Thursday, November 19, 2009

Alternator Load Dump by the pilot switching off the alternator circuit breaker

An interesting article by Femi G. Ibitayo of Zeftronics on how the pilot can damage the aircraft charging system by switching the alternator circuit breaker on and off in flight.

A load dump condition caused by the sudden removal of the battery from a rotating alternator may cause these types of damage. In a load dump condition the when the battery goes off-line, the output of a 12V alternator can suddenly increase to 30V or higher, especially when there is no ground on the voltage regulator or alternator controller.

The suddenly or intermittent opening of the battery relay may lead to a load dump condition. The freewheeling diode put across the coil of the battery relates in the aircraft serves as an inductive kickback suppressor or protection device. In this role, it also helps reduce relay contact chatter. Losing this diode may negatively impact the operation and the life of the battery relay. A negative going pulse of a magnitude higher than its forward current rating can damage the diode.

Load dump conditions can happen if the main alternator circuit breaker opens and closes suddenly while the alternator controller is still supplying current to the alternator’s field. Sometimes pilots will pull and push back in the alternator’s output circuit breaker at cruise speed. This is the point the alternator is capable of supplying very high and destructive voltage and current output. Pulling the breaker under this condition leaves the alternator’s stator with all the energy it produced pent up. Pushing in a breaker under this condition connects alternator to the bus and introduces the uncontrolled pent up energy on the system bus as very high voltage and current pulses. These pulses seek the best/easiest current paths and can damage electrical loads in the system. For this reason, we recommend to the pilots and mechanics that we deal with that they avoid pulling any main alternator circuit breaker while the alternator is rotating. The energy experienced in this situation is similar to what happens in a spark plug. To turn off a rotating alternator that has power to its field the best and safest practice is to turn off the alternator field switch first and then if necessary pull the main breaker.    read more...

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