I'm doing practice questions for an exam on electric machines and I'm doing a question on DC series motors. Instead of just putting the question here to get solved I'd really just like to know the correct way about using the equivalent circuit model. In DC shunt motors we could assume that field flux remained constant as long as the field coil current remained constant (this makes sense). Then we could use E = kphiω and T = kphi * armature current to evaluate the circuit. However, when the field coil is in series the flux is going to change at different loads/speeds as the induced motor voltage increases, thus changing the field current. What kind of information am I allowed to assume and when is basically my question? I apologize if this makes no sense...because it definitely doesn't to me right now.