From: Jon Elson
Subject: Re: (Avionics) How can this circuit produce an "inductive surge"?
Date: Fri, 27 Sep 2002 17:10:32 -0500
Organization: Washington University
NNTP-Posting-Date: Fri, 27 Sep 2002 23:13:23 +0000 (UTC)
X-Mailer: Mozilla 4.78 [en] (X11; U; Linux 2.4.8-26mdk i686)
> Jim Weir wrote:
> >That also is true. On engine startup, that magnetic field collapses in a big
> >way when the starter motor is de-energized after the engine starts.
> When the starter motor is de-entergised, it is also at the same time
> disconnected. How can the de-energisation of the starter motor produce
> a spike on the avionics supply? Especially as the battery is still
> connected into the circuit.
When the starter solenoid disengages, it is a mechanical switch, moving at
relatively slow speeds compared to electrical phenomena. The starter is a
series-wound motor, meaning that two large coils of massive wire are wound
around the motor's field poles, and connected in series with the armature.
When the solenoid is disconnected by the starter switch, the field in the
solenoid collapses, causing a large voltage to be produced, since all inductors
resist a change in the current in the winding. This current has to flow
through the winding, and there are only two paths. Either the insulation of the
windings break down and arc, or the switch contacts develop an arc,
dissipating the energy contained in the windings in the arc. When the energy
is dissipated, the flux in the solenoid is depleted, and a modest series of
impulses has been delivered to the electrical bus. But, these pulses will be
no greater in current than the solenoid requires when fully energized, probably
a few amperes.
When the solenoid flux is gone, the contacts in the solenoid begin to open,
and the process is repeated big time! If the engine is successfully started, and
the starter motor has freewheeled up to fairly high RPM at this point, the
starter motor current is modest, maybe 50 Amps or so. If the engine was
being cranked, but didn't start, and the pilot decides to give the starter a
rest, it is drawing several hundred amps at this moment, and the collapse of
the motor's field delivers a burst of pulses, beginning at several hundred amps,
to the electrical bus. Now, these pulses may be quite sharp and rapid, inducing
large fluctuations in voltage across the battery cables, due to the inductance of
the battery cables themselves. I'm pretty sure this is what does the damage -
large di/dt pulses, with edge rates of perhaps tens of volts/nS, reaching outrageous
voltage levels. One of the reasons this may be a greater problem in aircraft is
because the batteries are frequently placed to aid in weight balance of the
Car batteries are frequently about 18" from the starter motor. I've just about
never seen a battery this close on an aircraft.