Re: AC single phase motor speed control

The general electric rapsheet on their AC motor product line(s) indicates
that variations beyond +/-5-10% of voltage and/or voltage oscillation
frequency will cause the motor to function "with reduced efficiency," in
terms of rotational speed, torque generated, wear to insulation, and
thermally (or, in the case of increase of those values, just thermally and
insulation [-ally?], oh yes, and slip increases, which is what causes the
two previous issues ). They mention that a 25% reduction in voltage results
in around a 50% reduction in torque (actually it's a table, that's one of
the values I remember)and about a 40% reduction in motor speed, but
generates significant heat (conservation of energy, of course) and
potentially can result in wearing out winding insulation.
It does seem like using mechanical power transmission of some sort would be
less likely to kill the motor, but I have certainly learned a WHOLE lot
about the AC induction motor. I never did make it to BurningMan. But it's
been a fun trip just the same.
Someone pointed out to me that the capacitor start motors (which both of the
functional AC motors I have happen to be) use centrifugal switches which
will cause the motor to go into a high-torque "start mode" if the motor
slows, which sort of defeats the purpose (this is mentioned in the GE docs,
but not as any kind of a specific problem - just that the specific motor in
question uses a capacitor during startup and runs on induction once it's
going fast enough to open the switch - no mention at what speed the switch
disengages). Apparently running the motor in such a "start mode" too much
will kill the motor more or less immediately.
Ahhhnhhh well.
Thanks to everyone who contributed suggestions (unilaterally - "use a DC
motor" - variations from car starter motor to volkswagon generator from a
pre-1970 super-beetle/fastback) .
At this point, I'm not in a hurry about it anymore. So I'm going to do the
right thing and get a DC motor for the purpose.
Again thanks to everyone who gave The Suggestion, which I of course
stubbornly ignored for as long as I was able.
> From what I understand, you are hoping to control the speed of the motor by
> using a triac to vary the AC voltage applied to the motor.
> My response is to be careful and ensure that the motor is suitable for this
> type of operation.
> Most induction motors are psuedo synchronous machines and you can not vary
> the speed by varying the voltage applied. There are a small number of
> motors, generally integral fan motors, that have a high resistance rotor and
> are designed to be operated under variable voltage control.
> If you operate a standard motor under variable voltage control, you will
> find that in many cases it will run at full speed, or it will stall and you
> will draw a lot of current from the supply and cook the motor.
> Haveing expressed major caution on your proposed method of control, I am
> unable to offer an alternative. I believe that the DC motor is the best > solution.
> Best regards,
> Mark Empson
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> > My problem is this:
> >
> > I am putting the finishing touches on a novelty vehicle for an event > called
> > Burningman which is underway this week.
> >
> > All systems (soundeffects, audio amplification, relays for high voltage
> > transformers, gas-release valves, lights, steering servos, wheel > retraction
> > servos, fans, drive transmission) are fully functional and waiting for > field
> > testing.
> >
> > The DC motor that I was originally planning on using fell through, and all
> > that I have laying about the shop are a number of intermittent-duty > 115/230V
> > AC motors (all are around 6-700 Watts). The vehicle, with rider, > batteries,
> > etc, will probably weigh around 4-500 lbs (200 Kg, maybe) 1/2-3/4 HP seems
> > adequate for my purposes
> >
> > I have never tried to do an AC motor speed controller, but I have managed
> to
> > pull up a fair amount of info on the subject..
> >
> > At this point, I am considering using a triac/diac arrangement to supress
> > part of the modified sinewave generated by a 1000-Watt inverter that I > have
> > available. I will be shielding the motor and triac from surges using a
> > varistor. I am expecting the triac to get pretty warm, and am going to
> > monitor heat using a component thermometer that I have for the purpose as
> > well as heat sinking the bejeezus out of it.
> >
> > I have until Wednesday afternoon to complete this controller and my > ability
> > to troubleshoot the result will be EXTREMELY limited after that, as I will
> > be 80-miles away from the nearest electronics store of any caliber > > whatsoever.
> >
> > Getting a DC motor from Grainger would be prohibitively expensive just now
> > (and their non-bulk-purchase prices offend me, even had I the money). A
> > complex transmission-style mechanical speed control is entirely out of the
> > question.
> >
> > I have a small handful of Atmel 8051 microcontrollers available, but I am
> > far from expert with assembler, and right now, I can't really think of any
> > way to apply them to this problem, other than to use one to control the ON
> > state of the triac. FYI these microcontrollers can source/sink 15 X 20mA
> > (the Analog comparitor interface[s] will be used to read a pot for speed
> > control, and one other interface will control the ON state of the triac, > so
> > make that 12 X 20mA) and I have both 11.x Mhz and 20Mhz crystals. > >
> > Any thoughts/advice/alternative approaches? Would it be possible to do an
> AC
> > sort of PWM circuit (I guess that's what I'm talking about already, but
> > maybe faster so the resultant waveform is still somewhat sinusoidal, or
> > better yet, to simply generate a 60Hz pulse train at
> > full-positive/full-negative voltage but have a PWM-modified secondary duty
> > cycle at 0-Volts).
> >
> > I have (briefly) considered hacking into the inverter and varying supply
> > voltage and circumventing the low-voltage protection to vary the torque
> > output of the motor (thus the speed/acceleration under load), but I would
> > rather not make this inverter a permanent part of the vehicle. I will do
> > this, though, if necessary.
> >
> > Supply voltage can be anything from ~12-48 Volts, I have enough power
> > regulation silicon to choke a small nation, and plenty of 12volt > batteries.
> >
> > I am not going to be following this thread very closely, as I am SERIOUSLY
> > under the gun at the moment. Please direct responses directly to me at
> >
> >
> > I would appreciate the heck out of any ideas that seem a little less
> > hair-brained than what I am currently trying to do.
> >
> > Thanks,
> > dj
> >
> >
Reply to
DJ Bartlett
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