Conversion of Standard Induction Machines to Permanent Magnet Synchronous Machines with Higher Efficiency

To increase the efficiency of commercial induction machines, especially at part load and speed, these machines are converted into permanent magnet synchronous machines. The stators are not changed at all; only the rotors are modified. The appropriate size of the rotor iron and the magnets is found by a numerical optimization, using finite element software. The parameters that are optimized are: the magnet thickness (t(m)), the number of magnet segments per pole (N-ps) and the magnet pole angle (a(m)). The objectives are high efficiency, low magnet volume, low cogging torque and with sufficient mechanical power. The high robustness and reliability of the original induction machines remain high because the stators, the shafts and the bearings are not modified. The resulting synchronous machines still have a rather low torque to weight ratio, but they have low rotor losses, a good power factor and high efficiency. A financial analysis is made in order to assess if the payback time of the conversion is reasonable. Two induction machines with approximately the same outer dimensions (a 2-pole 3 kW machine and a 6-pole 1.5 kW machine) were converted into synchronous machines by different production techniques for the rotor. The efficiency maps were simulated and measured. For both machines, the peak efficiency increases by 5% and 10% respectively, and the average efficiency (between 0.5 and 1 times nominal torque, and 0.5 and 1 times nominal speed) by 4% and 14%, while the cogging torque and magnet volume are low. Conversion of induction machines with many poles to synchronous machines improves the efficiency much more than conversion of two-pole induction machines. Copyright (c) 2012 Praise Worthy Prize S.r.l. - All rights reserved.