Discussion
Starting a sensorless permanent magnet drive from rest is difficult because
Answer: D
Correct answer: the back emf that reveals the position is very small
The back emf is proportional to speed, so at standstill it is zero and near standstill it is buried in noise and inverter switching artefacts - exactly where the estimator needs it most. The usual remedies are to align the rotor to a known position by energising a phase, to start open-loop until the emf becomes measurable, or to use high-frequency injection which relies on saliency rather than motion. The magnets and the inverter are unaffected by being at rest.
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