Solving The Synchronization Problem of Line Start Permanent Magnet Motors

Abstract

Three-phase, line start permanent magnet motors (LSPMMs) are attractive candidates to significantly reduce the energy costs associated with motors that do not require adjustable-speed operation. In their present state, LSPMMs cannot start moment-of-inertia (inertia) loads equal to those specified in National Electrical Manufacturer’s Association (NEMA) MG1 for general-purpose induction motors. The inertia values specified in NEMA SM1 for LSPMMs are 15 to 32 times lower than the values mandated by the induction motor standard NEMA MG1. LSPMMs should comply with starting current limits imposed on NEMA Design B induction motors. Starting currents for LSPMMs have not received much attention in the technical literature but present a major hurdle to their widespread application unless LSPMMs are designed to meet this demanding requirement. A novel tapped-winding stator configuration is presented that applies an electromagnetic flux boost near the synchronization threshold to improve synchronization capability while complying with NEMA Design B current limits. The tapped-winding, flux-boost connection is configured for a brief time interval as the rotor accelerates towards synchronous speed. The temporary flux increase results in significant improvement to inertia synchronization capability.A combination of closed-form analysis, simulations, and experimental testing are presented to confirm the validity and effectiveness of this new technique. The transition between “normal” and “tapped” connections can be implemented using mechanical or solid-state switches. If the switching is conducted appropriately, the instantaneous peak transient (IPT) current can be limited to comply with NEMA standard requirements. Experimental results of an LSPMM incorporating the novel flux boost concept are presented for a prototype 4 pole motor rated 50 hp(37 kW). The objectives of the prototype motor are to verify the ability to: 1) comply with NEMA MG1 maximum starting current limits, 2) significantly exceed federally mandated efficiency requirements, and 3) synchronize an inertia value that meets or exceeds NEMA MG1 standard requirements for a comparably rated induction motor. Successful completion of the three objectives demonstrates the ability of super high-efficiency LSPMMs to become “drop-in” replacements for today’s induction motors.