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From: Gary Casey <glcasey@adelphia.net>
Subject: Re: Starter motors

The "electrical Magnets" to which Lorn refers imply that the motor is  
series-wound with starter current used to generate the field instead  
of using a permanent magnet(PM).  The fundamental difference in the  
two is the shape of the torque/speed curve with the PM motor having a  
straight line from a finite torque at zero speed to a finite speed at  
zero torque.  The torque curve of the series motor has essentially a  
hyperbolic shape that in theory goes to infinite torque at zero speed  
and infinite speed at zero torque (actually it never gets there  
because of other losses).  The point is that series motors typically  
have more torque at stall, but perhaps less torque at mid-speed.  PM  
motors work well in automotive applications since with their big  
batteries, short cables and small displacement the stall torque  
requirements aren't severe.  However, to get our large displacement  
engines to get past the first compression stroke takes a lot of low- 
rpm toque.  In order to get a PM motor to have enough stall torque  
they are usually designed so they draw more current (at stall - they  
are typically more efficient at high rpm) than a series motor.  I  
switched to a PM starter on my 4-cylinder and it always had a hard  
time with the first compression, but if it got past the first couple  
of compression strokes it would spin the engine pretty fast.  On my  
new 6-cylinder high-compression engine I used a B&C series-wound  
motor that weighs a couple of pounds more than the lightest PM  
starter.  I could not be more impressed with the starter and I would  
highly recommend this starter to anyone with a Lycoming engine.  I  
use mine without a separate starter contactor and just have a high- 
current pushbutton switch to control it - works perfect.

Gary Casey
ES # 157, N224SG

"I replaced my original starter with a Sky-Tech with permanent  
magnets (the standard model). I also started using a B & C BC100-1  
battery. The standard Sky-Tech uses a lot of starter current. The  
BC100-1 can supply it. I am very happy with the combination of  
starter and battery. I installed the battery in 2003 and in 2005  
mistakenly left the master switch on for 7 days. I am still using the  
same battery.

B & C also makes a light starter with electrical magnets. These  
magnets are supposed to last longer. Sky-Tech also makes a slightly  
more expensive model with the same magnets.

Lorn