Under what circumstances would semi-conductor (or high speed fuses) be used on a variable speed drive application? What purpose do they serve?

Semiconductor fuses are an energy limiting fuse using specially shaped silver elements in a silicon sand environment. Because of their energy limiting characteristics, they are able to protect some semiconductor devices from excess energy let through under fault current conditions. The energy limiting characteristic of semiconductor fuses is also useful for reducing the amount of damage done by an arcing fault. - effectively the fault current is reduced.

In the case of a VFD, there are two power semiconductor technologies used, the bridge rectifier and the output stage.
The bridge rectifier comprises power rectifiers that are connected directly to the supply and drive a capacitor bank, sometimes directly and sometimes via inductors. The nature of the design of the VFD means that the rectifier is less likely to be damaged by a short circuit on the output or in the motor. The rectifier is most likely to be damaged by a voltage surge or transient on the input. Because of the capacitive load on the rectifier, even a relatively low magnitude fast impulse can result in a high charging current trough the recitifers into the capacitors and this could damage them. The use of AC line reactors or DC bus chokes reduces the vulnerability of the rectifiers to such transients. The semiconductor fuses could be used on the input to provide some protection to the rectifiers but this is rarely done.
The charged capacitors are connected to a three phase inverter bridge, usually comprising IGBTs in modern designs. The output of the inverter is then connected via a cable to the motor. Modern IGBTs are very rugged and able to be controlled. If an overload occurs, it is generally possible to shut the devices OFF before they are damaged. The Capacitors however, do store up a large amount of energy and if any fault occurs, the energy is not limited, even if the AC supply is removed, until the capacitors are discharged. If there is an insulation breakdown, or an IGBT fails, then the resulting damage can be very severe and difficult to repair. The use of energy limiting fuses between the Capacitor bank and the output inverter will drastically reduce the damage, but will not protect the output transistors. The output devices are often damaged, not by overload currents, but by peak voltages due to arcing, switching or similar. Fuses will not prevent this.
As a rule, one or preferable two semiconductor fuses are used in the DC bus between the capacitors and the output bridge, but rarely used elsewhere.
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