University of L Aquila Department of Industrial and Information Engineering and Economics Permanent Magnet-assisted Synchronous Reluctance Motors for Electric Vehicle applications A. Ometto, F. Parasiliti, M. Villani 9 th International Conference Energy Efficiency in Motor Driven Systems EEMODS 15 Helsinki, September 15 th 17 th 2015
Electric Vehicles represent the most viable solutions to solve the problems associated with the traditional internal combustion engine motors and different typologies of electric motors are proposed. Moreover, the progress in power electronic makes it possible to realize direct-adjustable-speed drive machines with a wide operating speed range.
The strong demand of high performance electric motors for automotive application requires the use of: innovative and efficient design procedures, by specific tools and optimization processes; accurate choice of the materials and electrical steels; in order to fully satisfy the hard specifications and constraints in terms of performance, encumbrance, weight, reliability and cost.
The main requirements of electrical machine for traction are: high torque and power density; wide speed range; high efficiency over wide torque and speed range; wide constant power operating capability; robustness and reliability; reasonable cost.
Main requirements High Torque High Power Torque Power 0 base speed High speed speed
Types of EVs Motors - Induction motors They are widely accepted for EVs because of their low cost, high reliability, and freedom from maintenance. - PM motors Most EVs use PM synchronous motors and they are becoming more and more attractive and can directly compete with the induction drives. The advantages of PM motors are their inherently high efficiency, high power density and high reliability.
PM Rotor geometries (interior PMs) MP PM PM V-shape PM The key problem is their relatively high cost due to PM materials.
The recent increase of rare-earth PMs cost has led the manufacturers to choice low-cost motors. This has oriented the designers to investigate alternative solutions without penalizing the motor performance Magnetless motors or motors with low-cost PM Synchronous Reluctance motors (SRM) PM-assisted SRM
1. Synchronous Reluctance motors These motors with multi-barriers rotor structures have been obtained a great interest in brushless AC drives. Advantages: no winding and PM in the rotor ( cold rotor), low inertia, good acceleration performance, good flux weakening operation, low manufacturing cost. Disadvantages: low power factor; torque ripple.
Flux barriers Rotor Flux-barrier electr. steel Iron bridge Saliency ratio k s = L d /L q 5 8 The torque produced by the SRM is due to the anisotropy of the rotor. The number of rotor flux barriers affects the anisotropy, so as this number increases the reluctance torque component increases.
Prototypes of SRMs (by UnivAQ) 2 barriers 4 barriers Laminated rotors with flux barriers can be manufactured with normal punching tools at very low cost.
Electromagnetic Torque d-q axis theory can be used to analyze the electromagnetic performance of the SRM. T 3 2 p L d L q I d I q Reluctance Torque The Torque of motor can be varied by means of an accurate control of the d-q axis currents ( Vector control ).
Vector diagram of SRM Voltage equations (R 0): V V d q L L d q I I d q q-axis q d L q I q V L d I d I q I I d d-axis The voltage vector exhibits a large phase difference from the current vector and this means that the power factor (cos ) is low!
2. PM-assisted SRM In order to improve the operating performance of the SRM (torque density, power factor) it is useful to add proper quantity of permanent magnets into the flux barriers of the rotor core, and particularly cheaper PMs, such as Ferrite. In this case, the motor is called PM-assisted SRM. The PM-assisted SRM produces a torque 20 30% higher respect to the SRM (without PM). The amount of the Ferrite placed in the rotor core is limited by the geometry of the rotor and manufacturing cost which is considered as one of the design constraints.
PM-assisted SRM The use of the PMs in the flux barriers allows to reduce the q-axis flux (without affecting the d-axis one) and then to improve the torque and power factor. Conventional with PM 3
PM-assisted SRM PM-assisted SRMs become attractive for EVs applications: - Low cost of Ferrite; - Easy to handle; - High efficiency; - High power density; - Good power factor ( size of the Inverter).
Effect of inserted PM (by UnivAQ) CONFIGURAZIONI (? SRM = 55 ) Coppia (*) Nm? (Coppia) rispetto a 1 cos?? ( cos? ) rispetto a 1 Torque Nm Torque % cos Ripple % 26 1 26.59-0.711-4.0% 1 SRM - 0.71 31 +19 % 0.86 62 31.66 23.73% 0.867 22.03% 4.5% PM_ass 3 33 +27 % 0.90 8 33.34 30.27% 0.904 27.20% 4.0% PM_ass
Electromagnetic Torque 3 CT p [( Ld Lq ) Id Iq mag Id ] 2 (L q I q - mag ) q-axis V L d I d I q I mag I d d-axis The PM allows to reduce the angle between voltage and current vectors and this increases the power factor respect to conventional SRM.
Motor design The design of PM-assisted SRM for EVs requires the use of innovative and efficient design procedures, by using specific tools and optimization processes, in order to fully satisfy the specifications and the constraints on the encumbrance. Optimization procedures + Finite Element Analysis Objective: max Torque density max Efficiency combinations of more Obj.
Design Optimization procedure by FEA Preliminary design x2 x1 Optimization Algorithm x8 x7 x9 x3 x6 x5 x4 x10 X k F(X) Design variables (X) FEA yes Yes F(X k ) Optimized design Minimum? No no k = k+1
Design of PM-assisted SRM for EV: case study Specifications DC voltage supply V 500 Base speed rpm 4000 Torque @ base speed Nm 200 Output Power kw 83.8 Max speed rpm 12000 Torque @ max speed Nm 60 Axial core length mm 100 Outer stator diameter mm 240 Stator winding flat-wire PM-Ferrite Br=0.35 T; Hc=270 ka/m Cooling Liquid-cooled
Stator winding with flat wires (harpins) For this application ( high torque density motor) the stator winding with flat wires has been chosen. This solution requires rectangular slots. Stators with flat wires Advantages: - high slot fill factor (up to 0.80 0.85); - reduction of winding overhang; - high quality process.
Details of stator core with flat wires In this case, the phase resistance should be calculated taking into account the proximity and skin-effects that heavily depend on the frequency and flat-wire size. In co-operation with:
Cross-section of the optimized PM-assisted SRM 6 pole - 54 slots flat wires slot fill factor = 0.80 The iron bridges in the rotor core have been careful sized since they have impact on the motor performance and rotor robustness. Moreover, resin can be inserted in the flux barriers in order to improve the robustness of the rotor structure against the centrifugal forces at high speed.
Choice of the electrical steel High performance motor requires a right choice of the electrical steel and this is an important step during the sizing procedure. The requirements on electrical steels are: - low losses; - high permeability.
Different commercial non-oriented fully-processed materials have been tested and compared using the manufacturers data. 400-50 AP 530-50 AP 800-50 330-50 AP
Comparison of different electrical steels 800-50 530-50 AP 400-50 AP 330-50 AP Torque Nm 200 Speed rpm 4000 Frequency Hz 200 Output Power kw 83.8 Phase current Arms 164 161 161 163 AC Joule losses W 2337 2258 2258 2317 Core losses W 735 620 553 423 Efficiency % 95.4 95.6 95.7 95.7 Power factor 0.87 0.89 0.89 0.88 Bteeth ; Byoke T 1.82; 1.60 1.82; 1.60 1.83; 1.60 1.83; 1.61 The electrical steel 400-50 AP is the most suitable choice because combines low specific losses with high permeability and the motor presents good performance in terms of efficiency and power factor; the 400-50 AP has been preferred for this specific application.
Performance of the PM-assisted SRM T CU = 90 C T PM = 70 C 4000 rpm 12000 rpm Phase current Arms 161 161 Torque Nm 200 64 Output Power kw 83.8 80.4 AC Joule losses W 2258 2574 Power factor 0.89 0.86 Efficiency % 95.7 94.6
Flux density 200 Nm, 4000 rpm 64 Nm, 12000 rpm (T)
Torque and Power vs. Speed Torque Nm rpm Power kw CPSR rpm
Comparison with IPM synchronous motor The proposed PM-assisted SRM has been compared with a PM synchronous motor with Interior PM (NdFeB-N38SH) in order to evaluate the differences in terms of performance, weight and costs. PM-assisted SRM IPM 6 pole, 54slots Ferrite NdFeB
The comparison has been carried out considering the same overall dimensions and winding. In particular the two motors have: the same stator lamination (diameters and n. of slots); the same air-gap; the same number of turns and wire size; the same electrical steel (400-50 AP); the same temperatures of the winding and PMs.
Two different IPM motors have been proposed: IPM_1 IPM_2 with the same stack length of the PM-assisted SRM; with a reduce stack length (compact design) and the same current of PM-assisted SRM.
PM-assisted SRM vs. IPM-NdFeB (same stack length) PM-ass SRM IPM_1 PM Ferrite NdFeB Stack length mm 100 100 Outer stat. Diameter mm 240 240 Phase current Arms 161 150 4000 rpm Torque @ 4000 rpm Nm 200 200 Output Power kw 83.8 83.8 AC Joule losses W 2258 1945 Power factor 0.89 0.94 Efficiency % 95.7 96.2 12000 rpm Torque @ 12000 rpm Nm 64 73 Output Power kw 80.4 91.7 Current density A/mm 2 10.1 9.4
PM-assisted SRM vs. IPM-NdFeB PM-ass SRM IPM_1 IPM_2 PM Ferrite NdFeB NdFeB Stack length mm 100 100 91 Outer stat. Diameter mm 240 240 240 Phase current Arms 161 150 161 4000 rpm Torque @ 4000 rpm Nm 200 200 200 Output Power kw 83.8 83.8 83.8 AC Joule losses W 2258 1945 2131 Power factor 0.89 0.94 0.90 Efficiency % 95.7 96.2 95.9 12000 rpm Torque @ 12000 rpm Nm 64 73 79 Output Power kw 80.4 91.7 99.3 TRV knm/m 3 32.0 36.5 43.2
PM-assisted SRM vs. IPM-NdFeB Torque Nm IPM_2 IPM_1 SRM_Fe rpm Power kw rpm
Weight and Cost comparison (active materials) PM-ass SRM IPM_1 IPM_2 Stack length mm 100 100 91 Gross iron kg 45 45 41 Stator winding kg 6.2 6.2 5.9 PM kg 0.92 0.93 0.85 Cost (*): Gross iron Euro 40.5 40.5 36.9 Stator winding Euro 43.4 43.4 41.3 PM Euro 23.0 111.6 102.0 Total Euro 106.9 195.5 180.2-45% - 40% (*) Premium steel = 0.90 /kg; Cu = 7.0 /kg; Ferrite = 25 /kg; NdFeB = 120 /kg
Comments The IPM motors have higher power factors and this allows the inverter rating to be reduced. At high speed (12000 rpm) the IPM motors exhibit good performance with a power density (and TRV) higher than the synchronous Reluctance motor one. The PM-assisted SRM has excellent efficiency, very close to that one of the IPM_2, and good constant-power operating capability. The cost reduction for the PM-assisted SRM respect to IPM motors is mainly due to the lower cost of the PM in Ferrite. Ferrite PM has got a positive reversible temperature coefficient of coercivity, respect to NdFeB, and this increases the demagnetization strength as the temperature increases, leading to better dynamic performance of car.
Conclusions The Brushless motors are gaining a growing interest thanks to their power density capability, high efficiency and high reliability. Moreover, the progress in power electronic makes it possible to realize directadjustable-speed drive machines with a wide operating speed range. The demand of high performance electric motors for automotive applications requires the use of innovative and efficient design procedures, by using specific tools and optimization processes, and accurate choices of the materials and electrical steel. PM-assisted Synchronous Reluctance ensures good performance with high power density, high efficiency and reasonable cost and then it can be considered a strong potential for powertrains and an efficient alternative to IPMs and Induction motors.