Control Speed of 3 phase induction motor
There are commercial units that will do your speed control task very well. They allow you to increase and decrease speed of the motor. Typically they are VFD (variable frequency drives) that can alter both voltage and frequency. Here are some great ones I've seen used in production environments, though not cheap. 

If you have the skills you can hack together speed control of PFC motors, but only within a small range. If you are looking to drop the speed by only 10% or so then that may be possible using a Triac speed controller or a Zero Crossing SSR, but it's not for the feint of heart to implement. 
You can read this to get clues on how to implement.

The other technique I have seen used to lower the speed of PSC's is to do cycle dropping with an SSR. If you drop 1 cycle in 20, then 2 cycles in 20 and so on, your speed will drop. Range is typically limited to about -20%. You must be careful that the speed does not drop to the point where the Start capacitor switches back into circuit (if you have one).