Investigation into electrical resonance phenomena in the field circuit of wound-rotor synchronous machines

The field circuit of wound-rotor synchronous machines can be supplied in various ways, including brushed and brushless excitation systems. Regardless of whether the synchronous machine is equipped with a rotating exciter or not, the external excitation power source necessarily injects some high-frequency harmonics into the field circuit. The frequency of these harmonics can be particularly large if Pulse-Width-Modulated (PWM) converters are used as excitation voltage supply. This paper investigates the possibility that the excitation supply harmonics can produce electrical resonance phenomena resulting in possibly harmful overvoltages. The investigation is carried out through different approaches: by experiments on a low-voltage salient-pole 20-kVA laboratory prototype; by experiments on a large (multi-MW) round-rotor synchronous motor; through an analytical model. The results obtained from the analytical model are shown to well match measurements provided that a resistive parameter (affected by eddy-current loss effects) is properly calibrated. Another important conclusion is that the possible overvoltages due to electrical resonance would occur between internal points of the field (not between its terminals) and would not be thereby damped by conventional field circuit protection devices.