Explanation of the Perepiteia rotating machine and the accompanying theory concerning "Back EMF" By Natan Weissman Abstract The "Perepiteia" generator is demonstrated in a test setup driven by a motor taken from a Ryobi bench grinder. The acceleration behaviour of the system as a whole can be explained entirely by the behaviour of the grinder's induction motor. Meanwhile the hypothesis that back-emf is being managed in a novel way is disproved. Introduction In the demonstration videos for the Perepiteia, the audience's attention is directed to the Perepiteia generator, which causes even knowledgeable people to not pay attention to the grinder motor's contribution to the observed phenomena. This focus is a problem. A bench grinder is a load that requires very little starting torque, which leads to Ryobi's selection of a single-phase induction motor with characteristics appropriate to that application. In order to explain the behaviour of the joined induction motor - Perepiteia machine "system", it is essential to pay attention to the grinder motor. The first requirement is an understanding of the behaviour of induction motors in general. What follows is material excerpted verbatim from a white paper on induction motor behaviour on the website of a very reputable established motor manufacturer, Reliance. The paper is not biased. Reliance should have absolutely no vested interest for or against the Perepiteia, indeed, Reliance is probably unaware of the Perepiteia (whereas Ryobi might be) and the white paper probably predates the Perepiteia, since induction motor theory has been well known for decades, albeit by a very narrow circle of specialists. The average reader may find the jargon and mathematics of the paper somewhat daunting, since the webpage assumes a background in general motor theory, and circuit analysis using complex numbers. However, this page does nicely distill the math to a minimum, and presents correct conclusions about generic single phase induction motor behaviour. Below the material has been edited for brevity, but not in a way that distorts its message. The reader is of course strongly encouraged to visit the original page at Reliance's website. [begin excerpt from Reliance white paper] http://www.reliance.com/prodserv/motgen/b7097_2.htm Induction Motors
AC Induction Motor Equivalent Circuits Figure 2 The equivalent circuit for an AC induction motor can help visualize some of the motor characteristics. Figure 2a shows separate circuits for the stator and rotor, with the interaction between them modeled as a "transformer." This transformer has the unique characteristic of also changing the frequency of the signal! While the current in the stator is at the applied frequency of the motor power source, the rotor current flows at a frequency based on the slip of the motor. Rather than work with such a two part equivalent circuit having currents at different frequencies, the circuits of Figure 2a are typically modified to come up with a single circuit as shown in Figure 2b. Speed / Torque Curves As an AC induction motor is started, the values of resistance and reactance offered by the motor (or seen by the power source) will vary. At the instant of applying power to a stopped motor, the magnetic field is rotating much faster than the (stationary) rotor. This implies 100% slip, so r2/s is minimized. As a result, the current drawn at starting (locked rotor) conditions is quite high. Also it is common to design rotor slots which have dramatically different impedance at high slip (say 60 Hz for starting) versus at typically less than I2 Hz slip (normal running). This changes the values of both x2 and r2 from starting to running conditions. As a motor accelerates to speed from a standstill, the changing impedances result in a unique characteristic developed torque and current drawn during the time of acceleration. Depending on the design of the motor, a torque / current characteristic such as one of those shown in Figure 3 would typically result. The NEMA Design B motor is considered the most "general purpose" of these characteristic shapes, with Design C and D typically used for more "difficult to start" loads. Table 2 gives some ranges of characteristics for integral HP, 1200 and 1800 RPM motors.
Typical AC Induction Motor Speed / Torque / Current Curves Figure 3 As can be seen from all of these speed/torque curves, the current drawn by an AC motor in accelerating a load up to speed can be dramatically higher than the nominal running current. At the same time, the developed torque (during acceleration) may in some cases be less than the rated full load torque. Various methods exist to control the starting current drawn by an AC motor but the torque per amp seen during starting is always much lower than at running conditions. The nature of an AC induction motors acceleration to running speed is such that it can impose high stresses on the stator end turns and the rotor. The high current draw also stresses the upstream power system, including cabling, transformers, switchgear, etc. For this reason, there is often significant effort made to "control" AC motor starting and acceleration - both in terms of motor design as well as application. Efficiency and Losses Returning to the AC motor equivalent circuit of Figure 2b, we can identify three of the five basic component losses which exist in AC induction motors. The losses dissipated in the resistance of the stator and rotor windings, plus the core loss (eddy current and hysteresis losses in lamination steel) are modeled in the equivalent circuit. A fourth component loss is the friction and windage of the rotor, fan, bearings, etc. Finally, there is the "leftover category of stray load losses. These are losses which are a compilation of various less easily modeled losses, but are often a significant loss in highly efficient machines. The stray load losses include eddy current losses in the conductors, core losses due to flux distortion with load, etc. AC Induction Motor Efficiency vs. Load Figure 4
Since the friction and windage and core losses are essentially independent of load, while the other losses vary as the square of load (current), the efficiency of an AC induction motor falls off precipitously at light loads (see Figure 4). Adjustable Frequency, Variable Speed Operation For steady-state (as opposed to starting) operation, AC induction motors offer a reasonably linear torque per amp and high power factor characteristic. This is seen in Figure 5 as the part of the speed torque curve between "breakdown RPM" and "synchronous (no load) RPM." It is this portion of the AC induction motor range of operation within which adjustable frequency drives function. AC Induction Motor Speed Torque Curve Figure 5 [end excerpt from Reliance white paper] Explaining the Perepiteia demonstrations Behaviour of the Induction Motor Technical support staff at the Ryobi company in Anderson, South Carolina indicate that Ryobi is not a member of NEMA, and their motor assembly for small grinders is manufactured privately for them, so that it does not carry a formal NEMA classification. However, simple observation of the machine in action distinguishes it as having a Class A or B motor and definitely not class D. To demonstrate the Perepiteia machine in the University of Ottawa laboratory, a reduced AC voltage is applied using a variable autotransformer (variac). The voltage selected is barely enough to get the single-phase induction motor turning. Otherwise it is normal North American 60 Hz AC. Pause to study the speed-torque curve of Class A & B motors in Figure 3part2. Here there is no external mechanical load applied to that induction motor other than the Perepiteia generator. These initial conditions place the induction motor at or close to the torque minimum near 20% of synchronous speed (Figures 3part2 or 5). A typical demonstration has the Perepiteia generator's coils either open or shorted. If they are open, then the Perepiteia's contribution to the load is only due to its mechanical losses, which are small. If the Perepiteia's coils are shorted, there is a speed-dependent drag as well, but it isn't as great as one might think, because the gap between the Perepiteia's rotating magnets and its stator coils is fairly large (which contributes to the Perepiteia being a relatively inefficient, but otherwise conventional permanent magnet alternator). Either way, the load on the induction motor is not great while the machine is in operation at this low speed. This is also a very low efficiency regime for the induction motor. Consider Figure 4. Note that the x-axis here is load, not speed, so the operating point is near 0% load even if the speed is near 20%. The efficiency is also near 0%.
It bears emphasizing that this initial extremely inefficient operating point for the induction motor is nowhere close to a normal operating condition. It is also unstable. If the motor speed increases an infinitessimal bit, then it increases in torque (Figures 3part2 or 5), and decreases in its current consumption (Figure 3part1), which in turn implies an improving efficiency (Figure 4). The improving efficiency permits further acceleration while drawing less power, until the induction motor approaches the breakdown RPM. At or near this point, the acceleration ceases, and the induction motor establishes a steady-state mode of operation, often beyond the breakdown RPM but necessarily below the synchronous RPM. Note that just because the induction motor does accelerate over some range of operation - while drawing less power as it speeds up - one cannot extrapolate and conclude that it will keep speeding up forever (it doesn't), nor that its efficiency will keep increasing forever (that doesn't happen either). It can at most approach synchronous speed, and reach an efficiency of perhaps 90%. Behaviour of the Perepiteia Generator What about the Perepiteia generator's contribution to the above picture? True, the load represented by the Perepiteia generator increases as the motor speed increases, however, so long as that load doesn't fully consume the available torque from the motor, the motor can continue accelerating towards a stable operating point at a faster speed. Thus the acceleration is governed by the induction motor, not by the Perepiteia generator. Changes in Coupling between Motor and Generator What about the demonstrated differences between the ferromagnetic (iron or steel) shaft coupling vs. the non-ferromagnetic coupling (brass, plastic, etc)? Or what about the application of an external permanent magnet to the induction motor in the demo video that doesn't involve the Perepiteia at all? These phenomena are also explained by the nature of single-phase induction motors. One must remember that an induction motor is a rotating transformer. Its core is built of magnetically "soft" material, "soft" in the sense that it allows the direction of magnetization to change very easily and with minimal hysteresis losses. Nonetheless, a magnetic hysteresis curve is traced out with every reversal of the magnetization direction. Changing the externally applied magnetic field "biases" the magnetization in the induction motor's core, placing it in a slightly different efficiency regime. That shift can result in greater or lesser efficiency. The videos demonstrate some cases where the efficiency happens to improve at least marginally thanks to the magnetic bias. When the opposite happens, the result isn't very interesting: the motor simply stops. Thus, the acceleration behaviour of the Perepiteia in the demonstration videos can be explained entirely by the behaviour of the induction motor. No unconventional manipulation of "back EMF" is required to explain this. None. Visitors have gone to the laboratory at the University of Ottawa, and have seen the setup operating. Their observations are consistent with the present hypothesis. The inventor Mr. Heins and his colleagues have been asked whether they have attempted the same experiment with a DC motor. Visitors were told that this has indeed been tried, and that the Perepiteia didn't work when driven by a DC motor. This reinforces the hypothesis that the induction motor was solely responsible for the acceleration phenomenon, since if unconventional "back EMF" manipulation in the generator were responsible, it would have worked with a DC motor as well. Apparently the Perepiteia has not been tested using a dynamometer instead of a single phase induction motor, a test that would further confirm the present result. The Back-EMF Theory A separate issue is Mr. Heins' "back EMF" hypothesis, which claims that the direction of the magnetic flux in a transformer or rotating machine can be manipulated to reinforce rather than oppose the applied current and flux. The conventional result is predicted by Lenz's law and
Maxwell's theory. One may argue that just because the back-emf hypothesis isn't applicable to explaining the Perepiteia experimental results, that this doesn't mean that the back-emf hypothesis isn't correct in its own right. Fortunately the back-emf hypothesis can be subjected to a very simple test. Anybody who has tried to turn a conventional generator by hand while it is under load will have noticed that the torque required to turn it isn't uniform. The torque you have to apply increases and decreases periodically. This variation in the drag load is called "cogging torque". Try it sometime with a bicycle generator for example. If you can observe what happens inside the generator, you will notice that the number of "bumps" in the torque per rotation is equal to the number of poles. On the Perepiteia machine, the rotor poles are conveniently visible. They are the disk magnets on the rotor, and their path takes them past coils on the stator that are also perfectly visible. Concerning the cogging torque, the "back EMF" hypothesis makes a radically different prediction than James Clerk Maxwell's theory of electromagnetics. The Back EMF hypothesis claims that when a rotor magnet on the Perepiteia approaches a stator coil, it is drawn towards that coil, and when the magnet recedes from a stator coil, it pushes away from that coil. This is the exact opposite of the behaviour predicted by Maxwell's theory (more specifically, by Lenz' law) for all generators, which predicts the cogging drag described in the previous paragraph: When the magnet approaches a coil, it opposes the motion, and when it receeds from the coil, it also opposes the motion, but that opposition is nonuniform and is strongest when the magnet is closest to the coil. So, who is right in the case of the Perepiteia? Resolving this question is very easy to test by hand-cranking the Perepiteia, with it either decoupled from the induction motor, or with the motor simply not powered. Visitors to the lab at the University of Ottawa have performed this simple experiment. The answer is simple and unequivocal: the cogging behaviour of the Perepiteia is normal, exactly as predicted by Maxwell. Another telltale sign of whether "back EMF" was being manipulated would be to observe the phase of the flux in the cores beneath the Perepiteia's coils. This is not at all easy to do except perhaps with a Kerr effect microscope, however, observing the magnitude and phase of the current in the stator coils (which results from the flux) is very easy by means of an oscilloscope. This test has not been applied to the Perepiteia machine, but has been tried independently with a Perepiteia transformer, which is claimed to use the same anomalous back-emf mechanism to obtain perpetuum mobile levels of efficiency in excess of 100% (sometimes as high as several thousand %). A transformer was tested carefully by an engineer with a Ph.D. under various operating conditions recommended by the inventor himself, conditions under which the inventor reported efficiencies between one hundred and several thousand percent, and it was found that: (1) the magnitude and phase of the current in its coils was consistent with Lenz's Law and Maxwell's theory in general. (2) the efficiency of the transformer was poor, between 12% and 75% (vs. over 90% for typical conventional transformers). The poor efficiency was due in part to poor design, and in part to the fact that the specified operating conditions were beyond the unit's flux handling capacity, so the core was in saturation. (3) the saturation flux handling was poor for a transformer of its size, such that it wasn't able to carry anywhere near the currents that it should be able to handle. Conclusions The acceleration of the Perepiteia generator in all the configurations shown in the demonstration videos can be explained by the behaviour of the induction motor driving the Perepiteia. The acceleration phenomena cannot be observed when a DC/universal motor is used. There is no evidence that anomalous manipulation of Back-EMF takes place within the Perepiteia generator or the Perepiteia transformer. The behaviour of both Perepiteia devices is consistent with Maxwell's theory.