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Subject: Re: OFF TOPIC: Ohm's Law Correction
From: Brandon Jones
Reply To: mapug@shore.net
Date: Wed Dec 06 02:37:40 1995

Yes ohms law does not really apply in this circuit.
When the motor is stopped and the power is switched on then the
current flowing at that instant is limited by the dc resistance of
the windings.As the motor begins to turn it becomes a generator
that generates a back emf that tries to negate the supply source
so the current drops off very rapidly.As there is always some load
ie friction it never quite achieves this and a proportional current
is drawn.If you now load the shaft the motor slows and so less
back emf so more current drawn so more power into the motor.
This helps to speed up the motor again until a balance is achieved.
However it is not stable enough for a motor to track a telescope.
What Meade do in tracking mode is to put a low DC voltage across
the motor then if the motor slows down a short positive pulse of a
a few tens of milliseconds is added to the fixed DC voltage.
What this does is to give the motor a very quick injection of power
to speed up .The computer is speed checking the motor at a rapid
sampling rate.If the opposite happens then a negative pulse is
applied to the fixed dc level so doing the opposite.If the drive is
going over a sticky part of the gearbox or worm then a more pulses
will be applied.(It is the ticking noise that the drive motors make
from the pulses).The same happens if the scope is loaded ie more
pulses more power.If you go up a drive level to CNTR then the DC level is
increased by a fixed amount the motor goes faster pulsing still
applies.If you go up again then higher dc level only no
pulses,finally on slew the motor is flat out.
The motors chosen have a very good inherent slow speed ability
and will work from less than a volt up to 15 volts or more.
It is the gearbox that keeps the power level necessary in the motors
down to a very low level when tracking so only a minute injection
is necessary to keep the motors to speed.

Ok I will roll my skin back over my head now and go back to sleep

Brandon S. Jones