Ac Motor control

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Hi, I need an ac motor to put it to work on a clay mixer. Rotational speed of the blender must be adjustable between 1 and 24 rpm. Using a gearbox I can reduce 1420 rpm of a single phase 1.5 hp motor to 24 rpm but to reduce it up to 1 rpm I have to reduce motor speed without decrease of output power and torque of the gearbox. I have seen a video of a mixer in witch the operator just turns a switch to the right and left and easily that changes rotational speed of motor. any idea how can I do that? I want to use a cheep,user friendly and simple system. There is a circuit on this web page what do you think of this circuit?
http://www.electronica.mk/all_articles/Miscellaneous/2012/Variable_Speed_AC_Motor_U2008B/U2008B.html

thanks