Is this a tachometer?

brushes. That is

Wes, yes, I am 100% certain that it is a tachometer. 9.5 volts per

brushes. That is

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?

Wes

Yup, the big problem with DC tachs is keeping the brushes in good contact with the commutator. My guess is they use an odd number of slots/windings/commutator segments. With only two brushes, you'd still have one brush jumping across commutator segments every small amount of rotation. With 4 brushes, paired with their opposite brush as you do with a 4-pole DC motor, then there is always one of the pair making contact with the commutator. So, this should greatly reduce brush noise and produce a much cleaner tach signal. Some small tachs use a silver-plated commutator and sliver-wire brushes. These have limited life, however, like the Maxon motor/tachs.

Jon

brushes. That is

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

brushes. That is

My guess is a slight overlap of switching time between two sets of brushes so one switches while the other makes full contact to minimize glitches in the feedback signal -- which could cause glitches in the speed of the motor, and mar the finish of the workpiece.

Enjoy, DoN.

brushes. That is

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.

brushes. That is

You could use either a frequency to voltage converter, or a simple counter circuit. The F/V converter is a simple, single chip design, and cheap. The counter gives the option for accurate speed warnings or overspeed shutdown. Cost is probably five times the F/V converter and a meter movement.

That makes a lot of sense to me. Thank you for the explanation.

Wes

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)