Three-phase DC/AC power electronics converter systems used in battery-powered variable-speed drive systems or employed in three-phase mains-supplied battery charger applications usually feature two power conversion stages. In both cases, typically a DC/DC stage is attached to a three-phase DC/AC stage in order to enable buck-boost functionality and/or a wide input-output voltage operating range. However, a two-stage solution leads to a high number of switched bridge-legs and hence, results in high switching losses, if the degrees of freedom available for controlling the overall system are not utilised. If the DC/DC stage is used to vary the DC link voltage with six times the AC-side frequency, a pulse width modulation (PWM) of always only one phase of the DC/AC stage is sufficient to achieve three-phase sinusoidal output currents. The clamping of two phases (denoted as 1/3 PWM) leads to a drastic reduction of the DC/AC stage switching losses, which is further accentuated by a DC link voltage which is lower than for the conventional modulation schemes. This paper details the operating principle of a three-phase buck-boost converter system using 1/3 PWM and outlines an appropriate control system design. Subsequently, the switching losses and the voltage/current stresses on the converter components are analytically derived. There, a more than 66% reduction of the DC/AC stage switching losses is calculated without any increase of the stress on the remaining converter components. The theoretical considerations are finally verified on a hardware demonstrator, where the proposed modulation strategy is experimentally compared against several conventional modulation techniques and its clear performance advantages are validated.

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Analysis of a synergetically controlled two-stage three-phase DC/AC buck-boost converter

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The overwhelming trend within the industry today is to pair electric machines with variable-frequency drives. It is well known that the use of these drives can introduce high-frequency bearing currents during semiconductor switching events. This paper focuses on bearing discharge currents that gradually degrade the bearings by pitting their surfaces and can lead to premature failure. Common solutions include insulating the bearings, installing common-mode filters, or using a brush to ground the rotor shaft. These solutions are not universally scalable and may require periodic maintenance. This paper presents a new approach using noncontact (brushless) capacitive coupling. Here, a rotating capacitor is placed electrically in parallel with the bearing. At high frequencies, the impedance of the rotating capacitor is far lower than the bearing impedance, thereby shunting current around the bearings and reducing shaft voltage. The technique is experimentally demonstrated on a 3-hp (2.2-kW) induction machine. The experiment demonstrates a bearing current reduction factor of at least 8x.

Brushless Mitigation of Bearing Currents in Electric Machines Via Capacitively Coupled Shunting

Motor drive systems supplied by a fuel-cell/battery are especially demanding when it comes to the design of the inverter. Besides a high performance (high efficiency η and power density ρ), the inverter has to cope with the wide DC voltage variation of the fuel-cell/battery that supplies the motor drive. A promising three-phase inverter topology, denoted as Y-VSI, is presented in this paper. The Y-VSI is a modular three-phase inverter, and comprises three identical phase-modules connected to a common star “Y” point. Each phase-module is equivalent to a buck-boost DC/DC converter, which allows the AC output voltages to be higher or lower than the DC input voltage. Thereby, the Y-VSI effectively copes with the wide variation of the fuel-cell/battery voltage. Each phase-module can be operated in a similar fashion to a conventional DC/DC converter, independently of the remaining two phases. Accordingly, a straightforward and simple operation/control of the Y-VSI is possible. In addition, the Y-VSI features an integrated output filter. This allows for continuous/sinusoidal motor voltage waveforms, eliminating the need of an additional filter between the inverter and the motor. This paper details the operating principle of the Y-VSI, and comparatively evaluates two modulation strategies. In order to validate the proposed concepts, a Y-VSI hardware prototype is assembled within the context of a high-speed motor drive. In the investigated drive system, a fuel-cell supplies the Y-VSI, which in return controls a 280 krpm 1 kW electric compressor. The Y-VSI hardware prototype is compared against a state-of-the-art hardware prototype, which features two energy conversion stages. It is shown that the Y-VSI is ∆η = +2.3% more efficient and at the same time ∆ρ = +10% more power dense compared to the conventional inverter solution.

Three-Phase Sinusoidal Output Buck-Boost GaN Y-Inverter for Advanced Variable Speed AC Drives

The invention regards an method for estimating the end of lifetime for a power semiconductor device, such as an IGBT power module, comprising the steps of; establishing the temperature of the power semiconductor device, determining the voltage drop over the power semiconductor device for at least one predetermined current where the current is applied when the power semiconductor device is not in operation, wherein the end of lifetime is established dependent on the change in a plurality of determined voltage drops.

Method for estimating the end of lifetime for a power semiconductor device

This paper discusses the $\Delta$ -networks and other circuits designed to separate the conducted electromagnetic interference (EMI) into its common mode (CM) and differential mode (DM) components. The input impedances of CM/DM separators must be 50 $\Omega$ resistive in the measurement frequency range, and they must be independent of the values of the noise signals and noise source impedances. The conditions for achieving such input impedances are derived. It is shown that many of the proposed separators, including the $\Delta$ -network suggested in the International Special Committee on Radio Interference (CISPR) 16-1-2 standard, do not fulfill the input impedance requirement. This leads to unreliable CM and DM measurements and, consequently, to the oversizing of EMI filters and design by trial and error.

The Input Impedance of Common-Mode and Differential-Mode Noise Separators

Bearing currents cause a reduction of ball bearing's lifetime in electronically commutated permanent magnet motors. To compensate the lifetime decreasing factors, a tradeoff between special bearings and additional changes in the machine design have to be found. Stator grounding, for example, keeps the bearing voltage low and thus guarantees a long fan system's lifetime, however, results in a higher input filter effort. On the other hand, instead of reducing the bearing voltage, hybrid bearings could be deployed, which feature a high dielectric strength. The major disadvantage is that hybrid bearings are much more expensive than conventional metal bearings, thus the extra cost for the hybrid bearings has to be weighed up against the higher filter effort needed with stator grounding. In this paper bearing damage mechanisms are analyzed and the impact of stator grounding to avoid bearing currents in low power singlephase motors is presented. Based on the presented filter design procedure, the additional filter effort for stator grounded motors is determined, which shows that hybrid bearing are undesirable in mass application.

Impact of stator grounding in low power single-phase EC-motors

This Paper details two different contributions related to practical CM/DM EMI measurements. A first part investigates sources and implications of measurement errors that result for CM/DM separators in a practical measurement environment with a particular focus on the recently presented input impedance criterion for CM/DM separators. Furthermore, the realization of an active CM/DM separator, which features competitive separation capabilities (DMTR/CMRR > 51 dB and CMTR/DMRR > 47 dB for frequencies up to 10MHz), is presented.

Analysis and practical relevance of CM/DM EMI noise separator characteristics

A common approach for selecting a suitable EMI filter is detailed based on the example of a realized single-phase 500W PFC rectifier The presented work separates conducted EMI noise into Differential Mode (DM) and Common Mode (CM) components and derives expressions for the DM and CM insertion losses achieved if a given EMI filter is connected to a dedicated power converter With the use of hardware measurement results it is shown that the impedances of the EMI filter and the PFC rectifier have a considerable impact on the realized insertion losses, which, at particular frequencies, may be even less than approximate worst-case estimations proposed in CISPR 17

Practical characterization of EMI filters replacing CISPR 17 approximate worst case measurements

This work investigates sources of measurement errors that result for common mode/differential mode (CM/DM) separators in a practical measurement environment, with a particular focus on the recently ...

Sebastian Schroth

Papers

The Input Impedance of Common-Mode and Differential-Mode Noise Separators

Impact of stator grounding in low power single-phase EC-motors

Analysis and practical relevance of CM/DM EMI noise separator characteristics

Practical characterization of EMI filters replacing CISPR 17 approximate worst case measurements

Analysis and practical relevance of CM/DM EMI noise separator characteristics