VFD Drives

The system shall also be equipped with a facility which will restart the system in case of voltage dip over 20% of power interruptions for min. 2 seconds, with recovery of the voltage to its nominal value, with a facility to block the automatic restart if required by the operator. Upon restart the converter shall be capable of synchronizing onto a rotating motor and develop full acceleration torque within 10 seconds.

System Grounding Practice: Ungrounded, High Resistance or Solid Ground. The philosophy of the ground system for VFD input power is usually specified by the user and based on user concerns other than electrical noise. A solid grounded wye secondary system is a low impedance to the transient CM noise current and completes the return path back to the VFD input leads from the ground grid. Highest CM current magnitude occurs with this system but very little CM noise goes out into the PE grid beyond the transformer neutral connection in Fig. 8, so that CM noise is contained. An advantage of grounded secondary systems is that primary side line to ground high voltage transients are attenuated by typically 20 dB on the secondary side, thus reducing the amount of transient energy the VFDs Metal Oxide Varistor(MOV) transient protectors must handle.

What types of applications meet the three specific conditions detailed above? Applications that use dual fans or pumps are good candidates. The variable frequency drive can ensure that the two units are operated at the desired speed, and don't end up fighting each other or having one unit carry more than its design load level. Air handling systems, exhaust/supply fans, make-up air units, recovery wheels and fan arrays are also good candidates for an installation where one variable frequency drive controls multiple motors.