Converting an Induction Motor into a Permanent Magnet Synchronous Motor: - replacing the rotor in a used induction machine with a PM rotor
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- Institutt for elkraftteknikk 
Abstract In this article the possibility of exchanging the rotor of an IM with a high performance PM rotor to increase the motor s efficiency and PF is investigated. A standard industrial IM is used as a reference motor and has been rebuilt and tested. The motor has been investigated both electrically and mechanically to obtain all motor data. Analytical calculations complemented by FEM calculations have been used to calculate the motor s performance and to investigate the motor s dependency on the magnetic loading and the mechanical dimensions while driven by a converter. In the analytical calculations simplicity has been emphasized and compared to the FEM results the calculation methods match well. Two motors have been analyzed, one with a 1.5mm air gap and one with 2.5mm giving a high and a low synchronous reactance. For the voltage, flux density and torque calculations the values match within a few percents. The inductance calculation shows that the analytical and FEM methods give the same self inductance while the mutual inductance which is assumed to be 1.5 times the self inductance in the analytical calculation is 68.5% higher in the FEM calculation giving a 23.7% higher synchronous reactance. It is shown how small deviations in the air gap or magnet length cause ±10% change in the back EMF and magnetic loading. The combination of the chosen control strategy (MTPC) and the high reactive voltage drop in the PM motor leads to a demanded terminal voltage in excess of the converter s voltage limit for cases with a high back EMF and high reactance. The simulations show how the motor having a high synchronous reactance is more sensitive to the magnetic loading than the one with a low synchronous reactance. For a motor having a 10% higher back EMF than the reference motor and a high reactance, a 105rpm reduction in speed is needed to deliver the rated torque giving 7% reduction in the delivered power. A motor having a 10% lower back EMF than the reference motor and low reactance is able to deliver 14.7% higher power than the nominal staying within the converter s voltage limit and the motor s current limit. For the reference motor the voltage limit is reached before the nominal speed which limits the speed and power capability slightly. Made with low reactance the reference motor could have increased the power rating by 27.3%. The reference motor is built with arc shaped magnets which has drawbacks due to delivery time and price. Simulations for motors with rectangular magnets show promising results and in combination with a possible 66% magnet cost reduction this could be a good solution. The final test of the motor as a generator shows a 92.5% efficiency at full load and that the efficiency stays high for the whole speed and load range. The measured no-load back EMF of the motor is seen only to differ by 2V for the calculated one. The measured reactance at full load is 6.9% higher than the analytical and 15.2% lower than the FEM calculated. Hence both the analytical and FEM calculations coincide well with the measured values on the rebuilt motor.