Synchronous generator field excitation via capacitive coupling through a journal bearing

Wound field synchronous generators (WFSG) are the standard for back-up and utility scale power generation. Rotor field current and prime mover speed are the only control parameters required to regulate power conversion in a generator application. Maintenance costs may be minimized by adopting non-contact or “brushless” technologies to replace sliding slip ring connections. This paper presents a brushless excitation approach using ceramic insulated sleeve (journal) bearings with oil lubrication to form capacitively coupled slip rings, in contrast to more traditional inductive brushless exciters and rotary transformers. This capacitive power transfer (CPT) approach exhibits advantages including low weight, low volume and has a relatively simple construction using off-the-self components. Analysis, design and prototype construction of the CPT system are presented. Experimental results demonstrate that ~1.7nF of capacitive coupling transfers 340W to the rotor field winding of a 10kW 208V WFSG. Voltage regulation of a WFSG is demonstrated during steady state and 1 per unit load step changes yielding a NEMA-MG1 class G2 rating.