CRJ-200 Captain reported while flying a visual approach they experienced AC power loss resulting in landing with emergency power only.

Narrative

FO flying; CA monitoring on a VFR day into ZZZ. We were on a downwind leg for a visual approach; 190 knots at 5;000 ft. The first officer called for flaps 8 to begin configuring for the approach. I started the APU; then selected flaps 8. Once selected; we lost all AC power and the ADG (Air Driven Generator) deployed automatically. We received the EMER PWR ONLY EICAS warning message and did not have power for about 5 seconds. Then the APU generator came online and restored AC power. I directed my first officer to fly the vector ATC gave us and to maintain 200 knots. We [requested priority handling] with ZZZ approach. I ran the QRC/QRH for EMER PWR ONLY. I was able to restore power to both generators and transferred power away from the ADG. Once complete; I ran the QRH for subsequent cation messages MACH TRIM; STAB TRIM; and L WNDSHLD HEAT. I updated the flight attendant and passengers and let them know it would be a normal landing. We notified ZZZ approach we were ready to proceed inbound and landed without incident. ARFF (Airport Rescue and Firefighting) checked the airplane and I made a PA explaining their presence to the passengers. Once we arrived at the gate; we debriefed and I wrote up the discrepancy in the maintenance log.Cause: The cause was a mechanical failure of the AC power system. The ADG worked as it should.