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Over Voltage and Low Voltage – Inications, Cause, and Correction

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Aircraft Systems, Commercial Pilot, Flight Instructor, General Aviation, Instrument Rating, Private Pilot, Student Pilot

Hello,

I am trying to best understand the conditions that cause an Over Voltage and Low Voltage situation, as well what I should do about them when I receive such indications in-flight.

OVER VOLTAGE

Is an over-voltage indication (the over-voltage light, or the voltmeter showing very high) always accompanied by a high rate of charge on the ammeter? 

What is likely to cause an over-voltage situation? Is it just simply when the battery is fully charged, but the alternator doesn't stop charging it? 

If an over-voltage indication is given, what can I do about it? Am I just troubleshooting the voltage regulator? I know that re-setting the alternator circuit sometimes helps. If re-setting doesn't work, I suppose the alternator should be shut off (with the alt-fld switch, or maybe by pulling the alternator fuse?). 

Does the Over Voltage light in most small aircraft actually detect a high voltage? Or is it just simply detecting the absence of the alternator field (ie. the voltage regulator preventing an actual over-voltage situation)? In our club 172s, you can turn off the "Alt-Fld" side of the master to make the Over Voltage light illuminate. But what if the over voltage protection malfunctions and there is actually a high voltage present, will that light indicate that? 

LOW VOLTAGE

I'm fairly confident that a low voltage indication is usually accompanied by a discharge showing on the ammeter. Is this true? Are there situations where this is not true?

What is likely to cause a low voltage situation? Is it just when the alternator stops working? As in, when a belt breaks, or the electricity producing or regulating pieces malfunction?

If a low voltage situation occurs, what can I do about it? In the event of an alternator failure (belt maybe?), it just simply means I'm running off of the battery only, which will eventually die depending on load. So reduce load (turn things off) and land as soon as possible. 

FURTHER QUESTIONS

Is an ammeter a critical piece of equipment to diagnose these problems? Or can it be done all just simply with a voltmeter? 

Thanks for your help!

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1 Answers



  1. Skyfox on Nov 17, 2016

    Over voltage: Yes, an over voltage situation should be indicated by an excessively high rate of charge indicated on the ammeter. You may see it briefly after starting the engine as the alternator has to put the voltage back into the batter that was used to turn over the starter, but that should really not last very long unless the batter was very low. If you happen to notice it during cruise flight, that would be abnormal. An over voltage situation is most likely caused by the voltage regulator going bad. A bad regulator could cause over voltage, under voltage, or a wildly swinging voltage (shown by the ammeter acting like a metronome). You can try cycling the alternator side of the master switch off and back on in hopes of resetting the system and getting your voltage back to normal, but if that doesn’t work you’d have to switch off the alternator, continue on battery alone, and land as soon as you can. If you continue in an over voltage situation you will likely ruin your battery, possibly fry electronic components that don’t have adequate circuit protection, and I’d be concerned about wiring getting too hot and catching fire. I’m not sure how an over voltage light works, but it wouldn’t be an absence of the alternator field because that would cause an under voltage due to no charge being generated.

    Under voltage: An under voltage situation would be indicated by the ammeter showing a discharge. If you’re flight at night you’d notice the lights are a bit dim as well. An under voltage light would work the same as the alternator light in your car. In an under voltage situation you could try recycling the alternator half of the master switch off and on to hopefully regenerate the electrical field in the alternator and get things going again. A bad regulator could also cause an under voltage (although usually they go to over voltage when failing). If you don’t get it charging again, you’d want to land as soon as possible because the battery won’t last for very long. An under voltage could be due to the belt breaking, the components inside the alternator or voltage regulator breaking (ie. no electrical contact), a wire breaking, a blown fuse or popped circuit breaker, a fault master switch, etc.

    In either case, refer to the electrical problem procedures in the emergency section of your POH. And yes, an ammeter or a voltmeter could be used to monitor the charging system. An ammeter shows the flow of current and a voltmeter shows the voltage of the system and battery.

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