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The selected electric motor by BoschRexroth represented the best trade-off between speed, bandwidth and torque requirements. Its rated and maximum speed are respec- tively 2600rpm and 12000rpm, being therefore able to reach the desired 6000rpm. The bandwidth of the current loop is equal to 520Hz if its PI controller is calibrated as indicated by the producer’s specification for optimized per- formances. This leads to a maximum reachable bandwidth for the speed control loop of 260Hz. Consequently, the motor allows to reproduce all the harmonic components with their amplitude values up to 4000rpm, after this value 1st and 2nd harmonics will still be completely reproduced while 3rd and 4th will be attenuated but not eliminated provided we do not overcome the peak torque. The rated and maximum torque values are respectively 140Nm and 320Nm. As shown in Fig.4 to a speed of 6000rpm corre- sponds a maximum torque of around 200Nm which can be fully reproduced with a duty cycle of 10% on a period of 10 minutes, which is sufficient for the tests that will be performed. The values of rated and maximum torque define the maximum amplitudes that the electric motor
Fig. 5. Envelopes of harmonic contributions and compari- son with simulation results
is capable of replicating for each harmonic. Such analysis was analytically performed taking into account only the electric motor inertia and then compared to the results of the simulations that included the BDS effects. Each harmonic component was considered autonomously and its contribution was caluclated and simulated neglecting the other components. The analytical calculations provided therefore the envelopes of the allowed amplitude values that are shown in Fig. 5. In Fig. 5 is also shown the comparison between calculated and simulated values. It is clear how the torque required for operating the BDS represents a small portion of the overall required torque. It is important to notice that at low speeds the torque required for the accessories, and in particular the alterna- tor, become higher and no longer neglegible. This effect is particularly clear for what concerns the first harmonic.
3.3 BSG specifications
The definition of the BSG performances does not include an harmonic analysis but is detemined by the sole reach- able speed and torque values. The reachable speed is obtained through the transmission ratio between BSG and CS pulleys of the BDS and the maximum speed requested for the CS motor. In this case to 6000rpm of the CS speed correspond 14000rpm of the BSG. The torque that the BSG needs to provide is given by the performances required for the different operating modes that define the BSG functionalities. Said operating modes can be pided into the following groups:
• Standard engine driving mode with BSG working as traditional alternator
• Regenerative braking
• Electric traction mode
• Warm engine start, i.e. warm cranking
• Cold engine start, i.e. cold cranking
• Electric boost, i.e. traction with ICE on
The maximum torque value was obtained in the cold cranking operating mode and it is equal to around 40Nm. In Fig.6 can be seen the torque-speed characteristic re- quired for the BSG motor. Such requirement is completely fullfilled by the characteristics of the selected electric mo- tor.
4. CONCLUSION
In the present work a test rig for the characterization of Belt Drive Systems was developed. The design focused on