Motor Disconnects For VFD Applications–PART III

Motor Disconnects For VFD Applications–PART III

In the right applications, variable frequency drives (VFD) will improve the control and reduce the energy costs of your AC motor-driven equipment. To install a VFD safely, however, you may need to reconsider the type of motor disconnect you use. The traditional knife-blade style of disconnect, while compliant with the National Electrical Code (NEC), is not the best choice for modern VFD applications. The reasons why are both physical and electrical.

From a physical standpoint, the traditional NEMA-style blade disconnects are too bulky for IEC installations, which often group or combine motor controls and disconnects in a shared enclosure or in limited spaces. In these applications, the best alternative is a compact rotary disconnect.

The use of rotary disconnects, in non-metallic or stainless steel enclosures, continues to grow in popularity as more and more machine builders and OEMs adopt IEC devices.

From an electrical standpoint, VFDs require motor disconnects with an auxiliary contact that can provide crucial early break functionality. This contact signals the drive before disconnecting the motor, giving the drive time to power down in a controlled manner before the mains open.

Plenty of disconnects, both blade- and rotary-style, offer an auxiliary contact. But disconnects are not all created equal when it comes to protecting expensive VFDs.

For one thing, auxiliary contacts have speed differences based on their actuation methods. Cam actuated versions provide fast and predictable early break switching. Our onboard cam-actuated auxiliary contacts will provide a minimum, repeatable time gap of 20 ms between the opening of the auxiliary and the opening of the mains. This repeatability and speed provide an advantage over conventional auxiliary contacts with mechanical linkages whose actuation speeds can vary based on how slow or fast the switch handle is operated.

For another thing, auxiliary contacts aren't always included in the price of the base disconnect. In our case, auxiliary contacts are included as a standard feature. Both auxiliary contacts and early break capability can be options with other manufacturers at an added cost.

Read Part I and Part II of this series to learn more about how to select the best motor disconnect. Or download our catalog to see specifications for our full line of motor disconnects.