I have measured the voltage drop across the Triac and used the current to find the "ON" resistance but I want to be able to calculate it. I used the knee voltage chart but not sure I am calculating it correctly. My Case to Junction temp with my selected heat sink is about 140°C. My heat sink's thermal resistance is 15°C/W.

Due to size and cost I need to be able to accurately calculate the required heatsink. Once I find the power dissipated, calculating the heatsink is not a problem.

The current will be 13A and the voltage supplied to the motor 115V@50Hz.

The triac is a little fuse Q6016LH3 (Isolated Tab TO-220). http://www.littelfuse.com/~/media/electronics/datasheets/switching_thyristors/littelfuse_thyristor_qxx15xx_qxx16xhx_datasheet.pdf.pdf

Here is the circuit.

Any suggestion on calculating the heat dissipation or ways to improve the circuit would be appreciated.

David