This is accomplished by regulating the strength of the magnetic field within the generator. A separate generator control unit (GCU) or a voltage regulator is used for this task. Electrical connections of most starter generators are similar. There is either four or five terminal posts on the connection block and they are labeled A,B,D,E, and in some units C. In a short field-type starter generator or stand-alone generator, terminals B and E are large terminal posts with B connected to the aircraft bus and E connected to earth (aircraft ground).
Terminals A and D (the small ones) are used for control. Terminal C is used to supply a separate start winding in generators with five terminal posts. The GCU will supply a regulated voltage (frequently between 3 to 14 volts DC) to the generator terminal A. The result is a precisely-controlled magnetic field within the generator. Voltage on terminal A will have to be adjusted anytime engine speed changes or electrical load is altered.
With a constant electrical load on the generator by increasing the mechanical, excitation (increasing engine RPM) will increase the electrical output. So anytime speed is increased electrical excitation needs to be decreased. A similar situation exists when an engine is operating at a constant RPM and electrical circuits are energized or deenergized. By drawing more amperage from a generator, the electric potential or voltage would decrease. So as more systems are switched ON, the voltage on terminal A increases. Terminal D is an output from the generator to the GCU and is used to make GCU aware of the amperage flowing through the generator.
In the case of the starter/generators manufactured by Auxilec, the relationship of current flow to D wire voltage is 300 to 1. That is, if the generator is producing 300 amps, there will be about 1 volt DC at terminal D. If the generator is only delivering 150 amps the D lead will have about 0.5 volts. For a combined short field starter generator, this D lead voltage will also be used in the starting phase to control field weakening which regulates starting torque and speed. This is not used in devices utilizing a terminal C.
In the generating mode, the D wire is used to maintain load sharing in aircraft with multiple on-line generators. Should a generator control unit sense its generator with a lower D wire voltage than an adjoining generator, the result is that the GCU of the low generator boosts the A wire voltage until the D leads are operating at the same potential. Corroded or dirty connections on terminal D can lead to problems in parallel operation.
Aircraft using multiple generators to supply a common electric system often experience difficulties having generators maintain equal loads. Pilots sometimes add to this difficulty by describing a specific generator as weak. Should a small amount of corrosion exist on a D wire electrical connection, it will cause a small voltage drop. When this occurs, the GCU will see a somewhat lower input from the D lead and the GCU will respond by increasing field excitation. The result, at least from the cockpit, is the generator with the greater excitation will be delivering more amperage than its partner. Rather than having a problem with the "weak" generator the problem could very well be with the "healthy" unit. Before making any adjustments on a GCU to correct for load splits, it is worth while to inspect for signs of corrosion and even clean the electric connections.
Voltage fluctuations or spikes are a genuine concern. A frequent cause of this condition is brush bounce. This is where an abnormality in the commutator surface can cause the brush to momentarily open its electrical circuit. In this situation, arcing can occur creating further deformation to the commutator surface and rapid brush wear.
As the brushes wear, the tension on the retaining spring tends to decrease, which results in aggravation of the problem. Many generator manufacturers require inspection of brushes that require removal. The disturbance of the brush spring during this removal also has the potential of reducing spring tension. It is advisable to include a good visual check of the commutator during brush inspection.
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