Quote:
Originally Posted by Hieb
Perhaps someone can help me out here then (now that it's after the fact for us). I thought I had done the calculations correctly, but apparently not.
I had a 3.5 foot arm, total weight about 6 pounds. Using the 540 motor into the 64:1 gearbox, we then stepped down 28:10 and 70:10. The few times before shipping that we were able to test the arm it worked fine except for the backdrive. Then on ship day we fried the motor while driving it to the crate.
On Thursday we replaced the motor and swapped the 64:1 with a 125:1 gearbox. We proceded to fry 2 more motors before scrapping motors and switching to pneumatics (which were also abandoned for lack of time and a better strategy).
Also of note--we had a 20 amp breaker on the motor (stall current 42 amps) and in one practice match we completely drained a battery and the motor smoked with only 2 seconds left in the match.
Have I just messed up my calcs and completly abused these poor motors, or is there another issue here?
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The biggest issue with the 540 motors (from what I've heard) is running them to 'hold' position because they have poor heat dissapation when not moving (I believe that their cooling is directly related to the shaft moving). As such, when you are powering the motor to hold a position, it heats up very quickly and then lets the magic smoke out. This is compounded even more if you are at stall current.
Example of 'hold position': An arm that is extended and would backdrive quickly (due to gravity, etc) if the motor did not have current running through it. This can be checked by putting an ammeter inline while running the arm in 'hold' position.
The 20 amp snap-action breakers only trip if current is
ABOVE 20 amps for considerable time (the amount of time is dependent on the current since the breaker is a thermal device). They also quickly reset and reapply the current to the device, not giving the motors enough time to cool. Spikes can eliminate part of this problem because they have an integrated fuse, but if that fuse goes you lose all functionality of the device it controls.