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An Application Note for Using Industrial Switches with VFDs

Safety Disconnect Switches, Mechanical Interlocks and Variable Frequency Drives

Powerswitch® safety disconnect switches, LEV Series mechanical interlocks, as well as standard series mechanical interlocks from Leviton are all well suited for applications involving Variable Frequency Drives (VFDs).

VFDs are used extensively throughout many commercial and industrial applications to control motor and pump loads. The benefits include reduced energy consumption, extended equipment life and more efficient process control. However, there are some installation requirements that require additional consideration.

VFD Application Example

Article 430 of the National Electrical Code (NEC) requires that a disconnecting means for the motor be installed in sight of the motor and driven equipment. If the VFD is installed in sight, and has an integral disconnect, that would satisfy the NEC requirement. However, in many cases the VFD is located within a motor control center that is located outside of the “line of sight” requirement. In this case, a separate disconnect is required for the motor, which would be installed between the VFD and motor.

A standard disconnect switch is not appropriate for this application, as disconnecting the motor in this type of installation could damage the VFD, the disconnect switch, and/or the motor, resulting in costly repairs and create a potential safety hazard for workers. In order to avoid this situation, the disconnect switch should employ an early break, late make auxiliary contact. This would be used to signal to the VFD that the motor is being disconnected before actual power disconnect, and would signal reconnection to the VFD after power to the motor has been restored.

The Leviton Powerswitch line of safety disconnects features devices that come standard with an early break auxiliary contact, making them ideal for use with VFD installations. And, for cord connected equipment, LEV Series and standard series mechanical interlocks employ the same auxiliary contact arrangement.


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