Datasheet and boundary conditions
In the context of our study, we employed the Bosch Pedelec motor as our reference motor. The experimental setup involved the use of multiple motors, each differing solely in their winding configuration. Specifically, we had one motor with its original winding intact, while the others featured windings created through investment casting techniques.
To ensure consistency and a fair comparison, we decided to reuse the rotors from the original motor in all instances. Additionally, for the original copper-wound motor, we also opted to reuse the original wound stator laminations package. For the cast coil motor, which utilized aluminum windings, we procured a new stator laminations package based on the star/yoke principle to accommodate the unique coil design. It’s important to note that all stators were installed in universally identical housings to facilitate our test bench assessments. Within our internal identification system, we referred to the original motor as “RefMot.” We designated the motor with the substituted cast coil winding and nearly identical stator laminations as “RefMot C.” Lastly, the motor with the substituted cast coil winding and optimized stator lamination geometry was denoted as “RefMot G.”
This comprehensive setup allowed us to conduct a rigorous evaluation of the performance and efficiency differences among these various motor configurations.
Adjustments to make the coils pluggable
- a) A 90° angle was constructed at the bottom of the slot.
- b) The tooth tips were all mechanically connected to the respective neighboring tooth with the thinnest possible bridge (approx. 0.35 mm) (star- yoke principle).
- Same as with RefMot C and additionally c) thinner yoke and tooth whereas the outer diameter stayed equal
Simulation results
- Power and torque at 70 °C steady-state winding temperature and 500 rpm
- Ambient temperature 20 °C
- Weight and torque density refer to active part (stator and rotor)
- RefMot and RefMot C tested on test bench of independent institute
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Conductor material Cu 99,9
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Electr. conductivity 58 MS/m
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Slot filling factor 32%
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Volume stator lamination 57628 mm³
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Volume rotor lamination 26524 mm³
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Volume PMs 11270 mm³
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Volume winding 20000 mm³
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Mass stator lamination 438 g
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Mass rotor lamination 202 g
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Mass PMs 87 g
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Mass winding 179 g
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Mass activparts 905 g
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Continuous torque at 70°C (500 1/min) 1.01 Nm
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Torque density only active parts (500 1/min) 1.12 Nm/kg
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Continuous power at 70°C (500 1/min) 53 W
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Max efficiency 90.6
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Conductor material Al 99,7
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Electr. conductivity 34 MS/m
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Slot filling factor 64%
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Volume stator lamination 59622 mm³
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Volume rotor lamination 26524 mm³
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Volume PMs 11270 mm³
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Volume winding 41126 mm³
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Mass stator lamination 453 g
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Mass rotor lamination 202 g
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Mass PMs 87 g
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Mass winding 111 g
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Mass activparts 852 g
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Continuous torque at 70°C (500 1/min) 1.35 Nm
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Torque density only active parts (500 1/min) 1.58 Nm/kg
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Continuous power at 70°C (500 1/min) 71 W
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Max efficiency 90.8
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Conductor material CuAg0,2
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Electr. conductivity 56 MS/m
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Slot filling factor 80%
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Volume stator lamination 50666 mm³
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Volume rotor lamination 26524 mm³
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Volume PMs 11270 mm³
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Volume winding 65520 mm³
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Mass stator lamination 385 g
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Mass rotor lamination 202 g
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Mass PMs 87 g
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Mass winding 585 g
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Mass activparts 1259 g
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Continuous torque at 70°C (500 1/min) 2.11 Nm
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Torque density only active parts (500 1/min) 1.68 Nm/kg
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Continuous power at 70°C (500 1/min) 110 W
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Max efficiency 93.6
Simulation results – mechanical power
- I(max) is equal -> higher square diameter of cast coil leads to lower losses (higher efficiency)
- Cast coils generates more torque at higher speeds
Conclusion
The substitution of the conventional winding with a cast coil made of aluminum proved to maintain at least the same level of performance. On the other hand, the utilization of cast copper coils resulted in a notable enhancement in efficiency.
It’s worth noting that the cast coils need to be pluggable for practical applications, ensuring ease of maintenance and replacement when necessary.
While the star-yoke principle was employed for the stator laminations package in the cast coil motor, it’s important to acknowledge that this configuration slightly reduced torque due to the presence of the bridge between the teeth.
Our findings also suggest that merely substituting the conventional winding with cast coils might not yield the optimal results. To achieve the best performance with cast coils, it is advisable to coordinate the entire motor, including the rotor and stator, and, if necessary, the cooling system, during the design process. This holistic approach ensures a more integrated and efficient motor system.
