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.
> > >> 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 :) > > >> >>> What has really made today's speed control possible is permanent >>> magnet technology with modern alloys. No need for the old Ward >>> Leonard system, some still used on old elevators. >> Permanent magnets do simply replace the old wound fields in the >> motors-but the real speed control is due to the inverters providing >> variable frequency to (typically) induction motors which do not have >> magnets. >> ===================================================== >> I do not agree. An induction motor without a permanent magnet cannot >> be synchronous, it has to slip and the slip is load dependent. That's not >> speed control. > > True- an induction motor is not and never has been intended to operate > as a synchronous motor- but speed control by frequency changing is not a > new thing- what has made it reasonable is the advances in power > electronics. > However- applying a variable frequency source to an induction motor is > an effective speed control- It is necessary to make the applied voltage > proportional to frequency in order to avoid saturation and limit > magnetizing and stator current to a safe value. > Here is a reference- one among many- I haven't checked the equations > used but on a cursory glance they appear OK - the manufacturer of these > controls is well established. other references also deal with this. > > http://www.ab.com/support/abdrives/documentation/fb/1024.pdf > ======================================================= > Good grief, look at that! I was just saying above about negative slip > being a generator and you produce an article on the subject. > When I was a young man there was a huge disparity in the cost of > energy, North Sea Gas was a new thing and the coal industry on > the decline, coal-fired power stations made electricity expensive and > natural gas cheap. Stoves and boilers working on coal gas were > converted to natural gas nationwide. I had a friend who subcontracted > to make specialised ornate street lights in a small workshop with > four employees , a painter, a welder and two assemblers. To cut > a long story short, I and a mechanic coupled a used Rolls-Royce > engine to a large 3-phase squirrel cage motor, welded a water jacket > on the engine's exhaust, ran the engine on natural gas, drove the > electricity meter backwards and used the engine's water temperature > to heat the workshop. We even used the motor to start the engine.
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).
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.
> > > >> >> >> >> The brushless DC motors are essentially permanent magnet >> machines with switched poles and are inherently AC synchronous machines >> except for the discrete switching causing a rotating field rather than a >> polyphase rotating field. >> ========================================================= >> Quite so, the DC is electronically steered to the stator coils to >> produce a >> rotating field, we agree on that. >> >> >> The Ward Leonard scheme was really pre-power electronics and other >> schemes were used for AC machines. Early power electronics really >> started with Mercury arc rectifiers/inverters- I recall a late 1940's >> 5KW unit in a case about 6ft high and 2.5 ft x2.5 ft cross section with >> a transformer in the lower part and a glass octopus up top - pretty to >> watch the arc dancing over the cathode pool- and it worked. >> ======================================================= >> I saw the same used for welding in Chatham Dockyard, portable by >> being mounted in a panel van. >> >> >> >> In my mind, the development of SCR's etc is what made the revolution in >> speed control in that the ability to control speed became practical and >> economical. >> ============================================================ >> Yes, I recall the Swedish company Asea had a controller for a 500 HP >> commutator motor running a plastics extruder. Six thyristors, each about >> 2 inches in diameter, steering 3 phase AC to the DC motor. The production >> manager would tour the plant every morning at 9:00 am, pushing up the >> speed, and I'd follow behind him slowing it down again. If I didn't the >> 300 amp fuses would pop and I'd have to change them and they were >> bloody hot. He'd be hopping mad over an hours down-time, screaming >> and hollering to get it back up again. He watched the speed, I watched >> the current. He never realised I did more to keep his production going >> than he did, although my motive was to save myself more work so that >> I could study mathematics in the electrician's workshop. I was studying >> for my bachelor's at the time and when I got it I was outta there. >> The older extruders had hydraulic motors for speed control, they were >> not my problem, but they did have mercury switches in the heaters. >> Nasty, short circuit a heater and the switch would blow out before >> the fuse leaving broken glass and pools of mercury in the cabinet. >> >> >> >>> What I've not been able to discover is the speed control of trains on >>> 750V DC third rail, they sound like they have gear boxes. Maybe its to >>> reduce starting current. >> >> here is an excerpt. >> >> http://www.trainweb.org/tubeprune/Rolling%20Stock.htm >> >> "Series-parallel traction control using resistances was standard up to >> the 1990s. Resistance switching is achieved by the use of cam-operated >> contactors, the camshaft being driven by an air-operated, oil-damped >> engine. This system is known as the PCM (Pneumatic Camshaft Mechanism). >> All stocks so fitted use camshafts on each motor car, except for the >> 1967/72 tube stocks and C surface stocks which use a separate camshaft >> for series and parallel notching. The two camshaft system was >> introduced because of the more complex equipment required for rheostatic >> braking, which was introduced to Underground rolling stock at the same >> time. A larger single camshaft is used on the motoring and braking >> circuits of stock built since 1973. The 1992 and 1996 tube stocks have >> modern electronically controlled power systems known as chopper control >> and the 1995 Tube Stock has IGBT traction control. The 1995-6 stocks >> use 3-phase AC traction motors." >> >> It is likely that the 750V DC hasn't changes but the motors and their >> controllers have changed from the series resistance with series or >> heavily compounded motors at 750VDC to inverters and induction motors. >> Starting current would be limited electronically- to get a soft start. >> >> A recording of a train entering Baker St. station and departing sounds >> to me like all electric control. However, the only tube cars I rode on >> were between terminals at Heathrow and worry about catching my plane to >> Canada overrode any listening to the sound. Startup was very smooth >> which fits the soft start, constant torque and current run up to speed. >> ========================================================= >> That's the wimpy little Tube, I mean the mainline trains used by >> commuters >> to and from London and the coast. You can hear a frequency smoothly >> rise, suddenly drop and smoothly rise again, much like a car engine and >> gear change. I haven't heard it on the more modern Javelin trains. > > It could be a gear change- I don't know what the drives involved are > -DC? (series or compound?) AC?(induction or synchronous) individual axle > or wheel motors ala ASEA which don't favour gears) > OR > magic? (Potter or much better- discworld?):). > ===================================================== > 750V DC. > http://www.javelintrains.com/javelin99.jpg > The Javelins have both a third rail shoe and an overhead pantograph, > but this earlier stock has no pantograph. There is still no catenary supply > into London Bridge, Charing Cross or Victoria, the major commuter termini, > so third rail DC is still a must. The trouble with upgrading is it can't be > done all at once, trains must run still run on the old system as well as > the new. I do know from a train driver friend of mine that he was > pulling 3000 amps climbing out of Dover Priory and the train was > barely moving, and that was 40 years ago. Gears would solve that > problem, but I've never found out for certain if that was it. Oh well... > > -- This message is brought to you from the keyboard of > Lord Androcles, Zeroth Earl of Medway