How hard is it to drive a 3-ph brushless motor?
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From: Niklas Holsti on 9 Dec 2009 17:50 Jan Panteltje wrote: > On a sunny day (Wed, 09 Dec 2009 13:56:31 -0800) it happened Rich Grise > <richgrise(a)example.net> wrote in <pan.2009.12.09.21.56.29.114217(a)example.net>: >> >> Speaking of that, how long do geostationary comm satellites last? > > 15 years spec or so. > >> When >> one of them goes down, do they just scuttle it and launch a replacement? > > They let it burn up before the fuel to control its position is used up. That's done for low-Earth-orbit (LEO) satellites. Geostationary satellites are too high up. They are instead moved to an even higher "graveyard" orbit, a couple of hundred kilometers higher than the operational geostationary orbit (according to Wikipedia). >> I've never heard of anyone going up to repair one! There are plans to develop unmanned repair or life-extension vehicles, for example a vehicle that would grab an old comm satellite that is (almost) out of fuel and use its own fresh thrusters and fresh fuel to control the orbit and attitude of the comm satellite for a "second life". -- Niklas Holsti Tidorum Ltd niklas holsti tidorum fi . @ .
From: Jan Panteltje on 9 Dec 2009 17:54 On a sunny day (Thu, 10 Dec 2009 00:50:15 +0200) it happened Niklas Holsti <niklas.holsti(a)tidorum.invalid> wrote in <4b2029d2$0$3857$4f793bc4(a)news.tdc.fi>: >Jan Panteltje wrote: >> On a sunny day (Wed, 09 Dec 2009 13:56:31 -0800) it happened Rich Grise >> <richgrise(a)example.net> wrote in <pan.2009.12.09.21.56.29.114217(a)example.net>: >>> >>> Speaking of that, how long do geostationary comm satellites last? >> >> 15 years spec or so. >> >>> When >>> one of them goes down, do they just scuttle it and launch a replacement? >> >> They let it burn up before the fuel to control its position is used up. > >That's done for low-Earth-orbit (LEO) satellites. Geostationary >satellites are too high up. They are instead moved to an even higher >"graveyard" orbit, a couple of hundred kilometers higher than the >operational geostationary orbit (according to Wikipedia). Ah, yes, makes sense, thank you for the correction.
From: dagmargoodboat on 9 Dec 2009 18:08 On Dec 9, 5:02 pm, Peter <nos...(a)nospam9876.com> wrote: > Rich Grise <richgr...(a)example.net> wrote > > > A different option is to use a stepper directly, 0-13rpm, no gearbox. This > >> might do but a failure of the electronics could make it go round very > >> fast, > > >In what failure mode can this possibly happen? Shorted drivers will lock > >up the motor, open drivers will let the motor free-whee, kinda - steppers > >don't coast very well. If the micro locks up, no steps, no motion. I've > >worked with stepper motors before, and they have an inherent top speed - > >if you drive it too fast, it will just skip steps. > > >I could spec an appropriate motor and possibly gearbox, and write a uP > >program to drive it at a speed proportional to your input control voltage. > > The sort of thing I was thinking of was where you use e.g. a 10000rpm > motor with a 100:1 gearbox, so your max rpm is 100. If one achieved > the 13rpm max by limiting the control voltage, then a fault at that > point would cause the motor to go to max rpm. > > I agree that one could put a pulse rate detector which inhibits the > motor... > > I did look at steppers but most seem too big in diameter - I have a > limit of 35mm diameter. A micro-stepped stepper would be ideal: smaller, lighter, cheaper, more rugged, and mechanically more reliable than something with a gearbox turning 8k rpm. There are plenty under 35mm. Do you need super-smooth rotation or super-precise shaft positions? If not, the world's yer oyster. If so, maybe even then, too. -- Cheers, James Arthur
From: Dave Platt on 9 Dec 2009 18:08 In article <pan.2009.12.09.21.56.29.114217(a)example.net>, Rich Grise <richgrise(a)example.net> wrote: >Speaking of that, how long do geostationary comm satellites last? When >one of them goes down, do they just scuttle it and launch a replacement? >I've never heard of anyone going up to repair one! I haven't heard of them being serviced, either. "Scuttling" of the satellite is a bit tricky. Actually de-orbiting it (and having it burn up in the atmosphere) would require a huge amount of delta-V - far more than the remaining fuel in the station-keeping thrusters is going to be able to achieve. The current approach seems to be to move the satellite to a non-geostationary "graveyard" orbit, a few hundred kilometers above or below the geostationary altitude. This requires a much smaller delta-V than a de-orbit operation. http://www.orbitaldebris.jsc.nasa.gov/library/USG_OD_Standard_Practices.pdf -- Dave Platt <dplatt(a)radagast.org> AE6EO Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads!
From: Joerg on 9 Dec 2009 20:00
Peter wrote: > Paul Carpenter <paul(a)pcserviceselectronics.co.uk> wrote > >> Quite common to get 3 phase pattern by PWM or Timing Pattern generator >> and gate this with PWM clock/DC to change the RMS level (part of torque) >> and adjust 3 phase pattern frequency for RPM. Watching your torque and >> RPM relationships for load and current speed to avoid stall. The 3 phase >> pattern is easy to do even for reversing in software or hardware. >> >> I am seeing a customer tomorrow who have an ASIC solution for doing this >> used for missiles so long life time product. Let me know if you want a >> datasheet or other details. >> >>>>> And I would quite like some kind of closed loop motor speed control. >>>>> Even though the torque is small, the application needs the speed to be >>>>> within about 10%. This can be done with back-EMF; should not need a >>>>> tacho. >> +/- 1.3 rpm at 13 rpm or higher rpm at the actual motor? > > Unfortunately I did not make it clear enough that this is at the > output shaft of the gearbox. I am currently looking at a 600:1 gearbox > so the motor would run at ~ 8000rpm. > > Now that I have "converged" on building my own controller, using one > of the integrated chips that exist, the biggest problem is getting the > mostly American companies to talk to me. Micromo pushes me back to > their UK agent who does not reply. Last time I had a go at this > project (1/2008) the same thing happened. That would be a clear sign to move on and find another manufacturer. At least if this goes into production. Just imagine when a supply issue creeps up and you are afforded the same sort of "courtesy". -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM. |