Surge Protection for Variable Speed Drive

OVERVIEW

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 :

Location	System	Surge Protector Model #
VSD AC Input	120 to 240Vac (3W+G)	DS74US-240D
VSD AC Input	277 to 480Vac (3W+G)	DS74US-480D
VSD AC Input	600 to 690Vac (3W+G)	DS74US-600D

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:

Location	System	Surge Protector Model #
VSD AC Output	120 to 240Vac (3W+G)	M50-240D-A
VSD AC Output	277 to 480Vac (3W+G)	M50-480D-A
VSD AC Output	600 to 690Vac (3W+G)	M50-600D-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:

Location	System	Surge Protector Model #	Link
VSD Twisted Pair Wire	RS485, CAN Bus, Modbus	DLA-12D3
VSD Ethernet Port	RJ45 up to CAT6A	MJ8-C6A