Problem with stability of DC motor

[Barbarawi]

in my project , Single Phase Fully controlled rectifier with DC motor as a LOAD ..

the concept was to give a pulse to each 2-diagonal SCR in the H-Bridge .

the problem was for some 'Firing angle " , the motor goes to be Unstable 8-O 8-O !

why would the motor go in Instability region .. WHAT IS THE SOLUTION FOR THAT PROBLEM ?!

 

[FvM]

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.

 

[Barbarawi]

well , my problem was the DC motor is increase and decrease speed by itself .

also , if you want to know what the type of motor ! it was Separately-exited motor

more info. in the armature current once the current reach the maximum (1.07A) then it goes decreasing to almost Zero Amp.

here the figure below can describe the circuit design for my project

View attachment proj.jpg

 

[FvM]

I expect a wrong trigger scheme. The bridge is missing a free-wheeling diode.

 

[Barbarawi]

but everything in Simulation goes right .. just the Practical in Lab something went Unknown !!


to understand out project . you can see it here >> https://www.edaboard.com/threads/270615/

 

[chuckey]

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

 

[dick_freebird]

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.

 

[ALERTLINKS]

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.

 

[godfreyl]

feedback loop

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.

 

[ALERTLINKS]

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.

The other end of resistor and capacitor are grounded (common point) so it is effectively sensing voltage on the resistor.

 

[godfreyl]

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.

 

[ALERTLINKS]

If this is not for overload protection then it will not control speed to be constant but it will present a constant current control.