Design of an axial rotary transformer for wound rotor synchronous machines excitation winding feeding

In recent years, the use of permanent magnet (PM) machines has spread in industrial applications at the expense of wound rotor synchronous machines, since PM motors have a higher efficiency, guaranteed by the absence of rotor copper losses. Nowadays, the high cost and low availability of the rare earths used to realize the PMs make the interest in wound rotor synchronous machines grow again. The main issue connected to their use is the employment of a brushes-slip ring system to feed the rotating excitation winding. Since the slip rings rub the brushes, the system has to be frequently replaced. In this framework, a wireless power transmission between the fixed source and the spinning winding is proposed to remove the brushes-slip ring system. The use of a rotary transformer was proposed many years ago [1], but only recently, it has become possible to effectively use it thanks to the improvements in the switching frequency of power inverters that allow the increase in the working frequency and the reduction of the transformer size. In this case, the whole excitation system is composed of a DC-power supply, an inverter, the transformer and a rectifier (see Figure 1). The considered geometry is composed of two cup-shaped ferromagnetic cores axially aligned, with an airgap between them: one core is fixed while the other spins with the synchronous machine’s rotor. In each core, a winding is realised (see Figure 1), and the inverter feeds the primary one, arranged in the fixed core. The magnetic field produced by the current in the primary winding induces a voltage in the spinning secondary coil, realised in the rotary core. Such a voltage is rectified and applied to the synchronous machine’s excitation winding. With respect to a power transformer, the rotary one has a higher leakage inductance and a smaller magnetising inductance due to the airgap.