Synchronous Motor Drives Questions and Answers

Answer: C

Correct answer: the machine’s own voltages and currents

Sensorless schemes estimate the position from quantities the drive already measures. At moderate and high speed the back emf is estimated from the terminal voltages and currents and integrated to give the flux angle; near standstill, where that emf vanishes, high-frequency signal injection is used instead, exploiting the machine’s magnetic saliency. Removing the encoder saves cost, cabling and a failure point in hostile environments.

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.

Answer: A

An induction machine loads a weak supply with magnetising current as well as real power, whereas an over-excited synchronous machine helps to hold the voltage up. Its starting current is the difficulty rather than the advantage.

Answer: B

Correct answer: hold the power factor at a set value

The excitation controls reactive power, so an automatic regulator adjusts the field current to hold the power factor, or the reactive output, at whatever the network requires as the real load varies. Speed is fixed by the supply frequency and cannot be influenced by the field; the torque follows the mechanical load; and holding the torque angle at 90 degrees would place the machine exactly at its stability limit, which is the last thing a regulator should do.