The most RPM you can get from 60 Hz with a motor is 3,600 RPM. The next lowest is 1,800RPM. 3,150 RPM someone posted means 2 pole motor with a 12.5% slip at the rated load.
Induction motors are not inherently adjustable speed without changing frequency. You're changing transformer taps when you select different speeds on multi-speed motors like box fans and air handler motors. It slows down by increasing the slip between the rotor and the field. Power savings involving fans is also secondary. A fan that takes 1 hp to turn at 3450 RPM only uses 0.875 hp at 3350 RPM even though the RPM is only 4.6% lower. The significant saving in fans come from slowing them down while keeping in control.
As you slowly lower the voltage on a partially loaded standard motor, you'll hit a point where the RPM begins to plummet. Just ahead of here is where you get the best efficiency. It's not easy to use passively because you'll lose starting torque and maximum load at reduced voltage. Some VFDs include this kind of active control. If load is placed on the motor, it has to be able to quickly bring the voltage up to prevent stalling out.
A desktop PC is designed to keep within safe temperature with full CPU load across the entire rated ambient conditions. Very old computers ran fans at full speed. Modern systems use variable speed fan which basically never reach full speed unless you're running them at full load in a 90F room. This saves like 10-15W per computer which is significant given the quantity of them in corporate offices and the duration. They're doing something similar with condensing unit fans.
