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From: Colin Foster (no email)
Date: Sat Nov 03 2001 - 18:47:41 EST
From: <>
> In a message dated 11/1/01 9:52:28 AM Eastern Standard Time,
> writes:
>
> > There are no capacitors large enough to absorb the starter motor spike.
In
> > fact it is not a spike.
>
> The spike in question is caused by the magnetic fields in the starter
motor
> coils collapsing when the current is stopped.
>
> When current flows through a coil a magnetic field is created. When the
> current stops this magnetic field collapses, generating a high voltage
spike
> that can arc across the opening contacts of the solenoid and appear
> throughout the electrical system in various strengths. If you would like
to
> experience this spike personally, simply hold on to the bare wires, like
in a
> fuseholder, feeding an inductive load such as an electric pump and
interrupt
> the circuit. You will feel a bite when you open the circuit. This is the
> spike we are talking about.
>
> A large capacitor at the input terminals of sensitive equipment can do
much
> to absorb the spike. A real filter, such as available at Radio Shack or
car
> stereo shops, works even better.
>
> Norm
You didn't understand the messages, Norm.
1. It is true there is a large inductive spike from the starter motor when
the solenoid releases. That spike is, as you state, is generated by the
collapsing magnetic fields in the starter motor. Why is the spike so large?
Because the starter motor has just been disconnected from the battery and
since the current has NO WHERE TO GO, it appears as a very high voltage
spike. If the battery were still connected to it, or the starter motor was
connected to ground instead of being opened, then instead of a high voltage
spike you would have a current spike that decayed to zero. But the main
point is, all this happens after the starter motor has been DISCONNECTED
from the battery and all other circuits in the boat, car, or aeroplane. The
high voltage spike only appears across the starter motor. It is not going
to damage any electronics - you don't (normally) connect any thing to the
starter motor side of the solenoid except for the starter motor itself.
2. Of course you will feel it if you grab hold of it. So what - use an
insulated screwdrive if you need to short the starter solenoid terminals -
but there is no need for capacitors or Radio Shack filters or any filters -
the spike is going nowhere unless you grab it.
3. OK lets do the mathematics of how large a capacitor it would need, if for
some crazy reason you did want to suppress the spike. One contributor
suggested an aircraft style capacitor of 0.05 uF (from memory). Let's
assume that by the time the solenoid releases, the starter motor current has
dropped to 100 amps. This will be much less than the locked rotor current
and will vary depending on the size of the starter motor. The spike lasts
about 30 milliseconds typically. So the total energy to be absorbed is very
roughly 100 amps X 0.03 seconds or 3 amp seconds.
To absorb this energy would take a 3 farad capacitor rated at 100 amps, 25
volts. That is about 60,000,000 times as large as the 0.05 uF one
suggested, assuming you could find one and could fit it under the hood, so I
stick by my original statement that "There are no capacitors large enough to
absorb the starter motor spike".
4. Your suggestion of filtering the inputs to sensitive equipment is much
more practical than trying to suppress a spike at the starter motor that is
not doing any damage anyhow. Most modern electronics is already fitted with
input power filters so additional filtering is generally ineffective.
5. I think this subject has been beaten to death and is totally booring to
most readers. Lets drop it since I've had the last word.
Colin Foster,
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