"Don Kelly" wrote in message news:Gd9ts.13609$M14.firstname.lastname@example.org...
On 26/11/2012 3:06 PM, Lord Androcles, Zeroth Earl of Medway wrote: > "Don Kelly" wrote in message news:KtRss.8226$Mm3.email@example.com... > > On 25/11/2012 7:09 PM, Lord Androcles, Zeroth Earl of Medway wrote: >> Quite so, but reactive current is 90 degrees out of phase with the >> voltage >> so no power is involved. In larger motors a capacitor is added to remove >> the reactive current from the supply or additional losses will be >> incurred >> in the resistive conductor leading to the motor. >> Here a curious fact emerges. >> Mechanical losses can be overcome by driving the shaft with an internal >> combustion engine, removing slip, so we have a rotating field that an >> iron rotor has synchronous speed with and no current is induced in the >> rotor's squirrel cage. Opening the throttle on the gas engine, we again >> have slip but in a negative sense, the rotor is faster than the rotating >> field in the stator and we have a generator. >> I say this is curious because one would not normally couple a squirrel >> cage motor to an engine and expect it to generate. It shouldn't because >> there is no excitation current and no magnetic field in a squirrel cage >> rotor, so why should a voltage appear from the stator? > > It won't unless you have a source of excitation- I assume that you tried > it when the motor was connected to the grid so that the grid provided > the magnetizing current. Alternatively one can use shunt capacitors > (while the machine is still on the grid and then running up over > synchronous speed and disconnecting from the grid). While this works it > isn't, in my mind, all that satisfactory as you may not get the desired > voltage. > I believe induction generators are or were used extensively in rural > parts of Scotland -taking advantage of small streams. The motor would > drive a turbine and, when wanted the motor would be run up from the > grid, the gates opened and when the turbine brought it above synchronous > speed, it would generate -taking excitation from the grid. > Startup could be automatic and shut down simply needed shutting off the > water and disconnnecting the unit. The nice thing about an induction > machine, other than its simplicity, is that it will not contribute to a > fault (except for a very short transient)- excitation lost- generation > stops. > ====================================== > Consider: > No connection to the grid, engine RPM governor controlled, motor and > capacitor with a resonant frequency, it's a tank circuit. The smallest > residual magnetism in the rotor rapidly builds an oscillating excitation > current in the stator windings. Cheap and simple generator. ------------------------------------------------------------------ Essentially that is right- capacitive VAR's to match motor VAR's This is even mentioned in Wiki -------------- > >> >> >>> I can't imagine why anyone would want a variable speed fan in a >>> stove hood. >> That is a typical option- but being able to lower speed slightly does >> reduce noise. >> ====================================== >> So the chef can yell at the waiter, the dishwasher and the buss-boy >> more effectively? You must dine at very expensive restaurants if they >> can afford that luxury. :-) > > In my kitchen the chef is my wife and likes to hear the music from the > other room :) > =================================================== > I see... so rather than buy your wife her own kitchen radio/MP3 player > you'd prefer she put up with cooking smells and perhaps smoke from > the stove which only has a slow hood fan because the noise would > interfere with you listening to the stereo with your feet up in the other > room. You seem to have her properly house-trained, I was never able > to do that. ------------------------------ Not a case of preference- the typical unit here comes with speed control- Some of the time it is necessary to use the high speed and at other times a slower speed is quite satisfactory. Depends on what is cooking. ------------------
> > I've played around with this with grid excitation and with capacitors. > With grid excitation, as soon as the grid was disconnected, the voltage > collapsed to 0. With capacitors, once you do have some generation and > cut the grid connection, the voltage will rise or fall unless by luck > the capacitors were the right size. I have also hooked capacitors to a > synchronous machine and cut the field- some voltage drop but all went > well- again, whatever voltage resulted was dependent on the size of the > capacitors. (This is not something that I would NOT do with a large > machine -although the Russians have run alternators without field > current for periods of several minutes-using the transmission line > capacitance. Most US, European or UK machines would not do this (it's a > matter of internal reactances). > ================================================= > My good sir, the capacitors MUST be the right size. One doesn't > build a radio with any old capacitor and expect the desired frequency. > The alternative, of course, is a permanent magnet rotor with no > squirrel cage but that would be too boring... or, heaven forbid, a > a DC armature with slip rings and take the AC off the field coils. -------------------------- Yes, both frequency and voltage will be less stable than in the case of the grid supplying the excitation. Extra capacitance is needed if the load is not purely resistive. However there appears to be a plethora of electronic control schemes available. =================================================== You are shipwrecked on a small island. Washed up on the beach is a container in which you find a radio receiver which needs 120V 60Hz, a large desktop fan, a toolbox with tools any handyman might have including a soldering iron and solder, A 25 cc two stroke weed-wacker with a full tank. Drive the fan with the weed-wacker engine to power the soldering iron to modify the radio receiver to make it a Morse transmitter (SOS = ...---...), then power the radio TX and be rescued (or not, I could happily stay there but I'd like the option to change my mind as I might get annoyed by the sound of the engine while listening to my favourite music on the radio). (When I was in Florida I powered the front wheel of my bicycle with a 25 cc weed wacker... barely enough power but the 35 cc follow-up version would accelerate and carry me over the Punta Gorda bridge without pedalling - I had fun building that.) > > > > I recall one farmer who had a different approach- he needed 3 Phase but > was on a single phase rural system. He did have a large 3 phase motor so > he drove it from a single phase motor and connected the supply to one of > the phases- but drew from all 3 phases for his 3 phase needs. It worked > very well but the motor so used had to be considerably larger than the > one he wanted to drive. > ========================================= > No reason it should be, but what would he want 3-phase for except to > supply a 3-phase motor? Surely it would be less expensive to replace the > motor than buy two more? -------------------------- I doubt that this particular farmer bought (or did so at bargain basement prices). He probably got surplus equipment from military(likely WW2 vintage) or from the oil industry. I don't know what size of motor he wanted to drive but it may have been larger than the reasonably available single phase machines of that size. The second time I saw him, he had moved and his system wasn't working- I sorted it out but can't recall the advice I gave- possibly something to Later I found that the local utility also used this scheme as a stop gap measure in an area where there was single phase service but one customer needed 3 phase. These areas are remote (northern Alberta, Canada) and have population and energy density lower than anywhere in GB or Europe. Economics favour single phase in this case. Possibly Scotland may have a similar setup which is common in western Canada and US. ------------------------ Three phase here is 415V line to line, 230 volt line to neutral for domestic use. To keep the three lines balanced, 3-phase is delivered everywhere. If one house takes off red and neutral then the next gets yellow and neutral and the third gets blue and neutral. With all three taking the same power there is no current in the neutral back at the transformer. 10 hp single phase is never thought about as 3-phase is always available if that much power is ever needed in a domestic situation. That's why I can't get my mind around the situation you stated. If its 1 hp then use single phase because its a vacuum cleaner, over that we use 3 phase because its an industrial application. If the farmer needs 3 phase it has to be 5 hp or more and what else on a farm uses anything other than a motor? Heating a large greenhouse would still be 3 phase to keep the supply balanced. I just can't imagine anyone using low voltage single phase to drive grain to the top of a silo with a fan, it just wouldn't happen here. I did repair a 100 hp fan (rewound the rotor) that operated a kiln for drying and sterilizing Fuller's earth for a pig farmer once, so yes, a lot of power can be used on farms, but I've never seen anything other than a 3-phase grid. http://en.wikipedia.org/wiki/Fuller's_earth
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