Galvanic and/or optical isolation shall be provided between the Variable frequency drives power circuitry and control circuitry to ensure operator safety and to protect connected electronic control equipment from damage caused by voltage spikes, current surges, and ground loop currents.

Normal variable frequency drive operation will enable all of the connected motors to maintain a constant speed, but only if the correct type of motors are used. A standard induction motor tends to slip somewhat with respect to the line frequency as its load varies, so speeds won’t be synchronous. The solution is to use three-phase, inverter-duty synchronous induction motors as this ensures that the motor speeds will remain synchronous with the line frequency.

The best way to eliminate system noise is to attenuate it at the source (the VFD) before it gets out into the system grid and takes multiple high frequency sneak paths which are hard to track down in an installation. Past experience has shown Common Mode chokes on the VFD output and CM cores on the interface equipment are highly effective in ensuring fully operational tripless systems in medium to high risk installations. A Common Mode Choke (CMC) is an inductor with output Phase A, B and C conductors all wound in the same direction thru a common magnetic core. The CMC provides a high inductance and high impedance to any line to ground based capacitive noise current generated during the VFD's fast switching output voltage edges. Thus, the magnitude and rise times of these noise currents are substantially reduced below noise thresholds of affected equipment. The CMC is an optimal noise reduction technique since it does not affect the line to line power circuit while "choking" or high impedance blocking the ground based noise currents. As such, it takes up less physical space than an output line reactor. CMC's should be considered in installations with susceptible electronics. They may be used on retrofit situations, older systems with 3 wires in a conduit or preferably with the recommended shielded wiring practice to obtain maximum noise reduction benefit.