Hi, I want to connect an auxiliary 115V in-duct fan to my airhandler motor so that the in-duct fan comes on when airhandler motor comes on.
There is a red, purple and black wire going to the airhandler motor, and I can tap the in-duct wiring into any of them, but all three show 115V to ground even when the evaporator motor is not running. As a result, the in-duct motor is always on which I don't want.
Q,: Given that all three wires show 110V to ground, what makes the evaporator motor kick in when the thermostat triggers?
Thank you

Isn't your air handler 230 volts? You probably have two hot wires suppling power to the blower motor and your just seeing voltage through the winding on the other one. You shouldn't connect the 115 volt motor to one hot and the equipment ground. If you need to use the 115 volt motor find a second source that has a hot conductor, a neutral, and a ground. You should be able to purchase a separate 24 volt relay that you can hook up to the 24 volt G & C connections in your air handler. If your air handler is 115 volt, you should connect one line to the neutral and the other one to the line that's connected to the blower relay.

I agree with jack. But you need to make sure that you don't need a switch to adjust the speed on the blower, it may be too powerful and cause problems. Are you wanting to run this blower all the time when the blower in the airhandler is running or just for A/C?