electronic switching of 120v AC line to one of 4 taps.

[BlackICE]

I want to use a micro-controller to switch the speed of a blower fan that using a split phase motor with 4 taps for 4 different RPM settings. So I need to connect one side of the AC power line to 1 of the selected taps. Current load is rated at 7 amps. Peak is probably higher but not specified.

Using 2 spst relays in series works for 3 speeds. Probably using 2 dpdt can do 4 speeds, but I have to give it some thought. 4 triacs with opto isolators.   Any other ideas?

This will be used to control a typical blower inside of a HVAC system. Does anyone know the how the power consumption changes with RPM for this type of motor. The desire to control the speed is for lowering the noise and power used when maximum speed isn't needed.

[Ian.M]

Consider using pulse transformers for isolation as multiple TRIACs firing would be disastrous, and if the input side of a pulse transformer is shorted (by its half-bridge driver) its output side will be low impedance, reducing the risk of false triggering.  The TRIACs will need a generous voltage rating and probably RC snubbers across them, again to reduce the risk of false triggering.

[ledtester]

What's the motivation for using triacs over the relays?

If you decide to use triacs, I would first look into using commercially available solid-state relays -- which are essentially an optocoupler and triac.

[BlackICE]

I guess I will stick to relays. SSR cost more and need heat sinks. Also read they can fail closed which would be bad.