Real-time Simulation of Electric Motors

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1 Real-time Simulation of Electric Motors SimuleD Developments in the electric drive-train have the highest priority, but all the same proven development methods are not consequently applied. For example the simulation of the hybrid drive-train excludes the e-motor and its system environment such as the corresponding controllers. Too complex and too much effort, is the justification. SILVER ATENA produces a proof of the contrary and achieves with a real-time simulation of an electric machine using the Hardware-in-the-loop technique more precise system verifications. Despite the undoubtedly high complexity Software- and Control-Theory engineers can thus work more efficiently. With the development of the first Hybrid vehicles it was necessary to integrate the corresponding hybrid components, like the electric motor, high-voltage battery, inverter and the corresponding controllers into the simulation. The established methods of model in the loop-simulation (MiL) and Hardware in the loop (HiL) allow to verify the basic system functionality and to optimise the design. The exact simulation of the electric motor is essential to optimise for example critical phases like the handover between the electric motor and the combustion engine, such that it goes unnoticed by the driver. Another application of an electric motor simulation aside from the HiL-integration is the end-of-line test of inverter controllers. The simulation of an electric motor can be done on different levels: on mechanical and electrical level as well as on the level of the inverter. Simulation on the Mechanical Level A typical setup to simulate an e-motor is shown in (1). Here the inverter drives a real synchronous motor. A position sensor reports the current motor position back to the inverter. Depending on the currently flowing currents the motor generates a torque M M. To simulate the mechanical load M L on the motor, a second motor is used, which is connected to the first motor by a clutch. The external drive-train HiL provides the applied load torque. The resulting quantities like the torque M R, angle und angular velocity are reported back to the HiL. Using the real motor in this set-up is an advantage as well as a disadvantage. The advantage: The interaction between the motor and the inverter can be verified. A disadvantage is that a change of the motor parameters for optimising the use-case is hardly possible. Also the simulation of typical e-motor failures (like defect position sensors, short circuit of the coil windings or the temperature dependence of the coils) is almost impossible. In addition the security issues of such a set-up are not to be underestimated. Due to the high forces which occur on such a machine-bed, according security protections (like covering all moving parts and adequate foundations) are required. The whole set-up requires a lot of space and inflates the costs for setting up and maintaining such a simulator.

2 (1) Mechanical simulation of an e-motor using a machine-bed the electric simulation replaces the components shaded in grey Simulation on the Electrical Level On the electrical level the interfaces of the region shaded in grey in (1) are simulated. Here a real-time model of the e-motor is used, which based on the voltage patterns driven by the inverter and the load torque calculates the actual resulting torque and angular motor position. But this is not yet the full simulation. (2) shows the equivalent circuit of a synchronous motor. For a real simulation, in addition to the coil inductivities and resistances also the voltages induced into the stator coils by the rotating rotor the so called counter electromotive force needs to be simulated. The simulation on the electrical level offers compared to the mechanical simulation many advantages. Because there are no rotating parts in this setup, no elaborate security precautions are required. Since the complete functionality of the e-motor is realised now in a real-time model, the simulation of typical faults is easy to implement. Also all motor parameters can now be varied arbitrarily, which allows their optimisation to the required use-case. Furthermore the inverter test can be started now before the e-motor is available, which significantly reduces the time-to-market. Simulation on the Inverter Level Also on the level of the inverter a simulation of an e-motor is possible. An inverter typically consists out of a B6-bridge and the corresponding control-electronics. Through analysing the control signals sent to the B6-bridge a real-time model can calculate the reaction of the motor and generate the position signals and torque for the environment like on the electrical level. The advantages of this method are the low efforts required for the simulation; but on the other hand the verification of the power electronics is left aside. Moreover for this kind of simulation the inverter needs to be opened and modified, which is not possible during the later stages of development.

3 (2) Equivalent circuit of a synchronous motor Challenges Until recently the simulation on mechanical level or inverter level was used during development, because commercial simulators on the electrical level were not yet available. This is due to the various technological challenges one has to master to develop an electrical simulation of an e-motor. Based on the voltage pulses which are applied by the inverter to the three motor phases and the actual position of the rotor, the motor torque and the counter electromotive force have to be calculated. The calculation needs to be almost instantaneous to achieve the necessary required accuracy of the counter electromotive force. An electric rotor frequency of 2.5 khz corresponds to a sinusoidal period of 400 ms, i.e. the control of the counter electromotive force needs to respond within a few ms. Also the power electronics faces similar challenging requirements. It needs to provide counter electromotive force voltages of a few 100 Volts and short-term currents of up to 450 A. For this multiple half-bridges are used, which are clocked with a frequency of 100 khz, in a staggered way. Realisation of such a simulator needs know-how from many different fields, like real-time simulation, power electronics and control theory. SILVER ATENA currently develops the electric motor simulation SimuleD (Simulated Electric Drive) for inverter testing. SimuleD can simulate synchronous machines with a power of up to 100 kw. The simulation of a synchronous motor in this power class holds an additional challenge. To understand this, one has to take a look at the flowing energy between the inverter and the e-motor simulator as shown in (3). Energy Circulation During motor driven operation the energy flows from the three-phase supply network to the highvoltage/low-voltage battery simulations and from there over the inverter into SimuleD. This energy flow can now be either converted into heat by the e-motor simulator or fed back into the three-phase supply network. In the generator operation case the energy flow runs in the opposite direction. In this case the battery simulation either converts the energy into heat or feeds it back into the supply network. Feeding back the energy into the supply network requires a network which supports this, which is not the case everywhere. A conversion of 100 kw electrical power into heat is not acceptable nowadays from an ecological standpoint. To solve this problem SimuleD integrates in addition to the e-motor simulation the high-voltage (Lithium-Ion with 400 V) and low-voltage (12 V) battery simulation. This allows the circulation of the energy within SimuleD as shown in (4).

4 (3) Energy flow in a typical setup (4) Circulation of the energy during motor and generator operation within SimuleD In motor driven operation the energy flows from the inverter to the motorphase simulation and from there into an intermediate network. This network in turn supplies the battery simulation, which again delivers the energy to the inverter. In the generator case the energy flows accordingly into the opposite direction. Due to the limited efficiency of all components it is necessary to replenish the energy lost during one circulation. This is done by power supplies connected to the intermediate network. These supplies need to supply only about 10 % of the circulating power, which results in a very energy efficient system. Flexibility The e-motor simulator SimuleD, (5), distinguishes itself due to its great flexibility. It allows an easy customisation to different customer requirements. The power electronics is built as a modular system, such that the power of the simulated motor can be varied without difficulty in a range between 10 and 100 kw. Also the real-time model for the calculation of the motor behaviour and the simulated mechanical behaviour can be exchanged without problems. It is possible to integrate complete customer specific Simulink models. More complex models of the mechanical behaviour or the simulation of an asynchronous motor or a pm-machine are thus easily implemented. The simulator can be controlled and parameterised over a CAN interface. The CAN interface allows for an easy integration into existing HiL set-ups. To analyse the control behaviour of the inverter it is possible to capture certain signals (e.g. phase voltages and currents) or real-time model values with a rate of 300 ks and read them out via an Ethernet interface.

5 Security The issue of security is also of importance for the simulation on the electrical level, because the occurring voltages of over 400 V and currents of up to 450 A are life threatening for humans. In order that also a person with no technical experience can exchange the inverter, SILVER ATENA developed a security concept for protection: The inverter is mounted within a security enclosure, whose door cannot be opened during operation. The door is released only after all signals between SimuleD and the inverter are disconnected and have been discharged. (5) shows a picture of a complete SimuleD System including the security enclosure. Conclusion Using an e-motor simulation on the electrical level has a clear advantage over the other two possibilities presented the simulation on mechanical level and inverter level. It allows an accurate simulation of an e-motor and the mechanical behaviour and offers furthermore the possibility of an easy variation of the electrical and mechanical parameters. This allows already during the early development stages the parameters-optimisation and a thorough verification of the complete drive train. For the test and development of an inverter it is not necessary to wait for the availability of the e-motor. Due to the security concept also software and control-systems developers can perform simple and hassle-free a test of the inverter software and control-system algorithms. Overall the application of SimuleD allows thus to shorten the time-to-market decisively. (5) SimuleD-System including the security enclosure Product SimuleD Simulator Electric Drive Simulation of electric engines providing integration of hybrid engine control units. Contact product@silver-atena.de SILVER ATENA Electronic Systems Engineering GmbH Dachauer Str Munich Germany