"DoN. Nichols" wrote in message
...
On 2010-06-14, David Billington wrote:
Michael A. Terrell wrote:
Wes wrote:

[ ... ]

Interesting. The 4 brushes seem like overkill for a tach. I wonder
why a two brush
design wasn't good enough. EE's, got an explanation?



Lower contact noise from the commutator. This allows for a higher
degree of accuracy in the control loop.



Some years back a mate of mine mentioned he was involved in an
instrumentation project on a diesel engine in a bus or truck and they
used a synchronous motor, as found in mains timers etc, for the
tachometer. IIRC the AC generated was rectified but I can't recall what
they did with it after that, maybe a frequency to voltage converter. Do
you have any ideas as to the benefits or disadvantages of this sort of
tachometer. I can see that there would be no contacts so possibly
reduced noise but not sure what effect methods of converting the AC to a
DC voltage proportional to RPM would have in the system. I have put a
synchronous motor onto a scope and spun it to generate a AC waveform but
not tried it beyond that.

The DC tach provides pretty much a constant voltage for a given
speed -- even at a near standstill. I would expect the frequency of the
generated AC to be so low that even rectified and filtered, you would
get variation in the output voltage for a full rotation with a constant
speed.

And with rectified and filtered AC output, you would have much
slower response to variations in actual motor speed, so the ability to
control the speed would suffer.

I believe that the AC servo motors have a pair of Hall effect
sensors (at 90 degree spacing) and a rotating permanent magnet, and
analyze the two signals to determine the actual physical position, and
from rate of change in that determine the speed. A lot more electronics
to do the same thing -- but the advantage is that there are no brushes
to wear out.

Enjoy,
DoN.


typically not an AC generator but a selsyn for this application (did I
spell that right? special variant of a synchro resolver)