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Fuses Failed to Protect a Variable Frequency Drive
Copyright © 2006 Francis J. Martino

Question: I have commissioned a 300 H.P. AC Drive to drive a 200 H.P. motor. The aplication is a pump. The drive was supplied through ultra-fast fuses in order to protect the semiconductors.

The drive has been working properly for one month, but suddenly it has showed the fault code that means that there is no output in one phase. It seems that one output IGBT has blown. The cause of this fault was a blackout (main power supply missed).

Whatcould be the cause of damage in output IGBT after a blackout if none of the fast-fuses has blown? What considerations must I take in mind to avoid this kind of damage after a power supply lost?

Answer: Power supply failures normally come with transients that are generated by the utility power system.

A transient could have entered the drive that was high enough in voltage to puncture the IGBT but so short in duration that the fuses did not have time to respond to the subsequent current flow.

A similar malfunction is described here:
Transients: Incoming Power Disturbance

Fast-acting fuses are not intended to prevent damage to components. When the fuse blows, the fuse has blown because components have failed. The fast action and current limiting qualities of the semi-conductor fuses will give protection by limiting the fragmentation of failed components. Thus, the protection that a fuse gives is not to prevent a component failure but, rather, the fuse will keep other components from failing by protecting them from flying fragments.

The way to prevent damage from transients is to add surge protection. The surge suppressors supplied internal to VFDs consist of MOVs and capacitors which provide a minimal protection.

Frank




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