Connection of Gen. B and Master relay

Hi Eric,

Very helpful information, Thank you!  

It's always interesting how changing one thing can effect other things.  I will look into the new RS regulator.  

I very much appreciate your help!  Scott

Thinking through why Rotax wants the generator B output connected to the load side of the master switch is interesting.  It would seem that, as Eric suggests, they don’t want the load suddenly disconnected from the generator by turning off the master switch while the engine is running.  And by having the 30 amp breaker in the circuit you are given a method to disconnect if necessary, and this inherently feels more like an emergency procedure than a normal operation. That’s all good.  However, if you do turn off the master switch before stopping the engine you are disconnecting the battery and it’s natural buffering affect on the circuit, which could negatively affect sensitive electronics. Also, many of us are used to having a master switch that shuts off the airframe bus, which is not the case with the Rotax iS engines. To some extent, it feels like “pick your poison”. 

Agreed.  After many years of training that the first action to the smell of electrical smoke is turn off the master then bring items back one at a time as needed, it is odd to have a system that makes that not possible or desirable without the risk of further damage.

There is a way around this issue, but it's definitely not Rotax approved.  You would insert relays into any two of the three phase wires from the stator to the regulator (on a UL/ULS engine, you would interrupt one of two wires).  By cutting the stator feed to the regulator instead of the regulator's connection to the battery, there's no load dump on the bus when the system is turned off.

The easiest means to do this without cutting into the Gen B harness is to duplicate the existing connectors and fabricate a sub-harness to insert the relays.  See this post for all of the information about connectors.

The stator feed relays can be controlled manually with a switch, or by an overvoltage protection circuit, or both.  For overvoltage protection, see this thread on the AeroElectric-List forum, and scroll down to the post on Mon Apr 07, 2025 6:32 pm.

Finally, two comments:

First, I'll restate what I said at the top: this is not Rotax approved.  If you don't fully understand what you're doing, or you're not comfortable with the soldering and wiring tasks involved, or you're not fully cognizant of the risks of system mismanagement, then please do not attempt this modification.

Second, remember that the EMS doesn't monitor Gen B, so if you or the overvoltage system turn it off, there will be no notification to the pilot.  It is therefore mandatory that you add active notification of low voltage (~13V is a good trip point), either via an EFIS alarm or a warning light so that loss of battery charging is immediately apparent.

Thanks Eric, I'll check this all out.

Scott