Re: PLL?

From: 	Mike Harrison[SMTP:wwl-at-netcomuk.co.uk]
Reply To: 	wwl-at-netcomuk.co.uk
Sent: 	Wednesday, December 31, 1997 12:07 PM
To: 	Tesla List
Subject: 	Re: PLL?

On Wed, 31 Dec 1997 01:57:35 -0600, you wrote:

>From: 	Gregory R. Hunter[SMTP:ghunter-at-enterprise-dot-net]
>Sent: 	Monday, December 29, 1997 1:17 PM
>To: 	Tesla List
>Subject: 	PLL?
>Hello All,
>I've been reading the mail for some time regarding synchronous AC
>motors for RSGs.  Sounds like a lot of trouble to find the correct
>sort of motor, disassemble it, machine it correctly, etc.  I'm
>wondering if anyone has experimented with a phase-locked loop circuit
>to sync a RSG?  It seems like it would be a fairly easy matter to use
>a phase detector to phase lock a motor with respect to the AC mains
>cycle.  An electronic approach would offer certain advantages, such
>as a much wider selection of motor types to choose from, greater
>flexibility in contact geometry, and no disassembly & machine work on
>motor armatures.  I'm roughing out circuit ideas with pencil and
>paper while I wait out the endless English Winter.  If anyone has a
>schematic or even a good idea for a circuit, I'd love to see/hear it. 
>Happy New Year.
>Bored in gloomy East Anglia
I was thinking of doing exactly this when I get time (not for a month
or few probably!)
Help yourself to a few ideas I was chewing over - 
Have an 'in lock' output to control a relay to cut the transformer
supply when the gap is out of lock. For safety reasons, this should
require a manual reset.
Use a photo-interrupter to feed the speed back to the controller, but
use cheap 1mm polymer optical fibre to avoid RF problems - the actual
detector would be in a nicely shielded box with the rest of the
electronics. PLLs would be  sensitive to the sort of crud a spark gap
would produce - not sure if optical 'noise' from the spark would be a
I'm not a PLL expert, but I'd guess you probably want two loops - a
coarse speed-lock to get the speed roughly right, e.g. during ramp-up,
and a phase lock which is active when the speed is nearly right.
It may be possible to simplify this with a simple delayed switch-over,
i.e. ramp to the right speed, then switch to phase mode. This ought to
work as once the gap is up to speed, the load should be pretty
constant, and there should be no need to compensate for load changes
as you would need to do on a conventional speed controller.

I was looking at doing it with a PIC or two, to avoid sensitive
analogue circuitry, allow easy tweaking of parameters, speed ramp-up,
selection of feedback encoder pulses/rev,  phase adjustment, breaks
per rev etc. etc.,. but an analogue solution should work. 
It may be useful to split the hardware into two sections - the PLL,
which produces a 'motor power' output, maybe as serial data, and the
motor power controller. This would make it easy to produce different
versions for DC and AC motors, as well as segregating the noisy power
signals from the PLL

I think the biggest problem will be compensating for the mechanical
inertial to keep the loop stable - maybe a PID type of algorithm would
be useful?
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