Flux estimator

A flux estimator 1 for estimating an air-gap flux in a permanent magnet electric motor having a rotor and a stator, comprises a resistance compensation stage having a first multiplying digital-to-analogue converter 23, and an inductance compensation stage having a second multiplying digital-to- analogue converter 25. The flux estimator is configured to determine a first gain (Ku) for compensating for a stator resistance (Ra); and to determine a second gain (KLa) for compensating a stator inductance (La). The first gain is supplied to the first multiplying digital-to-analogue converter in the resistance compensation stage to generate a first output signal (VIAR) providing a scaled representation of the voltage drop across the stator resistance per phase. The second gain is supplied to the second multiplying digital-to-analogue converter in the inductance compensation stage to generate a second output signal (IA_LA) providing a scaled representation of the stator inductance per phase. An air-gap flux estimation signal (PSI_MON) is generated in dependence on the first output and the second output. The estimator may be used in a motor control unit (3); a control system (5); and a pump (4), particularly a Turbo Molecular Pump (TMP).