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Here is a simple circuit I use for a pillar drill. It suffers from speed variation due to mains fluctuations. And is only suited for AC/DC (eg brush) motors, and must not be used on AC Induction motors!

                       D1        D2
Live ---Motor----------!>!------!>!-------------
        with      !         !                  !
    Suppression   !        ---                100K
                  !   SCR  \ /        Diac     !
          Full   \         ---        !>!      !
          Power   !         ! \--220R-! !------!
                  !         !         !

My drill motor was only 500Watt so...

D1 is 1N5403 (50A peak), D2 is 1N4003
C is a 0.1uF (63v)
DIAC any 20v device (higher voltage = less control at low RPM.)
220R is optional depending on the sensitivity of the SCR.
SCR is a 3A 400V device (50A peak) 
it can be a Triac as D1 inhibits wrong way conduction.
 (mine used to blow up until I added the diode.)
Switch shorts out controller for full Power.
Multi turn low speed control is a preset pot in my design.

This circuit attempts to put DC out to the motor 50 times a second, every other half cycle (half power). How much DC depends on the preset setting & the motor back emf. Hence the power/torque has some feedback feature unlike just phase control, & enables variable torque loads to be handled much better, without dramatic RPM changes.



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This design was originally posted on Packet radio by John G8MNY - Jan 2001.