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Old December 29th, 2009, 20:29   #16
CDN_Stalker
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Regarding spikes on DC motors, I'm currently in the process of building a couple of instrumented wing pods (to be hung on the bomb racks of our T-33) that have custom made air pumps, run at 24VDC, and during testing, found that not only there was a 13A spike upon start up (settled to 5A while running), but there was a spike of close to 90A upon shut off across the coil. We swapped the relays out for a similar milspect type with diodes (both silicon and Zener types) across the coil, that cancelled out the cut off spike. Might be a handy thing for the PTW (as well as sealing the circuit board to prevent the common failure from moisture.)

Not sure if this is related, but diodes across the coils of switches of any type are very beneficial.

Lol, just noticed this thread is fucking old! Oh well, since it's very informative, is good to resurface once in a while to make it easier for those that actually search, to find it.
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Old December 29th, 2009, 20:42   #17
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FETs are themselves diodes, and some can be variable resistors as well. The PTW electronics are designed to absord the cemf spike of a denergized motor winding.

What has been know to kill them is intermittant winding failure on the 490 series of motors. This often precedes a complete winding failure, or more to the point, a failure of the folded-tab connection between the winding and the commutator. Modified motors have this joint soldered with silver solder, and the newest 490 motors have this done at the factory.

Also, modified motors have the comm trued on a lathe to keep the brushes from skipping on the comm points, creating arc damage and leading to motor failure. The brushes now are 30% silver content to keep them from being eaten up by arc damage as well.

I don't modify the motors anymore, Tackleberry in the UK does and can also rewind armatures for otherwise unrecoverable motors.
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