RL circuit: How to determine the equivalent resistance of the circuit?

So I'm working on an RL circuit and the solution is stated to be that R1 and R3 are in series, but I didn't think that was the case because they don't have the same current through them. Given solution says to add R1 + R3 in series and then use that equivalent resistance and add to R2 as if it were in parallel.

TL;DR: How do I find the equivalent resistance of the circuit below so that I can find the time constant for this RL circuit?

• In order to add R1+R3 in series you'll have to transform I1 into a voltage source. So I1 becomes say V1 with 600V and R1 is now in series with R3. See source transformation Commented Sep 28, 2018 at 0:33
• Thevenin's Theorem says this voltage can be zero. For DC resonance I1 can be omitted because it does not change the RL resonance value as it is not a 'frequency'. Add R1+R3 in series then add R2 in parallel. Now you have your RL values.
– user105652
Commented Sep 28, 2018 at 4:40