[walkman]

Looking at adding a B&C Over Voltage Module to a traditional externally regulated (ford style) alternator.

There is a note on the wiring diagram for teh B&C OVM on their web site that states that the ALT and BATT should come on and off together.

This seems to preclude the use of a traditional split rocker or indeed the suggested switch wiring in the AeroElectric book.

Anybody using this module?

B&C were no help.

 

[Rupester]

Yep ....

...but I have a 60amp Plane Power alternator. Had I known that my GRT system would light a big red warning light on hi/lo voltage, I wouldn't have installed the B&C device. Now when my voltage drops low, it's like Christmas ... I get a big yellow AND a big red light on the panel.

 

[humptybump]

Terry, I suspect the purpose of the B&C module is to "act faster than you can" to protect the avionics.

One thing confused me is "what is it actually doing". If I understand their diagram, it looks like it generates a short and assumes it will pop the Alternator Field CB ? That just doesn't seem right to me. If you understand the wiring better, I'd be interested in what it is doing.

 

[bcondrey]

It is indeed a crowbar circuit. If over-voltage is detected it will put a short across the circuit and pop the associated CB. Usually used in an alternator field circuit to take the alternator off-line.

 

[humptybump]

Thanks Bob. I guess I read the circuit correctly. It just surprised me. Still, the module would pop the CB much faster that I could pull it after noticing my idiot light.

 

[Captain_John]

I see what you are talking about on this wiring diagram.

http://www.bandc.biz/pdfs/OVM_wiringdiagram.pdf

You are using the ford style regulator.

My GUESS is that the OVM has a chance of "crowbar-ing" if the battery is not connected in the circuit. The battery will assist in keeping voltages in check.

Why don't you want to wire it as per the diagram?

Perhaps using their LR-3C regulator instead, as I am? Then you will be able to do the split master/alt.

Here is the diagram for the LR-3C:

http://www.bandc.biz/pdfs/LR3C_Wiring_and_Installation_RevC.pdf

Hope this helps!

CJ

 

[Doug]

Only speculating but I suspect that the issue is with switching the alternator on when the engine is running. How the alternator output voltage behaves in the next few milliseconds depends on the particular voltage regulator, alternator and wiring. Some installations may see a voltage surge that would again trip the over voltage detect thus re-opening the breaker.

Tieing master and alternator switches with a DPDT switch essentially just ensures the alternator is on during start.

I see this as a good configuration - avionics are protected irrespective of the regulator and alternator setup.

This is the configuration in my RV-6. I have had a loose connection to the voltage regulator - the OV circuit tripped each time I tried to close the breaker. Returned for a safe landing with plenty of battery reserve.

In practice with the split master I doubt it will be a problem. If it does trip when you turn on the alternator after start you know it has protected your avionics. I liked that the install was so simple - two leads to two terminals on the master(/battery) switch.

The LR-3c is a more expensive option, not sure if it is better.

 

[walkman]

inrush current

so you think the issue is inrush current bringing an already spinning alternator on line? Could be.

I plan on wiring on a DPDT Off-On-On with Batt coming on in the middle position and the alt field in the top position. This would operate functionally like a split master with a separate breaker for the alt field.

I never start with the alternator off line anyway...

 

[Doug]

The voltage regulator - alternator is a classic closed loop system and subject to potential instability due to phase errors or variations in the loop. Ie variations in alternator, wiring, load and the regulator design itself. Marginal instability would be seen as a voltage oscillation tapering off to the regulated voltage.

You are to a greater extent subject to the VR designers mercy as to the extent this has been managed but it is likely that with every one of our aircraft being a one offs without the benefit of putting a CRO on the system and testing, some may experience a degree of instability.

I have read stories on various sites about alternator failures when switched on routinely after engine start. I suspect this is indicative of just such a problem.

So rather than an inrush problem think of it as something like a PIO where the VR is the pilot!