voltage regulator questions

Would connecting a voltmeter to the alternator output tell me anything during a ground run? 

Walt: "I suppose a positive is with a good battery that holds a good voltage and doesn’t demand an immediate charge that the extra horsepower drain on the engine required for a charge is available for take off power."

Mike Miller: "At 12 amps and 12.7 volts you’re drawing 152 watts or 1/5 HP."

Unfortunately, the combination of a permanent magnet stator and a shunt type regulator doesn't work like that.  The regulator manages bus voltage by shorting excess stator current to ground.  The result is that nearly the full current capability of the stator is going one of two places at all times: either to airframe loads or to ground.

There are three conclusions we can draw from this.  First, the regulator's power dissipation -- and therefore temperature -- is inversely proportional to airframe loads.  Second, the stator is constantly fully loaded (remember, a permanent magnet stator's output capability is variable depending on engine RPM: IM §24-00-00 page 8).  Third, there's no hard limit on stator current output; once the limits of the stator's physics are reached, the bus voltage will decline to match battery voltage as current increases further, and eventually the overcurrent condition may overheat the stator's windings.

Pilots tend to think that reducing airframe loads (turning off lights, etc.) is best for the regulator, but with this type of regulator it's actually the opposite.  The more current that's consumed by airframe loads the less that's shunted by the regulator, reducing its power dissipation and temperature.  So, running as many airframe loads as possible -- within the published current limits at the present engine RPM -- is the kindest thing you can do for your regulator.  There's nothing we can do to make life easier for the stator.

Walt: "Would connecting a voltmeter to the alternator output tell me anything during a ground run?"

Not really.  Even with a partially shorted winding, the stator's open circuit voltage will be quite high and will fluctuate with engine RPM, just as you would expect.  The better measurement is to check resistance of the stator windings (across the two yellow wires).  This isn't easy to do accurately with a typical handheld meter because of the very low value, but you can get close.  Put your meter in its lowest ohms range (or in auto-range) and short the leads together (hold them together hard; you want to measure the probe wires, not the connection between them).  Let the meter stabilize, note the reading, then measure across the yellow stator wires (again, pinch the probes to the QC terminals hard).  Subtract the first reading (shorted leads) from the second (stator windings) to find the stator winding resistance.  It should be 0.1 to 0.8 ohms (MMH §74-00-00 page 7).  Next, measure from each yellow wire to the engine crankcase.  These should be open circuit (infinite resistance).

Eric, 

You’re basing the your argument on a fallacy. The Rotax 912ULS engines do not use a shunt regulator. I have no idea what the aftermarket SH regulator uses. 

Hi all

The Rotax injected 9 series engines have shunt regulators.  They are 3 phase.  The carb versions with single phase do not use a shunt type. 

Cheers

Rotax Wizard wrote:

Hi all

The Rotax injected 9 series engines have shunt regulators.  They are 3 phase.  The carb versions with single phase do not use a shunt type. 

Cheers

Interesting, mine is a ULS, so does it matter what regulator I put on it? I was considering the  ACV-1  

https://bandc.com/product/avc1-advanced-voltage-controller-14v-homebuilt/