Feedback Speed Controls for RSGs, etc.

Marco and other listees,

Many years ago I built a simple feedback speed control for an electric
drill.  It has worked well and I still use it. I don't think it will help
stabilize RSG speed much better than an open loop lamp dimmer does.  The
feedback increases duty cycle of power to a universal motor as the load
increases, maintaining fairly constant RPM.  But it does not apply braking
power to the motor when the load is reduced, so the motor just "coasts"
down in RPM.  With a big flywheel as in a RSG, you might save a bit of time
accelerating the motor up to speed, but not slowing it down.

Anyway, for what it's worth, I will try my hand at ASCII art with a circuit
for you.  It uses an SCR, not a triac. (Courier font).

   AC in O--------|---------------------------|         PARTS:
                  R1                          |         R1  3.3K  2 watt
           -------|                          \/  SCR    R2  1.0K  2 watt
           |      >                        ------       R3  150K  1/2 watt
          +|      <                         / |         C1  10 uF  75 volt
           C1  R2 ><---------|>|--|--R3--|-/  |         C2  .1 uF  100 volt
          -|      <           D2  |      |    |         D1,2 1 amp 200+
           |------|               |--C2--|    L         SCR  15 amp 200+
                  \/                          O 
                  -- D1                       A
                  |                           D  
   AC in O--------|---------------------------| 

Parts values aren't critical.  Enjoy.

--Steve Young  
> Because there is no feedback between the motor and the dimmer, if you
> change the speed you have to wait a couple of seconds before it
> to the new value. Speed controls used e.g. in electric drills DO HAVE
> feedback and I tried to get a schematic of one of those, with no result.
> That would be indeed the best solution.
> Again, never had any problem with EM or other disturbances affecting my
> "RSG dimmer".