Elektrotehniški vestnik 84(4): 181-188, 2017 Original scientific paper Magnetic-System Topology Selection for the High-Speed Electrical Machine with Interior Permanent Magnets Ismagilov F.R., Vavilov V.Ye., Ayguzina V.V.* Department of Electromechanics, Ufa State Aviation Technical University, Russia E-mail: vtipy@mail.ru *corresponding author Abstract. The paper presents a method enabling an optimal selection of the rotor magnetic-system topology. Using this method, the optimal magnetic-system topology of a 120 kW three-phase electrical machine (EM) aviation with a high-coercivity permanent magnet, 24,000-rpm rotational speed and 400 Hz output voltage frequency is determined. The developed bipolar magnetic rotor system allows achieving a maximum EM efficiency. To evaluate the computer modeling results, an experimental research on a full-size EM is carried out. The numerical difference between the experimental and simulation data is below 5 %. Keywords: magnetic system topology, high-speed electrical machine, permanent magnets Izbira topologije magnetov pri hitrem električnem stroju z notranjimi trajnimi magneti V članku predstavljamo metodo za optimalno topologijo magnetov. Na podlagi predlagane metode smo določili optimalno postavitev magnetov v trifaznem letalskem električnem motorju z močjo 120 kW, ki se vrti s hitrostjo 24.000 v/min in frekvenco izhodne napetosti 400 Hz. Razvili smo nov bipolarni magnetni rotorski sistem, ki omogoča večjo učinkovitost električnega motorja. Rezultate, pridobljene z računalniškim modeliranjem, smo preverili eksperimentalno z električnim motorjem. Odstopanje med eksperimentalnimi rezultati in rezultati simulacij je manj kot 5-odstotno. 1 Introduction Electrical machines (EMs) of the power supply systems are important in the modern transport industry. Ground vehicles have migrated to the hybrid technology direction [1-5]. The essence of these technologies is that the vehicle internal combustion engine is only used to rotate the electric generator which feeds the electric motors associated with wheels. Thus, the vehicle move is provided by converting the electrical energy into the mechanical energy, and the primary source for the electrical energy generation is the internal combustion engine. The vehicle fuel consumption is thus significantly reduced as the internal combustion engine works in one mode, which is optimal in terms of fuel consumption and efficiency. This reduces the environmental emissions and minimizes the vehicle noise. Obviously, the EM effectiveness depends on the entire vehicle efficiency. All these prerequisites form a significant market of high-efficiency EMs for the transport industry. Its need increases annually and so does also the competition between the EM manufacturers. Therefore, to meet customer requirements and to achieve a sustainable position in the market, the EM manufacturers should increase the EM power supply system effectiveness by decreasing the loss, increasing the power, reducing the mass and volume, and minimizing the cost. A solution to this problem is the use of high-speed EMs with a high-coercivity permanent magnet (HCPM). A significant number of articles and books is devoted to research these EMs [6-18]. Loss-reduction methods [69], design selection and calculation tasks of EM with HCPM and optimization solution task [10-12], selection of appropriate EM active materials, bearings [13-15] and cooling systems [16] are considered in literature. In [17, 18], the focus is on use of concentrated winding in EM with HCPM, to shorten the EM axial length. However, this increases the eddy-current losses in HCPM due to the EM spatial harmonics. In this paper, a method is proposed to solve the topology-selection problem of the EM bipolar rotor magnetic system with interior HCPM. The rotor magnetic system largely determines the EM effectiveness. This problem is not fully solved in literature [6-18], and it is interesting for EM manufacturers. The main focus of our paper is on the selection of an optimal bipolar magnetic-system topology selection of EM with HCPM. To solve the issue, a specific numerical example is used: a three-phase 120 kW 24 000 rpm EM with HCPM and 400 Hz output voltage frequency. Received 19 May 2017 Accepted 10 July 2017 182 ISMAGILOV, VAVILOV, AYGUZINA The developed methodology can be applied in different industries using EMs with HCPM. To achieve the maximum EM efficiency, a new two-pole rotor magnetic system is proposed. In contrast to known works [6-18], the optimal topology is determined by using the transient analysis, i.e. the magnetic field in the air gap is calculated by taking into account the demagnetizing field and distortions created by currents of the stator windings at different loads. This distinguishes our methodology from those defining the topology effectiveness by using the magneto-static analysis which may not be effective for the EM operation in the load mode. The paper also presents the results of our study of transient processes in EM and the impact of the magnetic system on them. 2 A METHODOLOGY TO DETERMINE THE MAGNETIC SYSTEM OPTIMAL TOPOLOGY Our determination of the magnetic-system optimal topology is based on the weight coefficient method. The effectiveness of any magnetic-system topology is determined by two functions: Q mm m * = 2 knFm m = 1 n Qmax 2 knFn n = 1 (1) (2) where Q is the topology estimation; kn is the weight * coefficient of the EM optimality criterion; F* is the relative optimality criterion with the maximum value; system depends on this criterion; rigidity is measured as a percentage of the voltage drop from the no-load condition to the 1.5 overload; - the cost and manufacturability. The most important criterion is the EM reliability. Each criterion is considered in relative units because of the different measurement units: F (3) * Fn = F y where Fn is the calculated criterion in dimensional units for a specific topology and Fy is the desired value of the criterion established in the technical task. The optimal magnetic system with interior permanent magnets for a 120 kW EM aviation with an output phase voltage of 115 V is considered. Table 1 presents desired criteria values (3) for a given EM. The values of these criteria are selected specifically for each EM type. Table 1. The EM criteria. Criteria Desired value F is the relative optimality criterion with the minimum value. The most efficient magnetic-system topology is with Qmin lower and Qmax higher than for a competing variant. Therefore, the main minimal effectiveness criteria for the transport industry are: - the mass and overall dimensions for the cars, spaceships and aircrafts with a limited space in which EMs are installed; - the permanent-magnet mass and rotor bandage thickness providing the rotor mechanical strength, airgap value and EM efficiency; - the permanent-magnet losses studied separately from the total losses; - the total harmonic distortion (THD) coefficient under no-load conditions; - THD coefficient under load conditions; - the EM thermal factor (a product of the linear current load and the current density in the EM windings); - the EM losses with the exception of the rotor losses; - rigidity of the EM external characteristics (the ratio of the voltage to current); the mass of the EM control EM mass, [kg] 24 EM power, [kW] 120 Total losses, [W] 1700 Output phase voltage, [V] 115 Rotor-bandage material Carbon Rotor-bandage thickness, [mm] 2 THD under no-load conditions 3 % THD under load conditions 5 % Rigidity of the external characteristic 15 % Rotor mass, [kg] 6 Rotor losses, [W] 80 Thermal factor, [ A/mm2 • A/sm ] 6000 Rotor cost, [$] 1000 3 Investigated topologies For the high-speed EM with HCPM, the use of a bipolar magnetic system is the optimal variant. This allows the minimum magnetization reversal frequency of the stator and hence the minimum stator-core losses and also meets special requirements for the output frequency of the EM voltage. For example, for some space applications, a generator of a 1000 Hz output-voltage frequency and a 60,000-rpm rotational speed is used. For some aviation EMs, the rotational speed of 24000 rpm and the output-voltage frequency of 400 Hz are required. The disadvantage of the bipolar magnetic system is the increased height of the stator back compared to other designs due to the magnetic-field line length. The following most applicable magnetic-system topologies of the high-speed bipolar EM with HCPM are investigated: MAGNETIC SYSTEM TOPOLOGY SELECTION OF THE HIGH-SPEED ELECTRICAL MACHINE WITH INTERIOR PERMANENT... 183 3.1 Solid cylindrical bipolar magnetic systems (topology A) The solid cylindrical magnetic systems (Fig. 1) can be effectively used in EMs with the rotor diameter below 60 mm. The higher rotor diameter values make the use of these magnetic systems ineffective due to the HCPM mechanical fragility and the complex provision of the rotor mechanical strength. Such magnetic systems are used in EMs with HCPMs at the rotational speeds of 30,000-1,000,000 rpm [19, 20-22]. 3.2 Assembled bipolar magnetic systems (topology B) The assembled bipolar magnet systems (Fig. 2) are used in EMs with the rotor diameter above 50 mm and mainly with the sectors magnetized radially or diametrically. For the bipolar magnetic system manufacturing technology, the magnetic field in the air gap can be inhomogeneous, leading to a significant high-harmonic manifestation and sinusoidal voltage distortion. The assembled bipolar magnet systems are effectively used only with the ferromagnetic rotor. Permanent magnets Figure 2. EM with assembled bipolar magnetic systems 3.3 Assembled bipolar magnetic systems with an optimal magnetization (topology C) Because of the disadvantages of the assembled magnetic systems with radial magnetization, another configuration of the rotor bipolar magnet system of the magneto-electric generator for aerospace applications is proposed (Fig. 3). Figure 3. EM with an assembled bipolar magnetic system and an optimal magnetization. In this case, the magnetization direction is simulated similarly to the entire cylindrical system (topology A), but the rotor is assembled. This minimizes the system cost and consequently the EM. Also the rotor mechanical stress is much smaller than in the solid cylindrical magnetic system. Thus, this magnetic system combines the advantages of the cylindrical and assembled magnetic system. Technologically, the magnetic system can be accomplished in two ways: - the permanent-magnet magnetization in the rotor on a magnetic shaft; - the desired permanent-magnet configuration is obtained from the magnetized prismatic HCPM by machining it. Using two these technologies solves the problem of setting up the proposed magnetic system to be used in practical implementations. 4 Computer simulation of the BIPOLAR MAGNETIC-SYSTEM TOPOLOGY To determine the optimal magnetic-system topology and to evaluate the proposed magnetic-system efficiency using the finite-element method, an EM computer model for each of the three topologies is created in the Ansys Maxwell software package. For the B and C topology, the external rotor diameter of the 120 kW EM is 100 mm. The number of the turns of the EM phase winding is 6. Based on calculations, at a diameter of 100 mm, the rotor mechanical strength for the A topology is practically impossible. Therefore, for this topology, the EM rotor diameter is reduced to 60 mm in order to maintain the same power for each topology, and the number of the turns is increased to 12. For each topology, the liquid is cooled. To allow for a comparison, the current density for each topology is 15 A/mm2. Table 2 shows the parameters for each EM topology. In the computer simulation, the distribution patterns of the magnetic field and the HCPM eddy-current losses are obtained for each of the studied topologies (Figs. 46). Fig. 7 shows results of the magnetic-field calculation and of the permanent-magnet loss analysis in EM. 184 ISMAGILOV, VAVILOV, AYGUZINA Table 2. The EM parameters for the A, B and C topology Parameter Topology A Topology B Topology C EM mass, [kg] 22 25.1 24 EM power, [kW] 120 120 120 Rotor diameter, [mm] 60 100 100 Outer stator diameter, [mm] 180 195 195 Active length, [mm] 125 120 120 Slot number 18 18 18 Output phase voltage, [V] 115 115 115 Number of the turns 12 6 6 RMS current, [A] 349 359 347 THD under no-load conditions, [%] 3 7 3 THD under load conditions, [%] 4 10 4 The current density j, [A/mm2] 15 15 15 Linear AC current load, [A/sm] 106700 38829 37531 Thermal factor j ■ AC , 16005 5820 5625 [ A/mm2 ■ A/sm ] Rotor mass, [kg] 3 6 6 Rotor losses, [W] 154 17 15 Stator-core losses, [W] 350 518 510 Stator-winding losses, [W] 2096 1109 1036 Total losses, [W] 2600 1644 1561 Voltage, [V] /A Voltage, [V] 140 140 rv B~WVi h \ \ I J Time, [ms] Figure 5. Voltage at the rated load for the investigated topologies. Losses. [W/mA3] ■ 6.14e6 _ 5.73e6 5.30e6 4.9le6 ■ 4.50e6 ■ 4.06e6 ■ 3.68e6 e) f) Figure 7. Distribution of the magnetic field and the permanent-magnet losses for various topologies: a) the magnetic-flux density for the B topology; b) the permanentmagnet losses for the B topology; c) the magnetic-flux density for the C topology; d) the permanent-magnet losses for the C topology; e) the magnetic-flux density for the A topology; f) the permanent-magnet losses for the A topology. MAGNETIC SYSTEM TOPOLOGY SELECTION OF THE HIGH-SPEED ELECTRICAL MACHINE WITH INTERIOR PERMANENT... 185 The analysis result shows that the C (the proposed design) and A topology have the required THD coefficient and the magnetic-flux density in the stator core is below 2 T. The value of the magnetic-flux density is ensured by using the Vacoflux cobalt alloy enabling the stator core to remain unsaturated. It should be noticed that the magnetic-system topology insignificantly affects the permanent-magnet losses. They are the same for the B and C topology, although the primary magnetic field in the air gap is significantly distorted for the B topology, and it is sinusoidal for the C topology. The reason for the equal permanent-magnet losses for these topologies are the unchanged zone of these topologies and the winding data. In the A topology, the slotted zone and the winding data are changed resulting in a 30 % permanent-magnet loss increase. Thus, the A topology is a perspective variant for EMs with the power below 30-40 kW and the rotor diameter below 60 mm. For more powerful EM, the use of such magnetic system is inefficient because of the high thermal load. A reason is the small rotor diameter limited by the rotor-bandage mechanical strength and the necessity of a significant linear current-load increase. This leads to an increase in the number of the turns of the stator windings, to a higher thermal load and lower low external characteristic rigidity (Table 2) and to increased winding losses. The THD coefficient of this topology is the same as for the C topology. The character of the magnetic field in the air gap is a sinusoidal. Therefore, it is recommended to use the A topology for EMs with the power below 30 kW and with the rotor diameter below 50 mm. For EMs with a higher power, the C topology is the most effective. Besides evaluating the effectiveness of various magnetic-system topologies under the rated operation mode, the magnetic-system topology impact on the transient processes is considered, too. The single-phase and three-phase short circuits in EMs are investigated. Fig. 8 shows the torque characteristics of EMs with a three-phase symmetrical short circuit. A single-phase short circuit is presented in Fig. 9. Torque, [H*m] K ft I A. A L \V /A