[archiver]

Posted by Al Skierkiewicz at 04/23/2001 8:05 AM EST


Engineer on team #111, Wildstang, from Wheeling High & Rolling Meadows High and Motorola.


In Reply to: Drill motor Demagnetization and lowered-torque woes
Posted by Joe Balint on 04/19/2001 12:06 AM EST:



Joe,
We have run across motor demag on motors we use here at work. They are a pancake/printed circuit armature design used in Ampex tape recorders. For some reason, the design of the magnet structure allows field to be drained off when the motors are disassembled for cleaning, armature replacement or brush replacement. It was astounding how much the field deteriorated after a few rebuilds. Since you are not disturbing the magnet structure, I don't think this is a problem. I would test bed the motors, out of the robot, with known good ones. This will eliminate bad electrical interface(circuit breakers, wire, connections). Under the load conditions you describe, it is possible to short a few windings (see Joe Johnson's post) in each of the many poles of the motor. We know that the circuit breakers reset very quickly and could be tripping on over current each time the bad pole comes around, thereby reducing the overall power output of the motor. Two ways to check the current on the motor is to scope the voltage across a small resistor in series with the motor, (1 ohm) or a current probe for a scope.(Hard to come by and expensive) The current should be fairly constant with no large peaks. (there will be spikes as the brushes move on and off the commutator) A repetive large peak would be a suspect short winding.
If the magnet structure is still suspect, a gaussmeter is the instrument of choice to check.
On our Ampex motors, when the magnet went bad we could stall it by hand, there was no doubt of the defect.
Al