Field-circuit-motion numerical models of the AC tachogenerator with rotor of solid conductor type

The paper deals with the finite element analysis of a simple and reliable speed transducer - the AC tachogenerator with rotor of solid conductor type. The coupling between the field equations, circuit equations and the rotor motion is considered in the numerical computation of eddy currents and global quantities that characterize the transducer operation. The diffusion of the electromagnetic field in the rotor, the speed - output voltage dependence, the power losses in the rotor and the power delivered by the 50 Hz supply source to the field winding of the transducer are evaluated for three rotor types: homogeneous copper or magnetic nonlinear steel rotors and a rotor consisting in a copper cup around a magnetic steel cylinder. The sensitivity of the transducer output voltage with respect the magnetic properties of the rotor and the airgap value is analyzed. The linearity of the output voltage - speed characteristic of the tachogenerator when the field winding is 400 Hz supplied and the upper limit of speed measurement is very low in comparison with the synchronous speed is studied.