REFLECTOR: How to protect the battery bus

[Brett Ferrell]

Hiroo,

I'm not an electrical engineer, so I don't know for sure, but it sounds like when the starter stopped spinning the in-rush currents trying to turn it went so high the shunt was blown.  I'd argue that you don't care what the current is when starting, so if you can't find a large enough shunt to handle the current, I'd consider putting a fuse or limiter on its line, or perhaps another contactor that drops the shunt out while the starter is engaged.

Brett
  ----- Original Message ----- 
  From: Hiroo Umeno 
  To: Velocity Aircraft Owners and Builders list 
  Sent: Friday, February 16, 2007 5:35 PM
  Subject: Re: REFLECTOR: How to protect the battery bus


  The shunt drives the ammeter.  In my case, even under the best of circumstances, the first rotation after the starter engages is painfully slow.  It usually takes about 2 seconds for the starter to get past the first compression stroke of the Franklin engine, then after that, it cranks fine.

   

  Last time, I held my finger on a bit too long hoping that the weakened battery can still overcome the compression stroke ending up in a blown shunt.

   

  Hiroo

   


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  From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] On Behalf Of John Dibble
  Sent: Friday, February 16, 2007 2:25 PM
  To: Velocity Aircraft Owners and Builders list
  Subject: Re: REFLECTOR: How to protect the battery bus

   

  I don't believe I have a shunt (I did not build).  Occasionally my starter will stop for a second during a hard start.  I find that if I hold the starter button, after a second it will start cranking much faster for some reason.  Also if I release the starter button and push it about 1/2 second later, it seems to crank much faster.  The only problem I've had is losing data in my RMI engine monitor and RMI encoder.  Now I have all instruments turned off until the engine is running. 
  John 

  Hiroo Umeno wrote: 

    Last weekend, I cranked the fickle Franklin one too many times.

    (Yes, I am still trying to find out why it simply will not run.  Long story.) 

    On the third cranking, the voltage on the battery must have dropped sufficiently that the starter motor stopped turning the prop.  I probably left my thumb on the starter button a tad longer than I should have. 

    The shunt blew and the whole electrical system went (except the backup battery system that ran the instruments (glad to know THAT works well). 

    That got me thinking.  Shouldn't there be a CB or something protecting the starter line?  I've heard that the starter line is the only line that should NOT have a CB on.  Yet I've managed to fry the shunt. 

    Any thoughts? 

    Hiroo

 

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