Synchronising AC signal generator frequency to motor RPM using encoder signal

I want to conduct an experiment where I need to apply an AC voltage having frequency synchronized to shaft rpm. The motor that drives the shaft has an encoder which I can use to measure the speed continuously and use as an input for the AC voltage frequency. I have a basic AC signal generator and tried to match it manually with the motor speed, but as the speed is never constant I observe small shifts in the speed and AC frequency and it never matches. I have NI-DAQ USB-61620 and I have already set it up to acquire the encoder Z index signal.

Any help or assistance to achieve this would be highly appreciated.

Thanks,

Ammad.