I have a blower out of an old furnace that I am turning into an exhaust fan for my finishing room. I have a yellow, black, white and purple plus 2 gray wires running to condensers.
The purple was taped off so that leaves me with black, yellow and white. White is neutral so the other 2 must be speed controls. I wired it to a on/off/on switch but each time I switch the power on the breaker goes. What simple thing am I missing?

Look at the end of the motor, many of them have a diagram to show connections for different speeds and also the primary connection.