Alternator slow to wake up

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Diamond_Dan
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Alternator slow to wake up

Post by Diamond_Dan »

I was wondering if anybody has seen this before - 2004 DA40 with the IO-360. Alternator was refurbished over a year ago at annual. When I fire it up and switch on the alternator, the current sits at +1 and the voltage ~24+V for about 2 minutes and then suddenly springs to life - ~+15A and 28+V. I was able to repeat this 3 times in a row last weekend. I got an alternator fail fault on the G1000 one of the times. I pulled a log from September and it wasn't doing that - the Voltage immediately starts ramping from +24 to +28 during warm-up. The major change in the past year was installing the SureFly in July for which I am having issues that I will share later after some more troubleshooting. I just wanted to see if there is any advice other than the usual check the connections, etc. when it goes in the shop.
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Rich
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Re: Alternator slow to wake up

Post by Rich »

I've never seen this and do have the Surefly. Mine comes o line immediately.
Something that comes to mind is be sure your alternator belt is properly tightened and in good condition.
Be sure to contribute the Surefly thread in the Lycoming forum where various tales of woe reside.
2002 DA40-180: MT, PowerFlow, 530W/430W, KAP140, ext. baggage, 1090 ES out, 2646 MTOW, 40gal., Surefly, Flightstream 210, Orion 600 LED, XeVision, Aspen E5
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Re: Alternator slow to wake up

Post by Rich »

I do have one interesting alternator behavior it might be worth mentioning. If I have the engine idling at something like 900 RPM (very rare), the voltage will be 27+ but output amps will jump around from something like 4-13. Normal amp draw is something like 8. Up beyond 100 RPM voltage is 28.2 or so and amps basically stable. But it has always been normal that even in flight indicated amps will commonly vary by maybe 2 with no change in what's on or off.
2002 DA40-180: MT, PowerFlow, 530W/430W, KAP140, ext. baggage, 1090 ES out, 2646 MTOW, 40gal., Surefly, Flightstream 210, Orion 600 LED, XeVision, Aspen E5
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Re: Alternator slow to wake up

Post by Diamond_Dan »

Update on this one. After squaring away the SureFly issues last winter, my mechanic checked out the charging system and couldn't find anything wrong with it. Of course he could not re-create the problem. He said next thing he would do is replace the voltage regulator if I wanted. Either the problem got better or I didn't notice it, but recently It is happening again. Maybe a cold weather problem? 1-2 minutes seems like forever for charging to begin. The belt is tight as well as the connections and the alternator is producing good current. I will have him replace the voltage regulator unless anybody has something else to try.
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Re: Alternator slow to wake up

Post by Diamond_Dan »

Hi one last call before I have the voltage regulator replaced. The alternator refurb shop my mechanic uses thought that the alternator might have a weak spring in it so we pulled it and sent it back. He replaced the brushes and bearings but thought it otherwise looked fine. Re-installed on the airplane and it still has the problem. I don't think it is a signal indication because the volts read low until the current comes alive. My mechanic's experience is that the solid state regulators usually either work or they don't. They are expensive enough to check a few things before replacing. I suggested to check the excitation voltage coming from the voltage regulator which we will do the next warm day.
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Re: Alternator slow to wake up

Post by Rich »

Ok, here's a bit of a shot in the dark. The VR has an adjustment screw to fine-tune the voltage target. Try turning that back and forth a bit to try to wipe away possible contamination or oxidation. Beyond that (or maybe in combination) it seems like something in the VR needs to warm up before it works properly.
2002 DA40-180: MT, PowerFlow, 530W/430W, KAP140, ext. baggage, 1090 ES out, 2646 MTOW, 40gal., Surefly, Flightstream 210, Orion 600 LED, XeVision, Aspen E5
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Re: Alternator slow to wake up

Post by Rich »

Here’s a curious behavior I’ve only recently noticed. There are two voltmeters in my plane: a Davtron and the VM1000. After starting in cold weather, at 1100 RPM the Davtron shows 28.3 and the VM1000 28.0. Current (shown in the VM1000) looks good at 8-9 amps. If I increase RPM to around 1350 or above, the VM1000 will start to show 28.3, agreeing with the Davtron. Reduce RPM back to 1100, VM1000 again shows 28.0.

All is normal during the flight, 28.3 on both. After landing, with everything warmed up the VM1000 will show 28.3 at 1100 RPM. :scratch:

Other indications in the VM1000 are all normal throughout.
2002 DA40-180: MT, PowerFlow, 530W/430W, KAP140, ext. baggage, 1090 ES out, 2646 MTOW, 40gal., Surefly, Flightstream 210, Orion 600 LED, XeVision, Aspen E5
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Re: Alternator slow to wake up

Post by Steve »

Rich:

For comparison, in my airplane, the VM1000 always reads 28.0 volts, ground or air, warm or cold. I replaced the Davtron clock about 15 years ago with an Aero 553 CO detector, which also displays voltage, IAT, OAT, cabin altitude, density altitude, local time, Zulu time, and of course, CO level. This instrument has always read 0.2 volts higher than the VM1000.

I haven't looked at the schematic, but a small difference like this could be due to where the voltage sense pickoff is for each instrument.
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Re: Alternator slow to wake up

Post by Rich »

Steve wrote: Sat Jan 22, 2022 11:52 pm I haven't looked at the schematic, but a small difference like this could be due to where the voltage sense pickoff is for each instrument.
So I did some more testing of this phenomenon. Maybe it's always been like this.

The Davtron and the voltage regulator are fed off the main bus. The low voltage and alternator failure annunciator lights are driven by the voltage regulator. The output from the alternator feeds the main bus. The alternator current sensor reads this feed to the main bus.

The VM1000 and various other things are fed off the essential bus. The operative thing here is that the battery is also connected to the essential bus. So what I'm seeing is a voltage drop due to current flow from the main bus to the essential bus, which varies according to the load on the essential bus vagaries of current draw by the various things when running.

Yesterday I tested this hypothesis.

Right after startup:
- 1100-1300 RPM: Davtron-28.3, VM1000 28.0
- 1400 RPM: Both read 28.3

After a few minutes of running:
- 1100 RPM: Both read 28.3

I then taxi to the fuel pumps and shut down. After restart, the same deal a before:
- 1100-1300 RPM: Davtron-28.3, VM1000 28.0
- 1400 RPM: Both read 28.3

In flight there is no more than .1 volt discrepancy, often none. So here's what I'm pretty sure is happening: The battery along with the other essential stuff), right after startup is sucking enough of the available amps across the intertie to cause that .3 volt drop. If I rev it a bit there is plenty of current to go around. After running a bit the battery has recovered and is drawing very little so the voltage drop disappears.

BTW, this business of the VM1000 being on the essential bus is worth noting in the context of a very low battery. Most of us are painfully aware that it's possible to start the plane with the battery in a low enough state that the essential bus CB will pop because of the amps it draws. When this happens, stuff fed off the main bus is fat and happy running off the alternator alone, while everything on the essential bus is running off the battery alone, sucking the battery down. All the stuff that we normally rely on to give us a warning of this state is ignorant of this condition. I can't speak to the G1000-equipped planes, but in mine the only indications that this has happened are the essential tie CB position (far from obvious) and (I now realize) the voltage indicated on the VM1000, which is tucked way out of sight on the right side of the plane. The moral is if your battery seemed low when starting be sure to check the VM1000 voltage indication, which in this situation would be far below 28 volts - more like 25-26 v or less. This condition would likely be apparent during runup, and certainly during the takeoff roll, when the alternator hits 40 amps and pops that CB.
2002 DA40-180: MT, PowerFlow, 530W/430W, KAP140, ext. baggage, 1090 ES out, 2646 MTOW, 40gal., Surefly, Flightstream 210, Orion 600 LED, XeVision, Aspen E5
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Re: Alternator slow to wake up

Post by Diamond_Dan »

My mechanic talked me out of replacing the voltage regulator. Says they will usually either work or not work. FWIW I haven't seen the same behavior although I did get my alternator rebuilt (prematurely) during the final troubleshooting. They didn't see anything unusual with it.

My idle RPM seems low although the engine won't stall. I am thinking this is a good thing, but there seems to be an RPM threshold low enough to where the alternator will cut out completely. If I bump the throttle it jumps back to normal +amps. Is this normal or maybe it really is time for a voltage regulator?
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