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Starter Circuit: Verner 7U Radial

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Eric Page

Joined: 15 Feb 2017
Posts: 245

Posted: Fri Jul 14, 2023 2:45 pm Post subject: Starter Circuit: Verner 7U Radial

I have a friend who's building a Kitfox with a Verner Motor Scarlett 7U radial engine.



http://www.vernermotor.com/page1.html#features18-24



The engine is shipped with a starter that features an integral solenoid.  The connections to the ship's wiring are as follows:



- One terminal on the motor case which is the ground lug.  This gets a fat wire to the forest-of-tabs through-bolt.



- Two terminals on the solenoid: One gets a fat wire carrying 12V from the switched side of the battery contactor.  This supplies current to the motor via an internal connection.  The other solenoid terminal receives 12V to activate the solenoid's coil.  The coil is grounded internally.



My friend's question is whether it is necessary to install a separate starter contactor to send 12V to the starter solenoid's control terminal, or if the start switch can control the starter solenoid directly.  The Z-diagrams show a starter contactor controlling a starter with attached solenoid, but neither of us can deduce why it's necessary to wire what are essentially two big relays in series.



If the separate contactor is necessary, then please explain:

1. Why?

2. Should it supply all power to the starter -- both motor current and control signal -- or, just the control signal?



Thanks!



Eric

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dj_theis

Joined: 28 Aug 2017
Posts: 56
Location: Minnesota

Posted: Sat Jul 15, 2023 4:23 am Post subject: Re: Starter Circuit: Verner 7U Radial

 	  Eric Page wrote: 	
		  

If the separate contactor is necessary, then please explain:

1. Why?

2. Should it supply all power to the starter -- both motor current and control signal -- or, just the control signal?

	




I think you are implying it appears redundant, and I agree. The only good reason I can imagine is if the starter does not have a separate terminal for solenoid acitivation.  



Ford motor company designed their starters with a single terminal and put the starter on the fender for years while GM always ran a fat wire to the starter with a secondary small wire from the ignition switch on a second smaller starter terminal.



Relays are (as you know) used to keep the fat wires out of the cockpit and simultaneously keep the wire length from the battery to the starter short.  Assuming an activation terminal on the starter, there is no advantage (that I can think of) in adding another relay in series.  The master contactor already provides a second contactor in series to the starter as a safety disconnect and utility to disconnect power in the case of a "stuck starter solenoid."



It will be interesting to hear Bob's response.



Dan

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Dan Theis
Scratch building Sonex #1362
Still working on the Revmaster Alternator improvement

user9253

Joined: 28 Mar 2008
Posts: 1921
Location: Riley TWP Michigan

Posted: Sat Jul 15, 2023 5:01 am Post subject: Re: Starter Circuit: Verner 7U Radial

Below is from http://www.matronics.com/archive/archive-get.cgi?AeroElectric-Archive.digest.vol-kt dated Dec 18, 2011.

 >   Is the likelihood of the starter contactor failing closed, great 

      > enough to warrant the use of two contactors in series with the starter motor?

      

          Not if you can climb out quickly and disconnect the battery

          without undue hazard. But the incidences of sticking starter

          contactors of the S704 variety are exceedingly rare.

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