Alternator cuts out over 1500 rpm ???

[N177MC]

This is a '65 M20E that I installed a Plane Power alternator and solid-state regulator under the STC. 

All was well for at least 6 months but now the alternator goes off line as soon as the RPM goes over 1500 rpm, or so, then comes back online as soon as the revs drop back down less than1500 .

I checked the VR and all looks well.  Then I made a small adjustment to the VR's pot to decrease the voltage, now the alternator goes off-line >2,000 RPM  and comes back when the revs drop back under 2,000 rpm.

There is no sign of an over-voltage relay on this plane nor in the electrical schema for this year model.

Anyone have any idea what would cause this ?

[Rmfriday]

need more details, is the field braker tripping? is there a over voltage event taking place?

done any trouble shooting yet? 

The fact that it is working and then "failing" at 1500 rpms leads me to believe it is a overvoltage event that is causing the issue but someone much better at this then me will chime in.

either way i would find the Power Plane trouble shooting guide and follow it to a T

good luck and keep us updated on what you find

[N177MC]

is the field braker tripping?  NO

is there a over voltage event taking place? THERE IS NO OVER-VOLTAGE RELAY

done any trouble shooting yet?  JUST ADJUSTED THE VR VOLTAGE AS DESCRIBED

either way i would find the Power Plane trouble shooting guide and follow it to a T    THE PP TT GUIDE DOES NOT COVER THIESE SYMPTOMES

[N201MKTurbo]

Have you measured the voltage at the VR when this happens? Most over voltage circuits are set to trip at about 16 volts. Most likely that is what is happening. The regulator is removing field current when it sees the bus voltage goes above its threshold. But that raises the question, why does it come back on line when you slow the engine? If there is no field current, then there is no alternator output, so the regulator wouldn't know to turn back on. I think most VR systems latch off in an overvoltage event and you have to cycle the field breaker to get it to reset.

With all that said, it would be good to know what the bus voltage is doing during all this.

[N177MC]

Looking at the Wiring Schema: THere's an optional Over-Voltage Relay , but I'm not sure if it's installed.

Where would it typically be installed ?

[N177MC]

34 minutes ago, N201MKTurbo said:

Have you measured the voltage at the VR when this happens? Most over voltage circuits are set to trip at about 16 volts. Most likely that is what is happening. The regulator is removing field current when it sees the bus voltage goes above its threshold. But that raises the question, why does it come back on line when you slow the engine? If there is no field current, then there is no alternator output, so the regulator wouldn't know to turn back on. I think most VR systems latch off in an overvoltage event and you have to cycle the field breaker to get it to reset.

With all that said, it would be good to know what the bus voltage is doing during all this.

 

I use a small digital volt-meter plugged into the cigar lighter to monitor the bus voltage. 

That's how I know when the alternator goes off-line and comes back on again.

[N201MKTurbo]

9 minutes ago, N177MC said:

 

I use a small digital volt-meter plugged into the cigar lighter to monitor the bus voltage. 

That's how I know when the alternator goes off-line and comes back on again.

OK, so what did the voltage do?

What was the voltage when the engine was off? What was the voltage just before it went off line? Was the voltage increasing with RPM or was it steady? What was the voltage when the alternator was off line? What did the Voltage do when it came back on line when you reduced the RPM?

[skykrawler]

What voltage is the system producing across the RPM range?

The regulator adjusts voltage to the alternator field (through the brushes to the rotating field) to control the alternator output.  Monitor the regulator field connection to the alternator and see if it goes to zero or low when the alternator goes "offline"

[Fly Boomer]

3 hours ago, N177MC said:

Anyone have any idea what would cause this ?

I think it unlikely that the regulator adjustment "slipped", and therefore needed to be changed.  I suggest not making adjustments until you diagnose the problem.

I googled "alternator offline high rpm" and got these which seem to be relevant:

https://vansairforce.net/community/showthread.php?t=88995

https://forum.cessna170.org/forums/viewtopic.php?t=8585

Might be some ideas in there.

[PT20J]

The over voltage protection is built into modern voltage regulators. It sounds like the regulator has a problem with the over voltage circuit. It sounds like it is tripping at too low a voltage and not latching. To troubleshoot,  I would measure bus voltage versus rpm. Also, most regulators have an output to indicate that the over voltage protection has tripped and I would monitor that.

 

 

 

 

 

[philiplane]

the over voltage cut out is built into the Plane Power regulator. It should cut out at 16 volts. These regulators are normally set to 14 volts from the factory, and often require no adjustment once installed. You'll need to monitor and record the voltage from idle, up to the point that it cuts out. You'll need this info in the event of a warranty claim on the regulator.

[Ragsf15e]

Just for fun, maybe make sure the belt is tight too…

[vik]

I never opened Plane Power voltage regulator, but if it is done right, the over voltage protection circuit should be separate from the voltage regulating circuit even though it resides on the same circuit board. The voltage regulating circuit controls the current (not voltage) in the field coil to maintain 14V (or whatever the target voltage is). If everything works correct, this should be enough to avoid high voltage as the field current would be reduced to maintain the correct voltage under any load/rpm.

If the current regulating transistor is shorted for example, the current thru the field coil will be constant, limited only by the resistors inside VR. Then, the alternator output voltage will depend linearly on the load and rpm. Once it gets over 16V at higher rpm, the over voltage protection circuit will step in and cuts the field current. The over voltage circuit can periodically try to release the current for few ms to check if the condition still present and if it see < 16V it will revert to normal operation. This might explain the alternator come back after the rpm drop.

This is the theory. Practice can bring something more interesting  or simple. Try resetting the circuit breaker that feeds VR first. They are known to drop the voltage under load. Given the symptoms, this is not likely in this case, but few seconds you spend on the test well worth it.

Vik