The problem of sensorless (local) speed regulation of a class of electrical machines is addressed and solved using a simple linear-time varying controller. The class, which contains permanent magnet synchronous motors, consists of all Blondel--Parks transformable machines, and of all machines whose magneto motive force can be approximated by a first harmonic Fourier expansion. The controller---which contains an internal model of the steady-state control---is able to asymptotically reconstruct the control signal necessary to achieve speed regulation, even in the presence of unknown but constant load torque. To prove global stability and boundedness of the unforced system we exploit the by now well--known passivity property of electro-mechanical systems. We work out in detail the problem of speed regulation for a permanent magnet synchronous motor, for which normalized simulations that illustrate the properties of the design are provided.