Shaded pole motor speed control
You first have to understand the speed of any drive is the result of the torque of the motor and the countertorque of the mechanism you want to drive. This is called the operating point. Changing the speed means moving the operating point.



These are (roughly) the hilly torque/speed characteristic of an asynchronous machine (as a shaded pole motor is) and a pump/fan.


If you placed an obstacle in the air channel of the fan, you made its characteristic steeper, so the speed of the whole drive would reduce about a tiny bit. I give this example to show you how speed control works.
If you reduced the voltage, the motor characteristic shrinks along the torque axis. Again, the speed of the whole drive would reduce only a tiny bit. If you reduced the voltage too much, the motor hadn't got enough torque to drive the fan. It never reaches the part of its characteristic right of the hill.
If you reduced the frequency, the motor characteristic shrinks along the speed axis. That's a great way to control the speed of the drive, but to avoid magnetic saturation of the iron parts, it had to be accompanied by a proportional voltage reduction for a given motor.
If you placed a resistor in row with the rotor windings of the motor, you made it's characteristic tilt left. Now voltage reduction will reduce the speed of the drive much more. That's what's done in motors designed for that kind of control, the ones usually used for fans, vacuum cleaners and so on. This is the so called squirrel-cage rotor. If your motor has such a rotor, reducing the voltage is the best way to control it.