Solid State Drive Control Questions and Answers

Answer: A

Correct answer: ripple current heats the capacitor internally

The capacitor has an equivalent series resistance, and the ripple current flowing through it dissipates \( I^2R \) inside the can. That heat raises the internal temperature, which accelerates the loss of electrolyte and shortens the life - so the ripple rating is a thermal limit rather than an electrical one.

Answer: B

The high-frequency common mode current that switching produces returns through the machine frame and its bearings unless it is choked off. Shaft grounding brushes and insulated bearings address the same problem from the other end.

Answer: C

Correct answer: protect the machine insulation on long cable runs

A fast switching edge travelling down a long cable reflects at the machine terminals, and the reflection can nearly double the voltage seen by the first turn of the winding. Repeated at the switching frequency, that overvoltage erodes the insulation. A reactor or dv/dt filter slows the edge so that the reflection does not build.

Answer: D

Current and speed loops, the modulator, the protection logic and the communications all run in software on one or two processors. That has made features such as sensorless control and adaptive tuning available at very little marginal cost.

Answer: A

Correct answer: once or twice per switching period

The current loop must run fast enough to control the current within the modulation, so it is synchronised to the switching period and samples once or twice within it. Sampling is timed to the middle of a switching state, where the ripple crosses its average, so the sample represents the fundamental without filtering.

Answer: B

Correct answer: a reading corrupted by the switching ripple

The current in a machine winding ripples at the switching frequency about its average, so a sample taken at an arbitrary instant catches the ripple rather than the mean. Sampling is therefore synchronised to the modulation, at the instant where the instantaneous current equals its average, which is why the timing matters as much as the transducer.

Answer: C

The command computed from this sample is applied in the next interval, which is a delay the loop has to allow for or it becomes sluggish. Predicting the state at the moment of application recovers most of the lost bandwidth.

Answer: D

Correct answer: an industrial fieldbus or Ethernet protocol

Modern drives are integrated into plant control through a digital protocol - Profibus, Modbus, EtherCAT, Profinet and the like - which carries speed and torque references, status and diagnostics on one connection. It replaces the analogue reference and relay contacts of older installations and allows far more information to pass.

Answer: A

The mean is \( 3V_{ml}/\pi = 3 \times 587/\pi = 560 \) V, and with a large capacitor it rises towards the peak of 587 V at light load. Drives are commonly specified with a link voltage of that order for a 415 V supply.

Answer: B

Correct answer: 396 V rms

Space vector modulation gives a maximum peak line-to-line fundamental equal to the dc link voltage, so the rms value is \[ \frac{560}{\sqrt{2}} = 396\ \mathrm{V\ rms}. \] This is about fifteen per cent more than sinusoidal modulation achieves, which is the practical reason space vector modulation is used.