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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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#81
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Flywheel on a rotary phase convertor
On Sat, 07 Jan 2006 06:46:58 GMT, Ignoramus20351
wrote: On Sat, 07 Jan 2006 00:21:44 -0600, Don Foreman wrote: I must say I respect Iggy's rather consistent practice of courteous and civil discourse. Thank you Don. If I barge into some interesting discussion with insults, am I really going to learn more? i That's up to you. I've learned some good stuff from some insufferably arrogant assholes. Please don't read that as an implied shot at Bob. Bob and I have corresponded for years. He ain't a bad guy most days. Hm, I wonder if there might be some others who have regarded me as an insufferably arrogant asshole along the way. Oh well, **** 'em if they can't take a joke. :) It's not hard to ignore the skeeterbites if there are good fish to be caught. |
#82
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Flywheel on a rotary phase convertor
On Fri, 6 Jan 2006 22:05:02 -0600, "Robert Swinney"
wrote: This may be going out on a limb, and if so, I'm quite sure Don is ready with a saw but here goes: When I said: "Sorry, Don. The lead between the 3rd leg terminals does not place them in parallel with each other because it connects from one end of one winding to the *other* end of the other winding. " This concept was immediatly poo-pooed by the "if its connected as a RPC, then it is in parallel". Later, then a definition of parallel was given as: "By this definition, if there are wires connecting each terminal of one device to a corresponding terminal of another device, they are in parallel -- regardless of what else might be connected to those terminals." Consider 2 wye motors connected as a RPC, seemingly in parallel. It appears to me that 2 wye motors connected in parallel by the definition of corresponding terminals, above, are not in parallel by that definition unless the "star" or neutral points within each motor are connected by a solid lead. Then, all corresponding points are connected. RPC connected motors do not meet the criteria. Kapeesh? Poo-pooed? That connotes fluffy dismissal. I flat disagreed. I do capish. Kapeesh indeed! Texas is obviously a long ways from Brooklyn, Ol' Son. Roger thet, big ol' ten-four. Yer waltz ain't quite in synch with mah foxtrot, but we ain't that fur apawrt. See recent post. They're still topologically connected in parallel per conventional definition, but I can see how one might regard them as functionally in series, particuarly if neutral floats as it must in an RPC. In any case, my chainsaws are put away for the MN winter and I'm way too lazy to pull a Swedesaw anymore. Bob (getting tired of all this)Swinney What, time for your nap? (ducking.....) |
#83
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Flywheel on a rotary phase convertor
That is really doubtful, Iggy. You seem to barge into discussions just to
see your name in print. Maybe, I am wrong. Bob Swinney "Ignoramus20351" wrote in message ... On Sat, 07 Jan 2006 00:21:44 -0600, Don Foreman wrote: I must say I respect Iggy's rather consistent practice of courteous and civil discourse. Thank you Don. If I barge into some interesting discussion with insults, am I really going to learn more? i |
#84
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Flywheel on a rotary phase convertor
Don, now you are beginning to get the idea, my poor teaching technique not
withstanding. Take another look at your line below where you say, "but I can see how one might regard them as functionally in series, particuarly if neutral floats as it must in an RPC." Yes, neutral *must* float in a RPC but it is still my contention that the idler and load of a RPC are not truly connected in parallel unless there is a solid connection between the 2 respective neutral points. That satisfies the definiton of corresponding points being connected, doesn't it?? Can we call your special definition of parallel as applied to RPC's, maybe, huh, "quasi parallel"? Bear with me for one more moment, please (courteously). We have a 3-phase source and wish to connect n numbers of 3-phase loads across it, in parallel. You'd have to agree there would be a "phase" connected to each of the 3 input terminals (nodes ?) of the loads. In other words, the connections looking into the loads would be in parallel, and connected across the 3-phase source, would they not? OK, if you're with me (I'm a poor teacher, I know), now disconnect the 3-phase source and look at the loads, say, call one of them an idler and the rest of them loads. Now the idler and load are not truly in parallel, by definition, because the lead between star points (neutrals) is not there. Now consider, a RPC (rotary phase converter) connected as in what I call "quasi parallel" for want of a better description. You might agree, the idler and load are not in true parallel because the solid lead between neutrals is missing. But they are connected as a RPC must be. Current from L1 to L2 "sees" the RPC as a series load. Current flow via the 3rd leg and the 2 line legs into the load, as connected in RPC fashion, results in circulating currents throughout that appear to the load as 3 phases. (emulation?) As you've said elsewhere, a RPC would not be possible if the neutrals were connected as in what I deem to be a true parallel connection. Therefore, it seems that in a RPC, the idler and load are not truly connected in parallel. Bob Swinney "Don Foreman" wrote in message ... On Fri, 6 Jan 2006 22:05:02 -0600, "Robert Swinney" wrote: This may be going out on a limb, and if so, I'm quite sure Don is ready with a saw but here goes: When I said: "Sorry, Don. The lead between the 3rd leg terminals does not place them in parallel with each other because it connects from one end of one winding to the *other* end of the other winding. " This concept was immediatly poo-pooed by the "if its connected as a RPC, then it is in parallel". Later, then a definition of parallel was given as: "By this definition, if there are wires connecting each terminal of one device to a corresponding terminal of another device, they are in parallel -- regardless of what else might be connected to those terminals." Consider 2 wye motors connected as a RPC, seemingly in parallel. It appears to me that 2 wye motors connected in parallel by the definition of corresponding terminals, above, are not in parallel by that definition unless the "star" or neutral points within each motor are connected by a solid lead. Then, all corresponding points are connected. RPC connected motors do not meet the criteria. Kapeesh? Poo-pooed? That connotes fluffy dismissal. I flat disagreed. I do capish. Kapeesh indeed! Texas is obviously a long ways from Brooklyn, Ol' Son. Roger thet, big ol' ten-four. Yer waltz ain't quite in synch with mah foxtrot, but we ain't that fur apawrt. See recent post. They're still topologically connected in parallel per conventional definition, In any case, my chainsaws are put away for the MN winter and I'm way too lazy to pull a Swedesaw anymore. Bob (getting tired of all this)Swinney What, time for your nap? (ducking.....) |
#85
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Flywheel on a rotary phase convertor
Don, in as much as you've railed against some of the definitions I used in
trying to explain my understanding of the RPC, I offer the following. Generators and consumers: Motor windings, (elements of a RPC for example) consume energy and give it up as torque. Those same windings also generate energy in the form of counter EMF. In essense current is flowing in 2 directions through the winding. In electronic parlance, you might say there is both a voltage rise and a voltage fall. I have sense and education enough to know average current flow in an AC circuit is zero, but you have tap danced all around the point, by even mentioning it. (Patronizingly, perhaps) Aggregate current: That which flows in a network as a result of everything going on in the network. Convoluted current: In a manner of speaking, that current which flows in the load side of a RPC. BTW much of the definition you question was duly addressed in the little paper I sent you awhile back. At the time your comments were to the effect the paper was well researched, well written, etc. Bob Swinney |
#86
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Flywheel on a rotary phase convertor
On Wed, 04 Jan 2006 11:04:20 -0600, Don Foreman
wrote: On Tue, 3 Jan 2006 23:14:07 -0600, "Don Young" wrote: Since the running idler and load motors are directly connected in parallel, wouldn't plug reversing with identical motors and no mechanical load have an equal chance of reversing either motor? When running free, it seems to me that either motor could be considered to be the source or load for the third phase leg. I tend to believe that the idler requires more mechanical inertia than the load to maintain the best functioning. Interesting! The relative impedances are also important here. The larger motor with lower impedance (and probably higher inertia) will govern. Look at the terminal voltage where the two third legs are connected. If the motors were perfectly matched, their effects would cancel and this terminal voltage would be zero. If they are not matched, the voltage (phase) of that terminal will be determined by the motor with the lower impedance, and the phase of this voltage determines (or indicates) the direction in which both motors turn. If an induction motor does not "generate", is induced counter EMF imaginary and the use of common induction motors as generators impossible? There are many ways to understand and describe how things work and I like to think of the RPC as simply a running induction motor with the magnetized rotor inducing EMF not only into the line energized windings (counter EMF) but also into the unenergized and phase displaced windings. Right, up to here. Note that, when disconnected and still turning, an induction motor still has voltage across its windings and loading this voltage with "braking" resistors will mechanically load the rotor. Only if the rotor has some significant permanent magnetism -- not usually the case. A squirrel cage motor DOES generate a back EMF and will continue to do so for several revolutions after all power has been removed even if it has zero permanent magnetism. In the case of a 2 pole motor, at the instant of disconection, the induced circulating currents in the short circuited rotor inductance provide a diametrical NS field which decays at at the rotor L/R rate for several tenths of a second. The voltage generated by this decaying field is is easily observed with an oscilloscope - the initial voltage is close to full supply voltage. Jim |
#87
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Flywheel on a rotary phase convertor
On Sat, 7 Jan 2006 17:21:30 -0600, "Robert Swinney"
wrote: Don, now you are beginning to get the idea, my poor teaching technique not withstanding. Take another look at your line below where you say, "but I can see how one might regard them as functionally in series, particuarly if neutral floats as it must in an RPC." Yes, neutral *must* float in a RPC but it is still my contention that the idler and load of a RPC are not truly connected in parallel unless there is a solid connection between the 2 respective neutral points. That satisfies the definiton of corresponding points being connected, doesn't it?? Can we call your special definition of parallel as applied to RPC's, maybe, huh, "quasi parallel"? Bear with me for one more moment, please (courteously). We have a 3-phase source and wish to connect n numbers of 3-phase loads across it, in parallel. You'd have to agree there would be a "phase" connected to each of the 3 input terminals (nodes ?) of the loads. In other words, the connections looking into the loads would be in parallel, and connected across the 3-phase source, would they not? OK, if you're with me (I'm a poor teacher, I know), now disconnect the 3-phase source and look at the loads, say, call one of them an idler and the rest of them loads. Now the idler and load are not truly in parallel, by definition, because the lead between star points (neutrals) is not there. It has finally occurred to me that the neutral is what's causing the confusion. Nearly all three phase machines are three-terminal devices. There may be a physical neutral in a Y-connected machine, but it's rarely used. We agree that the neutrals would not be connected with Y-connected idlers and loads. If they were delta-connected, there'd be no neutrals to connect. Consider a delta-connected idler and load(s). There are wires connecting each terminal of the idler to corresponding terminals on each load. The machines are connected in parallel. Each winding of the idler is in parallel with a corresponding winding in the load(s). There are only three nodes in this circuit. Now connect mains to two of the three terminals. We haven't broken any connections, so the idler and load(s) are still connected in parallel, each winding in the idler is still in parallel with a corresponding winding in the load(s). There are still only three nodes in the circuit, with power fed to two of them. The power line is connected across one winding and one phase. We might not know what the potentials across the other two phases might be, but it's clear that the voltages across corresponding phases of the two machines are the same. They're in parallel. Now consider Y-connected idler and load(s). The winding not tied to mains on the idler is in series with the corresponding winding on the load. There are still 3 nodes, the two that mains are connected to and the one between the two third windings. Are these machines still in parallel? I assert that they are. The confusion comes from looking at those windings that are connected in series and referring to neutral. In a Y-connected machine, a winding is not a phase. A phase is from terminal to terminal whether the windings within are delta or Y connected. Each phase in a Y connected machine has two series-connected windings from one terminal to the other. Each winding is a member of two adjacent phases, and each phase has two windings in series. If you draw a circle around each *phase* (not winding) of a Y-connected machine, from terminal to terminal, you see that the idler phases are indeed connected in parallel with the load phases, whether or not there are any power lines connected. Let's leave the power off for a moment. You see not just one, but three loops of four windings in series -- two idler windings and two load windings. But each phase in the idler is still in parallel with it's corresponding phase in the load. Now connect real threephase power to the terminals. I think you've agreed that in this situation the idler and load are still in parallel. Phase currents are currents into a terminal, and are the same as line currents when the system is driven with threephase power. Each winding has two phase currents flowing thru it, so the net current in any winding is the vector sum of these two currents. Now remove the threephase feed and connect a single phase power line to two terminals, or one phase, and try to figure out what's going on in the other two phases with the terminal between those two phases (on both idler and load) connected to nothing else. We see two windings in series between neutral of idler and neutral of load. But the same situation is true with the other windings! You can go from neutral to neutral via three routes, each thru one winding in the idler and a corresponding winding in the load. If you drew circles around the *phases* (pairs of windings) before, you'll see that each phase in the idler is still connected in parallel with a corresponding phase in the load. It's tempting to think of a phase as line to neutral thru just one winding, because that looks easier to understand. But it's incorrect unless the neutrals are actually connected because the voltages from the third node to the other two nodes, the other two phase voltages, do not depend only on the windings connected to the third node. Similarly, the current thru the wire connecting the third nodes does not depend solely on the voltage from third nodes to their respective neutrals unless those neutrals are tied together or otherwise held at some known potential from one neutral to the other. |
#88
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Flywheel on a rotary phase convertor
On Sat, 7 Jan 2006 18:11:10 -0600, "Robert Swinney"
wrote: Don, in as much as you've railed against some of the definitions I used in trying to explain my understanding of the RPC, I offer the following. Generators and consumers: Motor windings, (elements of a RPC for example) consume energy and give it up as torque. Those same windings also generate energy in the form of counter EMF. In essense current is flowing in 2 directions through the winding. So my ammeter needs two needles? In electronic parlance, you might say there is both a voltage rise and a voltage fall. Whoa! My voltmeter needs two needles too? Just kidding, Bob. I know what you mean: sometimes its easier to think of voltages and currents as vector sums of component voltages and currents, I have sense and education enough to know average current flow in an AC circuit is zero, but you have tap danced all around the point, by even mentioning it. (Patronizingly, perhaps) I know you do. In the following sentence I suggested that perhaps you meant power flow which made sense in context. Aggregate current: That which flows in a network as a result of everything going on in the network. Well, in that context the aggregate current in a balanced threephase system is zero though the currents in various branches are certainly non-zero. Convoluted current: In a manner of speaking, that current which flows in the load side of a RPC. BTW much of the definition you question was duly addressed in the little paper I sent you awhile back. At the time your comments were to the effect the paper was well researched, well written, etc. I didn't recall the definitions, but I do recall the comment and I wouldn't have said it if I didn't mean it. That must have been two computers ago because I can't find your writeup in this machine or the last one. |
#89
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Flywheel on a rotary phase convertor
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#90
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Flywheel on a rotary phase convertor
On Sat, 07 Jan 2006 22:39:02 -0600, Don Foreman
wrote: Each winding has two phase currents flowing thru it, so the net current in any winding is the vector sum of these two currents. True for delta connection, not for Y. In a Y each phase voltage is the vector sum of two winding voltages. |
#91
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Flywheel on a rotary phase convertor
Unfortunately when I learned about electric motors, I was taught that
an electric motor GENERATES a back emf because there is a magnetic field which is cutting conductors. So my view of say a motor is somewhat different from yours. If you have an electric motor and you increase the load, the speed drops slightly, the back emf drops, and the current drawn goes up. If you decrease the load, the speed increases, the back emf goes up, and the current drawn goes down. If you decrease the load until it is negative, ( ie mechanical power is being applied to the motor ) , the back emf goes up until it is more than the applied emf, and the current drawn goes negative. That is current goes into the mains. That is an induction generator. Works whether the motor is a single phase motor or a three phase motor. If you have three phase motor and get it running on single phase power, things are a bit more complicated. But you still get a back emf generated, and on the terminal that is unconnected to the power there is a back emf, but there is no forward emf. So you can draw current from that terminal. Consider this. If you get a three phase motor running on single phase, you can use it to produce mechanical power. So the way I think of a RPC is as a three phase motor running on single phase, with some of the mechanical power being used to drive an induction generator. I am a bit confused by your statement about overdrive from the AC mains. Mostly by the word "overdrive". As I see it a RPC is connected to the AC mains, so I think it would be excited by overdrive from the AC mains. However an induction generator does not have to be connected to the AC mains in order to work. You can use a gasolene engine to drive an induction motor and generate electric power with no connection to the mains. It just is just sensitive to the amount of power you draw and does not regulate the voltage at all well. To further confuse you, you can build a very nice RPC by using a single phase motor to drive a three phase motor via a belt drive adjusted so mechanical power is going into the three phase motor. Now if you apply single phase power to the three phase motor, it will act as a three phase induction generator. If you do this use an adjustable pulley on one of the motors and measure the current drawn by the single phase motor. Adjust the pulleys so the current drawn by the single phase motor is close to but below rated nameplate current when the RPC is driving whatever load you are going to drive. As you might suspect such a RPC produces voltages that are very closely balanced. So I still analyse a RPC as an induction generator. Trying to analyse it as some sort of transformer, I have a lot of problems figuring out one ever gets anything that is not still in phase with the original single phase mains. And how one calculates what the phase angle is going to be. I amy not be able to convince you that this is a valid way to analyse RPC's, but it works for me. Dan Robert Swinney wrote: Dan sez: " In my opinion you need to realize that a RPC is an induction generator." Dan, I know you have some experience with induction generators so I'll ask you to respectfully consider that: An induction motor is a consumer, not a generator. As you know true induction generators (induction motors) have to be excited by overdrive from the AC mains in order to generate. Tht is not done in any fashion in a RPC. The RPC is a load on the mains, not a supplier to the mains. Again, I'll say, we need lose the idea of a RPC being a generator. Think of it as more of a converter; well, that's part of it's name now isn't it? Bob Swinney wrote in message oups.com... Robert Swinney wrote: IMO, you need to lose the thinking of a RPC as being a form of generator. Bob Swinney As far as flywheels are concerned, a flywheel will keep the slip angle from changing as quickly. So a RPC without a flywheel will draw power from the mains more quickly when the load is increased. Score points for that side. On the other hand, a RPC with a flywheel will draw power from the flywheel when the load is increased as well as from the mains. So score points for the other side. In the real world, it does not make much difference as the change in speed of the RPC should be slight, and therefore only a small amount of power can be drawn from the flywheel. Having a flywheel would help with an undersized RPC when the load motor is plugged. Dan |
#92
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Flywheel on a rotary phase convertor
On Wed, 04 Jan 2006 21:59:47 -0600, Don Foreman
wrote: On 4 Jan 2006 13:22:13 -0800, jim rozen wrote: When the converter is operating of course there is one special lead that breaks the symmetry - it's missing the line connection. An electrican would say that the absence of that line connection does not change the fact the two sets of windings are in parallel. A EE looks at the entire network as a system, including the incoming power. He says parallel means all nodes have the same number of connections. Jim He does? In circuit analysis, a set of two-terminal networks are regarded as "in parallel" if they are each connected to the same pair of nodes so the voltage across them is identically the same. Similarly, a set of n-terminal networks are in parallel if they are connected to the same set of n nodes so the various inter-terminal voltages on each n-terminal network are identically the same for corresponding pairs of terminals. This is regardless of whatever else might be connected to, between or among those nodes and regardless of any external symmetry or lack thereof. Some nodes may well have more connections than other. By this definition, if there are wires connecting each terminal of one device to a corresponding terminal of another device, they are in parallel -- regardless of what else might be connected to those terminals. Just to stir things up a bit further I register my support for BOTH the parallel and non parallel camps - it just depends on how you look at the circuit operation. If we regard the idler plus load motors as two connected passive three terminal networks the there can be no doubt that this is a straightforward parallell connection. No playing with words on the way any further conections are made (e.g single phase power input) can alter this. However,when the motors have rotating rotors, BOTH motors accept input power on two of their terminals and BOTH try to deliver output power to the third linked phantom phase connection. The direction of the power flow through the phantom phase link then depends on the relative mechanical loading on the two rotors. In the two limiting conditions of both motors idling or both motors loaded to the same fraction of their rated HP, each motor is self sufficient and little or no third phase power flows through the link. In the normal case of one idling and one loaded, the power flow is from idler to load motor. It is equally valid to regard this current as a circulating current within a parallel connected system or as the current developed in the series connection of an idler generator to its load motor Jim |
#93
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Flywheel on a rotary phase convertor
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#94
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Flywheel on a rotary phase convertor
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#95
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Flywheel on a rotary phase convertor
Robert Swinney wrote: Dan sez: " In my opinion you need to realize that a RPC is an induction generator." Dan, I know you have some experience with induction generators so I'll ask you to respectfully consider that: An induction motor is a consumer, not a generator. As you know true induction generators (induction motors) have to be excited by overdrive from the AC mains in order to generate. Tht is not done in any fashion in a RPC. The RPC is a load on the mains, not a supplier to the mains. Again, I'll say, we need lose the idea of a RPC being a generator. Think of it as more of a converter; well, that's part of it's name now isn't it? Bob Swinney I am sorry but thinking of RPC's the way I do just seems to make sense. When I learned about motors, I learned that they generate a back emf because they have a rotating magnetic field and conductors that cut the magnetic field. And if you increase the load on a motor, it slows slightly and the back emf drops and the current rises. And if you decrease the load the back emf increases and the current decreases. Now consider decreasing the load even more, so that the load is negative. ( putting mechanical power into the motor trying to make it run faster than synchronous speed ) The back emf increases and the current goes negative. That is current is being supplied by the motor to the mains. So as I see it a motor can work from locked rotor to being driven. ( Don't try locked rotor for very long unless you have a AC servo motor ) Same physics for all cases. Now this happens whether the motor is a single phase motor or a three phase motor. Now what happens when you have a three phase motor and run it on just one phase? After you get it started, it will run on single phase power. You still have a rotating magnetic field, and windings for three phases. So the rotating magnetic field generates a back emf in all the windings. So you consume power from the single phase, but generate three phase power. Incidently you do not have to have an induction motor connected to the mains in order for it to work as an induction generator. It just will not work with a large variety of loads. Another by the way. One of the better ways to make a RPC is to connect a single phase motor to a three phase motor using an adjustable belt drive. You monitor the current going into the single phase motor and adjust the drive so the current is at or below name plate current when driving the three phase load. Recommended ( by me ) for driving three phase machines as surface grinders that are sensitive to unbalanced three phase power. In that case it is pretty obvious that you have a three phase induction generator being driven by a single phase motor. You may find this way of analysing a RPC as weird, but it works for me. I have problems understanding RPC's as transfomers that produce a voltage that is not in phase with the input voltage. And it lets me think about how a flywheel would affect a RPC. Dan |
#96
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Flywheel on a rotary phase convertor
To offer a little food for thought on the original topic, here's a
picture of the rotor from the motor which I'm proposing to use as an idler: http://www.mythic-beasts.com/~cdt22/lse_rotor.jpg I dismantled the motor to give it a good clean, repaint the rusty bits and check the bearings. That's a 10" ratchet in the picture and the rotor weighs 30 lbs, so it's pretty sizeable. Gunner and Jim may well be right when they suggest that the rotor itself will be plenty adequate as a flywheel. By the way, the motor is rated at 2.5 hp at 940 rpm. Best wishes, Chris |
#97
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Flywheel on a rotary phase convertor
Robert Swinney wrote: An induction motor is a consumer, not a generator. As you know true induction generators (induction motors) have to be excited by overdrive from the AC mains in order to generate. Tht is not done in any fashion in a RPC. The RPC is a load on the mains, not a supplier to the mains. Again, I'll say, we need lose the idea of a RPC being a generator. Think of it as more of a converter; well, that's part of it's name now isn't it? Bob Swinney In school I was taught that a motor generates a back emf because there is a rotating magnetic field whose flux cuts the windings in the motor. When a motor is running and you increase the load, the motor slows slightly and the back emf drops so the net voltage increases causing the current to increase. If you decrease the load the speed increases, the back emf rises, net voltage drops and current drops. If you decrease the load until it is negative, the back emf increases until it is above the input emf, and the current goes negative. That is current flows from the motor to the mains. Now this works for both single phase and three phase motors from locked rotor to being an induction generator. ( Don't try locked rotor for very long unless you have an AC servo motor ) It even works for a three phase motor running on just one phase. In that case the motor consumes single phase power, but still generates back emf in all windings. Which results in generating three phase power. Because the back emf is less than the mains voltage, the voltage is not balanced. But this can be improved by either adding capacitors. So now you can at least see how I analyse RPC's. It isn't the only way, but it works for me and maybe Don Young and Pentagrid. ( Speak up if you disagree ). As an aside issue, an induction generator will work without being connected to the mains. You can google and find some references to this as regards using an induction motor driven by a small gas engine for operating ham field stations. It is load sensitve. And as another aside, you can build a fine RPC using a single phase motor connected to a three phase motor via a adjustable belt drive. Two terminals of the three phase motor are also connected to the mains. You adjust the belt drive so the current drawn by the single phase motor is at or below name plate rated current while the RPC is supplying three phase power to whatever needs three phase power. I happen to think this approach is good for things as surface grinders where good three phase power is needed. Dan |
#98
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Flywheel on a rotary phase convertor
According to Don Foreman :
[ ... ] It has finally occurred to me that the neutral is what's causing the confusion. Nearly all three phase machines are three-terminal devices. There may be a physical neutral in a Y-connected machine, but it's rarely used. We agree that the neutrals would not be connected with Y-connected idlers and loads. If they were delta-connected, there'd be no neutrals to connect. I've been wondering why nobody has considered the motors as black-box devices, with only the three terminals leading to the outside available for consideration. Under those conditions, there is really no way to tell a Wye from a Delta connected motor, if you allow that the windings could be special to make them work from the same voltages. And -- you could analyze the circuits from that basis (it is commonly done when sufficient information about the internals is not available, or when the details about the internals don't really *matter* to the functionality. Enjoy, DoN. -- Email: | Voice (all times): (703) 938-4564 (too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html --- Black Holes are where God is dividing by zero --- |
#99
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Flywheel on a rotary phase convertor
I hope this message actually gets posted. My last two attempts failed.
I am hoping the problem has something to do with cookies and posting through Google. When I learned about motors, I was taught that a motor generates a back emf because there is a rotating magnetic field cutting the conductors in the winding. When a motor is running the back emf is close the applied voltage and the amount of current drawn is proportional to the net emf / the inductance and resistance of the winding. Increase the load on the motor and the speed decreases, back emf goes down, net voltage goes up, and current goes up. Decrease the load and the speed increases, net voltage goes down and current goes down. Decrease the load some more until it is negative ( mechanical power going into the motor ) speed increases, net voltage goes negative ( back emf is larger that applied voltage ) and the current goes negative ( power goes into the mains ). This works from locked rotor to induction generator for single phase and three phase motors. Just don't try the locked rotor for very long unless you have an AC servo motor. Now when you have a three phase motor running on single phase power, it still works. A back emf is generated that keeps the net voltage across the terminal connected to the mains from being very large. But you also have a back emf generated in the windings that connect to the terminal that is not connected to the mains. Not quite as large as the emf from the mains, but nearly as large. So you have single phase power being consumed and three phase power being generated. This may not be the only way to analyse a RPC, but it works for me, and I think it works for Don Young and Pentagrid. Speak up if you disagree. A couple of aside issues. An induction generator will work without being connected to the mains. Google enough and you will find some web sites that talk about using an induction motor and a lawn mower type engine to power field ham radio stations. Such a generator is load sensitive. Also a rather nice RPC can be made using a single phase motor to drive a three phase motor using an adjustable belt drive. Both motors are connected to the mains and the belt drive adjusted so the current to the single phase motor is at or below the nameplate current when supplying three phase power to the load. I think this type of RPC will supply three phase power that is more balanced and therefore suitable to run things as surface grinders that are sensitive to harmonics in the power. Dan Robert Swinney wrote: An induction motor is a consumer, not a generator. As you know true induction generators (induction motors) have to be excited by overdrive from the AC mains in order to generate. Tht is not done in any fashion in a RPC. The RPC is a load on the mains, not a supplier to the mains. Again, I'll say, we need lose the idea of a RPC being a generator. Think of it as more of a converter; well, that's part of it's name now isn't it? Bob Swinney |
#100
Posted to rec.crafts.metalworking
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Flywheel on a rotary phase convertor
I hope this message actually gets posted. My last two attempts failed.
I am hoping the problem has something to do with cookies and posting through Google. When I learned about motors, I was taught that a motor generates a back emf because there is a rotating magnetic field cutting the conductors in the winding. When a motor is running the back emf is close the applied voltage and the amount of current drawn is proportional to the net emf / the inductance and resistance of the winding. Increase the load on the motor and the speed decreases, back emf goes down, net voltage goes up, and current goes up. Decrease the load and the speed increases, net voltage goes down and current goes down. Decrease the load some more until it is negative ( mechanical power going into the motor ) speed increases, net voltage goes negative ( back emf is larger that applied voltage ) and the current goes negative ( power goes into the mains ). This works from locked rotor to induction generator for single phase and three phase motors. Just don't try the locked rotor for very long unless you have an AC servo motor. Now when you have a three phase motor running on single phase power, it still works. A back emf is generated that keeps the net voltage across the terminal connected to the mains from being very large. But you also have a back emf generated in the windings that connect to the terminal that is not connected to the mains. Not quite as large as the emf from the mains, but nearly as large. So you have single phase power being consumed and three phase power being generated. This may not be the only way to analyse a RPC, but it works for me, and I think it works for Don Young and Pentagrid. Speak up if you disagree. A couple of aside issues. An induction generator will work without being connected to the mains. Google enough and you will find some web sites that talk about using an induction motor and a lawn mower type engine to power field ham radio stations. Such a generator is load sensitive. Also a rather nice RPC can be made using a single phase motor to drive a three phase motor using an adjustable belt drive. Both motors are connected to the mains and the belt drive adjusted so the current to the single phase motor is at or below the nameplate current when supplying three phase power to the load. I think this type of RPC will supply three phase power that is more balanced and therefore suitable to run things as surface grinders that are sensitive to harmonics in the power. Dan Robert Swinney wrote: An induction motor is a consumer, not a generator. As you know true induction generators (induction motors) have to be excited by overdrive from the AC mains in order to generate. Tht is not done in any fashion in a RPC. The RPC is a load on the mains, not a supplier to the mains. Again, I'll say, we need lose the idea of a RPC being a generator. Think of it as more of a converter; well, that's part of it's name now isn't it? Bob Swinney |
#101
Posted to rec.crafts.metalworking
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Flywheel on a rotary phase convertor
I hope this message actually gets posted. My last two attempts failed.
I am hoping the problem has something to do with cookies and posting through Google. When I learned about motors, I was taught that a motor generates a back emf because there is a rotating magnetic field cutting the conductors in the winding. When a motor is running the back emf is close the applied voltage and the amount of current drawn is proportional to the net emf / the inductance and resistance of the winding. Increase the load on the motor and the speed decreases, back emf goes down, net voltage goes up, and current goes up. Decrease the load and the speed increases, net voltage goes down and current goes down. Decrease the load some more until it is negative ( mechanical power going into the motor ) speed increases, net voltage goes negative ( back emf is larger that applied voltage ) and the current goes negative ( power goes into the mains ). This works from locked rotor to induction generator for single phase and three phase motors. Just don't try the locked rotor for very long unless you have an AC servo motor. Now when you have a three phase motor running on single phase power, it still works. A back emf is generated that keeps the net voltage across the terminal connected to the mains from being very large. But you also have a back emf generated in the windings that connect to the terminal that is not connected to the mains. Not quite as large as the emf from the mains, but nearly as large. So you have single phase power being consumed and three phase power being generated. This may not be the only way to analyse a RPC, but it works for me, and I think it works for Don Young and Pentagrid. Speak up if you disagree. A couple of aside issues. An induction generator will work without being connected to the mains. Google enough and you will find some web sites that talk about using an induction motor and a lawn mower type engine to power field ham radio stations. Such a generator is load sensitive. Also a rather nice RPC can be made using a single phase motor to drive a three phase motor using an adjustable belt drive. Both motors are connected to the mains and the belt drive adjusted so the current to the single phase motor is at or below the nameplate current when supplying three phase power to the load. I think this type of RPC will supply three phase power that is more balanced and therefore suitable to run things as surface grinders that are sensitive to harmonics in the power. Dan Robert Swinney wrote: An induction motor is a consumer, not a generator. As you know true induction generators (induction motors) have to be excited by overdrive from the AC mains in order to generate. Tht is not done in any fashion in a RPC. The RPC is a load on the mains, not a supplier to the mains. Again, I'll say, we need lose the idea of a RPC being a generator. Think of it as more of a converter; well, that's part of it's name now isn't it? Bob Swinney |
#102
Posted to rec.crafts.metalworking
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Flywheel on a rotary phase convertor
I hope this message actually gets posted. My last two attempts failed.
I am hoping the problem has something to do with cookies and posting through Google. When I learned about motors, I was taught that a motor generates a back emf because there is a rotating magnetic field cutting the conductors in the winding. When a motor is running the back emf is close the applied voltage and the amount of current drawn is proportional to the net emf / the inductance and resistance of the winding. Increase the load on the motor and the speed decreases, back emf goes down, net voltage goes up, and current goes up. Decrease the load and the speed increases, net voltage goes down and current goes down. Decrease the load some more until it is negative ( mechanical power going into the motor ) speed increases, net voltage goes negative ( back emf is larger that applied voltage ) and the current goes negative ( power goes into the mains ). This works from locked rotor to induction generator for single phase and three phase motors. Just don't try the locked rotor for very long unless you have an AC servo motor. Now when you have a three phase motor running on single phase power, it still works. A back emf is generated that keeps the net voltage across the terminal connected to the mains from being very large. But you also have a back emf generated in the windings that connect to the terminal that is not connected to the mains. Not quite as large as the emf from the mains, but nearly as large. So you have single phase power being consumed and three phase power being generated. This may not be the only way to analyse a RPC, but it works for me, and I think it works for Don Young and Pentagrid. Speak up if you disagree. A couple of aside issues. An induction generator will work without being connected to the mains. Google enough and you will find some web sites that talk about using an induction motor and a lawn mower type engine to power field ham radio stations. Such a generator is load sensitive. Also a rather nice RPC can be made using a single phase motor to drive a three phase motor using an adjustable belt drive. Both motors are connected to the mains and the belt drive adjusted so the current to the single phase motor is at or below the nameplate current when supplying three phase power to the load. I think this type of RPC will supply three phase power that is more balanced and therefore suitable to run things as surface grinders that are sensitive to harmonics in the power. Dan Robert Swinney wrote: An induction motor is a consumer, not a generator. As you know true induction generators (induction motors) have to be excited by overdrive from the AC mains in order to generate. Tht is not done in any fashion in a RPC. The RPC is a load on the mains, not a supplier to the mains. Again, I'll say, we need lose the idea of a RPC being a generator. Think of it as more of a converter; well, that's part of it's name now isn't it? Bob Swinney |
#103
Posted to rec.crafts.metalworking
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Flywheel on a rotary phase convertor
I hope this message actually gets posted. My last two attempts failed.
I am hoping the problem has something to do with cookies and posting through Google. When I learned about motors, I was taught that a motor generates a back emf because there is a rotating magnetic field cutting the conductors in the winding. When a motor is running the back emf is close the applied voltage and the amount of current drawn is proportional to the net emf / the inductance and resistance of the winding. Increase the load on the motor and the speed decreases, back emf goes down, net voltage goes up, and current goes up. Decrease the load and the speed increases, net voltage goes down and current goes down. Decrease the load some more until it is negative ( mechanical power going into the motor ) speed increases, net voltage goes negative ( back emf is larger that applied voltage ) and the current goes negative ( power goes into the mains ). This works from locked rotor to induction generator for single phase and three phase motors. Just don't try the locked rotor for very long unless you have an AC servo motor. Now when you have a three phase motor running on single phase power, it still works. A back emf is generated that keeps the net voltage across the terminal connected to the mains from being very large. But you also have a back emf generated in the windings that connect to the terminal that is not connected to the mains. Not quite as large as the emf from the mains, but nearly as large. So you have single phase power being consumed and three phase power being generated. This may not be the only way to analyse a RPC, but it works for me, and I think it works for Don Young and Pentagrid. Speak up if you disagree. A couple of aside issues. An induction generator will work without being connected to the mains. Google enough and you will find some web sites that talk about using an induction motor and a lawn mower type engine to power field ham radio stations. Such a generator is load sensitive. Also a rather nice RPC can be made using a single phase motor to drive a three phase motor using an adjustable belt drive. Both motors are connected to the mains and the belt drive adjusted so the current to the single phase motor is at or below the nameplate current when supplying three phase power to the load. I think this type of RPC will supply three phase power that is more balanced and therefore suitable to run things as surface grinders that are sensitive to harmonics in the power. Dan Robert Swinney wrote: An induction motor is a consumer, not a generator. As you know true induction generators (induction motors) have to be excited by overdrive from the AC mains in order to generate. Tht is not done in any fashion in a RPC. The RPC is a load on the mains, not a supplier to the mains. Again, I'll say, we need lose the idea of a RPC being a generator. Think of it as more of a converter; well, that's part of it's name now isn't it? Bob Swinney |
#104
Posted to rec.crafts.metalworking
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Flywheel on a rotary phase convertor
I hope this message actually gets posted. My last two attempts failed.
I am hoping the problem has something to do with cookies and posting through Google. When I learned about motors, I was taught that a motor generates a back emf because there is a rotating magnetic field cutting the conductors in the winding. When a motor is running the back emf is close the applied voltage and the amount of current drawn is proportional to the net emf / the inductance and resistance of the winding. Increase the load on the motor and the speed decreases, back emf goes down, net voltage goes up, and current goes up. Decrease the load and the speed increases, net voltage goes down and current goes down. Decrease the load some more until it is negative ( mechanical power going into the motor ) speed increases, net voltage goes negative ( back emf is larger that applied voltage ) and the current goes negative ( power goes into the mains ). This works from locked rotor to induction generator for single phase and three phase motors. Just don't try the locked rotor for very long unless you have an AC servo motor. Now when you have a three phase motor running on single phase power, it still works. A back emf is generated that keeps the net voltage across the terminal connected to the mains from being very large. But you also have a back emf generated in the windings that connect to the terminal that is not connected to the mains. Not quite as large as the emf from the mains, but nearly as large. So you have single phase power being consumed and three phase power being generated. This may not be the only way to analyse a RPC, but it works for me, and I think it works for Don Young and Pentagrid. Speak up if you disagree. A couple of aside issues. An induction generator will work without being connected to the mains. Google enough and you will find some web sites that talk about using an induction motor and a lawn mower type engine to power field ham radio stations. Such a generator is load sensitive. Also a rather nice RPC can be made using a single phase motor to drive a three phase motor using an adjustable belt drive. Both motors are connected to the mains and the belt drive adjusted so the current to the single phase motor is at or below the nameplate current when supplying three phase power to the load. I think this type of RPC will supply three phase power that is more balanced and therefore suitable to run things as surface grinders that are sensitive to harmonics in the power. Dan Robert Swinney wrote: An induction motor is a consumer, not a generator. As you know true induction generators (induction motors) have to be excited by overdrive from the AC mains in order to generate. Tht is not done in any fashion in a RPC. The RPC is a load on the mains, not a supplier to the mains. Again, I'll say, we need lose the idea of a RPC being a generator. Think of it as more of a converter; well, that's part of it's name now isn't it? Bob Swinney |
#105
Posted to rec.crafts.metalworking
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Flywheel on a rotary phase convertor
In article .com, DoN. Nichols
says... I've been wondering why nobody has considered the motors as black-box devices, with only the three terminals leading to the outside available for consideration. Basically everyone one does. The fun starts when you hook the three black box wires up to something else! Any efforts to explain rotary converters that invoke currents flowing from the motor's 'neutral' are doomed to failure I would say. Jim -- ================================================== please reply to: JRR(zero) at pkmfgvm4 (dot) vnet (dot) ibm (dot) com ================================================== |
#106
Posted to rec.crafts.metalworking
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Flywheel on a rotary phase convertor
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#107
Posted to rec.crafts.metalworking
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Flywheel on a rotary phase convertor
Wow, Dan! Nine posts. I am flattered (Really !) you went to such trouble
to share your opinion with us. I am very sorry you apparently were responding to my first reply to you, which was in error. Maybe your server didn't deliver my second reply, the one in which I attempted to correct the error in the first. I am sorry to have thrown such confusion into this already overlong thread. Anyway FWIW, my 2nd reply in an attempt to correct the errorious one went like this: {"Sorry, Dan - Make that overdrive via overspeed from the prime mover to make an induction generator. The induction generator (one made from a common induction motor) will generate when excited by the mains and when its rotor is driven by external means to a speed exceeding that of the motor's synchronous speed. Slip is said to be negative under these conditions."} Please note, the operative change made here was to insert "rotor driven by external means to a speed exceeding that of the motor's synchronous speed" in place of "overdrive from the AC mains". Again, I am very sorry for that mistake and for you to have gone to the trouble of posting 9 times in order to straighten me out on the matter. As someone once said, "I'm afraid I've already told you more than I know". Please understand, the following quote from McGraw-Hill EE Handbook, 10 Edition, Sec. 18-116 is the source of all my knowledge re. induction generators; it goes: """Induction Generators. Any induction motor, if driven above its synchronous speed when connected to an a-c source, will deliver power to the external circuit. This generator action is easily visualized from the motor-circle diagram. (Fig. 18-17)corresponding to the lower half of the circle in which the current vector is directed below the OV line. [pls. adivse and I can scan the figure to your direct e-mail addy, if you like] A unique feature is that the power factor of the output is fixed in value by the generator characteristics and is always leading, independent of the external circuit. The explanation is that the generator draws all its excitation from the system and so must receive a definite amount of lagging kilovoltamperes for a given voltage and load current. For this reason, induction generators alone cannot supply a power system but must always operate in parallel with synchronous machines or with capacitors. They are therefore, no more helpful in system stability than the addition of parallel reactors with a rating equal to the generator magnetizing reactance. An induction generator delivers an instantaneous 3-phase short-circuit current equal to the terminal voltage divided by its standstill reactance, but its rate of decay is much faster than that of a synchronous generator of the same rating, and its sustained short-circuit current is zero. Since an induction generator must have a laminated rotor, to provide for the slip-frequency rotor magnetic field, its construction is not adapted to as high speeds as synchronous machines employing solid steel rotors. For these various reasons, induction generators have found few practical applications, their chief use being perhaps, in variable-ratio frequency converter sets, where the induction end of the set operates as a motor or a generator depending on the direction of power flow through the set.""" Dan, I'm confused over the paragraph where you said: "To further confuse you, you can build a very nice RPC by using a single phase motor to drive a three phase motor via a belt drive adjusted so mechanical power is going into the three phase motor. Now if you apply single phase power to the three phase motor, it will act as a three phase induction generator. If you do this use an adjustable pulley on one of the motors and measure the current drawn by the single phase motor. Adjust the pulleys so the current drawn by the single phase motor is close to but below rated nameplate current when the RPC is driving whatever load you are going to drive. As you might suspect such a RPC produces voltages that are very closely balanced." It seems to me you may be describing an idler driven by a pony motor, similar to systems in which the pony is disconnected after the idler has come up to speed. Could this be the case you're describing, except that the pony motor is not disconnected after the starting interval? Bob (easily confused) Swinney |
#108
Posted to rec.crafts.metalworking
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Flywheel on a rotary phase convertor
On 9 Jan 2006 05:35:43 -0800, jim rozen
wrote: In article . com, says... I hope this message actually gets posted. My last two attempts failed. I am hoping the problem has something to do with cookies and posting through Google. Coming through fine here Dan. Jim Yup - All six of 'em, and the other three you thought didn't, too... Ain't Technology wonderful? ;-P "There is nothing that can go wrong click can go wrong click..." -- Bruce -- |
#109
Posted to rec.crafts.metalworking
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Flywheel on a rotary phase convertor
Robert Swinney wrote: Wow, Dan! Nine posts. I am very sorry for the multiple posts. I kept getting messages that said " Server Error try again in 30 minutes " or it would seem to accept the message but would not show up when I would read the thread. Apparently the messages were buffered somewhere and all of them eventually got posted. Bob, the first reply was before your post that clarified what you meant to say. It seems to me you may be describing an idler driven by a pony motor, similar to systems in which the pony is disconnected after the idler has come up to speed. Could this be the case you're describing, except that the pony motor is not disconnected after the starting interval? Bob (easily confused) Swinney I am more or less describing exactly what you said above. Except when you use a pony motor you may have the pony motor only bring the three phase motor to say 80 % of its rated speed. If you apply power to the three phase motor while leaving power on the pony motor, the pony motor will be driven faster than is synchronous speed until you disconnect it. I think this is how Jim Rozens RPC is done, where his sytem throws the belt when you apply power to the idler. You might have the pony motor directly connected to the three phase motor. In that case there would be little or no mechanical power transferred between the two motor when power is applied to both. So I am describing a subset of an idler driven by a pony motor, where the drive is adjusted so that the pony motor turns the idler at a speed which is faster than the idler would run by itself. The pony motor may or may not be large enough to drive the idler above its synchronous speed but mechanical power is still going to the idler ( actually not an idler anymore, but a driven motor ). Again I apologize for the multiple posts. None of my posts showed up here until today. Dan |
#110
Posted to rec.crafts.metalworking
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Flywheel on a rotary phase convertor
In article , Robert Swinney says...
It seems to me you may be describing an idler driven by a pony motor, similar to systems in which the pony is disconnected after the idler has come up to speed. Could this be the case you're describing, except that the pony motor is not disconnected after the starting interval? Dan's comments there are about deliberatly driving the idler motor at above slip speed. It will behave like an alternator at that point and deliver the mechanical power that is entering the shaft to the electrical load. Jim -- ================================================== please reply to: JRR(zero) at pkmfgvm4 (dot) vnet (dot) ibm (dot) com ================================================== |
#111
Posted to rec.crafts.metalworking
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Flywheel on a rotary phase convertor
Jim and Dan . . . .
Hmmmmnnnn ... I never thought of it that way. If I may describe what I think it was you said: You have a pony motor driven RPC, connected as in other RPC's described in this thread, where the single-phase pony and 2 terminals of the 3-phase idler and 2 terminals of the 3-phase load are all connected across a single-phase source. The pony, an induction motor, is belted such that it mechanically drives the idler ( 3-phase motor) at a speed greater than synchronous speed. If I "see" it correctly you'd have a RPC (includes load) which runs at a speed somewhere beyond normal synchronous speed of the 3-phase load motor. Jim's comment, "It will behave like an alternator at that point and deliver the mechanical power that is entering the shaft to the electrical load" seems to be spot on." Well! Knock me over with a rubber dick, err, make that rubber duck. Seems to me that ought to work. Id'd bet it is kind of tricky to get the right belt, or gear, ratio between the pony and idler. I suppose, though, it would just refuse to "generate" if everything was not properly sized. BTW - how would you know if it was functioning as an induction generator ? If the load motor was running at greater than its normal synchronous speed - it'd be generating, right?? Bob (learns something new every day) Swinney PS: Hey, why not use a 3-phase pony to get things started and then, and then, then . . .connect the 3 terminals of the load back to the 3 terminals of the pony. Vee-I-ola! Perpetual motion, Naw! "jim rozen" wrote in message ... In article , Robert Swinney says... It seems to me you may be describing an idler driven by a pony motor, similar to systems in which the pony is disconnected after the idler has come up to speed. Could this be the case you're describing, except that the pony motor is not disconnected after the starting interval? Dan's comments there are about deliberatly driving the idler motor at above slip speed. Jim -- ================================================== please reply to: JRR(zero) at pkmfgvm4 (dot) vnet (dot) ibm (dot) com ================================================== |
#113
Posted to rec.crafts.metalworking
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Flywheel on a rotary phase convertor
In article , Robert Swinney says...
Well! Knock me over with a rubber dick, err, make that rubber duck. Seems to me that ought to work. Id'd bet it is kind of tricky to get the right belt, or gear, ratio between the pony and idler. That's why dan suggested a varispeed pulley setup in his post. I suppose, though, it would just refuse to "generate" if everything was not properly sized. Yes, if it weren't line excited. But it is. BTW - how would you know if it was functioning as an induction generator ? If the load motor was running at greater than its normal synchronous speed - it'd be generating, right?? Generating on the third leg. I think the idea would be to make a rotary converter that somehow would have better balance or lower impedance by deliberatly driving the rotor above synchronous speed. Jim -- ================================================== please reply to: JRR(zero) at pkmfgvm4 (dot) vnet (dot) ibm (dot) com ================================================== |
#114
Posted to rec.crafts.metalworking
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Flywheel on a rotary phase convertor
Robert Swinney wrote: Well! Knock me over with a rubber dick, err, make that rubber duck. Seems to me that ought to work. Id'd bet it is kind of tricky to get the right belt, or gear, ratio between the pony and idler. I suppose, though, it would just refuse to "generate" if everything was not properly sized. BTW - how would you know if it was functioning as an induction generator ? If the load motor was running at greater than its normal synchronous speed - it'd be generating, right?? Actually it is dead easy to get it adjusted. Just monitor the current going to the " pony " motor. Adjust the pulleys so the current is at or slightly below the rated full load current on the nameplate. The pony motor is then putting out its rated horsepower. Where is it going? Got to be going into the three phase motor. With the three phase " idler " connected to the mains, you won't notice much if any difference in speed. Most four pole motors run at about 1750 rpm when they are being used as a motor. 50 rpm slip. To drive at 1850 rpm, -50 rpm slip, would take the same horsepower going in as the motor is rated to put out. Actually a bit more to take care of windage and other losses. So the drive ratio is very close to 1 : 1 Dan |
#115
Posted to rec.crafts.metalworking
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Flywheel on a rotary phase convertor
On 9 Jan 2006 18:28:27 -0800, jim rozen
wrote: BTW - how would you know if it was functioning as an induction generator ? If the load motor was running at greater than its normal synchronous speed - it'd be generating, right?? Generating on the third leg. I think the idea would be to make a rotary converter that somehow would have better balance or lower impedance by deliberatly driving the rotor above synchronous speed. Jim I think it'd be generating on all three phases; there would be a component of current in the line-connected phase that flows against the applied voltage, feeding power back into the mains. This current component would be in quadrature with the magnetizing current it draw from the mains. I think it would have better balance because the sign or direction of the IZ drop wrt the emf would be the same in all three phases. This is not the case in a self-excited idler. |
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