AC Drives FAQ (part 4): Why semiconductor (or high-speed fuses) are used on a variable-speed drive (VSD, VFD) application

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Explain why semiconductor (or high-speed fuses) should be used on a variable-speed drive (VSD or VFD) application?

Semiconductor fuses are a type of power-limiting fuse using specifically-shaped silver elements in a silicon/sand environment. Because of their power-limiting characteristics, they can protect some semiconductor devices from excess power. The power-limiting characteristic of semiconductor fuses is also useful for reducing the amount of damage done by an arcing fault; hence,the fault current is effectively reduced.

For Variable Frequency Drives (VFDs), there are two power semiconductor technologies employed: bridge rectifier and the output stage.

The bridge rectifier is made up of power rectifiers which are connected directly to the supply and drive a capacitor bank, either directly or via inductors. By design, then, the rectifier of the VFD is less likely to be damaged by a short circuit on the output or in the motor. The rectifier is susceptible to damage by a voltage surge or input transients. Due to the capacitive load on the rectifier, even a relatively low-magnitude fast impulse can cause high-charging current through the rectifiers into the capacitors (which may damage them). The use of AC line reactors or DC bus chokes reduces the vulnerability of the rectifiers to such transients. The semiconductor fuses may be used on the input to give some protection to the rectifiers, though this is not a common practice.

The charged capacitors are connected to a 3-phase inverter bridge, usually made up of modern IGBTs. 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 happens, it's normally possible to turn OFF the devices before damage occurs. The capacitors, however, store 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 power-limiting fuses between the capacitor bank and the output inverter will significantly 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 events. Fuses can't prevent this.

As a rule, one or, better yet, two semiconductor fuses are used in the DC bus between the capacitors and the output bridge, but rarely used elsewhere.

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