Voltage & Alternator Failure

[shdw]

Typical system: 28 volt/60 amp alternator with a 24 volt/35 amp battery.

If I remember right, the low voltage light illuminates when below 26.5 volts.

Question 1: Is 26.5 volts right for the system listed? If not, what are the variations?

Question 2: Would it be safe to then assume that if the alternator fails the low voltage light will always illuminate?

Question 3: It has been a while since I have flown, but some older 172s and 152s do not have low voltage lights, correct?

Question 4: If an aircraft only has a voltmeter and no low voltage light, is there any way to tell an alternator failure? Is this a configuration that even exists?


Thanks fellas.

 

[Roger, Roger]

Typical system: 28 volt/60 amp alternator with a 24 volt/35 amp battery.

If I remember right, the low voltage light illuminates when below 26.5 volts.

Question 1: Is 26.5 volts right for the system listed? If not, what are the variations?

Somewhere around there should be about right. It's often pretty inexact.

Question 2: Would it be safe to then assume that if the alternator fails the low voltage light will always illuminate?

No. Here are a few things that would cause that to not be the case:
1. Burnt out bulb in the undervoltage light.
2. Failure in the ACU's circuitry that is supposed to turn on the light.
3. Wiring to the light disconnected (in many of the older Cessnas, there are 2 quick disconnects [power and ground] that allow removal of the instrument panel without taking the light out of the panel.)

Question 3: It has been a while since I have flown, but some older 172s and 152s do not have low voltage lights, correct?

I'd have to look in some of the manuals at work. I know that the voltage light in some of the older Cessnas is called an "overvoltage light" but it illuminates when the alternator is turned off so...I'd need to read through how that system is supposed to work.

Question 4: If an aircraft only has a voltmeter and no low voltage light, is there any way to tell an alternator failure? Is this a configuration that even exists?

It is a condition that exists. The way to tell is that if the alternator is running, system voltage should be 28.x volts. If the alternator has failed, system voltage should be 24.x volts(for an aircraft with a 28 volt alternator) immediately after the failure, dropping pretty quickly into the 23.x range. As the battery continues to drain, voltage will drop. Once you get below about 20 volts (IIRC), the battery is pretty well dead. Your alternator is set at a higher voltage than the battery voltage (28 vs 24 in newer aircraft) because the higher voltage allows the alternator to charge the battery.

 

[wjmiller3]

Pretty much what Roger said, but the light in the later model Cessnas will not come on till battery voltage gets below 24 volts. So you will lose the ALT and not see the low volt light for several minutes if you have a good hot battery.

 

[shdw]

It is a condition that exists. The way to tell is that if the alternator is running, system voltage should be 28.x volts. If the alternator has failed, system voltage should be 24.x volts(for an aircraft with a 28 volt alternator) immediately after the failure, dropping pretty quickly into the 23.x range. As the battery continues to drain, voltage will drop. Once you get below about 20 volts (IIRC), the battery is pretty well dead. Your alternator is set at a higher voltage than the battery voltage (28 vs 24 in newer aircraft) because the higher voltage allows the alternator to charge the battery.

This brings on a new question. Mind you I have only been in one aircraft with a voltmeter (piper arrow) and that was about 2 years ago. From what I recall, turning on more equipment caused the meter to rise, almost as if it recognized the amps used, not the volts used. Any insight?

Thank you for the other information, cleared that right up.

 

[shdw]

May have answered my own question. I just found my arrow POH and read the electrical section. They define it as an ammeter that registers load on the system instead of charge/discharge.

Wouldn't that be a load meter though.

If that is the case than a voltmeter is what I always thought it was: a system that shows volts used.

To summarize:

Voltmeter: Displays volts on the system.
Ammeter: Shows charge/discharge of amps on the system.
Load meter: Shows amps used by the system.

 

[Roger, Roger]

May have answered my own question. I just found my arrow POH and read the electrical section. They define it as an ammeter that registers load on the system instead of charge/discharge.

Wouldn't that be a load meter though.

If that is the case than a voltmeter is what I always thought it was: a system that shows volts used.

To summarize:

Voltmeter: Displays volts on the system.
Ammeter: Shows charge/discharge of amps on the system.
Load meter: Shows amps used by the system.

How well do you know your DC electricity?

 

[shdw]

How well do you know your DC electricity?

It means direct current....

But I have google!

My electrical system knowledge is spotty. Some areas great and others not. So go ahead and explain whatever you intend on explaining and I will do research to try and understand it. I have no issues researching to expand knowledge, so bring it on.

 

[tlove482]

No. Here are a few things that would cause that to not be the case:
1. Burnt out bulb in the undervoltage light.
2. Failure in the ACU's circuitry that is supposed to turn on the light.
3. Wiring to the light disconnected (in many of the older Cessnas, there are 2 quick disconnects [power and ground] that allow removal of the instrument panel without taking the light out of the panel.)

4. If the battery is dead.

 

[trafficinsight]

A "zero center ammeter" as in the Cessnas and the loadmeter in the Pipers are essentially the same, one just doesn't read discharge. If the loadmeter is reading zero you can assume that the alternator is not working.

 

[ppragman]

Typical system: 28 volt/60 amp alternator with a 24 volt/35 amp battery.

If I remember right, the low voltage light illuminates when below 26.5 volts.

Question 1: Is 26.5 volts right for the system listed? If not, what are the variations?

Question 2: Would it be safe to then assume that if the alternator fails the low voltage light will always illuminate?

Question 3: It has been a while since I have flown, but some older 172s and 152s do not have low voltage lights, correct?

Question 4: If an aircraft only has a voltmeter and no low voltage light, is there any way to tell an alternator failure? Is this a configuration that even exists?


Thanks fellas.

Question 1:

Welllllll, in the 207 the low voltage light in the 207 comes online when the voltage drops below "normal" as the POH puts it. This is either the result of idling super low, or an overvoltage of 31.5 Volts or more which has been detected by the over voltage sensor and kicked the alternator offline. Kind of counter intuitive, I know, I don't remember at which voltage on the low side the light illuminates. Thats not really something you can do that much about anyway. Some of the cessnas call that light an "over voltage" light too, even though its the same thing, and means under volt.

Aside: I always think of the movie Apollo 13 when I think about this system, "Main BUS B undervolt, ACK! WERE VENTING SOMETHING!"

Question 2:

In the cessnas no, because if you look at your electrical diagram there's a line from the alternator control unit to the the gauge, then to the circuit breaker, so you could essentially 4 points of failure, the acu goes TU, the wire goes TU, the light gets burnt out, or the INST circuit breaker gets popped, but if the INST circuit breaker gets popped, you'll know it because you'll lose your fuel quantity, CHT, and Oil Temp gauges too.

That being said, having had numerous failures of the alternator in the 207, I can say with confidence that the light always kicked on, however I noticed that the ammeter was reading a discharge before I noticed the light. That should be part of your scan.

Question 3:

If I recall, yes, that is the case, I think the A model 172s lack the light, but I could be wrong.

Question 4:

You're talking about the load meter on the pipers right? Looking through the cherokee 6 maintenance manual now, I can see that even in here they call it an "ammeter" even though its technically a load meter. When I've had alternators kick offline in the cherokee (only happened once for me now) the load meter indicated zero load, even though I was probably drawing about half capacity. This is because either the alternator circuit breaker popped, or the overvoltage relay kicked the thing offline. Regardless, the design of the cherokee electrical system is set up so that the load meter isn't energized when the alternator is offline, so its not showing anything at all, and the needle rests at the stop all the way over to the left. If you want to know if the thing is fully off, or your just not drawing that much current, cycle the alternator switch and see if there's any change in the needle.

Cessna differs in their design philosophy, and has the ammeter wired so that whenever the battery is on, the ammeter receives current, and can measure it, if the current is less than the calibrated "zero" then you see a discharge, otherwise you show a charge.

Certainly if anything in there is wrong, a mechanic will correct me.

 

[shdw]

Aside: I always think of the movie Apollo 13 .

The only thing I think of from that movie is Budweiser banging the champagne glass.

As for the rest, thanks a bunch. You too traffic, nice to know a load meter (or whatever piper likes calling it) does show 0 in a failure.

 

[jynxyjoe]

Apollo 13, I always thought it was funny that it was a undervolt they saw but an overvolt that caused it.

A part designed by beech aircraft for a 28v system was put under a 65 volt system an blew the innards apart. Great job NASA.

Also, an interesting tibit that a lot of my students found helpful. In DO-160 and SAE-5414,16,18 you'll find the avionics of an aircraft must work from 31V to 18V DC. So if you are in a newer aircraft with the real volt meter, and you see it coming up around 18.5 and lower, you know you've got little time left.

Also, another tip, if you can shut down the system quickly after the alternator/generator failure, you can save the batt power for the next time you start it up so you can turn on the lights at the field or whatever you need from the avionics. The thing to avoid, is to let things run while you knock out the checklists, then suck down so much juice that when you go to start it later, the sudden draw of starting (current rising) drops the voltage down and the circuitry calls it quits. The problem is, you may still have 19.5 volts on the 30 amp hour battery, but once you kick the system on it draws so much current to initialize the system that you drop below that magic 18v and the system shuts off on you. It's weird for some to understand, but if you are around that 19V it is better to leave the system on sometimes, than shut it down and try to use the remaining bit of power later. Get her shutdown quickly if you can or return immediately for a landing if possible.