Short circuit protection in industrial motor drives
The overall market trend for industrial motor drives is the ever-increasing demands for greater efficiency and reliability and stability. Power semiconductor device manufacturers continue to seek breakthroughs in conduction losses and switching times. Some trade-offs to increase the conduction loss of insulated gate bipolar transistors (IGBTs) are: higher short-circuit current levels, smaller chip sizes, and lower thermal capacity and short-circuit withstand times. This highlights the importance of gate driver circuits and overcurrent detection and protection. This paper discusses the successful and reliable implementation of short-circuit protection in modern industrial motor drives.
Short circuit in industrial environment
Industrial motor drives have a relatively harsh operating environment and can experience high temperatures, AC line transients, mechanical overloads, wiring errors, and other unexpected conditions. Some of these events may cause large overcurrents to flow into the power circuit of the motor drive. Figure 1 shows three typical short circuit events.
among them:
1 is the inverter through. This may be caused by incorrectly turning on two IGBTs of one of the inverter arms, which may be due to electromagnetic interference or controller failure. It may also be caused by one of the IGBT wear/faults on the arm, while the normal IGBT remains switched.
2 is a relative phase short circuit. This may be caused by a performance breakdown, excessive temperature or an overvoltage event that causes an insulation breakdown between the motor windings.
3 is a phase line to ground short circuit. This may also be caused by a performance breakdown, excessive temperature or an overvoltage event that causes an insulation breakdown between the motor windings and the motor casing. In general, the motor can absorb very high currents for a relatively long period of time (milliseconds to seconds, depending on the size and type of the motor); however, the IGBT, the main part of the industrial motor-driven inverter stage, is short-circuited. The tolerance time is in the order of microseconds.
IGBT short circuit withstand capability
The IGBT short-circuit withstand time is related to its transconductance or gain and the thermal capacity of the IGBT chip. Higher gains result in higher short-circuit currents within the IGBT, so it is clear that lower gain IGBTs have lower short-circuit levels. However, higher gains also result in lower on-state conduction losses, and trade-offs must be made. The development of IGBT technology is contributing to the increase in short-circuit current levels, but the tendency to reduce short-circuit withstand time. In addition, advances in technology have led to smaller chip sizes, reduced module size, but reduced heat capacity, resulting in further shortened tolerance times.
In addition, it has a large relationship with the IGBT collector-emitter voltage, so the parallel trend of industrial drive tends to higher DC bus voltage levels further reduces the short circuit withstand time. In the past, this time range was 10 μs, but in recent years the trend has been in the direction of 5 μs 3 and under certain conditions down to 1 μs.
In addition, the short-circuit withstand time of different devices is also quite different, so for IGBT protection circuits, it is generally recommended to build more margin than the rated short-circuit withstand time.
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