There's no unequivocal meaning of the term "stability" in this relation, I fear. Also "DC motor" doesn't clearly designate a motor type.
P.S.: Reviewing your previous posts, I get the impression that your problem isn't specific to SCR bridge with motor load. Did you succeed to operate a SCR control circuit in the meantime? In any case, you should clearly describe what you are doing now and which problems you observe.
The motor requires a capacitor across it to suppress the switching transients as the brushes cross commutator slots as well as four diodes clamping both ends of it to within the Vcc lines. I would try a .01 disc cap.
Frank
In the distant past I designed some SCR controllers and
one issue can be the torque (current) compensation in
a constant-speed controller - too much current gain (to
compensate I*R) and you'll get runaway or at least give
the speed loop another thing to fight. Check this by
seeing if the RPM (not measured as armature voltage,
but directly) increases rather than decreasing slightly
with applied braking torque.
A simulation with wrong motor params could easily give
you a happy result that is not borne out on the bench.
47uF is a very high value. Try with 0.47 or 1 uF. Do multiple sampling and averaging in code.
It is about feedback loop response time. You can also restrict PWM rise and fall for motor speed control more slowly and gradually.
I don't think the feedback works as intended. Look where the top of that 10K resistor (RC) connects to in the main schematic. What is is supposed to be sensing?
Perhaps the 1Ω resistor (REXT) is supposed to be a current-sense resistor but the circuit is not monitoring the voltage across that resistor. Instead it is monitoring the voltage between one end of that resistor and ground.
Perhaps the 1Ω resistor (REXT) is supposed to be a current-sense resistor but the circuit is not monitoring the voltage across that resistor. Instead it is monitoring the voltage between one end of that resistor and ground.
Oops, I missed the ground connection to the resistor.
Now I'm wondering what the power source is. T2 and T3 short V1 directly to ground. It's fine for a floating supply as shown, but if it's connected to mains without an isolation transformer, it will short live to earth.