Controlling variable frequency drive shock hazard is an important part of proper VFD maintenance and management. The risk of electrical shock stems from the manner in which motor drives assume control of motor voltage supply.
Electrical Shock Origination & Control
When a drive is energized but disabled, and the motor is stopped, it’s NOT open…there is no air between the line voltage and the motor leads. To safely work on the motor, the line input to the VFD must be opened. Some newer drives have a feature called “safe torque off,” which also is NOT a contractor. When the “safe torque off ” is active and the motor is stopped, the power connection is NOT open…there is no air between the line voltage and the motor leads. Controlling variable frequency drive shock hazard can be accomplished by opening the line input to the VFD. “Safe torque off ” simply interrupts the gate power to the output transistors so that rotation is not possible. The transistor and the diodes still connect the motor leads to the line voltage, which can cause a shock hazard unless the line input to the VFD is opened.
Arc Flash Implications
The electrical shock potential from a VFD is distinct but potentially related to arc flash. Arc flash, also sometimes referred to as “flash over”, stems from an arc fault, which is a severe instantaneous electrical discharge that results from a low-impedance connection through air to ground or another voltage phase change in a electrical systems. A gaseous fire ball is created filling the space immediately in front of an open cabinet. Arc flash is deadly and all precautions must be made to prevent it.
Controlling variable frequency drive electrical shock conditions is a #1 priority at Invention House. We take great precaution when building and working on custom AC motor drive solutions for our OEM and military clients.
Consult the users guide, website FAQ page or the help center of your VFD manufacturer before working on any VFD.