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Adjustable Speed Drives, Part 3

Adjustable Speed Drives, Part 3What are the NEC requirements for Variable Frequency Drives (VFDs) and other types of Adjustable Speed Drives (ASDs)?

You will find these in Part X of Article 430 (430.120 through 430.130). These requirements are the minimum for safety, not instructions on how to select, install, or program a drive for the desired outcome in a given application. Engineering considerations may compel you to exceed them.

The circuit conductors included as part of a drive system must have an ampacity of at least 125% of the rated input current to the power conversion equipment [430.122(A). The catch here is power conversion equipment can have multiple power ratings and thus multiple corresponding input currents [430.122(A) Informational Note]. Before you size the circuit conductors, nail down which power ratings will be used in your specific application.

Often, overload protection is built into the drive; if that’s the case, you don’t have to provide additional overload protection [430.124]. Drives are required to provide protection against motor overtemperature conditions [430.126(A)]; don’t confuse this protection with the overload protection.

Where do you place the motor disconnect? You can place it in the incoming line to the conversion equipment (this is actually a good choice for many reasons); make sure the rating of the disconnect is at least 115% that of the conversion unit [430.128].

New with the 2014 NEC is an entire subsection about short circuit and ground fault protection [430.130]. Read it carefully.

« Part 2 | Source: Mark Lamendola | Mindconnection