Should Drives Be Specified In Division 15 Or 16? (August 2000)
The motor control center (MCC) was first introduced at the turn of the century. MCCs allow the designing engineer to maximize the usage of valuable space by putting motor controls and branch circuit protection in a common enclosure. MCCs also allow the enduser to consolidate wiring, simplify installations, and keep the electrical controls out of the manufacturing environment. With the advent of solid-state electronic controls, it seemed natural to incorporate these new controls into the MCCs. However, MCCs were not designed to house electronics. In the case of vfd’s and other heat-producing, solid-state devices, there are several potential problems that the designer should understand.
Voltage reflection may occur when the length of the motor cable is greater than or equal to the critical length. Vcable (sometimes referred to as propagation factor) is the speed that the pulse travels from the drive to the motor in feet per microsecond. The value of Vcable depends on the type of conduit or cable tray and other details of the cable installation. Vcable is often estimated as 500 ft per microsecond (µsec); tr is the rise time of the output pulses from the vfd under consideration.