# Common Mode Voltage Behavior of a 3 Level ANPC Inverter on Vehicle Auxiliary Drives

Maximilian Weigelt, Fabian Lorenz, Ralf Werner, Daniel Paul

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Maximilian Weigelt, Fabian Lorenz, Ralf Werner, Daniel Paul. Common Mode Voltage Behavior of a 3 Level ANPC Inverter on Vehicle Auxiliary Drives. The 26th European Conference on Power Electronics and Applications, GDR SEEDS France & EPE Association, Mar 2025, Paris, France. 10.34746/epe2025-0102 . hal-05088987

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# Common Mode Voltage Behavior of a 3 Level ANPC Inverter on Vehicle Auxiliary Drives

Maximilian Weigelt¹, Fabian Lorenz¹, Ralf Werner¹, Daniel Paul²

¹Chemnitz University of Technology, 09126 Chemnitz, Germany

²thyssenkrupp Dynamic Components, 09120 Chemnitz, Germany

## Acknowledgements

The authors appreciate the support of the Department of Materials and Surface Engineering at Chemnitz University of Technology by providing surface analysis of roller bearings.

## Keywords

«multi-level inverters», «common mode voltage», «bearing current», «automotive application», «PMSM».

## Abstract

EV drives are mostly made up of PMSM's driven by 2-Level voltage source inverters. The resulting common-mode voltage can significantly shorten the service life of the bearings. The application of a 3-level ANPC inverter on a 10kW AC compressor is described including comparative measurements of both inverter types.

## Introduction

The concept of modern main and auxiliary drives of electric vehicles consists predominantly of permanent magnet synchronous machines (PMSM) controlled by voltage source inverters in a 2-level topology. The operating points of these drives cover a wide range of speed, torque and DC bus voltage which requires highly dynamic inverters. Their fast-rising voltage pulses and high switching frequencies cause parasitic current flows within the drive system. If the shaft is not grounded, which is the case for roller bearings and other lubricated machinery under movement, the common mode voltage in particular can cause considerable voltage levels on the shaft due to the voltage division over the stray capacitances of the machine [1]. If the shaft voltage is high enough to cause a local breakdown of the insulating

lubricant film the charge of the capacitor formed by rotor and machine frame discharges in a current peak. This effect commonly known as electric discharge machining (EDM) causes a surface degradation of adjacent parts by locally melting up the material [2]. Even though the defects are very small they accumulate over time and form larger craters which can often be found in the races of steel roller bearings. Such pittings can significantly shorten the service life of the drive unit and must be reduced to a tolerable level.

<figure><img src="image_2.png" /><figcaption>Fig. 1: EDM damage of rotor bearing (inner race)</figcaption></figure>

The reduction of common mode effects on mechanical parts can be achieved in several ways. On the one hand common mode filters can be installed between inverter and machine but they are bulky and heavy and reduce the strictly limited installation space. Another approach is to break the current paths for example by installing ceramic bearings or non-conductive frame parts, but since a complete insulation of the drive train is often impossible such measures just shift the problem to the driven equipment. A dedicated grounding of the shaft is also possible but involves additional wear and tear. The use of a 3-level topology reduces the common-mode voltage substantially and can help to overcome this problem [3]. This article compares the application of 2-level and 3-level inverters on a selected drive for air conditioning compressors in Hybrid/EV cars in the power range of 10kW.
<figure><img src="image_3.png" /><figcaption>Fig.2: Drive structure of the investigated AC compressor prototype</figcaption></figure>

The design and control strategy of the investigated ANPC inverter are described in section II. The measurement setup used to compare the results of the voltage across the bearings for both inverter topologies at different points of operation is described in section III followed by selected measurement results in section IV.

## Three level hybrid ANPC inverter

### Topology and principle of operation

General observations on NPC and ANPC converter topologies and their control methods have already been made in previous works [4], [5]. Specifically, for the PMSM a Si/SiC hybrid ANPC inverter topology is used, as this topology enables the introduction of a third voltage level with the same number of fast-switching power electronic components regarding the previously used 2-level topology. Figure 3 shows the circuit diagram of one phase of the three-level hybrid ANPC inverter. The half bridge of one phase consists of two SiC-MOSFETs (S₂, S₃) and four Si-IGBTs (S₁, S₄, S₅, S₆). A hybrid modulation technique is used, where four Si-IGBTs (S₁, S₄, S₅, S₆) are controlled with the fundamental frequency of the machine and the two SiC-MOSFETs (S₂, S₃) are controlled with the rated switching frequency of the inverter. Due to the switch arrangement, all 6 switches only have to block half the DC link voltage. This arrangement leads to four possible switching states (P, O⁺, O⁻ and N), which are shown in Table I.

<figure><img src="image_4.png" /><figcaption>Fig.3: Circuit diagram for one phase leg of the three-level hybrid ANPC inverter</figcaption></figure>

<table><caption>**Table I: Switching states of the three-level hybrid ANPC-inverter**</caption><thead><tr><th>State</th><th>Output</th><th>S<sub>1</sub></th><th>S<sub>2</sub></th><th>S<sub>3</sub></th><th>S<sub>4</sub></th><th>S<sub>5</sub></th><th>S<sub>6</sub></th></tr></thead><tbody><tr><td>P</td><td>+0,5 V<sub>DC</sub></td><td>1</td><td>1</td><td>0</td><td>0</td><td>0</td><td>1</td></tr><tr><td>O<sup>+</sup></td><td>0</td><td>0</td><td>1</td><td>0</td><td>1</td><td>1</td><td>0</td></tr><tr><td>O<sup>-</sup></td><td>0</td><td>1</td><td>0</td><td>1</td><td>0</td><td>0</td><td>1</td></tr><tr><td>N</td><td>-0,5 V<sub>DC</sub></td><td>1</td><td>1</td><td>0</td><td>0</td><td>0</td><td>1</td></tr></tbody></table>

Regarding the switching states of one phase shown in Table I, the current paths shown in Figure 4 result. Based on the NPC topology, there are usually 3³=27 voltage vectors available. Due to a further redundancy of the O-state of the hybrid ANPC topology, there are 4³=64 possible voltage vectors. To control the hybrid ANPC inverter, a modulation of the voltage space vectors is required, which is described below.
<figure><img src="image_5.png" /><figcaption>Fig. 4: Current paths for the switching states of the three-level hybrid ANPC inverter</figcaption></figure>

## Three-level space vector modulation

As already described in the previous section, a hybrid modulation method is used for the hybrid ANPC inverter. Based on the switching states of one phase shown in Figure 4 the space vector diagram shown in Figure 5 results for the ANPC inverter in the three-phase compound. The space vector diagram consists of 24 active voltage vectors which are composed of 12 small vectors, 6 medium vectors and 6 large vectors. In addition, there are 3 zero vectors (PPP, NNN, OOO). Based on the position of the large vectors, the space vector diagram is separated into six main sectors (sector I to VI). Furthermore, each main sector is divided into four subsectors (subsector A to D). The implementation of the space vector PWM first requires the determination of the main sector and its subsector in which the reference voltage vector is located. Secondly, the nearest vectors are determined in order to generate a suitable switching pattern. The relative switch-on times of the power switches are then calculated using the switching pattern. Specifically, the relative switch-on times for the hybrid ANPC topology refer to the fast-switching SiC MOSFETs. The

starting point for the calculation of relative switch-on times is the definition of the reference vector $V_{ref}$, which corresponds to the three-phase voltage.

$$ \vec{v} = \frac{2}{3}(V_{ao} + V_{bo} \cdot e^{j \frac{2\pi}{3}} + V_{co} \cdot e^{-j \frac{2\pi}{3}}) \quad (1) $$

By using the Clark transformation, the three-phase abc-system is transformed to the 2-dimensional $\alpha$-$\beta$ frame which is used in sector identification.

$$ \begin{bmatrix} V_{ref,\alpha} \\ V_{ref,\beta} \end{bmatrix} = \frac{2}{3} \begin{bmatrix} 1 & -0.5 & -0.5 \\ 0 & \sqrt{3}/2 & -\sqrt{3}/2 \end{bmatrix} \begin{bmatrix} V_a \\ V_b \\ V_c \end{bmatrix} \quad (2) $$

The reference voltage vector can be located in any main sector (I-VI) and its subsector (A-D). For example, considering that the reference vector $V_{ref}$ is located in subsector B of the main sector I, the nearest three voltage vectors for this subsector are $V_1$, $V_2$ and $V_8$ like shown in Figure 6. After identifying the nearest three vectors, the following expressions can be developed for the switch-on time calculations of the corresponding vectors where $T_s$ is the switching period, $t_a$, $t_b$ and $t_c$ are the switch-on times for the voltage vectors $V_1$, $V_8$ and $V_2$.

$$ \vec{V}_{ref} T_s = \vec{V}_1 t_a + \vec{V}_8 t_b + \vec{V}_2 t_c \quad (3) $$

$$ T_s = t_a + t_b + t_c \quad (4) $$

<figure><img src="image_6.png" /><figcaption>Fig. 5: Three-level space vector diagram</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Fig. 6: Three-level space vector in main sector I and subsector B</figcaption></figure>

By solving equation (3), the switch-on time of the corresponding voltage vectors can be computed by (5).

$$ 
\left. 
\begin{aligned}
t_a &= T_s - 2\sin(\theta) \\
t_b &= 2\sin\left(\theta + \frac{\pi}{3}\right) - T_s \\
t_c &= T_s - 2\sin\left(\theta + \frac{\pi}{3}\right)
\end{aligned}
\right\} (5)
$$

<figure><img src="image_8.png" /><figcaption>Fig. 7: Gate signals of one phase</figcaption></figure>

## Experimental Setup

To measure the shaft voltage under operation the prototype machine was modified and equipped with ceramic bearings to achieve a complete electrical insulation of the shaft. The pump of the compressor was removed to eliminate additional grounding paths of the shaft. The load machine

was connected by a jaw clutch which is also insulated due to its polymer center. The shaft voltage was tapped under rotation by a slip ring using a graphite-brass contact which was installed at the outer surface of the clutch.

<figure><img src="image_9.png" /><figcaption>Fig. 8: Measurement setup with slip ring on electrically insulated shaft</figcaption></figure>

A hybrid ANPC inverter with the design specifications shown in the Table II is used to control the electric machine. This prototype is freely programmable and can be operated as 2-level inverter as well as 3-level inverter to get comparable results using the same hardware.

<table><caption>Table II: Design specification ANPC inverter</caption><thead><tr><td>Parameter</td><td>Specification</td></tr></thead><tbody><tr><td>DC-Link voltage</td><td>565 V</td></tr><tr><td>DC-Link capacitor</td><td>50 μF</td></tr><tr><td>max. switching frequency</td><td>100 kHz</td></tr><tr><td>Output current</td><td>35 A (RMS)</td></tr><tr><td>Output frequency</td><td>3,3 kHz</td></tr></tbody></table>

Taking the hybrid ANPC topology into account, the power switches only have to block half the DC link voltage. The prerequisite for this assumption is that an appropriate three-level modulation technique is used. As this inverter system is also operated as a two-level inverter for test purposes,
the full DC link voltage must be taken into account when dimensioning the power switches. The power module F3L11MR12W2M1_B74 from Infineon is used to meet this requirement. The hybrid ANPC inverter includes a full-ceramic DC-Link capacitor of 50 µF. To measure the phase currents, the current sensor GO 20-SME/SP3 is used. The CAD model of the ANPC power PCB is shown in Figure 9.

<figure><img src="image_10.png" /><figcaption>Fig. 9: CAD model of the ANPC power PCB</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>Fig. 10: Three level ANPC inverter prototype</figcaption></figure>

The central part of the hybrid ANPC inverter is the real-time system. For this purpose a controlcard evaluation board of the microcontroller C2000 Delfino F28388D from TEXAS INSTRUMENTS is used. The implementation of the control algorithm and the programming of the C2000 F28388D microcontroller is done by MATLAB/Simulink. Due to this modular and freely programmable inverter system, different modulation techniques and control algorithms can be tested.

## Measurement Results

The resulting shaft voltage under operation was measured for a hybrid three-level and standard two-level PWM method. Two distinct points of operation (OP) were chosen which represent the operational range of the machine. While OP1 focusses at low output voltage of the inverter OP2 takes high load and thus high output voltage into account. The test conditions and operating points are listed in Table III.

<table><caption>Table III: Test conditions</caption><thead><tr><td>Parameter</td><td>Specification</td></tr></thead><tbody><tr><td>DC-Link voltage</td><td>420 V</td></tr><tr><td>switching frequency</td><td>15 kHz</td></tr><tr><td>OP 1</td><td>speed: 1000 rpm<br>phase current: 5A</td></tr><tr><td>OP 2</td><td>speed: 6000 rpm<br>phase current: 30A</td></tr></tbody></table>

The following figures show the results in direct comparison for both points of operation. The upper plot shows the measured output phase voltages of the inverter while the lower plot shows the measured voltage on the shaft and the common mode voltage, which is calculated according to equation (6).

$$V_{CM} = \frac{1}{3}(V_a + V_b + V_c) \quad (6)$$

Fig. 11 shows a snapshot of the 3-level modulation at OP1. The inverter is using $V_1, V_2$ and $V_0$ to create the reference voltage vector $V_{ref}$ (according to Fig.6). The resulting common mode voltage is significantly smaller compared to 2-level modulation (Fig.13) due to the fact that one phase voltage remains zero, that means $V_c$ is connected to the neutral point over the whole switching period. A similar result can be found in comparison of Fig.12 and Fig.14, which represent the high load operation. The amplitude of $V_{CM}$ in 3-level mode is smaller because never all phases are switched on $+V_{DC}/2$ or $-V_{DC}/2$ at the same time, which, on contrast, is the case with 2-level mode at maximum modulation.
<figure><img src="image_12.png" /><figcaption>Fig. 11: phase voltage, common mode voltage and shaft voltage for 3-level operation at OP1 (1000rpm, 5A)</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>Fig. 13: phase voltage, common mode voltage and shaft voltage for 2-level operation OP1 (1000rpm, 5A)</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>Fig. 12: phase voltage, common mode voltage and shaft voltage for 3-level operation OP2 (6000rpm, 30A)</figcaption></figure>

<figure><img src="image_15.png" /><figcaption>Fig. 14: phase voltage, common mode voltage and shaft voltage for 2-level operation OP2 (6000rpm, 30A)</figcaption></figure>
The tests carried out show that a significant reduction in the common-mode voltage amplitude can be achieved by using a 3-level ANPC modulation instead of a 2-level modulation.

## Conclusion

The shaft voltage caused by common mode effects in inverter driven machines are a serious problem in main and auxiliary drives for electric vehicles. A significant reduction of the common mode voltage can be achieved by using a 3-level ANPC inverter instead of a 2-level inverter. The comparison of both control strategies was carried out on the basis of a selected AC compressor prototype using a PMSM drive. Measurement results are presented as well as the machine structure and the 3-level topology.

## References

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[2] “ABB drives - Technical guide No. 5 - Bearing currents in modern AC drive systems,” 2011.

[3] E. Robles, M. Fernandez, J. Zaragoza, I. Aretxabaleta, I. M. De Alegria, and J. Andreu, “Common-Mode Voltage Elimination in Multilevel Power Inverter-Based Motor Drive Applications,” *IEEE Access*, vol. 10, pp. 2117–2139, 2022, doi: 10.1109/ACCESS.2021.3137892.

[4] T. Bruckner, “Loss Balancing in Three-Level Voltage Source Inverters applying Active NPC Switches.”

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