How to control a blower using the analog output from a PID controller?
Induction motors are called "asynchronous" because they don't go an exact speed based on the ac frequency, not because they can be easily speed controlled using voltage/current limiting instead.


For a thorough explanation of the motor type, the Wikipedia Article is a great place to start.  
In general terms, induction motors can run with fair efficiency/control between 90-100% of "synchronous speed" (at 50hz, that's 3000rpm divided by the number of coil 'poles' in your motor, which is likely 1), and act as generators between 100-110% of "synchronous speed."  


The further from 100% of synchronous speed the motor is turning/being turned, the more current it will draw/generate (without external current limiting) to produce torque towards recovering to its synchronous speed.



Thus, for best speed control of an induction motor, you'll need some form of VFD. Luckily, however, being single-pole &amp; only 100W, you could use a rectifier+capacitor to store the power you'll need, then a timing circuit &amp; "Logic-Level Gate" Power MOSFET to generate the needed frequency quite easily (could use as few as 4 total components added to your circuit).