not a flywheel. If the motor is big enough and has enough copper and iron to make the transformation the motor RPM doesn't matter. Agreed. But there's also the idea that a 1800 RPM motor will have lower windage losses than a same size 3600 RPM motor. Windage loss, and bearing friction loss, are the two major non-electric loss mechanisms for the idler. The electric loss mechanisms are winding resistance and flux leakage (same as for a transformer). Fitch demonstrated that a flywheel doesn't help. In fact you want the lowest rotating mass that permits low winding resistance and flux leakage. That's because you *want* the idler to respond as rapidly as possible, ie draw more current from the 1 ph line, when the load varies. It is that current which creates the rotor flux which allows the idler to work as a RPC. I have been following this thread and experimenting a bit on my own. I now believe that a RPC isn't doing much until the load motor has a significant load. Both motors are pretty much running on single phase. At some point, the load motor starts to slow down(ie slip) and the RPC motor is still running at full speed and starts suppling some power to the load motor on the third leg. Maybe my theory is all wet, but I have tried connecting up three different motors without balancing caps and I see almost no current in the third leg when the motors are unloaded. Your theory is not all wet. Assume the idler and load motor are identical. When both motors are running unloaded, with the same slip, they have the same back EMF on their wild legs, so there is no voltage difference to cause current to flow in the wild leg. When the load motor is loaded, its slip increases, its back EMF falls, and a potential difference is created between the idler wild leg voltage and the load motor wild leg voltage which then forces current to flow between the idler and the load motor. Obviously, wild leg current reaches a maximum as load on the load motor reaches maximum. In a well tuned system (ie a system with good voltage balance), this current should be the same as the currents in the other two legs (within a few percent anyway) at full rated load. Note this is at the *load* motor's rated load. If you were running a 1/2 hp load motor off of a 20 hp idler, max wild leg current would occur when the load motor is mechanically loaded to 1/2 hp, the idler hp rating is irrelevant as long as it is significantly larger than the load motor's hp. Of course during load motor starting, slip starts out infinite, ie the back EMF of the load motor is zero, so maximum current flows between the idler and the load motor. This is exactly what you want for starting a 3 ph motor. Again, in a reasonably well tuned system, the currents on all 3 legs of the load motor should be equal during starting. But if you don't require maximum starting torque, the idler's voltage imbalance can be rather large and the motor will still start, though not as rapidly. That's why folks can get away with using an untuned converter. In summary, during starting, the load motor back EMF is small, so significant current flows on the wild leg, properly starting the motor. While running at speed with no load, wild leg current will be small, for identical idler and load motors it will be zero. As load increases, wild leg current increases, until at the limit when the load motor is running at full rated load, it is drawing full rated current on the wild leg, same as it is on the other two legs. Gary |
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The B field does not come from rotation. It takes current to produce a B field. Since there are no permanent magnets present, rotation alone cannot produce a B field. Given that there is a B field, it takes energy to keep it rotating if it induces voltage (hence current) in a winding somewhere that therefore produces a counter field and counter torque. Calling it a "rotary transformer" doesn't clear anything up. What is a rotary transformer and how does it work? Transformers work by inducing voltage from a varying B field. Induction motors (and generators) work with a rotating B field of constant magnitude, the rotation causing the field linking a stationary coil to vary with time. One method of analyzing single-phase motors and unbalanced polyphase motors is by the method of symmetrical components, which is a collection of constant B fields rotating in one direction or the other but all at the same speed, whose resultant sums to the actual field. A complete electrical understanding requires a description, by whatever method, of how the field (or contrived fields as in symmetrical component analysis) vary with time at the location of each winding and in the rotor. Once this is known, the exchange of electrical power and mechanical power (instantaneous torque * speed) can be described by the interactions of the fields with currents. This has been done. Fitch found it and sent it to me some years ago. It is a mathematical nightmare! On 31 Aug 2004 21:55:20 -0700, jim rozen wrote: In article , Don Foreman says... Sure it does - it takes energy to provide the rotating B field that spins with the rotor's squirrel cage. The energy is in the kinetic energy of the rotor. That seems to be pretty consistent with the 'rotary transformer' approach to converter operation. Jim |
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In article , Don Foreman says...
The B field does not come from rotation. It takes current to produce a B field. Since there are no permanent magnets present, rotation alone cannot produce a B field. My exact comment was "rotating B field." Given that there is a B field, it takes energy to keep it rotating if it induces voltage (hence current) in a winding somewhere that therefore produces a counter field and counter torque. Exactly. Calling it a "rotary transformer" doesn't clear anything up. What is a rotary transformer and how does it work? A rotary transformer could be thought of as a permanent magnet that rotates inside the three windings on the stator, which are wound to produce the correctly phased voltages when the *rotating* B field sweeps over them. You might say that this is nothing more than a permanent magnet three phase generator and you would be right. Now the PM rotor has to be replaced with a 'squirrel cage' that can be found in any common induction motor. The currents that flow in the buss bars of this device make it behave like the PM version mentioned above. So one simple explaination is that the primary is the circuit inside the squirrel cage in the rotor, which generates a B field. The B field is in motion with respect to the stator windings, so even though its *magnititude* is constant, there is a voltage impressed on the secondary (stator windings) because of the relative motion. This story is somewhat incomplete because something has to be exciting the rotor's current, or it would rapidly decay away and the rotor would come to a standstill. I like to think that the power is coupled from the one excited winding in the stator, to the rotating field that the rotor generates, to the other two windings in the stator. So granted it's a bit of a convolution, but I like to think of the primary as the one driven winding, and the secondary as the combination of all three windings. The *rotating* B field of the armature's conductive buss bars is the item that couples them together. How do you model one of the old Alexanderson Alternators? There you have a set of driven coils coupled to another set of coils that see only the B field of the first coils. The field is chopped with a variable reluctance (iron vanes on a rotor inerposed between the two) so that the secondary current is modulated at the chopping frequency. That's another kind of 'rotary' transformer, albeit a much simpler one to comprehend. Even though the excitation is dc, the field is still varying and it works as a transformer. Transformers don't have to have alternating current flowing in the primary winding to create the time-varying B field seen by the secondary. In most cases of course you *do* need to have the secondary see a time-varying field for the voltage to be induced (faraday's law) but the mathmatical difference between, say, a field generated by alternating current in a stationary coil, versus a field generated by direct current in a winding that is in motion relative to the secondary, is nearly inconsequential when analyzing the transformer action. All that matters is that the field seen by the stator windings changes over time. It does not matter *how* the change is effected. The term 'transformer' is typically reserved for a device where all the windings are mechanically fixed relative to each other. So 'rotary transformer' is a fairly novel and unconventional term I admit. It does a pretty good job of describing the energy flow in a rotary converter though - inductive coupling of two different circuits via a time-varying B field. Transformers work by inducing voltage from a varying B field. Induction motors (and generators) work with a rotating B field of constant magnitude, the rotation causing the field linking a stationary coil to vary with time. One method of analyzing single-phase motors and unbalanced polyphase motors is by the method of symmetrical components, which is a collection of constant B fields rotating in one direction or the other but all at the same speed, whose resultant sums to the actual field. A complete electrical understanding requires a description, by whatever method, of how the field (or contrived fields as in symmetrical component analysis) vary with time at the location of each winding and in the rotor. Once this is known, the exchange of electrical power and mechanical power (instantaneous torque * speed) can be described by the interactions of the fields with currents. This has been done. Fitch found it and sent it to me some years ago. It is a mathematical nightmare! I am sure that if Fitch took the time to do a complete analysis, it is a) correct and b) extremely intricate. Even though there is a great deal of hand-waving in the 'rotary transformer' explaination, it still provides a good seat-of-the pants understanding for those who either cannot understand the math of a full treatment, or who don't want to take the time to do so. As they say, it will be left as an exercise to the reader to figure out which camp I reside in! :^) Jim -- ================================================== please reply to: JRR(zero) at pkmfgvm4 (dot) vnet (dot) ibm (dot) com ================================================== |
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"Don Foreman" wrote in message ... I like your explanation of induced EMF in the load's wild leg varying with load motor slip speed, Gary. That clearly and concisely shows how the load motor draws additional third-phase power under load. I'm sure not inclined to dispute Fitch's observations from data. . I will, however, try to understand what that data tells us. He may have found that *additional* flywheel makes no observable difference. I'd bet that he did his experiment with an idler at least 1.5 times as large (HP) as the load motor. I rather doubt that he built an induction motor with a rotor of negligable mass. The mere fact that an induction motor draws many times rated load current to reach speed in a period long compared to 1 cycle suggests that the kinetic energy in the rotor is considerably greater than the integral of rated line power over one cycle or fraction of a cycle. Variation in rotor speed (kinetic energy) over 1/60th of a cycle would therefore already be difficult to observe. The effect of adding more mass would then be even more difficult to observe. One would need a high resolution (fraction of RPM) speed sensor with a very high sample rate to observe the ebb and flow of kinetic energy over each cycle. Some clarification of the term "rotary transformer" might be helpful. Simply stating that it is a rotary transformer doesn't clear anything up for me! What exactly is a rotary transformer? If the rotating mass is irrelevant, then how is rotation relevant? If the wild leg is generating power during periods when little or no power is available from the mains each cycle, where does that power come from if not from kinetic energy stored in the idler rotor? If that *is* where it comes from, then I contend that idler motor rotor mass is indeed relevant, though I could sure see how adding more mass may not make any noticable difference. "..the induction motor may be considered as a transformer whose secondary winding is permitted to rotate. Consequently, much of the analysis applied to the transformer may be modified and utilized in the analysis of the induction motor." From my old "Energy Conversion" text book. Yes, the math becomes involved-much more than I can recollect after so many years! |
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Jim, more concerned with a dead squirrel, sez:
"Um, maybe in some non-intuitive theoretical treatment - but from a topological sense the motor windings *are* in a parallel circuit, at least for delta wired motors. Granted the incoming line is across two of the paralleled windings of course." Nope! Draw out a capacitor-assisted rotary phase converter...not just 2 ordinary three-phase motors in parallel. Granted, two phases are across the line, and as such are in parallel. For the other "phase" assume a direction of current flow from the 1st motor's "Y"center point through the end of the 3rd leg. Then look at that same point on the other motor and 'viola', the difference will strike you -- dead squirrel between-the-eyes fashion! Jim, you are right that two 3-phase motors are in parallel such as in your own non-balanced RPC. But when you connect them as a capacitor-assisted RPC, the convoluted 3rd leg current is forced to emulate an ordinary parallel connection. This is more obvious if you consider that each set of windings in a 3-phase motor acts as both consumer and generator. The generator aspect is augmented by capacitor action to creat an aggregate current that does, in fact, flow in the same direction as if the motors were truly in parallel. Bob Swinney "jim rozen" wrote in message ... In article , Robert Swinney says... load motor are in parallel. BAM! Don hit me right between the eyes with "dead squirrel" reasoning which proved they were *not* in parallel. Where's the dead squirrel? Jim -- ================================================== please reply to: JRR(zero) at pkmfgvm4 (dot) vnet (dot) ibm (dot) com ================================================== |
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On Tue, 31 Aug 2004 22:58:21 -0500, Don Foreman
wrote: Fitch is *very* busy designing his new house at the moment, but he'll be done with that eventually and have time again for thinking about other things. Glad to hear that and all the best wishes to him, since I took the jump to retired life, I still haven't been able to determine how I EVER had time to work for a living! Gerry :-)} London, Canada |
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On Wed, 01 Sep 2004 11:19:05 -0500, Don Foreman wrote:
I like your explanation of induced EMF in the load's wild leg varying with load motor slip speed, Gary. That clearly and concisely shows how the load motor draws additional third-phase power under load. Thanks. If you look a bit closer, you can see that the back EMF determines the load current drawn by the windings of any motor, not just the wild leg of a 3 ph motor running off a converter. I'm sure not inclined to dispute Fitch's observations from data. . I will, however, try to understand what that data tells us. He may have found that *additional* flywheel makes no observable difference. I'd bet that he did his experiment with an idler at least 1.5 times as large (HP) as the load motor. I rather doubt that he built an induction motor with a rotor of negligable mass. No, he didn't, but he did put a *heavy* flywheel on it, and noted no improvement. In fact, the converter can't respond as quickly to changes in load with a heavy flywheel attached. (And we want it to, since the majority of the energy in, and all of it that is passing through, the system is electrical, not mechanical.) The mere fact that an induction motor draws many times rated load current to reach speed in a period long compared to 1 cycle suggests that the kinetic energy in the rotor is considerably greater than the integral of rated line power over one cycle or fraction of a cycle. True, but the lighter the rotor, the less power needed to change its speed (and hence its slip). Since that slip is mechanically coupled for all bars of the squirrel cage, more power can be immediately drawn from the primary feed if the rotor mass is low. Unless we can draw this power, we can't pass it along to the load. In electrical terms, a lower mass rotor lowers the source impedance of the RPC, and a lower impedance source can provide a stiffer output, ie less sag under changing load. Variation in rotor speed (kinetic energy) over 1/60th of a cycle would therefore already be difficult to observe. The effect of adding more mass would then be even more difficult to observe. One would need a high resolution (fraction of RPM) speed sensor with a very high sample rate to observe the ebb and flow of kinetic energy over each cycle. Or simply watch the voltage and current on a dual channel scope, then, if you have a good scope, integrate the two to get instantaneous real and reactive power. Some clarification of the term "rotary transformer" might be helpful. Simply stating that it is a rotary transformer doesn't clear anything up for me! What exactly is a rotary transformer? If the rotating mass is irrelevant, then how is rotation relevant? Well, you have 3 sets of stator coils, and one rotating squirrel cage made up of a number of bars. The latter are all energized in parallel, so the electrical phase is the same on all of them at any instant. But their positions aren't the same, and the whole thing is turning. This creates a mechanically rotating B field which sweeps past the fixed position stator coils inducing a back EMF in them (all of them, including the ones hooked to the wild leg, ie the "transformer" secondary). *Some* of the stator coils (primary) are also energized by line current from utility power. This is electrically time varying, producing its own electrically rotating B vector. This vector is what induces currents in the squirrel cage in the first place to allow it to produce its own rotating B field. The rotor is just an intermediate between primary and secondary which has the interesting time varying property of mechanically displaced (phase shifted) synchronization with the primary field. If the wild leg is generating power during periods when little or no power is available from the mains each cycle, where does that power come from if not from kinetic energy stored in the idler rotor? If that *is* where it comes from, then I contend that idler motor rotor mass is indeed relevant, though I could sure see how adding more mass may not make any noticable difference. It is important to realize that a RPC (or more conventional transformer) has mostly *reactive* currents circulating in it. These are inductively reactive, so the current lags voltage. (If the coils were lossless, the lag would be exactly 90 degrees.) These reactances consume no power, but they do store considerable energy in their magnetic fields. It is this energy which is transferred from input to output of any transformer, whether rotary or not, as the fields rise and collapse. Negligible mechanical energy is exchanged. What's different about a rotary transformer is that the coils are in different spatial relationships at different points in time. This means that the geometry of the stators and rotor are such that inducing and induced currents are of differing phase (time). So even though the primary is going through a voltage zero, the secondary is seeing a rising voltage induced by the collapsing rotor field, and vice versa. In other words, the B field induced into the rotor doesn't instantly die when the primary goes through a zero. That field is still collapsing since it is inductively lagging, as it mechanically approaches a secondary coil. The collapsing field induces a current in the secondary which lags the current in the primary by the *mechanical* phase (time) difference between stator locations. A non-rotating transformer can't do this phase shift, but a rotary transformer can, and does. That's how it is able to produce 3 ph from 1 ph. But all the energy being transferred is *electromagnetic*, the mechanical rotation is just there to provide the phase shift. The ratio of real power to reactive (imaginary) power in the system varies with load. But the system stored energy is constant (at least until you overload it enough to drive it into saturation). In the steady state, ie after sync speed is achieved in the idler, energy out equals energy in less system losses. Same as for any transformer. Gary |
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On 1 Sep 2004 08:44:35 -0700, jim rozen wrote:
In article , Robert Swinney says... load motor are in parallel. BAM! Don hit me right between the eyes with "dead squirrel" reasoning which proved they were *not* in parallel. Um, maybe in some non-intuitive theoretical treatment - but from a topological sense the motor windings *are* in a parallel circuit, at least for delta wired motors. Granted the incoming line is across two of the paralleled windings of course. Where's the dead squirrel? Topologically, the two windings are in parallel as viewed from the outside. But from the *point of view of the current*, they are in series. In other words, current has to flow through one then the other to close the loop. That's a series circuit. (Note that you always use the POV of the current when determining whether a circuit is series or parallel. Nothing else makes any electrical sense.) The wild leg winding in the RPC can be pictured as a battery, and the winding in the load motor can be pictured as a light bulb. The bulb won't burn unless you have a closed loop for the current, and you do. It is the *other* leg connecting the *other* ends of the windings. Of course in reality the "battery" is producing a time varying voltage, and the "bulb" actually represents the mechanical output of the load motor. But none of that changes the fact that the two coils are in series from the POV of the current. Gary |
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On Wed, 1 Sep 2004 13:14:42 -0500, "Robert Swinney" wrote:
Jim, more concerned with a dead squirrel, sez: "Um, maybe in some non-intuitive theoretical treatment - but from a topological sense the motor windings *are* in a parallel circuit, at least for delta wired motors. Granted the incoming line is across two of the paralleled windings of course." Nope! Draw out a capacitor-assisted rotary phase converter...not just 2 ordinary three-phase motors in parallel. Granted, two phases are across the line, and as such are in parallel. For the other "phase" assume a direction of current flow from the 1st motor's "Y"center point through the end of the 3rd leg. Then look at that same point on the other motor and 'viola', the difference will strike you -- dead squirrel between-the-eyes fashion! Jim, you are right that two 3-phase motors are in parallel such as in your own non-balanced RPC. But when you connect them as a capacitor-assisted RPC, the convoluted 3rd leg current is forced to emulate an ordinary parallel connection. This is more obvious if you consider that each set of windings in a 3-phase motor acts as both consumer and generator. The generator aspect is augmented by capacitor action to creat an aggregate current that does, in fact, flow in the same direction as if the motors were truly in parallel. Capacitors aren't required for this to be true. The RPC is mechanically unloaded, the load motor isn't. So the back EMF on the wild leg winding of the RPC will be greater than the back EMF of the load, thus you have a potential difference, and current will be forced to flow from the higher potential to the lower potential. No tricks with capacitors are required for this to happen. Capacitors serve a different purpose entirely in a tuned RPC. They're there simply to equalize the phase to phase voltages at the RPC. They do that by forming LC phase shifters which offset some of the inductive lag with capacitive lead so that you get 120+120+120=360 instead of 167+135+58=360, which you might see in a really badly tuned converter. Gary |
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The capacitors serve as a source of reactive power. |
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In article , Gary Coffman says...
Capacitors aren't required for this to be true. The RPC is mechanically unloaded, the load motor isn't. So the back EMF on the wild leg winding of the RPC will be greater than the back EMF of the load, thus you have a potential difference, and current will be forced to flow from the higher potential to the lower potential. No tricks with capacitors are required for this to happen. OK but from a circuitry standpoint the two windings on the two motors (delta configured both) are in parallel. Yes there is current that flows in the circuit of course. Jim -- ================================================== please reply to: JRR(zero) at pkmfgvm4 (dot) vnet (dot) ibm (dot) com ================================================== |
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In article , Gary Coffman says...
Topologically, the two windings are in parallel as viewed from the outside. But from the *point of view of the current*, they are in series. Yep. Consider a "parallel" tuned rf tank circuit. A capacitor and coil connected to each other. They call it a parallel circuit but of course the internally circulating currents are what give it the interesting properties. We have a nomenclature impass here. :^) Jim -- ================================================== please reply to: JRR(zero) at pkmfgvm4 (dot) vnet (dot) ibm (dot) com ================================================== |
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In article , Gary Coffman says...
*Some* of the stator coils (primary) are also energized by line current from utility power. This is electrically time varying, producing its own electrically rotating B vector. This vector is what induces currents in the squirrel cage in the first place to allow it to produce its own rotating B field. Obviously the term 'transformer' is most often used to describe stationary devices with no mechanical moving parts. But thinking of a rotary converter as the primary, excited winding coupled to the three output windings via the rotating B field of the armature makes it look a great deal like a transformer. The time-varying B field in this case is coming from a mechanically moving object rather than a stationary winding with a time varying current in it. The fly in the ointment is that the primary winding and the secondary windings share a common element. This makes it even tougher to wrap ones mind around. Jim -- ================================================== please reply to: JRR(zero) at pkmfgvm4 (dot) vnet (dot) ibm (dot) com ================================================== |
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In article ,
jim rozen wrote: :In article , Gary Coffman says... : :Capacitors aren't required for this to be true. The RPC is mechanically :unloaded, the load motor isn't. So the back EMF on the wild leg winding :of the RPC will be greater than the back EMF of the load, thus you have :a potential difference, and current will be forced to flow from the higher :potential to the lower potential. No tricks with capacitors are required :for this to happen. : :OK but from a circuitry standpoint the two windings on the two motors delta configured both) are in parallel. Yes there is current that :flows in the circuit of course. Topologically in parallel, yes. Consider that when you connect a light bulb across a battery, they too are topologically in parallel, but that isn't a particularly useful way to look at the circuit. -- Bob Nichols AT comcast.net I am "rnichols42" |
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On Tue, 31 Aug 2004 22:58:21 -0500, Don Foreman
wrote: With some trepidation, I have to say that I don't think it's "just a rotary transformer". Energy must be stored and released somewhere to create a third phase, because a third phase provides power when there is none available from the mains (during zero crossings). One way to check this (and perhaps prove me wrong) would be to make measurements of phase currents (with no "tuning" capacitors) with various loads with and without additional mass (flywheel) on the rotor of the idler motor. It would also be interesting to observe rotor speed on the idler as load is increased on the driven motor. If it decreases, that would suggest that some exchange between mechanical power and electric power is taking place in the idler, with the process of generating the third phase creating countertorque that slows the idler rotor. Fitch is *very* busy designing his new house at the moment, but he'll be done with that eventually and have time again for thinking about other things. A slightly different way of looking at this vexed problem may help. It doesn't directly clarify the operation of a real real rotary phase converter with all its losses and second order effects but it does at least give a reasonably convincing (to me anyway) insight into the way it works. Consider a 2 pole 3 phase wound LOSSLESS squirrel cage motor supplied with single phase power to one winding. There is no mechanical load so there are no losses of any kind. Once the rotor is spun up to operating speed, the stable operating condition is with the rotor spinning at 2 pole synchronous speed. This necessarily means that the rotor is diametrically magnetised as a single N-S magnet as a result of the induced current that is circulating in in the superconducting rotor bars. Because these are perfect conductors this current CANNOT decay so that the rotor behaves exactly as a rotating permanent magnet. The rotating magnetic field produced by this magnet induces equal voltages into each of the three phase windings and it is this rotating field that produces the true balanced three phase output voltage pattern. In the case of the energised winding, in this completely lossles system, the induced voltage (the back EMF) has risen to be both equal to, and in phase with the supply voltage so that no current is drawm from the supply. If a load current is taken from either or both of the unenergised windings the current in the windings produces a magnetic field that tries to slow down the rotor. It doesn't succeed because the motor is operating in a synchronous mode. What happens is that there is a slight change in the angular position of the rotor so that the voltage maximum of the induced back EMF is no longer exactly coincident with the supply voltage maximum. This shift causes just enough supply current to flow to cancel the drag forces generated by the load currents. In this lossless motor there are no voltage changes resulting from voltage drops in the winding resistance so the output remains at three voltages all equal to the supply voltage and 120 deg apart. Exactly the same correction mechanism applies if a mechanical load tries to slow down the rotor. This means a lossless motor delivers balanced three phase out independent of variations in both mechanical and electrical loading. It's interesting to note that this explanation does not require any direct transformer action between the windings, all the energy transfer is via the rotor. A factor,so far ignored, is that the torque generated by input supply currents fluctuates between zero and a maximum value twice per cycle of the input waveform. Because of this there must be enough mechanical inertia in the rotor to continue the rotation through the low current parts of the cycle. This is only matters if the inertia is so low that the input torque fluctuations produce significant instantaneous rotary velocity changes at twice supply frequency. In practice the rotor inertia of the commonly used motors so large that this is not a problem. Any variation large enough to matter would show itself as a modulation distortion of the phantom phase voltage(s) waveform at twice supply frequency. Jim |
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In article , Robert Nichols says...
Topologically in parallel, yes. Consider that when you connect a light bulb across a battery, they too are topologically in parallel, but that isn't a particularly useful way to look at the circuit. No, they're in series. The example I gave before was what is classically known as a parallel tuned tank circuit. The internal current of the two devices are flowing as though they are in series, but the combination, as connected to the outside world, are in parallel. Your light bulb example doesn't work, because there is noplace for current to flow besides in the single loop battery - load. In the "parallel" tank circuit, as in the rotary converter, each node in the circuit has more than one other connection. Jim -- ================================================== please reply to: JRR(zero) at pkmfgvm4 (dot) vnet (dot) ibm (dot) com ================================================== |
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On 1 Sep 2004 11:03:20 -0700, jim rozen
wrote: (snip) So granted it's a bit of a convolution, but I like to think of the primary as the one driven winding, and the secondary as the combination of all three windings. The *rotating* B field of the armature's conductive buss bars is the item that couples them together. Thanks! I certainly agree that it is "a bit of a convolution", but I see nothing wrong with it -- and now I know what you mean by "rotary transformer" ! All that matters is that the field seen by the stator windings changes over time. It does not matter *how* the change is effected. The term 'transformer' is typically reserved for a device where all the windings are mechanically fixed relative to each other. So 'rotary transformer' is a fairly novel and unconventional term I admit. Yes. My problem was making sense of an explanation using that term. I am sure that if Fitch took the time to do a complete analysis, it is a) correct and b) extremely intricate. Even though there is a great deal of hand-waving in the 'rotary transformer' explaination, it still provides a good seat-of-the pants understanding for those who either cannot understand the math of a full treatment, or who don't want to take the time to do so. Well....maybe your pants, cuz you already had the term in your pocket! The math is easier for me to understand, but whatever works! |
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On Thu, 02 Sep 2004 04:04:47 -0400, Gary Coffman
wrote: On Wed, 01 Sep 2004 11:19:05 -0500, Don Foreman wrote: I like your explanation of induced EMF in the load's wild leg varying with load motor slip speed, Gary. That clearly and concisely shows how the load motor draws additional third-phase power under load. Thanks. If you look a bit closer, you can see that the back EMF determines the load current drawn by the windings of any motor, not just the wild leg of a 3 ph motor running off a converter. Yup. I'm sure not inclined to dispute Fitch's observations from data. . I will, however, try to understand what that data tells us. He may have found that *additional* flywheel makes no observable difference. I'd bet that he did his experiment with an idler at least 1.5 times as large (HP) as the load motor. I rather doubt that he built an induction motor with a rotor of negligable mass. No, he didn't, but he did put a *heavy* flywheel on it, and noted no improvement. In fact, the converter can't respond as quickly to changes in load with a heavy flywheel attached. (And we want it to, since the majority of the energy in, and all of it that is passing through, the system is electrical, not mechanical.) OK with the first sentence. Did he observe and report what you assert in the second sentence? You note that he noted no improvement. Did he note any degradation? The mere fact that an induction motor draws many times rated load current to reach speed in a period long compared to 1 cycle suggests that the kinetic energy in the rotor is considerably greater than the integral of rated line power over one cycle or fraction of a cycle. True, but the lighter the rotor, the less power needed to change its speed (and hence its slip). Since that slip is mechanically coupled for all bars of the squirrel cage, more power can be immediately drawn from the primary feed if the rotor mass is low. Unless we can draw this power, we can't pass it along to the load. In electrical terms, a lower mass rotor lowers the source impedance of the RPC, and a lower impedance source can provide a stiffer output, ie less sag under changing load. Variation in rotor speed (kinetic energy) over 1/60th of a cycle would therefore already be difficult to observe. The effect of adding more mass would then be even more difficult to observe. One would need a high resolution (fraction of RPM) speed sensor with a very high sample rate to observe the ebb and flow of kinetic energy over each cycle. Or simply watch the voltage and current on a dual channel scope, then, if you have a good scope, integrate the two to get instantaneous real and reactive power. Some clarification of the term "rotary transformer" might be helpful. Simply stating that it is a rotary transformer doesn't clear anything up for me! What exactly is a rotary transformer? If the rotating mass is irrelevant, then how is rotation relevant? Well, you have 3 sets of stator coils, and one rotating squirrel cage made up of a number of bars. The latter are all energized in parallel, so the electrical phase is the same on all of them at any instant. But their positions aren't the same, and the whole thing is turning. This creates a mechanically rotating B field which sweeps past the fixed position stator coils inducing a back EMF in them (all of them, including the ones hooked to the wild leg, ie the "transformer" secondary). *Some* of the stator coils (primary) are also energized by line current from utility power. This is electrically time varying, producing its own electrically rotating B vector. This vector is what induces currents in the squirrel cage in the first place to allow it to produce its own rotating B field. The rotor is just an intermediate between primary and secondary which has the interesting time varying property of mechanically displaced (phase shifted) synchronization with the primary field. If the wild leg is generating power during periods when little or no power is available from the mains each cycle, where does that power come from if not from kinetic energy stored in the idler rotor? If that *is* where it comes from, then I contend that idler motor rotor mass is indeed relevant, though I could sure see how adding more mass may not make any noticable difference. It is important to realize that a RPC (or more conventional transformer) has mostly *reactive* currents circulating in it. These are inductively reactive, so the current lags voltage. (If the coils were lossless, the lag would be exactly 90 degrees.) These reactances consume no power, but they do store considerable energy in their magnetic fields. It is this energy which is transferred from input to output of any transformer, whether rotary or not, as the fields rise and collapse. Negligible mechanical energy is exchanged. A conventional transformer may have relatively very little reactive current when operating at full load. In fact, this is a necessity in high-current high-freq switchmode power xfmrs and great pains are taken to achieve very tight coupling so as to minimize stored energy. A fully loaded induction motor has power factor considerably below 0.5, which says that power converted from electrical to mechanical is more than power being stored in the magnetic fields. I'm not saying you're wrong about RPC's, mind you. I don't know. It would be interesting to measure the inductances of a typical 3phase motor to see if the stored energy is consistent with your assertion. Your assertion *is* consistent with (but not proven by) the observation that idlers should be at least 1.5 times the rating of the load, since induction motors run at very low power factors at light load. What's different about a rotary transformer is that the coils are in different spatial relationships at different points in time. This means that the geometry of the stators and rotor are such that inducing and induced currents are of differing phase (time). So even though the primary is going through a voltage zero, the secondary is seeing a rising voltage induced by the collapsing rotor field, and vice versa. In other words, the B field induced into the rotor doesn't instantly die when the primary goes through a zero. That field is still collapsing since it is inductively lagging, as it mechanically approaches a secondary coil. The collapsing field induces a current in the secondary which lags the current in the primary by the *mechanical* phase (time) difference between stator locations. A non-rotating transformer can't do this phase shift, but a rotary transformer can, and does. That's how it is able to produce 3 ph from 1 ph. But all the energy being transferred is *electromagnetic*, the mechanical rotation is just there to provide the phase shift. The ratio of real power to reactive (imaginary) power in the system varies with load. But the system stored energy is constant (at least until you overload it enough to drive it into saturation). In the steady state, ie after sync speed is achieved in the idler, energy out equals energy in less system losses. Same as for any transformer. All true ... and all based on your initial thesis that all (or nearly all) energy storage is in magnetic fields. None of the above argument addresses that question but builds on an assumed answer to it. Again, I'm not saying your wrong. Gary |
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On 2 Sep 2004 08:14:10 -0700, jim rozen
wrote: We have a nomenclature impass here. :^) We certainly have a nomenclature disparity! I'll agree with Gary that the wild legs are in series as regards current flow, and further argue that they are therefore also in series topologically. I sent Bob Swinney a 3D CAD model to support my wild-legs-in- series topology assertion without regard to current flow. As Clinton would have said, "define parallel....." G |
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On Thu, 02 Sep 2004 23:50:16 -0500, Don Foreman wrote:
On 2 Sep 2004 08:14:10 -0700, jim rozen wrote: We have a nomenclature impass here. :^) We certainly have a nomenclature disparity! I'll agree with Gary that the wild legs are in series as regards current flow, and further argue that they are therefore also in series topologically. I sent Bob Swinney a 3D CAD model to support my wild-legs-in- series topology assertion without regard to current flow. As Clinton would have said, "define parallel....." G Yeah. Draw a tank circuit like this: x--------------x | | | ) = ) | ) | | x--------------x and it looks like a parallel circuit. But draw it like this: x---------||-------))))---------x | | x-------------------------------x and it looks like a series circuit. But electrically they are the same circuit. Gary |
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In article , Don Foreman says...
No, he didn't, but he did put a *heavy* flywheel on it, and noted no improvement. In fact, the converter can't respond as quickly to changes in load with a heavy flywheel attached. (And we want it to, since the majority of the energy in, and all of it that is passing through, the system is electrical, not mechanical.) [gary] OK with the first sentence. Did he observe and report what you assert in the second sentence? You note that he noted no improvement. Did he note any degradation? I think the experiment of interest would be to somehow create a rotor with a much lighter than normal mass, and see how *that* behaves. It is pretty apparent that the ratio of energies is what's being discussed the the kinetic energy of the rotor vs some other energy stored in a magnetic sense. Possible the moment of the squirrel cage field in the applied field of the excited stator winding. That's won't be exactly right but it would give an upper bound on the stored magnetic energy. Jim -- ================================================== please reply to: JRR(zero) at pkmfgvm4 (dot) vnet (dot) ibm (dot) com ================================================== |
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I passed circuits 1 in college; a little help would be appreciated.
Given the following assumptions: 50 hours per year run time on home phase converter 220v 1/2 hp 1 phase motor owned w/ extra caps reasonable wiring skills $.00013/kwh If one buys a reasonably priced used 3 phase motor to make a phase converter for a bridgeport or lathe in the home shop How much more expensive would it be to run it by using a 220v 1 phase motor belt driving the 3 phase motor as a generator? This as compared to using the 3 phase motor as a rotary phase converter. |
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A tank circuit is usually a parallel circuit because the inductor and
cap are both in shunt from driving point to AC ground. A "series" resonant circuit is driven on one end and grounded (or connected to a load) on the other end. See modifications to your drawings below. The O is a voltage or current source. On Fri, 03 Sep 2004 03:44:13 -0400, Gary Coffman wrote: Yeah. Draw a tank circuit like this: ---- x--------------x | | | | | ) O = ) | | ) | | | |---- x--------------x and it looks like a parallel circuit. But draw it like this: x---------||-------))))---------x O x-------------------------------x and it looks like a series circuit. But electrically they are the same circuit. Gary |
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In article , Don Foreman says...
A tank circuit is usually a parallel circuit because the inductor and cap are both in shunt from driving point to AC ground. Right. Topologically they are in "parallel" when viewed from the outside world. As a combination their properties have specific, well understood behavior when connected to an outside circuit. But those properties exist because of the circulating currents of the series combination. The currents that circulate internally are what make the pair of practical interest. Indeed you don't need to connect them to any external circuit, simply bringing a grid dip meter (did I just date myself?) near will show what is going on internal to the pair. The terms "series" and "parallel" work great when demonstrating light bulbs and batteries in a grade school text, but the descriptions get kind of more complicated with things like rotary phase converters. I think the only way to do this rigorously is the way it's already been done, writing out the loop and mesh equations and solving them explicitly. Jim -- ================================================== please reply to: JRR(zero) at pkmfgvm4 (dot) vnet (dot) ibm (dot) com ================================================== |
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In article ,
jim rozen wrote: :In article , Robert Nichols says... : :Topologically in parallel, yes. Consider that when you connect a light :bulb across a battery, they too are topologically in parallel, but that :isn't a particularly useful way to look at the circuit. : :No, they're in series. : :The example I gave before was what is classically known as :a parallel tuned tank circuit. The internal current of :the two devices are flowing as though they are in series, :but the combination, as connected to the outside world, :are in parallel. : :Your light bulb example doesn't work, because there is :noplace for current to flow besides in the single loop :battery - load. OK, I over simplified. Here's a circuit: ---------- +--------| Device 1 |--------+ | ---------- | | | | | | ---------- | +--------| Device 2 |--------+ | ---------- | | | | | | ---------- | +--------| Device 3 |--------+ ---------- Without knowing anything about the devices, tell me what's in series and what's in parallel. If I tell you that device 2 is a battery and the other two devices are lamps, does this change your answer? How about if I now substitute a battery charger for device 1? My point (and I'm in agreement with Jim's tank circuit example here) is that topology alone can't always tell you whether you have a series connection or a parallel connection. Until you pick a point of view it may be impossible to make the distinction. Even for the fairly trivial case where device 2 is a battery and the other devices are lamps, there can be times when you need to treat the battery and one of the lamps as parallel elements, e.g., to answer the question, "If the battery has a finite internal resistance, what is the equivalent source resistance seen by the lamp in position 3?" -- Bob Nichols AT comcast.net I am "rnichols42" |
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