A short circuit must be cleared by an IGBT within about
Answer: C
The device's short-circuit withstand time is of that order, after which it fails, so protection has to act far faster than any fuse. That is why the gate drive itself carries the detection rather than leaving it to the controller.
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Soft turn-off is used when clearing a short circuit in order to
Answer: D
Correct answer: limit the overvoltage from the circuit inductance
Interrupting a short circuit current abruptly generates a voltage across the stray inductance proportional to the rate of change, and at short circuit currents that voltage would exceed the device rating. Soft turn-off reduces the gate drive so that the current falls more slowly, keeping the overvoltage within what the device can block.
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Semiconductor fuses in a drive are provided to
Answer: A
A device that has already failed short must be isolated before the link capacitor empties into it and ruptures the module, so the fuse protects the surroundings rather than the device. Electronic detection protects the healthy devices.
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The junction temperature of a device in a drive is estimated from
Answer: B
Conduction and switching losses give the heat, and the junction-to-case, case-to-sink and sink-to-ambient resistances give the rise. Adding them to the ambient gives the junction temperature that has to stay within the device's limit.
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A device dissipating 60 W through a total thermal resistance of 0.8 K/W from junction to ambient sits at a junction temperature of
Answer: C
Correct answer: 48 K above ambient
\[ \Delta T = P R_{th} = 60 \times 0.8 = 48\ \mathrm{K}. \] The thermal circuit is exactly analogous to an electrical one, with power as current and temperature difference as voltage. If the ambient is 40 degrees the junction sits at 88, which must be compared with the device limit of typically 125 or 150.
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Reducing the thermal resistance from case to heatsink is achieved by
Answer: D
Air is a poor conductor, so the microscopic gaps between the two flat surfaces dominate unless they are filled with paste or a pad and the module is clamped evenly. It is a routine assembly step that is easy to get wrong.
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Forced air cooling of a drive is preferred to natural convection when
Answer: A
Moving air over the fins multiplies the heat a given heatsink can remove several times over, which allows a much smaller assembly. The fan is then a wearing part whose failure has to be detected before the devices overheat.
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The dc link capacitor of a drive is precharged through a resistor in order to
Answer: B
An uncharged capacitor looks like a short circuit to the rectifier, and the resulting surge would damage the diodes and trip the supply. A resistor shorted out by a contactor once the voltage has risen is the usual arrangement.
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The dc link capacitor of a drive must also be
Answer: C
It holds a lethal charge for minutes after the supply is removed, so a discharge resistor and a positive means of verifying zero volts are essential. Warning labels and a stated waiting time appear on every drive enclosure.
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Electrolytic dc link capacitors limit the life of a drive because they
Answer: D
Electrolyte escapes slowly through the seal, and the rate roughly doubles for every ten degrees of temperature, so a hot cabinet shortens the life sharply. Film capacitors and the matrix converter's absence of a link are the two ways round it.
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