In several installations the generator manufacturer has installed the brushes so they trail the commutator. In this case, if the generator drive shaft is rotated opposite the normal direction, brush damage can occur. Many operators elect to replace their own brushes in the field. This is usually not a problem as long as brush spring tensions are tested and adequate brush seating techniques are used.
One nonrecommended technique used for brush seating is after installation of two brush sets (180 degrees apart) in a starter generator to complete two successive ground power starts. After the second start, the brushes are seated. Unfortunately, implementing this technique results in excessive arcing between the brushes and the commutator during the first start. The reason the brushes are so well seated after the second start is the arcing causes the commutator surface to resemble a rough cut file. So much for extended brush life. Attention should always be paid to the manufacturer's recommendations prior to conducting any maintenance on a starter generator.
Aircraft using separate starters can incorporate brushless generators or rectified alternators. There are several distinct differences in the internal operation of these devices as compared with a unit using brushes. One primary difference is that a brushless unit will include a permanent magnet generator (PMG). Anytime the shaft of this device is rotating, the PMG has relative movement with a series of coils. This means that even if the cockpit switch is selected "off" there will still be excitation power available when the engine is in operation. Frequently, three phase AC power is supplied from this PMG to a generator control unit. Here the excitation power is converted to DC and is then metered into the main excitor winding within the generator case.
The magnetic field produced by this stationary excitor winding works in conjunction with (usually three) main power coils installed on the rotor. Output from each power coil is AC and is directed through a diode circuit to change the AC into a rippled DC. Feeder cables are used to connect this output into the aircraft electrical distribution system. The generator control unit will also monitor this output and adjust the regulation system according to voltage deficiencies or surges.
Numerous avionic/airframe system problems arise from anomalies in the power systems. Most autoflight systems, as well as electronic flight instrument systems (EFIS), are voltage sensitive. In the event of voltage drop below a specific threshold, the autopilot may disconnect or the EFIS could black out.
In one situation, an aircraft had gone in for a complete interior refurbishment and new avionics. Afterwards, on extended duration flights, the autopilot would periodically trip off but would always reset. Numerous technicians replaced various components in the autopilot circuit. Nothing solved the problem. Eventually, a technician flew with the aircraft on an overseas mission and happened to notice that when the flight attendant switched on the new oven in the galley, the autopilot tripped.
The technician was able to duplicate this condition time after time. The current draw associated with turning on the oven was causing a momentary voltage drop on the distribution system resulting in the autopilot trip. Relocating the oven power supply to a bus that was supplied by more than one generator solved the problem.
In other cases, the ripple produced by brush bounce or regulation malfunctions can cause various computers to see voltages that might be out of tolerance for a computer sensor. The computer then signals the flight crew that it has sensed a failure. Ground power units (GPU) are also not exempt from spiking electrical power systems. In many aircraft the battery(ies) can be brought on line with the GPU and serve as a filter. When using a questionable GPU on an electrically sensitive aircraft, connecting a lead acid battery in parallel with the GPU will provide some protection. It is a good idea to periodically test each GPU for excessive voltage ripple.
When troubleshooting electronic problems, even those associated with self diagnosing systems, consider the power source before replacing too many components.