How hard is it to drive a 3-ph brushless motor?
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From: Robert Sneddon on 9 Dec 2009 19:54 In message <ba49cbaa-f1da-44d5-8106-90fb5711b563(a)z10g2000prh.googlegroups.com>, dagmargoodboat(a)yahoo.com writes >A micro-stepped stepper would be ideal: smaller, lighter, cheaper, >more rugged, and mechanically more reliable than something with a >gearbox turning 8k rpm. The original poster specified high reliability over a period of years; I don't see a 600:1 gearbox lasting that long without maintenance or replacement. It will also exert a significant frictional load on the motor that will change over time due to wear and lubricant expiry, causing more control problems down the line. In addition a DC motor capable of running at up to 8000rpm continuously for years on end will be very expensive; what are the MTBF figures for such motors? 10,000 hours, 50,000 hours? A stepper motor gets rid of the requirement for a gearbox; fewer moving parts are a good thing, but the long-duration operation means whatever the prime mover is it will have to be a hi-rel part and they are never cheap whether it is a stepper motor or a DC brushless motor. -- To reply, my gmail address is nojay1 Robert Sneddon
From: Joerg on 9 Dec 2009 20:04 Peter wrote: > Rich Grise <richgrise(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... > If you can't trust the controller or control signal and bad things can happen when it's over-revving it is almost a must to do that. > I did look at steppers but most seem too big in diameter - I have a > limit of 35mm diameter. 35mm? That's huge, especially since you wrote earlier that you don't need much torque. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: -jg on 9 Dec 2009 20:40 On Dec 10, 2:47 am, z...(a)ds1.com (Peter) wrote: > 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. can you define 'small' ? The ideal candidate would seem to be micro-step controlled Stepper motors. See Allegro and OnSemi, for chips that do all the drive, and current ROM handling for Micro stepping. Steppers will give very precise average speed, but will still step, so you need to define how much non-linear phase velocity is tolerable. -jg
From: Jamie on 9 Dec 2009 21:17 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. 8000 RPM into a gear box is a little much.. I suppose the gearbox you have in mind is spec'd out for this ? Oh well, we have turbo pumps that do 50k RPM :)
From: Spam on 9 Dec 2009 21:24
On Wed, 9 Dec 2009, dagmargoodboat(a)yahoo.com wrote: > > 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 > Perhaps a BLDC motor, like the one found in any old hard drive might do the trick ... and the back-emf commutation method should work ... Cheers, Rob Sciuk |