How do I calculate firing angle of single phase RL circuit?

Thread Starter


Joined May 19, 2009
Hi. How do I calculate the firing angle of single phase ac/ac full wave if i am only given resistive load 1.5 ohm, Vs 120v, 60 Hz and oP 7.5Kw? Thanks Guys


Joined Mar 6, 2009
There is no "simple" way of solving this but it can be done numerically.

If the power at firing angle 0 degree is Po and the power at firing angle α is Pα

then it can be shown that

Pα/Po=k=1-α/Pi + (1/(2*Pi))*sin(2α) [with α in radians]

Since you are given Pα=7500W and you can work out Po (thereby giving you the required value of k) you can then solve for the value of α using some numerical approximation technique (or graphical method) or a programmable calculator (if you have one) might have an equation solver function.