On 25/11/2012 3:02 AM, Lord Androcles, Zeroth Earl of Medway wrote: > "Don Kelly" wrote in message news:1ogss.6280$Mm3.firstname.lastname@example.org... > > On 24/11/2012 6:35 PM, Lord Androcles, Zeroth Earl of Medway wrote: >> "Don Kelly" wrote in message news:LGess.24063$Sm5.email@example.com... >> >> On 23/11/2012 11:58 PM, Lord Androcles, Zeroth Earl of Medway wrote: >>> "Don Kelly" wrote in message news:ExZrs.12238$nO.firstname.lastname@example.org... >>> >>> On 22/11/2012 7:49 PM, Timothy Sutter wrote: >>>> Don Kelly wrote: >>>> >>>>> Timothy Sutter wrote: >>>> >>>>>> Archimedes Plutonium wrote: >>>> >>>>>>> Timothy Sutter wrote: >>>> >>>>>>>>>> and, it really does seem as if >>>>>>>>>> the little whizzers =DO= -have- "flight paths" >>>>>>>>>> and that they are -not- simply in >>>>>>>>>> 10000000000000000000000000000000000000000 >>>>>>>>>> places at the same time.... >>>> >>>>>>>>> just look at these images... >>>>>>>>> http://en.wikipedia.org/wiki/File:Various_Spirograph_Designs.jpg >>>>>>>>> see, you don't just see the cloud of uncertainty >>>>>>>>> you see distinctly flight paths... >>>> >>>>>>>> the thing about the spirograph images is that they are =flat= >>>> >>>>>>>> and the atom travails in -volume- and so, 3D images >>>>>>>> and you really would wonder if the shapes of snowflakes -are- >>>>>>>> sort of like the shapes of certain electronic flight paths... >>>>>>>> "but isn't i true that no two snowflakes are alike?" >>>>>>>> have you really looked at all of them? >>>> >>>>>>>> STOP >>>> >>>>>>> Hi Tim, I will stop for 3d volume. >>>> >>>>>>> It has been a long time since I took apart an electric motor of its >>>>>>> windings of copper wire. >>>> >>>>>>> Tell me, are the windings close to being spherical in all? And are >>>>>>> the >>>>>>> windings of 1 long copper wire or are they of 2 long copper wires or >>>>>>> more? >>>> >>>>>> i just happen to have the motor of >>>>>> an olde box window fan in the basement. >>>> >>>>>> it's a lot of copper wire and these things called >>>>>> "bushings" that seem to be copper as well. >>>> >>>>>> it doesn't look too much like this one >>>>>> but it resembles it a little bit. >>>> >>>>> The box fan motor is likely a single phase shaded pole induction >>>>> motor. >>>>> Such a motor will have no commutator as shown in your permanent magnet >>>>> DC motor (it also will not have permanent magnets and the rotor >>>>> will be >>>>> quite different in general). >>>> >>>> i'm pretty sure i said they didn't look too much alike >>>> but had a little bit of similarity, and, i still say that now. >>>> >>>> my new fan has a fairly small motor with a diameter >>>> of about 5 inches and no exposed copper, but my >>>> old fan motor was larger and you could see blobs >>>> of copper wiring -somewhat- like that scooter motor. >>> Your new fan, considering its size may be a "brushless DC" motor - it >>> too will have coils. >>>> >>>> just seeing the copper coils is a similarity. >>> Yes- there is a similarity- the same as the similarity to a solenoid and >>> a transformer. seeing copper coils in a transformer >>>> >>>> >>>>> The "bushings" are "oilite (sic?)" bearings which are >>>>> typically copper or a copper alloy which is sintered and holds oil. >>>> >>>> my old fan may have had this little felt tipped >>>> front end where you had to oil every so often. >>> That figures. >>>> >>>>> They are cheaper than ball bearings but don't generally last as >>>>> well.. However, these bushings have nothing to do with the >>>>> electrical/magnetic operation of the motor. >>>> >>>> it's possible that these old bushings were carbon >>>> and would crud up after a while and you'd have >>>> to clean up the crud. >>> Doubtful. However graphite has been used as a lubricant. Sintered bronze >>> is common. >>>> >>>> maybe i'll dig it out and take it apart unless >>>> i already disposed of it in an enVIromentally safe manner. >>>> >>>>> As for Archie's question- No- the windings are not spherical at all. >>>> >>>> no, many coils are sort of round or cylindrical and not spheres. >>>> i have an olde starter motor that may be a -little- bit more >>>> like the scooter motor, but i'm, not in the mood to take >>>> it apart right now, as, it is greasy and secure where >>>> it is on its little shelf. >>> Cylindrical is common and practical."sort of round" is meaningless. >>> >>>> >>>> >>>>> As usual he is off in his own little world -where facts are not >>>>> important. >>>> >>>> >>>> well, yon Pluto does -ask- if the windings are spherical >>>> Pluto doesn't exactly tell me what sort of motor >>>> i have and then tell me how it's constructed. >>> If I am wrong- let me know. >>> Fan- typically 120V (240V in UK) -assumed not to be a new fan (you said >>> old(e)) so what is typical? A form of small induction motor-with coils >>> on the stator. If it is a DC motor, then the construction will be >>> essentially the same as the scooter motor with coils on the rotor and a >>> commutator. The presence of a commutator is a give-away. >>>> >>> -----rant snipped------ >>> >>> ====================================================== >>> If DC then even 30 years old it'll be brushless, floppy drives have come >>> and gone long ago, Don. >>> http://en.wikipedia.org/wiki/File:Floppy_drive_spindle_motor_open.jpg >>> The fan on my computer is variable speed, it speeds up if the processor >>> gets hot and slows to reduce noise. >>> It's difficult to imagine any young engineer designing a commutator in >>> 2012 or any entrepreneur investing in one when it can be made on a chip. >>> I have a TV on a dongle and if you want power just look at the microwave >>> oven, it's a low to high frequency converter at over one horsepower. >>> >>> -- This message is brought to you from the keyboard of >>> Lord Androcles, Zeroth Earl of Medway >>> >>> >> I think that we may be involved in a case of terminology. >> To me, a box fan is a fan, run from a 60Hz 120/240V system or a 50Hz >> 240V system, which is in a square enclosure and is used for room >> cooling. Many are portable and will have shaded pole induction motors >> and have some speed control (3 settings typically) which may simply be >> switching steps on an autotransformer or by control of a triac. These >> are cheap and reliable- although bearings can be a problem. This is what >> I assumed that he was looking at and trying to compare to a commutator >> motor. >> >> I would use "case fan" for the ones used in a computer and, yes, these >> are 'brushless DC' with electronic commutation of what is essentially an >> AC synchronous machine or a stepper. My comments did not refer to these >> but the resemblance that was mentioned is only that all have windings. >> >> As to the conventional commutator machine- these are still in use and >> are dying out as better power electronic switching makes it possible to >> eliminate the mechanical commutator which limits the practical upper >> size of DC machines and is a major maintenance problem. >> I do note that they are still in use in many applications -e.g. the >> series DC motor used as a car starter- where the inherent behaviour is >> preferable. >> I also note that large inverter drives supply AC machines- mostly >> induction machines. >> >> I am also quite prepared to say that I don't actually know what kind of >> fan and wha was its use -so that I just referred to what I call a "box >> fan" as above. >> ==================================================== >> Ok, we are almost completely in agreement. >> I've never come across a triac or autotransformer controlled induction >> motor though. That would be like trying to control a fluorescent lamp. >> Multi-speed ceiling fans with induction motors have two windings, >> one of which can be switch configured as a 4-pole or 8-pole (2-pole >> is much too fast for a ceiling fan) and the other a 12-pole in a >> 24-slot stator. > That is true for ceiling fans but there are some room fans with speed > control for which there is no coil switching. I had one which I had to > take apart to get at the bushings and soak them in oil -got an extra > year out of it- and it is definitely a shaded pole motor with no coil > switching. I didn't take apart the control section in the stand but I > would expect something involving a small autotransformer with taps or a > triac. I have a small blower in a fireplace that has a similar motor and > I installed a triac dimmer rated for motor loads and this works well. I > can set a lower voltage limit so that it won't stall. An exhaust fan > over the stove is similar. I expect that the R/X ratio of the motor is > such that the peak torque is near standstill. > > ====================================================== > Triacs should only be used when the load is resistive.
Triac dimmers for incandescent lights are not rated for inductive loads but the one I got is rated for motor loads. It was a bit more expensive than the usual dimmer but I have had no problems. Here is a circuit reference: www.ie.itcr.ac.cr/rsoto/.../carga%20inductiva%20con%20triac.pdf
> At standstill (say you lock the shaft of a squirrel cage induction motor) > one can view the stator as the primary of a transformer and the rotor > as the secondary with a shorted turn. That is max torque.
Not necessarily- a rough estimate is that the slip at maximum torque equals the R/X ratio so that only with R/X=1 will the max torque be at standstill. Most motors have an R/X in the order of 0.1
As the rotor > accelerates the difference between the rotor RPM and field RPM of the > stator (known as the slip frequency) gradually reduces to zero at > synchronous speed and the torque vanishes, the motor is then just > an inductive load like a transformer with no load on the secondary.
It wont run at synchronous speed but must slip even at no load to overcome mechanical losses. In a sense it is a transformer (and can be modeled as such at any speed) and at synchronous speed all you have is the exciting current which is of the order of half the full load current-entirely reactive.
> 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. > >> 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 > > > > 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?):).