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Adjustable speed drives (ASDs) solve many application problems, but pose additional NEC requirements. All of Parts I through IX of Article 430 apply to these, except as modified or supplemented by Part X (which occupies a page and a half).
A common type of ASD is the Variable Frequency Drive (VFD); we’ll refer to VFDs going forward. Before we get into the Code requirements, why would you install a VFD?
It won’t multiply torque (for that, you need a gearbox), but it does allow you to adjust the speed across a range rather than in discrete steps. To change the output speed of a gearbox, you must change the gears. You can program a VFD to vary speed in time based on the settings provided.
It’s often a good solution to use a gearbox to provide the desired torque, combined with a VFD to get the desired speed or set of speeds.
One of the drawbacks of a large motor is it plays havoc with your distribution system. Obviously, it adds a major inductive load and the resulting phase shift drops your power factor (you can fix this with power factor capacitors installed at the motor). When a big motor starts across the line, the results aren’t pretty; you get wild swings in voltage and current in response to that big inrush current demand. A power factor-corrected, harmonics-corrected VFD solves these and other power quality problems.