hofer powertrain GmbH

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1 Berlin, Your Partner for energy-efficient powertrain systems hofer powertrain GmbH A company of hofer AG Oberboihingen Nürtinger Strasse 78 info@hofer.de

2 Content Introduction Electrical machines Challenges of the electrical drive system Conclusion 2

3 Introduction The e-mobility will play a major role in the near future Especially electrical axle drives for hybrid- and electrical vehicles are in focus nowadays Because an electrical axle drive can be considered as a high dynamic torque-source/sink, special attention is required during the operation mode as well as in the failure mode The mass production costs for an electrical axle drive and the availability of the related material will be very important 3

4 Introduction Electric drive system for a typical axle drive 4

5 Introduction Typical performance characteristic This kind of characteristic can be achieved by using different electrical machines. 5

6 Electrical machines General requirements on electrical machines Electrical machines integrated into the powertrain have to fulfil the following requirements: low cost low weight / small installation space high efficiency long durability / low wear high availability of the material low influence in case of failure low noise and vibration level high grade of protection 6

7 Electrical machines Three-phase machines Electrical three-phase current machines are particularly suitable for use in hybrid and electrical vehicles. Three-phase machines Asynchronous machine Synchronous machine with cage rotor with permanent magnet excitation with separate excitation 7

8 Electrical machines Asynchronous machine Block circuit diagram for such a drive Rotor design of an asynchronous machine with squirrel cage Conductor bars Short circuit ring Rotor Electrical drive based on asynchronous machine 8

9 Electrical machines Permanent magnet excited synchronous machine Block circuit diagram of a permanent excited synchronous machine PSM Rotor design of permanent excited synchronous machines Permanent magnets Permanent magnets Bandage Rotor Electrical drive based on permanent excited synchronous machine 9

10 Electrical machines Current excited (separate excited) synchronous machine SM Rotor design of a separately excited synchronous machine Electrical drive based on a separately excited synchronous machine 10

11 Electrical machines Utilization of electrical machines Important equations 1. Equation: f s n S Pmech = V 2π f s = Bˆ Aˆ P mech : Mechanical power fs: Specific tangential force n s : Rotor speed B: ˆ Flux density peak value Â: Electrical loading peak value Fast running machines therefore have a smaller size than machines with lower speed at the same performance 2. Equation: f f s s π D 4 V = T 2 l = T D: Diameter of the rotor l: Length of the rotor V: Rotor size T: Torque The size of an electric machine will be determined by the required torque, if will be constant. f s 11

12 Electrical machines Development process for an electrical machine requirements E-drive expert knowledge EM analytical calculation boundary conditions power electronic battery installation space FEM mechanic calculation FEM electro magnetic calculation thermal simulation structure dynamic A. acoustics no system simulation yes prototype requirements fulfilled EM-control strategy power electronics control test bench vehicle In addition to the above mentioned performance simulation also a dynamic simulation concerning the dynamic behavior of the electric drive especially in case of failure is required. This simulation is very important regarding the influence on the vehicle dynamic. 12

13 Electrical machines Optimized control of the electrical machine In principle all mentioned electrical machines can be used. But especially for axle drives a high speed asynchronous machine in connection with a transmission with a high gear ratio is well suited for the following reasons: Very high robustness No safety issues in case of failure Needs aluminum inside the rotor instead of rear earth magnets, therefore no problem with availability of magnet materials Low cost solution compared to other solutions. Therefore well suited for mass production High overload capability Well suited for high speed applications No problems with acoustic noise Very low torque ripple compared to other solutions 13

14 Electrical machines Optimized control of an ASM Map for optimal efficiency Control for optimal efficiency in complete speed range Air gap flux must be adjusted permanently to the driving situation (dynamic- efficiency mode, selection via CAN) feed forward Best voltage usage in field weakening (Optimal Pulse-Patterns ) _ref Motor/Generator/ passive mode is_max Flux on line optimization rd_ref isd_ref - isd_real + isd 2 is = isd + isq 2 Usd Uu 2 Uv feed forward is Us Usq 3 Uw ϕ rd = f ( U, T, ω) dc req _ref 1 n rd isq_ref - isq_real + isq ϕ rd Rotor flux isq 2 iu U dc dc-link/battery voltage Temp Temp. compensation n_ref Rs Rr isd Speed Observer 3 iv n_obs T req ω Reference torque Rotor speed n_obs 14

15 Electrical machines Optimized control of a PSM/IPM Control of IPM Machine with consideration of non-linear E-Machine Parameters Parameter Definition strategy Ld, Lq, Psi_p Best voltage usage in field weakening (Optimal Pulse-Patterns ) 15

16 Electrical machines Optimized control of the electrical machine Typical simulation of an electrical drive including a battery model Typical system simulation including electrical machines, power electronics and battery based on a driving cycle (NEDC) Torque PSM Tq_req_psm N U_dc PSM Tq I_ph P_dc I_dc_psm Tq I_ph P_dc I_dc Tq [Nm] Tq_req_asm N 2.5 Gear N_isg U_dc ASM Tq I_ph P_dc I_dc_isg Tq I_ph P_dc I_dc t[s] Battery model U_batt - U_batt = 270 V R_batt = 0.4 X R_batt I_dc I_dc_psm + + I_dc_isg Idc [A] t[s] I_dc PSM 16

17 Challenges of the electrical drive system Electrical axle drives can be applied in hybrid vehicles as well as in pure electrical vehicles. In a hybrid vehicle normally only one axis is electrified. In a pure electrical vehicle all axis, even all wheels can be separately driven by an electric drive. B G E E G B Front wheels E: Electrical machine Chassis B: Brake B G E E G B Rear wheels G: Gear - Box Electrical vehicle with all wheel drives With this configuration it is possible to influence the longitudinal dynamic as well as the transversal dynamic of the vehicle. With this concept it is possible to have a specific acceleration or brake torque on each wheel, so called torque vectoring. Furthermore it is possible to compensate over steer or under steer with a brake intervention but also with a power boost in a range of some mille seconds. 17

18 Challenges of the electrical drive system System behaviour in normal mode of the electrical drive As earlier mentioned only an axle drive will be considered. Chassis Axle bearing Stator EM Gearbox Rotor EM - EM T Drive shaft c Vehicle - F Mechanical model of an axle drive This model can be used for different investigations: Acoustic noise Bonanza and jerk effect 18

19 Challenges of the electrical drive system Acoustic noise The main reason of acoustic noise of an electrical axle drive is normally the torque ripple of an electrical machine. The torque ripple depends on the kind of the electrical machine but also on the design of the machine. The torque ripple will be transferred via different ways: From the rotor shaft to the gearbox From the stator housing via axle bearings to the chassis In both cases the torque ripple can generate acoustic noise. Furthermore the stator housing of the electrical machine can also generate structure borne noise caused by radial magnetic forces. 19

20 Challenges of the electrical drive system Dynamic drive control The main task of the dynamic drive control is the active damping of oscillations within the drive train. The following boundary conditions have to be taken into account: On the one hand, an oscillation excitation by the driving torque of the electric drive shall be compensated. This is the case if a fast torque demand is required e.g. "Tip in". On the other hand also outer disturbances, e.g. caused by the wheels have to be considered. Chassis The reduction of the oscillations, shall not lead to a noticeable reduction of the torque dynamics which the driver perceives as being negative. B B G E E G G E E G B B Front wheels Rear wheels In case of failure the vehicle dynamic should not be influenced noticeable. 20

21 Challenges of the electrical drive system Dynamic drive control Classical approach T ref filter - feed-forward control controller T* em control plant inverter + ASM T* disturbance drive train T S Ω wheel Ω Rotor filter Block circuit diagram of the controlled system with a classic control concept 21

22 Challenges of the electrical drive system Dynamic drive control State variable control In the control engineering the state variable control represents an alternative to the classical methods. Starting point for this is a description of the system to be controlled in the state space in which a linear time-invariant behavior is presupposed. x(0) + z (disturbance variable) T ref VV preliminary filter T em - BB x (t) 1 S A x (t) CC Ω R measurement RR Model of an axle drive with state variable control 22

23 Challenges of the electrical drive system Safety relevant aspects Possible failures within an electric drive 23

24 Challenges of the electrical drive system Safety relevant aspects by using a PSM/IPM for an axle drive Three-phase short circuit Two-phase short circuit n= const B G E E G B Front wheels Chassis Braking torque Oscillation torque B G E E G B Rear wheels Idle speed / no load operation Dragging torque 24

25 Challenges of the electrical drive system Safety relevant aspects By using an electrical axle drive based on an asynchronous machine or a separately excited synchronous machine there are no special measures in case of failure required By using an electrical axle drive based on a permanent magnet excited synchronous machine (PSM/IPM) the following measures are required in case of a threephase/two-phase short circuit in order to avoid safety problems with the vehicle: Disconnecting the mechanical drive train by a fast switching clutch Disconnecting the terminals of the machine or disconnecting the star point of the threephase winding of the machine 25

26 Conclusion Some important items have been considered in order to avoid serious problems with the mass production of hybrid and electrical vehicles It is very important to consider the total vehicle system behavior in the operation mode as well as in the failure mode of an electrical axle drive It was shown, what kind of an electrical axle drive has a big potential for the future to fulfill the economic and technical requirements of hybrid- and electrical vehicles 26

27 Contact Your contact: Dr. Heinz Schaefer tel fax Visit us at: 27