Control of an AC Brushless Exciter to Regulate Power Factor on an ASD-Fed Synchronous Motor: An Increased Efficiency Approach

Where application requirements demand the need for high power at low speeds, fewer than 900 revolutions per minute (r/min), synchronous machines are commonly used in place of induction machines because of various performance benefits. Induction motors draw magnetizing current from the source, and the total reactive current diminishes the power factor and efficiency in highpole- count applications. By substituting a salient-pole synchronous motor (SM), the power factor can be controlled to near unity. A three-phase ac–ac brushless exciter with a rotating rectifier is an option to supply the field excitation on the rotor. In this article, a high-power adjustable-speed drive (ASD)-fed SM is presented as well as an ac pulse-width modulation (PWM) chopper as the source to the brushless exciter. The motor’s field V curve is shown along with the schematic of the system. This article includes the performance of the voltage and current of the main stator as well as the exciter regulation of the power factor. The differences between brushed versus brushless and dc versus three-phase ac excitation are described, including ac motor drive starting. A brief description of SM rotor design considerations for across-the-line starting versus adjustable-speed drive (ASD) starting is provided.