Starter Relay/Contactor Wiring on Rotax 912ULS

[mwtucker]

Almost every light aircraft electrical diagram I've seen shows the starter solenoid on the output (i.e. switched side) of the master relay/contactor. In this configuration, starter current flows from the battery positive terminal through the master relay/contactor and then through the starter relay/contactor to the starter. If you look at the Rotax 912ULS Installation manual (page 20 of section 24-00-00), you will see that the starter relay/contactor is connected directly to the battery positive terminal and the control voltage to energize the starter relay/contactor comes from the main 12 volt bus (which is switched by the master). Both of these approaches require the master switch to be on in order for the starter to operate. I am leaning toward the Rotax approach because there will be fewer connections and less voltage drop when the high starting current flows from the battery to the starter. In the common approach there are connections (i.e. small voltage drops when the high starter current flows) at both the master relay/contactor (2 connections) and the starter relay/contactor (2 connections) as well as the 2 contacts in each relay/contactor. Is there any reason to wire these relays/contactors differently than as shown in the Rotax installation manual?

Thanks in advance for your comments.

Mike

 

[Mich48041]

Everything that you wrote is true. But how significant is the voltage drop when the starter is wired like most aircraft?
There are hundreds of Van's RV-12s with Rotax engines all wired with the battery contactor and starter contactor wired in series.
And they start just fine. If the starter contactor welds shut, the battery contactor can be opened.

 

[mwtucker]

Thanks Joe. Good points.

 

[hgerhardt]

The only drawback I can see is you'll have an always-hot fat wire firewall-forward which you can't switch off to mitigate potential fire starters in case of an impending crash landing.