Heat Pumps and Electric Heating - Which motor wire is common

I am trying to figure out which motor wire to loop an A50 current sensor around to operate a new Aprilaire model 400 humidifier. Everything that I have read says the "white" or "common" motor wire. The heat pump is a 10 year old Trane XE1200 model #TWV036B140A1 and there is no white wire going or coming from the motor, and the wiring diagram has faded to the point that it is unreadable. The only wires I can see are a red and blue that come from the run capacitor and go to the speed terminals and a black (so it looks) wire that comes from the capacitor and goes to the motor along with a yellow. Can someone tell me which is common and should the sensor go before or after the capacitor. I would love to get a new wiring diagram if someone can put me in the right direction. I have found a diagram for an XE1200 at http://www.aireng.com/clientuploads/Coils_etc/ycp.pdf, but the model #'s do not match. Thanks.
Ray

It appears you have 2 problems going on. First, you are probably trying to hook up the humidifier to an electric furnace(air handler for your heat pump). The blower wiring is typically 220v not 110v so you will not have a common wire that you are looking for. The other problem is that the humidifier type you are installing, will not properly work on a low temp air heat pump system unless you are always using you aux heat. The proper unit would be a steam type humidifier . The unit that I use in these applications is a Skuttle model 60,
http://www.skuttle.com/601.html

Hope this helps