Solid State Drive Control Questions and Answers

Answer: C

Correct answer: 160 W

Switching loss is the energy per event multiplied by the number of events per second: \[ 40 \times 10^{-3} \times 4000 = 160\ \mathrm{W}. \] Note that it is proportional to frequency, which is the fundamental constraint on how fast an inverter may be switched - the harmonic benefit of a higher frequency is paid for in heat.

Answer: D

Correct answer: 400 W

Conduction loss is the on-state voltage times the current times the fraction of the time the device conducts: \[ 2 \times 400 \times 0.5 = 400\ \mathrm{W}. \] With the 160 W of switching loss the device dissipates 560 W in total, and it is that total which the thermal design must remove.

Answer: A

Correct answer: 3 kW at the permitted temperature rise

The heatsink must remove the whole 3 kW while holding the junctions below their limit, so the thermal resistance from junction to ambient must be low enough that the product of loss and resistance leaves a margin. Removing 3 kW at any temperature rise would be trivial and useless - the rise is the whole of the requirement.

Answer: B

Correct answer: about 10 kW into its enclosure

\[ 0.02 \times 500 = 10\ \mathrm{kW}. \] Two per cent sounds negligible until it is expressed in kilowatts, and all of it appears as heat inside the cabinet. The room ventilation or air conditioning must remove it, which is why the drive’s loss is part of the building services calculation and not only the drive’s own.

Answer: C

Correct answer: 97 to 98 per cent

A modern voltage source drive loses two or three per cent at rated load, divided between conduction and switching in the semiconductors with a small contribution from the magnetics and the control supply. The figure falls at light load, where the fixed losses become a larger share, which is worth remembering when a drive spends its life part-loaded.

Answer: D

Correct answer: alternating current to direct current

The rectifier is the input stage, converting the incoming ac supply into the dc that charges the link. The inverter at the other end converts that dc back to ac at whatever frequency the machine requires. Splitting the conversion in two through a dc link is what makes the output frequency independent of the supply.

Answer: A

Switching the dc link voltage onto the machine terminals in a controlled pattern synthesises an alternating supply of whatever frequency and amplitude is wanted. It is the stage that gives an induction machine its variable speed.

Answer: B

Correct answer: direct current at one voltage to direct current at another

A chopper is a dc to dc converter, switching a device on and off to control the mean voltage delivered. In a drive it appears as the braking chopper that dumps regenerated energy into a resistor, and in dc drives as the means of controlling armature voltage.

Answer: C

Correct answer: between the rectifier and the inverter

The dc link is the reservoir between the two conversion stages: the rectifier charges it from the supply and the inverter draws from it to feed the machine. Its capacitance holds the voltage steady against the current pulses each stage demands, and it is what decouples the input frequency from the output.

Answer: D

Switching rather than throttling is what makes the efficiency high, since a device that is fully on drops little voltage and one that is off carries no current. Dwelling in between is what destroys devices, which is why transitions are made as fast as the circuit allows.