>Bob- Just read your thoughts on starters being engaged. Assuming that
>there is power applied to the starter due to some fault in the system, and
>assuming the starter is not actually engaged on the flywheel, the starter
>would freewheel, right?
Not exactly. On all exept the old bendix geared Prestolite pigs
the starter pinion gear is pushed out to engage the engine ring
gear by the same solenoid actuator that closes the electrical
circuit between battery and motor. If the engine is running, the
starter is essentially free-wheeling because the overrunning clutch
prevents the rapidly spinning pinion shaft from feeding engergy
back INTO the slower running starter.
Assuming electrical power is applied to the motor but NOT providing
torque to the engine (for what ever reason) then the armature indeed
climbs to some practical, terminal velocity. With the permanent
magnet starters, terminal velocity is quite sedate . . . when
BACK EMF in the armature (wires rotating rapidly in a FIXed field)
approaches applied voltage (BATTERY) the speed increases no further.
For series wound starters like the B&C and the Prestolite, there is
no theroetical limit for no-load speed EXCEPT that friction and
windage provides a modicum of load that prevents the armature
speed from going clear to the moon. None the less, the terminal
velocity for these starters is MUCH higher than for the PM
motor devices and overspeed damage is much more likely.
>In this case, is there likely to be any damage
>done to the starter itself? If so, how long would you suppose this
>condition could prevail without causing damage? Would the type of starter
>be a factor?
Sure, and for the reasons I've outlined above. Now . . . someone
is CERTAIN to suggest the PM motor starter is a better choice
because it's overspeed failure probability is lower. Folks should
know that the REASON B&C choose to stay with series field motors
is because field flux in a series wound motor goes UP as load
on the motor goes up . . . they peform better in heavy torque
applications like cranking engines. This means the motor
IS LESS EFFICIENT (heating losses in series windings not present
in fixed magnets) but efficiency is not an issue for a motor that
runs 3-8 seconds per flight cycle. It's ability to get the engine
running over the widest envelope of conditions (temperature,
battery condition, engine tune-up, etc). The decision NOT to
go PM was not a trivial consideration . . . PM motors are cheaper
to build. But their inrush currents are higher which increases
probability of contactor sticking . . . EVERY design decision
is a trade-off.
So, the long answer to your short question is, "yes, unloaded
starters are stressed pretty hard by overspeeding operation -
some starters worse than others." But I would add that the
likelyhood of a starter coming to grief for this reason does
not figure strongly in choices of design for reasons of
performance under normal operating conditions.
Bob . . .
AeroElectric Connection
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