[DeTomaso] Hot Start Issue
SOBill at aol.com
SOBill at aol.com
Wed Dec 17 19:07:35 EST 2014
FWIW,
The Pantera OEM Ford starter motor has two windings: a field winding which
creates a fixed location magnetic field and an armature which causes the
direct drive starter pinion to rotate in that magnetic field. Both windings
are composed of many turns of copper wire.
The coefficient of electrical resistance for copper wire is +0.004/degree
C. The resistance of copper increases with increasing temperature. The
electrical resistance of the field winding and the armature winding will
increase with increasing temperature. For a 50 degree C temperature rise from
ambient, the resistance of each winding will increase by approximately 20%.
Since each winding is fed by a direct current from the battery the current in
each winding will decrease by approximately 20%.
The torque of the starter motor is proportional to the product of the
field current and the armature current. A 20% reduction in the field current
concurrent with a 20% reduction in the armature current will result in a
starter motor with 64% of it's room temperature torque. This problem will be
compounded by BTDC ignition firing (which try to spin a slowly cranked engine
backward), cable losses due to a remotely mounted battery (which reduce
the voltage at the starter), an increased engine compression ratio (which it
hard to spin the engine), and increased "drag" in a hot engine (which makes
it hard to spin the engine).
Modern starter motors use very strong, rare earth material, permanent
magnets for the fixed magnetic field and gear reduction mechanisms to allow the
armature to spin faster than the starter pinion. Both of these design
features makes life much easier for the starter motor. The magnetic field is
stable with temperature, the battery supplies current only to the armature,
and the armature has more "leverage" via the gear reduction.
Ford recognized that the starter motors were becoming marginal. When the
Duraspark system was introduced, the White wire in the Duraspark harness was
activated whenever the starter was engaged. The White wire retards the
Duraspark ignition timing to make the engines easier to crank.
The main thing is to keep the main thing the main thing.
Do the best you can with what your have where you are.
Have fun today!
SOBill
-------------- next part --------------
FWIW,
The Pantera OEM Ford starter motor has two windings: a field winding
which creates a fixed location magnetic field and an armature which
causes the direct drive starter pinion to rotate in that magnetic
field. Both windings are composed of many turns of copper wire.
The coefficient of electrical resistance for copper wire is
+0.004/degree C. The resistance of copper increases with increasing
temperature. The electrical resistance of the field winding and the
armature winding will increase with increasing temperature. For a 50
degree C temperature rise from ambient, the resistance of each winding
will increase by approximately 20%. Since each winding is fed by a
direct current from the battery the current in each winding will
decrease by approximately 20%.
The torque of the starter motor is proportional to the product of the
field current and the armature current. A 20% reduction in the
field current concurrent with a 20% reduction in the armature current
will result in a starter motor with 64% of it's room temperature
torque. This problem will be compounded by BTDC ignition firing (which
try to spin a slowly cranked engine backward), cable losses due to a
remotely mounted battery (which reduce the voltage at the starter), an
increased engine compression ratio (which it hard to spin the engine),
and increased "drag" in a hot engine (which makes it hard to spin the
engine).
Modern starter motors use very strong, rare earth material, permanent
magnets for the fixed magnetic field and gear reduction mechanisms to
allow the armature to spin faster than the starter pinion. Both of
these design features makes life much easier for the starter motor. The
magnetic field is stable with temperature, the battery supplies current
only to the armature, and the armature has more "leverage" via the gear
reduction.
Ford recognized that the starter motors were becoming marginal. When
the Duraspark system was introduced, the White wire in the Duraspark
harness was activated whenever the starter was engaged. The White wire
retards the Duraspark ignition timing to make the engines easier to
crank.
The main thing is to keep the main thing the main thing.
Do the best you can with what your have where you are.
Have fun today!
SOBill
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