Re: Basic Stamp Controlled Spark Gap

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jim Lux by way of Terry Fritz <teslalist-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>

 >  > >Speed control -  H-Bridge push/pull type circuit
 >  > >using power MOSFETS and an output from the stamp.
 >  > >This and the tachometer feedback will keep the
 >  > >motor running at a consistent speed by varying
 >  > >the frequency of AC. This should compensate for
 >  > >any desyncronizing drag of the disc. How fast
 >  > >it rotates is yet to be determined. Initially
 >  > >planning on 1800RPM with eight electrodes for
 >  > >up to 240BPS.
 >  >
 >  > Since the motor is unidirectional, all you need is
 >  > one switching device..
 >  > you don't need 4 quadrant operation (i.e. reverse or
 >  > braking)...
 >
 > You hook a synchronous AC motor up to a special H
 > bridge and you can feed it a pseudo-sinewave of
 > whatever frequency you want (up to a point)
 > It's RPM is a function of the frequency you
 > give it. The down side is that the motor has the most
 > torque at 60Hz (it's designed for that...)
 > and you have have a 120V DC supply :) (not hard..)

Much harder than you need... two easy alternatives, based on the fact that
you've got a speed control....
1) Use a DC motor... 90V is a standard voltage.. others are available... for
instance, my electric lawnmower has a bridge rectifier off the AC line to
run it.  A SCR phase control is probably good enough.. the mechanical
inertia of the system is such that the bandwidth/response time of the loop
probably doesn't warrant something fancier.
2) Use an induction motor (PSC is probably best) and put N+1 electrodes on
the disk. 5 instead of 4, and run it at 80% of sync speed (i.e. 2880 RPM
instead of 3600...) Most of your load on the motor is windage losses, and
they go as the square of motor speed, so variable voltage control works
quite well.


 >
 > It may also be possible to vary the phase this way
 > by stretching just one of the sinewave 'pulses' (H
 > bridge makes a sinewave out of a series of hi-low
 > pulses)

Making variable speed motor controllers for single phase motors is very
non-trivial... if you want to get the enhanced performance from doing so. If
you don't care about the performance (efficiency), then why not do something
simpler... DC motors

 >
 > I've since changed my design thoughts on this one :)
 > I found a 'digital potentiometer', which makes
 > controlling a triac and all that jazz (light
 > dimmer/speed control/phase shift) available.
 >
 >   > >Phase control - R/C type servos will rotate the
 >  > >'fixed'
 >  > >pair of electrodes (connections to the power
 >  > supply)
 >  > >anywhere from 0 to 90 degrees in fractional degree
 >  > >increments.
 >
 >  > Why not shift the rotor motor speed and phase...
 >  > rather than moving the
 >  > electrodes. no extra servos needed. Imagine you've
 >  > got the motor synced up
 >  > at 1800 rpm and at phase=0... if the motor is slowed
 >  > to 1799 rpm, it will
 >  > roll through 360 of phase shift relative to power
 >  > line in 1 minute.. 6
 >  > degrees/second.. so, if you wanted to run at 18
 >  > degrees behind, you just
 >  > run at 1799 for 3 seconds.. in reality, this is just
 >  > a phase locked loop,
 >  > and you don't need the step functions...
 >
 > Yep, it all depends on what the basic stamp can do
 > and how quickly it can process the timing signals.
 > I've seen that the faster ones can send pulses that
 > are mere microseconds long, so it shouldn't be too
 > difficult.

It's more running the control loop in the stamp... However, a 5-10 Hz loop
update rate is probably fast enough...and you can do a lot of computations
in 100 milliseconds, even on a stamp.

 >