Dash 8-300
Aircraft Equipment Problem Critical; Flight Deck / Cabin / Aircraft Event Smoke / Fire / Fumes / Odor
General Evacuated; General Flight Cancelled / Delayed; General Maintenance Action
Aircraft
With passengers [boarding] the aircraft; I got a signal from one of the ramp crew members outside as well as a Gate Agent came on board and stated that I needed to come outside immediately because of smoke coming from the Number 2 engine. I quickly exited the aircraft and upon seeing the right hand side of the Number 2 engine I could see smoke coming from the DC starter/generator air exhaust port. I immediately went back inside the aircraft and shut off the APU; battery switches and took the airplane off GPU power. I instructed the gate agent to deplane immediately and went outside to analyze the condition of the starter/generator with power to the airplane turned off. Since there was still smoke coming from the exhaust port; I instructed the ramp crew to call ARFF just in case. I then assisted the Flight Attendant and Gate Agent in deplaning the aircraft through the main cabin door. Shortly thereafter; ARFF; as well as a on field Mechanic showed up to the airplane and assisted with the issue.  After a few minutes; the smoke subsided and the Mechanic confirmed it was the starter/generator that was causing the smoke. ARFF was dismissed and the aircraft was written up accordingly. On a side note; this exact same scenario happened about 3 years ago while I was a First Officer on a Q200. Somehow; the DC starter/generator will short out while on ground power causing the starter to remain engaged and subsequently burn itself out. That was also on the Number 2 engine side.
The reporter stated that Maintenance found the electrical components on the top of the generator to be melted; but the exact cause of the short circuit remains unknown.
When accepting the aircraft from previous flight crew we noticed the odor of smoke and my First Officer discovered the Number 2 DC generator vent had smoke coming from it.  I proceeded to shut the aircraft down while the incoming Captain called for the fire trucks to come to aircraft. By the time of their arrival the smoke had mostly stopped; we had them open the cowling and check to see that nothing else was on fire; we determined it was sufficiently out and had Maintenance proceed with the inspection. It turned out the starter/generator was stuck in the starter mode caused it to overheat and burn up.  We subsequently canceled the flight waiting for Maintenance to get us a new generator.
DHC8-300 inbound and outbound Captains report smoke was coming from the starter/generator air exhaust port. Power is quickly removed from the aircraft and the passengers evacuated through the main cabin door.