Ed de C. Posted October 8 Report Posted October 8 Good evening folks - Another squawk I noted for the shop during annual (Ovation 3) is that the B&C backup alternator shows 29.9V in flight at 2300 rpm. That seems too high according to Savvy and the shop. Question is what could be allowing that. The main alternator (and its voltage regulator) runs voltage around 28.3V or so. A brief backstory: I checked in flight voltage because I noticed a few months ago that the low-voltage indicator light, when the backup alternator was energized, would no longer extinguish at 2000 rpm as it would in the past (this is during runup on the ground). Amps would go positive as the rpm increased, but the low-voltage light would not extinguish. The engine data trace showed that back-up alternator voltage would rise linearly as rpm increased. I flagged the situation to the shop. Consensus seems to be that the in-flight voltage of 29.9V is a bigger concern than the runup low voltage light not extinguishing. Question is what's the likely culprit here? Our sense is that we have a voltage regulator problem. But, one mechanic working on the plane says there is one voltage regulator controlling voltage for both alternators. If so, why would we have different controlled voltages depending on the alternator engaged? Thoughts appreciated. Ed Quote
carusoam Posted October 8 Report Posted October 8 Hey Ed… expect that each alternator has its own VR voltage regulator… some installations have one device that contains two VRs in one box… adjusting the VR’s output is usually a simple procedure to follow… the 24V system charges near 28V… there is an upper limit that should be easy to look up for your collective systems… The fancy B&C alternator should have a booklet of info to go with it… possibly found in the POH. If not both the alternator and VR can be looked up on the BandC.com website. Battery, electronics, high and low voltage warnings… Look up the best charging voltage for your favorite batteries you have on board… if you are fortunate, you have a pair of Concordes back there… For maintenance, the Concordes charge between 28.0 and 28.8V… either way, charging at too high of a voltage is bad for battery health… PP thoughts only, not a mechanic… - a - Quote
EricJ Posted October 8 Report Posted October 8 57 minutes ago, Ed de C. said: Good evening folks - Another squawk I noted for the shop during annual (Ovation 3) is that the B&C backup alternator shows 29.9V in flight at 2300 rpm. That seems too high according to Savvy and the shop. Question is what could be allowing that. The main alternator (and its voltage regulator) runs voltage around 28.3V or so. A brief backstory: I checked in flight voltage because I noticed a few months ago that the low-voltage indicator light, when the backup alternator was energized, would no longer extinguish at 2000 rpm as it would in the past (this is during runup on the ground). Amps would go positive as the rpm increased, but the low-voltage light would not extinguish. The engine data trace showed that back-up alternator voltage would rise linearly as rpm increased. That means the regulator isn't regulating, since the output is proportional to rpm. 57 minutes ago, Ed de C. said: I flagged the situation to the shop. Consensus seems to be that the in-flight voltage of 29.9V is a bigger concern than the runup low voltage light not extinguishing. Question is what's the likely culprit here? Our sense is that we have a voltage regulator problem. But, one mechanic working on the plane says there is one voltage regulator controlling voltage for both alternators. If so, why would we have different controlled voltages depending on the alternator engaged? Thoughts appreciated. Ed The usual architecture with a backup alternator is that each alternator has its own regulator, and the main regulator output voltage is set a little higher than the backup. e.g., if the main regulator is set to 28.3V (like yours), then the backup would be set to 27V or something like that. The difference should be enough to reliably keep the backup alternator from producing power until/unless the main alternator fails. If the backup regulator is set to 27V and the main regulator is putting out 28.3V from the main alternator, the backup regulator will essentially turn off the backup since the system voltage is already too high from its perspective. If the primary alternator fails as soon as the voltage drops to 27V the backup regulator will try to keep it there with the backup alternator. So there should be two regulators, and the backup alternator shouldn't be putting out any power if the main alternator is putting out more than whatever the backup regulator is set to. 1 Quote
carusoam Posted October 8 Report Posted October 8 Interesting note about alternator output… at low rpm, while taxiing… their output is pretty low. You may get a low voltage light on the panel… they will be providing maximum output possible… but at 700rpm of the engine… the alternator will be providing a pretty low output… increasing rpm, the low voltage light extinguishes… but taxiing speed starts to increase… dragging the brakes, nobody likes… O1 experience only, no BU alternator… -a- Quote
dkkim73 Posted October 8 Report Posted October 8 @Ed de C., From the Acclaim (M20TN) service manual MAN180 (which might parallel the Ovation's?) I found: 24-00-00 - GENERAL Main Alternator System A 28 volt, 100 AMP alternator (90 amps max. output) utilizing a transistorized voltage regulator/over voltage control supplies electrical power for the main alternator source. Two 24 volt, 10 AMP HR storage batteries are installed in the tailcone. The alternator, during normal operation, supplies power in conjunction with the se- lected battery when the Master Switch and Alternator Field Switch is - ON. The circuit breaker panel is on the right subpanel. The electrical system is capable of sup- plying current for simultaneous operation of multiple ra- dios, anti-collision lights and navigation lights. Standby Alternator System An optional standby system provides 30 volts, with 20 amps of power to support continued flight in the event of primary alternator failure. The Standby alternator is installed on the accessory pad location. Panel mounted equipment includes a STBY ALT ON annunciator and two standard pull type circuit breakers (1 amp and 5 amp) and a STBY ALT master switch. Appropriate plac- ards are provided for each panel mounted device. The regulator is mounted in the RH nosegear wheelhouse area on the M20TN Aircraft. If the primary alternator fails in flight, the pilot activates the standby alternator. If the current requirement is over 26.5 volts when the standby alternator is activated, the annunciator will flash. The pilot is advised to choose which equipment he needs for the given flight conditions by simply keep- ing the total load below the flashing point of the annun- ciator. This will reserve battery energy for transient loads during approach (refer to the M20TN POH/AFM for specific equipment limitations). Loads may be beyond the flashing point of the annunciator for up to 5 minutes without damaging the standby alternator. This fits with @EricJ's description of the preferential-loading behavior (with differentially-set voltages on two different regulators). I would *guess* that given the commonalty of the Acclaim/Ovation designs, this might be similar to your plane. Might be worth a quick PDF search of your Maintenance Manual. HTH D Quote
Ed de C. Posted October 8 Author Report Posted October 8 Thanks dkkim73 - Sure enough, the Ovation service manual in this area is nearly identical to the Acclaim. Key thing: looks like 30V is by design for the backup alternator (text below). (Mine is regulating to 29.9V). It seems I sent my shop on a wild goose chase, which I just fixed. The data logging at runup shows steadily climbing volts and amps as rpm rises through 2000, indicating the backup alternator and its regulator seem to be working properly. Only question remaining is why the blinking low volts light on the panel annunciator is not extinguishing at 2000 rpm as it used to. At runup and 2000 rpm, volts shows 26.2V. Progressing to 2200rpm increases volts to 27. The in-flight test was at 2500rpm and generated 29.9V. I'll collect more data on the indicator light once the plane is out of annual. The service manual says the low voltage blinking indicator light should extinguish at 26.5V and if not, it can be adjusted. Ah, checking prior runup data, I see that backup alternator voltage at 2000 rpm has been dropping over time. In Feb 2024 it was 27.4V, May: 26.8V, June: 26.4V, July: 26.2V. That would explain why I'm noticing the indicator light behaving differently recently. Question is, why would the voltage be dropping over time? "Standby Alternator System An optional standby system provides 30 volts, with 20 amps of power to support continued flight in the event of primary alternator failure. The Standby alternator is installed on the accessory pad location. Panel mounted equipment includes a STBY ALT ON annunciator and two standard pull type circuit breakers (1 amp and 5 amp) and a STBY ALT master switch. Appropriate placards are provided for each panel mounted device. The regulator is mounted in the RH nosegear wheelhouse area on the M20R Aircraft. If the primary alternator fails in flight, the pilot activates the standby alternator." Ed Quote
Will.iam Posted October 8 Report Posted October 8 Have you check the alternator belt for proper tension? A loose belt under load might be slipping giving the lower voltage output. Quote
LANCECASPER Posted October 8 Report Posted October 8 4 hours ago, Ed de C. said: Thanks dkkim73 - Sure enough, the Ovation service manual in this area is nearly identical to the Acclaim. Key thing: looks like 30V is by design for the backup alternator (text below). (Mine is regulating to 29.9V). It seems I sent my shop on a wild goose chase, which I just fixed. The data logging at runup shows steadily climbing volts and amps as rpm rises through 2000, indicating the backup alternator and its regulator seem to be working properly. Only question remaining is why the blinking low volts light on the panel annunciator is not extinguishing at 2000 rpm as it used to. At runup and 2000 rpm, volts shows 26.2V. Progressing to 2200rpm increases volts to 27. The in-flight test was at 2500rpm and generated 29.9V. I'll collect more data on the indicator light once the plane is out of annual. The service manual says the low voltage blinking indicator light should extinguish at 26.5V and if not, it can be adjusted. Ah, checking prior runup data, I see that backup alternator voltage at 2000 rpm has been dropping over time. In Feb 2024 it was 27.4V, May: 26.8V, June: 26.4V, July: 26.2V. That would explain why I'm noticing the indicator light behaving differently recently. Question is, why would the voltage be dropping over time? "Standby Alternator System An optional standby system provides 30 volts, with 20 amps of power to support continued flight in the event of primary alternator failure. The Standby alternator is installed on the accessory pad location. Panel mounted equipment includes a STBY ALT ON annunciator and two standard pull type circuit breakers (1 amp and 5 amp) and a STBY ALT master switch. Appropriate placards are provided for each panel mounted device. The regulator is mounted in the RH nosegear wheelhouse area on the M20R Aircraft. If the primary alternator fails in flight, the pilot activates the standby alternator." Ed On the #1 alternator that's not a good trend you might be on to something there. Has someone who knows what they are doing checked the coupler - that's a known issue. On the backup alternator I think you have a solution (30v backup - good!) looking for a problem (there is none). But I'm sure they'll be happy to keep the clock running while you have them chase it down . . lol Quote
Ed de C. Posted October 8 Author Report Posted October 8 Thanks William - on the Continental 550, the backup alternator runs off one of the rear accessory drive pads. So, it is gear/spline driven, no slippage there. Lance, not sure what the coupler is. The issue is with the backup alternator, not the main alternator. Main alternator shows no voltage anomalies, but it does exhibit a whine during taxi (that goes away when I turn off the field switch). I've asked the shop to look into that, since I have been told that alternator whine can be caused by a diode problem. Ed Quote
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