In recent years, consumer demand on electric car is generating new challenges, mainly focused on the problem of autonomy and safety. This depends on the overall efficiency and power density in the powertrain. To achieve very high-power densities, even with low available current and voltage, additional effort was taken to the design of the ModulED’s motor, in order to offer a both safe and strong technology.
To reduce motor’s active weight and to improve its efficiency, the main goal for the EM (E-motor) development was to save more than 50 % magnet mass compared to existing PMSM (Control Permanent Magnet Sync Motor) with similar power. Many iterations were needed to satisfy all the previously defined drive requirements, described in the following list of main attainment and highlights:
- Very high efficiency over a wide speed and torque range (Table 2);
- Great magnetic, mechanical and thermal simulation accuracy compared to measurements;
- High peak power over a wide speed range;
- Sinusoidal back EMF;
- Low torque ripple;
- Very good overload capability, potential for even more torque and power with higher inverter currents;
- Well balanced and stable mech. high-speed rotor design;
Figure 1: Main motor dimensions
- Very good cost potential due to low active weight and the magnet mass reduction;
- Patented rotor shapes;
- Impressive peak and continuous power density due to high-speed motor design and new winding technology (Figure 7);
- 3ph or 6ph star- and 6ph independent-connection is possible (1 x 3ph; 2 x 3ph; 1 x 6ph;)
- Patented stator winding technology (Form litz wire);
Figure 2: Form litz wire section (micrograph)
All the previous defined requirements were tested on the BRUSA test bench with a 3ph inverter (see Figure 3). Tests with the integrated motor in the housing with 6ph-GaN-inverter (see Figure 2 and Figure 3) and transmission will be done in the test bench of PUNCH Powertrain.
To validate the final e-motor design, tests were done on a BRUSA test bench with the Buried Permanent Magnet Motor BPMM1 (BPMM1: NdFeB magnet rotor; BPMM2: Injected magnet rotor). The battery voltage was set to VBatt = 320 VDC and the modulation factor of the 3ph BRUSA inverter is Umod = 0.95. This corresponds to a DC voltage of 320 VDC x 0.95 = 304 VDC.
The 450 ARMS with 3 phases or the 225 ARMS with 6 phases do not yet represent the limit of the machine. Initial tests with higher currents have shown that the motor has potential up to 217 Nm and / 221 kW at 400 VDC and a modulation factor of 0.95. From the torque per amp measurements the linearity of the motor also with higher inverter currents was tested and passed.
The magnet weight of the ModulED motor is significantly reduced from 2 to 2.5 kg for conventional lower motor speed designs to 1.35 kg which is a great achievement.
With a high-speed motor design, it is possible to realize great power density improvements without compromising the motor efficiency or the peak power and continuous characteristics. In Figure 7, the continuous power is linear for appr. 90 sec at 143 kW, before the derating of the inverter starts.
In Figure 8, the continuous power measurements show the Formed Litz Wire potential. With a conventional water jacket cooling and a low flow rate, the maximum thirty minutes power is 104 kW at 320 VDC and 12’000 rpm. The temperature of the rotor is higher than the stator at higher motor frequencies. For further continuous power improvements another cooling concept like oil spray could further improve the thermal capability of this motor. Also, the temperature distribution would be better balanced and higher specifications in power and torque could be met. However, the given specifications were met in every single case even without rotor oil cooling.
The motor can deliver 90 s the full current of 450 Arms. This measurement shows the margin of thermal performance of the litz wire stator. The rotor isn’t the limiting factor for short time full power operation points.
The measured peak efficiency is 97.4%, shown in Table 2, which is 0.4% higher than the proposed target of 97%, see Table 1. More important than the peak efficiency is the wide torque and speed area of a high efficiency. This is very important for the WLTC efficiency. The required torque and power in this cycle is quite low. Depending on the gear ratio the torque is below 70 Nm in 2nd gear about 40 Nm. With an optimized gear switching strategy the total vehicle efficiency could be optimized.
In the end, all initial requirements of the ModulED project were met or even exceeded qualified on the BRUSA test bench. Optimizations could be made thanks to the potential improvements identified during the latest developments and will be explored during future tests with integrated motor and transmission, planned on the test bench of PUNCH Powertrain.