Joule losses of magnets in permanent magnet synchronous machines - case concentrated winding machine Hanne Jussila Lappeenranta University of Technology 1
Joule losses of permanent magnets Eddy current losses in the rotor permanent magnets are caused by three different reasons A winding stator produces a significant amount of current linkage harmonics generated by flux densities travelling across the permanent magnets, thereby causing eddy currents. These are called winding harmonics. The slot openings cause flux density variations that induce eddy currents in the permanent magnets. These are called permeance harmonics. And frequency-converter-caused time harmonics in the stator current waveform cause extra losses in the rotor. 2
Prototype machine with concentrated windings The prototype machine is a two-stator-one-rotor axial flux permanent magnet machine, 37 kw, 2400 min -1. The stator winding is a doublelayer fractional slot concentrated winding, where the number of slots per pole and phase q = 0.4. The two stators are electrically connected in series. -A A A -A B C -B -C -B -C C B -C -B B B C C -CA -A -A A-B 3
Prototype machine External diameter of the stator stack Internal diameter of the stator stack Thickness of rotor and PM 274 mm 154 mm 16 mm 4
Magnets The magnet material type is NdFeB 495a by Neorem Magnets. The construction of NdFeB magnets varied from just one bulky magnet per pole to segmented magnets where there were 20 magnet segments per pole either in radial or tangential direction. As the rotor is of special construction, there are no iron losses under the magnets. 5
A radial flux 12-slot 10-pole machine The 2D modelling of the axial flux machine can be carried out by a radial cutting plane at the arithmetic mean radius, which is then developed into a 2D radial flux machine. If the arithmetic mean radius is used, the magnet width to pole pitch ratio in an axial flux machine should be constant at different radii and the stator should not be skewed. 6
PM Joule losses calculated by the 2D FEA for a 12-slot-10-pole machine PM Joule losses 1800 1600 1400 1200 1000 800 600 400 200 0 2D FEA, slotting effect 2D FEA, pulsation effect 1 2 5 10 20 Number of segments 7
PM Joule losses calculated by the 2D FEA for a 12-slot-10-pole machine % of total PM Joule losses 120 100 80 60 40 20 Pulsation effect Slotting effect 0 1 2 5 10 20 Number of segments 8
PM Joule losses calculated by the 2D FEA for a 12-slot-10-pole machine PM Joule losses 3500 3000 2500 2000 1500 1000 500 0 2D FEA, slotting + pulsation effect 2D FEA 1 2 5 10 20 Number of segments 9
No-load measurements Measured 2D FEA 3D FEA Rotor equipped with EPM (V) EPM (V) EPM (V) radially segmented magnets 220 222 - tangentially segmented magnets 227 - - bulky magnets 220 229 226 Rotor equipped with radially segmented magnets tangentially segmented magnets bulky magnets Measured losses 630 680 2000 2D FEA losses + measured mechanical losses 630 - (not possible to calculate with 2 D) 2400 Rotor frame (no magnets) 170-10
Loss division at no load at 2400 min -1 in the generator mode Rotor equipped with No-load loss (MEAS) Mechanical loss (MEAS) Iron loss (FEA) Loss in PM (FEA) Loss in PM = No-load loss - Mechanical loss - Iron loss radially segmented magnets 630 170 350 110 110 tangentially segmented magnets 680 170 350-160 Bulky magnets 2000 170 350 1900 1500 11
Load measurements The machine was driven as a motor supplied by an ABB M1 frequency converter and loaded with a DC motor drive to achieve a rated output power of 37 kw. Rotor equipped with radially segmented magnets tangentially segmented magnets bulky magnets Measured losses (at rated load) 1250 1300-12
Loss distribution at load at 2400 min -1 in the motor mode Rotor Equipped with Load loss (MEAS) Mechanical loss (MEAS) Iron loss (FEA) Copper Loss (MEAS) Loss in PM (FEA) Loss in PM (+ additional loss ) = Load loss - Mechanical loss - Iron loss - Copper loss radially segmented magnets 1250 170 330 560 100 190 tangentially segmented magnets 1300 170 330 560-240 13
Conclusion This motor type emphasizes the losses in bulky magnets, but the motor can be operated with segmented magnets with a good efficiency. There is a 50 W difference in the loss results of the tangentially and radially segmented magnets. In practice, it is impossible to say which segmentation produces the smallest losses as the difference may result from a measurement uncertainty or it may be caused by differences in the motor assemblies. 14
Thank you for your attention! 15