It will surprise absolutely no one that the power grid in many parts of the US is stressed, particularly when more extreme weather events occur. Power sags, brown-outs, and even black-outs can occur. Normally, variable speed drives (VSD’s) are quite capable of handling these low-voltage conditions before they become damaged; worst case, the VSD shuts down and the process is interrupted, which admittedly can be an expensive problem. On the other hand, power surges, depending on their causes and characteristics, can be a much more serious problem for VSD’s.

Surges can be transient or sustained, and result from internal or external sources. Their magnitude is a consequence of the duration of the surge and the power available to feed it, which in turn results from the impedance available in the supply system. Transient surges, defined as occurring for less than one millisecond, are of greatest concern because they are harder to detect and interrupt before equipment damage occurs. Even high-speed fuses may not clear quickly enough to prevent electronic component damage if the current let-through is high enough, and coordination with upstream over-current devices is more difficult, meaning the extent of process interruption is harder to limit.

Most surges result from internal (i.e. inside the Point of Common Connection (PCC) of the facility with the utility supply) factors, such as load switching during normal operations. Because internal surge events are often damped significantly by the distribution system wiring and components, their magnitudes are often not a concern other than for particularly sensitive equipment. Note that there are exceptions to this – for example, a surge caused by a short-circuit in a line supplying multiple parallel-connected motors – which must be considered. Often of greater concern, however, are externally caused transient surges such as utility supply switching and lightning strikes. Depending on your facility’s location, and the load characteristics of its neighbors, the impedance may not be high enough to limit the magnitude of the event, and severe damage to VSD’s and other connected equipment can occur.

There are a few ways to reduce the impacts of such surges on your components. One is to use properly rated Surge Protective Devices (SPD’s), formerly referred to as Transient Voltage Surge Suppressors (TVSS’s). These are intended to limit over-voltage by discharging surge current within their rating. Depending on classification, they can be installed on the line side of the facility’s service entrance disconnect, on the load side, at the point(s) of utilization (i.e. downstream of distribution panels), or in combinations of these locations. They are typically designed with single-use, replaceable interrupting elements, usually metal oxide varistors, although some higher-capacity lightning arrestors use components designed to be subjected to a number of surges before failure. At Joliet Technologies, we source SPD’s from ABB and Raycap, among others. See standard UL 1449, 3rd edition, for additional information.

Two other methods of protection for VSD’s are to install AC line reactors, or  to separate the drive physically from the supply system by feeding it through an isolation transformer. While line reactors are usually less costly, require less physical space to install, and provide a good measure of surge protection,  an isolation transformer can provide more robust surge protection and is sometime chosen for its superior harmonics reducing capabilities. The choice is often one of available space and capital outlay. Note that many drives are provided with DC link reactors as standard equipment; while these are effective at smoothing DC current and reducing harmonics, they do not protect sensitive rectifier bridges from voltage surges, so the risk of surges to the equipment must be factored in when deciding whether to purchase optional AC reactors.

We can provide more information specific to your application; contact us at or check out our web site ( And please feel free to contribute to our Comments section; your real-world experiences could be invaluable to our readers. See you next week!

Jay Baima - Author


Jay Baima
Joliet Technologies