Re: Motor speed control help
This form of speed control, can be finicky. Your trigger
circuit is playing against the line (60Hz) phase. The
SCR (or TRIAC) trigger event -must- be applied at an
angle prior to the zero-crossing because the zero-
crossing is the only way these things shut off. Applying
before the peak can also be bad. You would like the
range of control to be from peak to zero and limit the
travel.
You may want to change the 0.1uf cap to 0.15, to
get to a time constant more like the original design.
The DIAC breakover voltage might be too large a
portion of the available line, to get you a good
trigger event. This may be part of why you need so
much potentiometer to start it.
Do you have a 2-channel 'scope that can show you
how the trigger phase and input AC waveform line up?
R1*C1 should be setting your max conduction angle
(min delay) and (R1+R2_full)*C1 your min angle
(max delay, equals next-zero-crossing or 8.3mS).
I haven't touched SCR motor controls since the '80s
(early '80s), and don't remember much more than
this. Or maybe, not even.