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From: Steve Thomas <steve@stevet.net>
Subject: Re: [FlyRotary] FWD: Alternator Voltage Spikes 
Date: Tue, 2 May 2006 14:33:25 -0700
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>

Marc,

You should post this on the aeroelectric list.  I'll bet it will  
generate a LOT of discussion.

Best Regards,

Steve Thomas
SteveT.Net

________________________________________________________________________

On May 2, 2006, at 2:17 PM, Marc Wiese wrote:

> Subject: Alternator Voltage Spikes
>  I have been noticing all the comments about voltage spikes during
> starting/shutdown. One poster mentioned that a battery couldn't put  
> out 200
> volts, so I thought I would toss my 2 cents in. I worked for  
> several years
> as an IC design manager at Texas Instruments in the automotive  
> group where
> we designed ICs for automotive regulators. I will assure you that  
> charging
> systems for cars (and airplanes) do indeed put out spikes well over  
> 200
> volts when the alternator is rotating very slowly. This voltage can  
> easily
> cause oxide failure, known as 'punch-through', which shorts VCC and  
> Ground
> inside the IC. Once that happens, heavy current flows through the  
> IC causing
> the smoke stored in the IC to escape. ICs don't work once you let  
> the smoke
> out! :-) To understand how a 12 volt battery/alternator can put out  
> a high
> voltage, you have to understand that in an inductor (i.e. the  
> windings of
> the alternator), the voltage across the inductor is equal to L*di/ 
> dt, or the
> Inductance of the winding times the rate of change of the current  
> through
> the inductor. If you attempt to instantly stop or start the current  
> through
> an inductor, di/dt will become very large, and the voltage will  
> increase to
> whatever level necessary to collapse or create the magnetic field  
> around the
> inductor. When an alternator is turned very slowly, there are  
> points where
> the windings are open circuited (or routed through high resistance  
> paths)
> which causes voltages to rise to the level that breaks down the  
> primary
> protection - around 200 volts. In an automobile, there is a highly  
> reliable
> circuit that disconnects the radio bus from the charging bus during  
> start
> and shut-down to protect equipment from these spikes. This is what  
> Cessna
> attempted to do with some aircraft, but the circuit they used is  
> somewhat
> crude and potentially unreliable, so people disconnected its output  
> that was
> supposed to control the avionics power relay, and rewired this  
> relay to be
> controlled by a simple switch on the panel.
> Marc Wiese
>
>
> --
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