Flight Crew Commercial; Flight Crew Instrument; Flight Crew Flight Instructor
Aircraft Equipment Problem Critical
Air Traffic Control Provided Assistance; Flight Crew Became Reoriented; Flight Crew Returned To Departure Airport; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Overcame Equipment Problem; General Flight Cancelled / Delayed; General Maintenance Action
Aircraft
ALTR FAIL master warning occurred for approximately 1 second during the downwind vector setting up for the ILS XX into ZZZ. Approximately 10 minutes later while approaching minimums on the ILS XX ZZZ; the ALTR FAIL master warning came on continuously. I instructed the student to go-around and fly the missed approach procedure as normal. After a positive rate of climb was established; I silenced the warning and started working on the checklist. The alternator amps were positive when I checked them. The checklist failed to bring the alternator back online without a master warning activation; so I deactivated the alternator and completed the recommended load shedding procedures with the exception of the lighting bus due to being in close proximity of ZZZ1. BATT AMPS was -7. We flew the ILS XYL into ZZZ1 due to poor inflight visibility. The flight was completed without incident; [company] was notified; and aircraft was written up.Reviewing the POH; my best guess as to the cause of the alternator failure is an alternator voltage between 30.5 and 32. The alternator was still providing power with a positive amperage indication while the master warning was active; which leads me to believe that the over-voltage relay was not tripped. Something else caused the master warning to activate. There was no burning smell or other indication of trouble.
GA instructor pilot reported alternator failure and elected to return to the departure airport.