# Design and Verification of Multiphase Multilevel Traction Inverter
## 3 Verification of the TNPC Converter
### Test Procedure  
First, the inverter's switching pattern (Figure 13) was verified using a 16-channel logic analyzer from Saleae (San Francisco, CA, USA). The measured signals shows PWMs to the transistors according to the SPWM modulation. The signals labelling corresponds to the MOSFETs configuration shown in Figure 1. When reference sine wave is in positive half, the TOP and OUT MOSFETs are switching in complementary, MID is turned on and BOT is turned off. Consequently, when reference is in negative half, the BOT and MID are switching in complementary, OUT is turned on and TOP is turned off.  
The TNPC inverter was verified at different switching frequencies to properly measure the prototype power losses and efficiency. The selected switching frequencies were 10 kHz, 20 kHz, and 30 kHz. The output frequency was chosen to be 50 Hz and was constant during all the measurements. The converter input voltage and modulation index were changed during the measurements to measure different voltages and output power levels. The output power versus modulation index at different input voltages can be seen in Figure 14.  
The maximum output power is 19,739 W. The available load limits this output power because it cannot dissipate more power. Figure 15 shows the dependency of the efficiency on the output power of the inverter:  
Figure 12: Test set-up with RL load.  
Figure 14: Output power dependency on the modulation index at f\({}_{\rm sw}\) = 20 kHz.  
Figure 13: Switching signals of the power transistors.  
Figure 15: Efficiency of the prototype at different switching frequencies.  
Due to the high efficiency of the converter prototype, the water cooling was not connected to the water system. Cooling during this test was only passive with modules connected to the water block. Temperatures at the modules are given in Table 6.  
From the data above, the modules can only be passively cooled to an average output power of approximately 30 kW. Then, water cooling must be connected to cool the semiconductors properly. Figure 19 shows the output voltages and currents of the TNPC inverter at a switching frequency of 20 kHz and output power of 20 kW.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Switching Frequency** & **Average Module Temperature at 20 kW** \\ \hline
10 kHz & 30.71 \({}^{\circ}\)C \\
20 kHz & 46.25 \({}^{\circ}\)C \\
30 kHz & 53.58 \({}^{\circ}\)C \\ \hline \hline \end{tabular}
\end{table}
Table 6: Measured temperature of modules at the different switching frequencies.  
Figure 19: Output waveforms at 20 kW.  
Figure 18: Efficiency map at a switching frequency of 30 kHz.  
The RMS phase current value measured by the YOKOGAWA (Tokyo, Japan) was 21.68 A and by the on-board current sensor 21.51 A. According to the YOKOGAWA, this measurement proves to be accurate for the onboard measurement. The DSP calculates the value of moving RMS online for all six phases. Figure 20 shows a summary of the measured voltages and currents. The output RMS voltage at phase was only 188 V due to the low value of the DC\(-\)BUS. If the DC\(-\)BUS voltage is 565 V and the modulation index is set to 1, the phase voltage reaches 230 V RMS.  
As seen in Figure 19, the apparent output power of the converter was 24.06 kVA. The active power was 19.48 kW. The output power factor during this test was 0.813.  
The thermal image of the converter when delivering 20 kW at the output is shown in Figure 21.  
Figure 21: TNPC inverter temperature at 20 kW.  
Figure 20: Output variables at 20 kW.  
Due to the power limit of the power resistor at the testing load, the maximum output power that could be safely reached was 20 kW. Figure 22 shows the temperature of the actively cooled power resistors.  
The maximum temperature is around 283 \({}^{\circ}\)C. This temperature is reached at the end of the load, where airflow from the cooling fan is limited.