effect on the voltage of reducing RPM of a motor?

Probably 0 volts because to reduce the RPM you would have to load the fan down pretty well. You would then burn up the motor and pop the breaker. Hence 0 volts.

Agree with opal and phil up there. 120V is a AC motor. RPM is constant regardless of voltage. If you want the fan to spin slower, you may need some sort of mechanical reducer: gears or belt & pulley. Or variable frequency drive: changing frequency (i.e. 60Hz) can change RPM.

If it was a DC motor (12V, 24V) then you can reduce RPM by reducing the voltage applied.

Or are you trying to reduce speed by outside force and see how the voltage responds? Then likely it would be answers above: amp load goes up and motor may fail sooner.

If the voltage applied is 120 it does not matter how you slow the fan down the voltage will still be 120.

All you will do if you slow down the fan is that the current will increase as there will be a decrease in back emf and the fan will burn out. {as most moters will if they are slowed down]

Be careful, as some motors cannot be controlled by voltage change, they'll just burn up or blow breakers.