I have 5 motor drives using the LMD18200 drivers for driving 24- 850ma motors. I have now replaced 5 out of 10 of the LMD18200 on these units. I have the brake signal tied to ground and only switch the direction pin and turnon /turn off the PWM pin after a short delay. I have the boot strap capacitors in place, the 220uf cap is across the power supply but is several inches from the LMD. 2 of the failures have been definitely been where the motor leads are 70 feet from the controller to the motor.
I just did measurements this week on the output voltage from the LMD. The is a little LC oscillation when the output shuts off coming back from the motor but there are no transients greater than 24 volts. I have put in opto- couplers on the control pins and bias these at 5 volts. The 24 power supply is negative grounded.
These are all on Solar tracking motor drives - brushed motors. I tested the circuit on the bench driving motors with no problems.
I attached pictures of the on and off transitions at the terminals of the LMD18200 when driving one of the motors with 60 leads attached. 2438.Asche Solar LMD18200 Output Voltage.pdf
Thanks for any tips.
Not clear as to 'what-is-where' in the "... 70 feet from the controller to the motor ..." statement.
Is the LMD18200 70 feet away from the motor, or is the control logic output 70 feet away from the LMD18200 which is mounted at the motor?
Where is power supply mounted, adjacent to LMD18200?
The LMD18200 is fairly rugged device, but if the LMD18200 is indeed remote from the motor, the 70 feet of wire on each output will require a serious look at inductive transients on the output pins.
There was nothing revealing in the waveforms provided.
As a guess ... I would think that failures are most likely from electrical overstress of some type ... and I would start looking for voltage transients with respect to the LMD18200 ground pin. Most likely would at be the output pins. As a precaution I would add four external Schottky diodes to the outputs to help clamp any transients. Yes, there are internal diodes in the LMD18200 but they are not particularly efficient.
Should these be about a 40V 1 or 2 amp schottky diode?
The 70ft of wire is between the LMD18200 and motor. Lead distance from power supply to LMD's is about 6 inches.
40V 1A should be OK.
At 70ft from LMD18200 output to the motor I would suggest that twisted pair be used.
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