Design and Analysis of an Electrically Excited Synchronous Motor for a Compressor Application - A Performance and Cost Comparison of an Electrically Ex cited Synchronous Motor and a Permanent Magnet Synchronous Motor

Abstract

Abstract Permanent magnet synchronous motors (PMSMs) are commonly used motors. However, to reduce the usage of rare earth metals magnet-free motors are desirable. The electrically excited synchronous motor (EESM) utilizes field windings instead of magnets in the rotor and most research regarding EESMs has been focused on traction applications. This thesis investigates the possibility of using an EESM for an air compressor application that continuously operates in the constant torque region. The performance and cost of the designed EESM were compared to a reference PMSM. A rotor for an EESM was designed and optimized to reach high continuous torque, using the finite element method to study electromagnetic and thermal properties. The stator used for the EESM was the same as for the reference PMSM. However, using only stator cooling the EESM had to be approximately 87% longer than the reference PMSM to reach the nominal torque in continuous operation. It was shown that high continuous torque, without significant increases in stack length, requires more efficient rotor cooling techniques. The currents in the EESM were optimized using a copper loss minimization algorithm. However, contrary to the reference PMSM, it was not possible to optimize the currents such that high continuous torque, maximum efficiency and high power factor were obtained simultaneously. Furthermore, the maximum efficiency of the long EESM during continuous operation in the nominal operating point was slightly more than 2.8 percentage points lower than for the reference PMSM. Regarding the material cost the price levels and the outer dimensions of the motors matter. However, it was shown that the eco cost is lower for an EESM than for a PMSM, even if it is substantially longer. Overall, it was concluded that it is not feasible to use an EESM if the cooling method and outer dimensions have to be the same as for the reference PMSM. However, if the motor can be made a little longer, a slightly lower efficiency can be accepted and more efficient rotor cooling is implemented an EESM may be used as a more sustainable alternative to a PMSM for the investigated application.