Surge Protection for Variable Speed Drive


Today’s increased reliance on sensitive electronics makes surge protection an important topic for most industries. The Insurance Institute for Business & Home Safety study found that $26 billion dollars was lost due to non-lightning power surges. In addition, there are about 25 million lightning strikes in the US each year that cause between $650M to $1B in losses according to the Insurance Information Institute, State Farm©.

A Variable Speed Drive (VSD) also known as Variable Frequency Drive (VFD) is the main component of any moving part in the electrical equipment. This device allows for the control of speed to almost any electrical motor, which otherwise would always be a constant speed. This is done by adjusting the output frequency to the electrical engines. VSDs are found in a multitude of applications: Pump panels, HVAC compressors, conveyors, CNC machines, blowers, machine tool speed, etc. Therefore, they are found across multiple industries, such as Water Waste Treatment, Oil & Gas, and Industrial Machinery.

Directly installed inside a UL 508A control panel (known as package drive system) and spread across a Motor Control Center (MCC), Variable Speed Drives are critical to the overall operation of the system. Usually sitting between the incoming power to the panel and the motor, these devices are continuously prone to electrical transients. Based on solid state components, their surge withstand capability is extremely limited, hence the need for a global surge mitigation solution avoiding costly downtime.

This section will identify the UL 1449 5th Edition Surge Protective Devices (SPD) to efficiently protect the power input and output of the drive, as well as UL 497B Listed Surge Protection Devices to protect the communication between the VSD and the Programmable Logic Controller (PLC), usually done through RS485, Modbus, CAN bus or more recently Ethernet ports.

VSD Power Input | VSD Power Output | VSD Communication I/O

VSD Power Input

The main AC incoming will be the primary location to protect the drive against electrical surges and overvoltage. There can be a multitude of AC input voltages for the VSD, but the most common drives will run through a three-phase AC network without any Neutral, split into 3 voltage ranges : from 120Vac to 240Vac, 277Vac up to 480Vac and 600Vac up to 690Vac. DC Drives do exist as well, but are less common with voltages ranging up to 1,000Vdc. The common surge protectors used below :
LocationSystemSurge Protector Model #Link
VSD AC Input120 to 240Vac (3W+G) DS74US-240D DS74US-480D
VSD AC Input277 to 480Vac (3W+G) DS74US-480D DS74US-480D
VSD AC Input600 to 690Vac (3W+G) DS74US-600D DS74US-480D

VSD Power Output

The AC power output of the drive to the motor, runs on the same voltage as the VSD input. Only the frequency varies. It's obvious the exposure to transients feedback from the field. However, the output of the VSD is very commonly discarded in terms of surge protection, the main reason being the presence of Temporary Over Voltages (TOV). Regular MOV-based SPDs will not provide the robustness required to handle these events. This is why hybrid solutions (such as CITEL “/G” or “VG” Technologies) are highly recommended. The common surge protectors used below:
LocationSystemSurge Protector Model #Link
VSD AC Output120 to 240Vac (3W+G)${ITEM_LINK#2178920}<-No Item, #2178920 M50-240D-A${ITEM_LINK#2178920}<-No Item, #2178920 M50-A
VSD AC Output277 to 480Vac (3W+G)${ITEM_LINK#2178948}<-No Item, #2178948 M50-480D-A${ITEM_LINK#2178948}<-No Item, #2178948 M50-A
VSD AC Output600 to 690Vac (3W+G)${ITEM_LINK#1505198}<-No Item, #1505198 M50-600D-A${ITEM_LINK#1505198}<-No Item, #1505198 M50-A

VSD Communication Ports

The drive output frequency shall respond to the actual need for the engine to run faster or slower. This information is provided to the VSD by PLC, which interface between the different sensors in the field on the variable speed drive. Traditionally, the communication between the VSD and the PLC was done through 3 main protocols : RS485, CAN Bus, Modbus. Nowadays the deployment and Ethernet cable with RJ45 connectors is a lot more efficient. The common surge protectors used below:
LocationSystemSurge Protector Model #Link
VSD Twisted Pair WireRS485, CAN Bus, ModbusDLA-12D3 DLA-24D3
VSD Ethernet PortRJ45 up to CAT6AMJ8-C6A

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