Setting the max motor current too low can cause you grief if, on occasion, the motor needs that current to get things moving. Esp if you have the restart tries set for three. Motor starts, faults, waits x seconds, repeat, repeat. All the while jerking the dung out of things and you wonder what the heck is going to break first, not that I've had any personal experience of that sort.:slaphead:
Any hints on how the max motor current should be set? Experience? Dart board? Cow pie bingo?
Tom,
It's going to depend a little on the details of the specific VFD. Some ask you for the motor current or motor OL current, which is the nameplate FLC, and from that it determines the OL tripping curve. Those that don't have UL listings as a motor protective device will have a simpler "Maximum Motor Current" setting that is more like a "shear pin trip" that trips the instant the current hits that value. That's the type that gives you headaches like you mentioned. Even drives that have the OL curve protection capability will still have that, but it will be based on the current capacity of the VFD itself. So on a "Constant Torque" rated drive, that is almost always 150% for 60 seconds, and on a "Variable Torque" drive is is usually 110% for 30 seconds. These are hard limits of the DRIVE'S output capacity. Better quality CT rated drives will also have a little overhead to allow for Break Down Torque of 200% of FLC, but only for 2-3 seconds. Those will have a separate trip function called "Hardware Over Current", which means you were about to kill the drive transistors...
One tactic to avoid the headaches is to over size the drive, ie if you have a 100A motor, buy a 200A drive, because 150% of the 200A drive is 300A for 60 seconds, rather than 150A. If you can't do (or afford) that, step 2 is to look for a drive with active stall prevention, torque limit or current limit (all names for similar functions). That will work to override a speed setting in favor of keeping the current below the trip thresholds. So if you set the accel time to 10 seconds but then set the current limit to 100A and the motor hits 100A in 5 seconds as it accelerates, it overrides your accel time and lets the motor accelerate as best it can with 100A. It results in longer acceleration (or RE-acceleration after a step change in load) than you may have set the drive for, but it avoids the tripping hassle. If the motor CANNOT accelerate at 100A, and that's the wall of the VFD rating, then your drive is too small for the application.
