VFD Practices
Variable Frequency Drive problems seem to be common these days, and for a variety of reasons. Failures or frequent drop outs due to component failures are seen often, and the reasons are worth noting.
Manufacturers of drives and soft starters have reduced costs in some cases by eliminating internal fusing, leaving sensitive electronics vulnerable to failure and to non-compliance with code branch circuit requirements, especially on drives below 200HP.
Some manufacturers claim that the IGBT in the drive unit itself affords a measure of self protection. The problem with this approach is that transient voltages higher than the IGBT maximum voltage rating will irreversibly damage the IGBT, eliminating its self-protection characteristics and causing destruction of the IGBT. In this condition, the drive cannot shut down on its own.
Capacitors can be damaged from a shorted IGBT. Experience with VFD failures, including that of third party repair facilities, confirms that shorted IGBTs are a common area of failure in a VFD. The resulting low impedance current path generates excessive heat and pressure that can cause a violent case rupture.
The rectifier section of a drive is built with diodes or SCRs, and while rectifiers have good withstand ability to over voltage conditions, they are very susceptible to damage from over current levels, constituting another potential failure point.
Circuit breakers provide overload protection but they lack the fast acting characteristics of a high speed fuse. Only purpose designed fuses can provide the fast acting, current limiting values that are compatible with semiconductors such as diodes, SCRs, GTOs and SSRs commonly used in solid state motor controls.
Typical let through levels by type of protection over time
What is needed to assure better protection levels for drives is a fuse that combines the low I2T (thermal energy) of a semiconductor fuse along with branch circuit protection, enabling one fuse to address a range of drive circuit applications.
Class HSJ fuses in particular offer some interesting characteristics that go beyond typical Class J fuses in addressing these problems -
They combine current limiting functionality with high interrupting ratings - typically to 200k AIR - in a fast acting fuse design.
This class of fuses meets both the NEC requirements for branch circuit protection and power electronics protection in one device.
Due to this unique design, customers who deploy HSJ Class fuses may be able to reduce inventory, using one fuse for both branch circuit protection and power electronics protection.
Considering the options, and the risk inherent in an inadequately protected VFD circuit, consideration should be given to whether VFDs are adequately protected.
Find out more about high speed, high interrupting level fuses for VFDs and other sensitive electronics Amp-Trap-High-Speed-Class-J-Fuses-HSJ-Brochure.
