New Generation Voltage Regulators
By Electrosystem's Technical Service Group
General aviation aircraft electrical systems may be one of the least understood aircraft subsystems when it comes to maintenance and troubleshooting. A sound understanding of system functional characteristics combined with application of the latest design alternator and voltage control units can significantly compliment diagnostic skills and reduce downtime. The following provides an overview of both the functional and the diagnostic advantages offered by the latest design voltage control units, with an emphasis on system troubleshooting and problem resolution.
High frequency voltage control units
One of the more classic complaints concerning the early solid-state variable frequency, "bang-bang" voltage regulator designs is loosely referred to as the "nervous needle" syndrome. Nervous needle describes an ammeter or load meter whose indicator needle seems to vibrate making it difficult to read the current. Another similar complaint is flickering panel lights.
To remedy the problem, a new generation, high frequency regulator should be installed. Because the frequency of these new regulators (1,000 Hz) is well beyond the normal operating frequency of the alternator, the alternator doesn't follow the individual correction pulses, thereby, providing smoother alternator operation. Once the high frequency regulator is installed, and the "jittery" output current has been removed from the alternator and the battery, belt, and battery service life should be greatly improved.
If a high frequency regulator exhibits a rapidly swinging ammeter needle (sometimes referred to as float), the condition may be due to a high resistance contact (loose connections, corrosion) in the Alternator Control Switch or within the circuit breaker, which is in series with this switch. Short them out one at a time and see if the situation improves. As always, a good practice is to keep a voltmeter attached to the bus at all times until the system is repaired and returned to service.
Ground fault protection
Another nuisance problem is when a regulator becomes disabled by either a "flying short" (flying shorts are caused by centrifugal force) in the alternator rotor or from a regulator field lead being faulted to ground during installation. In other cases, this field may short to ground from vibration or insulation failure.
When this happens, the power output transistor in the solid-state voltage regulators would probably be damaged beyond repair from the direct short to ground. The failure mode of the early design regulators is to have the main power transistor suffer a "melt down" and the traces on the PC board burned open. New voltage regulators are now designed with Ground Fault Protection circuitry (GFP) and an annunciator LED mounted on the side of the case to indicate a "ground fault." This provides the maintenance technician with a tool to diagnose this type of failure and immediately identify a ground fault.
If a ground fault takes place either within the body of the alternator or in the aircraft wiring of a plane equipped with a new generation regulator incorporating GFP, the voltage regulator will not be damaged and the GFP circuitry will take the electrical system off-line instantaneously, precluding aircraft wiring or avionics damage. If the ground fault is intermittent in nature, cycling the alternator switch or master switch each time the fault occurs can restore normal operation of the electrical system.
This is not a permanent solution to the problem. Analysis should be conducted to locate the source of the ground fault and a permanent fix to the problem should be implemented.
The ground fault diagnostic procedure suggests that the system be shut down. The field wire should then be temporarily removed from the alternator field terminal and insulated. Restart the engine and run at the highest speed practical on the ground.
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