Calculation of PWM-induced rotor-can losses in flooded generators

Improving availability of tidal turbines has been identified as a key area to lower the levelized cost of energy from tides. One of the approaches suggested to achieve high reliability is to use a flooded permanent magnet (PM) generator. Flooded PM generators are designed to operate with a seawater filled stator-rotor gap in the generator. Using such generators will prevent frequent maintenance required for the high pressure rotary mechanical seals between the turbine shaft and the nacelle enclosure. Flooded generators are conventional generators with additional stator and rotor-can material to protect the active parts of the generator against water ingress. Normally, stator-can is made of an electrically nonconductive material to prevent excessive losses from the PM field. On the other hand, choice of the rotor-can is more flexible. If the rotor-can of the flooded generator is made of stainless steel or any other electrically conductive material, this may give rise to substantial losses in the rotor-can due to the space and time harmonics of the stator magneto-motive force. The time harmonics arise from the pulse-width modulated inverter used to control the torque/speed of the generator. This paper aims at quantifying losses in the rotor-can of the flooded generator due to the time harmonics of the stator current. Results show that the rotor eddy current losses due to the space harmonics are not always negligible. However, losses due to the time harmonics, under normal operating conditions, can be assumed to be negligible.