A variable speed drive comprises an output for delivering a drive voltage to an electric motor, power inverter, a drive controller, and a current sensor. The drive controller includes a PWM generator, a control law module, and a state variable estimator for estimating a state variable of the electric motor. The module computes a target voltage signal based on state variable estimates provided by the estimator and outputs the target voltage signal to the PWM generator. The generator approximates the target voltage signal with a PWM control signal, controls the inverter using the control signal, computes, based on the deviation between the control signal and the target voltage signal, an estimation support signal, and outputs the estimation support signal to the estimator. The estimator estimates a state variable of the motor based on the estimation support signal and the drive current, and outputs the estimate to the module.