I can build this into my new programmable inductor module, for self-test. With a 1% cap, it should be plenty good. L range will be
750 mH down to maybe 10 mH, part of simulating solenoids and torque motors and such.If I add a switch to open R3 and run some current through R1, it can measure series resistance too.
What's R1 (1G to +5) for?
Are torque motors actually predominantly inductive? Just because it has coils doesn't necessarily mean it is so.
Jeroen Belleman
It can be switched in, and R2 switched out, to make an ohmmeter. Then the opamp is a DC amplifier into an ADC.
One case is used in some jet engines, for controlling fuel flow I think. 750 mH and maybe 200 ohms..
Unlike steppers, they seem to have no detent torque and I suspect L doesn't change much with angular position. Gotta machine a test setup and verify that.
If the brushes short windings sometimes, it could get weird.
At first sight, if they're like brushed DC motors, the equivalent circuit would be a series combination of an inductor, a resistor and a voltage source, function of rotational speed. There will be some torque ripple and commutator noise, but there's no telling how important that may be.
Jeroen Belleman
They behave about like 2-phase stepper motors. DC applied to the two windings sets an angular position, not a speed.
I did some tests on that maybe 10 years back. I can't remember the details, but I am pretty sure there is a significant change in the inductance vs position
Are there specs on that one? I didn't see anything like a manual.
They won’t have nice linear ferrite cores but nasty lumps of iron so measured inductance could depend on test frequency, AC amplitude, amount of DC flowing, rotor position, temperature etc etc etc. Precision is futile?
Why would you need brushes in a torque motor?
Not futile but probably difficult to achieve. Lots of variables to measure and correct for, and non-linear correction equations.
This is charmingly bizarre. There are 41 (prime number!) of commutator segments. If I pick one arbitrarily, and call it #1, and go clockwise with an ohmmeter, resistance goes up to about 8 ohms in steps and goes back down to about 1 ohm at commutator 17, 26, and 34.
This is an NOS motor from ebay. It's an older style with brushes and alnico magnets.
Imagine designing this 50 years ago without a computer.
afaict there's a video explaining all the fucntions and equations
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