Here is link with some diagrams of the Electrical Power Distribution and Control system.
http://klabs.org/DEI/Processor/shuttle/sp-504/section_4/section_4_10_electrical_power_distribution_control.htmAC power is distributed to motors through motor controllers called "Motor Control Assemblies". There are ten MCAs that feed power to 250 (!) AC motors in the orbiter.
EDIT: It turns out that the fuel cell contains its own built-in three-phase ac "motor controller" for the coolant pump. The problem must, as a result, be internal to the fuel cell.
Here is the description of the orbiter's AC system.
"Alternating Current Generation and Distribution
Each forward avionics bay contains three power static inverter modules, which are connected in a phase-locked array to produce 117/200-volt, 400-hertz, three-phase, y-connected, four-wire ac power. Direct current input power to drive the inverter arrays is furnished by the respective forward PCA's, which contain circuitry to limit in-rush current to an acceptable level when the highly capacitive inverters are activated. Output current is limited to 20 amperes by circuitry within the inverters. The inverters are synchronized by an internal oscillator.
The output of each three-phase inverter array is monitored and controlled by an inverter distribution and control assembly (IDCA). Relays within the IDCA's connect the inverter arrays to the respective three-phase buses. These relays can be controlled manually by the crew, remotely from the ground during checkout, or automatically by internal circuitry in the event of an overload or an overvoltage. This automatic disconnect feature may be inhibited by the crew during critical mission phases to avoid disconnects caused by spurious signals or transients. Normal C&W monitoring and alarms will continue to operate when in the "inhibit" mode.
Each of the three redundant three-phase ac buses is isolated, is capable of supplying nominal power of 2.25 kilovolt-amperes, and is grounded to structure in a single point. All current is confined to the bus wiring except for some navigation equipment which uses an internal chassis ground. No provisions have been made for cross-tying the ac buses to accommodate inverter failures. Power reliability for critical ac loads is obtained either by providing a switch which allows access to more than one bus or by providing duplicate hardware operating off separate buses.
Ten motor control assemblies (MCA's), three each in the fore and aft avionics bays, and four in the midbody area, provide power and control to motors and other three-phase and single-phase loads in the Orbiter. Approximately 250 three-phase motors are required to drive deployment/retract mechanisms, latches, actuators, motorized valves, positioning devices, etc. Remote switching capability is provided by three-phase hybrid relays, which can be controlled by MDM commands."
- Ed Kyle