Isolation Transformers and VFDs

Informative discussion regarding VFDs and Isolation Transformers

Interposing a drive isolation transformer between the VFD and its power source offers several benefits. Isolation ensures that no direct electrical connection exists between source and load — but that's true for any transformer other than an autotransformer. What makes the drive isolation transformer unique is the placement of grounded electrostatic (Faraday) shielding between and around primary and secondary windings. This shielding provides up to a million-fold decrease in the capacitive coupling involved in transferring common-mode voltage disturbance. Without such shielding, that capacitance allows the passage of high-frequency noise and transient voltage spikes through the transformer.

Common-mode transients are those appearing between the ground and neutral of the ac system. Although those two parts of the circuit are normally bonded together at one point, they cannot be presumed to be at the same potential throughout an entire power system. Common mode transient disturbances arise from switch mode power supplies, drive operation, arc welders, lightning, or even from normal operation of such equipment as stepper motors. Some isolation transformers can also block “normal-mode” transients appearing between line and neutral.

Consider one application in which a daily utility company powerup of a substation capacitor bank in an industrial park causes a transient voltage spike — amplified by reflection from onsite capacitors in a nearby plant. Assume that one facility in the park has several small drives rated at 7.5 hp. Normal mode transients can cause these drives to shut themselves off, resulting in costly process downtime. An isolation transformer can prevent such disruption.

Note: Installing an Isolation Transformer to feed one or more drives provides a good grounding point on the XO tap of the transformer secondary. This would be a good solution when installing VFDs on a high-resistive or ungrounded power distribution system.