Mailing List flyrotary@lancaironline.net Message #49231
From: Ed Anderson <eanderson@carolina.rr.com>
Subject: Coil Current Draw
Date: Thu, 26 Nov 2009 09:35:47 -0500
To: 'Rotary motors in aircraft' <flyrotary@lancaironline.net>

 

Hi Jeff,

 

It appears the primary coil resistance can typically range from 0.8 – 1.4 ohms for an ignition coil.  So picking 1 ohm as a nominal value.  The stabilized current flow for one coil  would be 13.8 /1 = 13.8 amps – not too surprising for what is almost a dead short.  But, we know the coils are not on continuously - fortunately.  IF all four coils were on continuously they  would draw 4 * 13.8 = 55.2 amps. 

 

Steve Bose provide me some ignition data from the EC2 which indicated that at approx 3000 rpm the period between firings is 20 msec with a dwell (on) time of approx 3.7 Msec.  So that gives a duty cycle of approx 3.7 / 20 = 0.185 or 18.5 %.  That indicates for a single coil a DC current flow of approx 0.185 * 13.8 =  2.55 amps.

 

Taking that 0.185 * 55 =  10.175 amps as the outside limit on the total current for all four coils.  Now this is a DC resistive analysis approach.  The time it takes for the current to build in an inductor, building and collapsing magnetic fields and their effects would indicate somewhat less current flow than this purely DC resistive type analysis.

 

I found some information at the MSD (ignition) forum where the company replied to a question stating their “Blaster Coils” drew approx 1/2 amp average per 1000 rpm when used on a 4 cylinder engine.  So if we applied that to the Renesis coils at 3000 rpm, we would have 1.5 amps each or a total of 4*1.5  = 6  amps at 3000 rpm.  Now, not certain if that relationship holds linearly for the higher rpm or not.  Probably drops off a bit due to less dwell time to charge the coils – but that is speculation on my part.

 

So I would say that Al’s 6 -  8 amps sounds right to me.

Ed Anderson

Rv-6A N494BW Rotary Powered

Matthews, NC

eanderson@carolina.rr.com

http://www.andersonee.com

http://www.dmack.net/mazda/index.html

http://www.flyrotary.com/

http://members.cox.net/rogersda/rotary/configs.htm#N494BW

http://www.rotaryaviation.com/Rotorhead%20Truth.htm

 



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