Motor Overvoltage Research Articles

Inverter With Paralleled Modules to Extend Current Capacity and Combat Motor Overvoltage in SiC-Based Adjustable Speed Drives

This article proposes a quasi-three-level (Q3L) PWM-based module-parallel inverter to address two major issues of deploying SiC MOSFETs in high-power cable-fed adjustable speed drives. Firstly, the maximum output power of SiC inverters is limited due to the lack of high current rating SiC devices, which cannot satisfy the demand of high-power applications such as heavy-duty electric vehicles and more electric aeroplanes. The second issue comes from the fast-switching speed of SiC MOSFETs, where power cables act like transmission lines under steep rising/falling voltages (high <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$dv/dt$</tex-math></inline-formula> ), with back and forth voltage reflections that result in serious overvoltage oscillations at motor terminals. To address these issues, a SiC module-parallel inverter is adopted with elevated current capacity, where the phase voltage is maintained at the mid-point of a coupled inductor connected to the output nodes of each paralleled half-bridge module. By actively controlling the switching delay between the paralleled half-bridge-legs, Q3L PWM waveforms are generated at the inverter output nodes which can mitigate the motor overvoltage due to the voltage reflections through power cables. An active current control technique is also proposed to facilitate the current balance between the paralleled half-bridge-legs. The proposed Q3L PWM-based module-parallel inverter is experimentally verified using a SiC-based cable-fed motor drive system. The results show that the proposed approach can extend the current capabilities of SiC devices as well as mitigate the motor overvoltage, enabling the adoption of SiC devices in high-power motor drives.

Motor Overvoltage Mitigation by Active Cancellation of Reflections Using Parallel SiC Devices With a Coupled Inductor

Motor Overvoltage Mitigation by Active Cancellation of Reflections Using Parallel SiC Devices With a Coupled Inductor

Mitigation of Motor Overvoltage in SiC-Based Drives Using Soft-Switching Voltage Slew-Rate ($dv/dt$) Profiling

In silicon carbide (SiC) device based motor drives, the high-voltage slew-rate ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dv</i> / <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dt</i> ) associated with the fast-switching transitions results in excessive motor overvoltage, due to the reflected wave phenomenon, which increases the motor winding insulation stress and causes premature failure while raising electromagnetic interference (EMI) problems. This article proposes a soft-switching voltage slew-rate profiling approach to mitigate the motor overvoltage in SiC-based cable-fed drives. The proposed approach optimizes the rise/fall time of the output voltage according to the cable length, without altering the switching speed of the SiC devices. Since increasing the switching rise/fall time using conventional approaches, such as increasing the gate resistance, results in an increased switching power loss, the proposed profiling approach is implemented using a soft-switching inverter. The optimum rise/fall time that can significantly mitigate the overvoltage is derived using frequency- and time-domain analysis. The auxiliary resonant commutated pole inverter (ARCPI) is adopted as an example of the soft-switching inverter to experimentally verify the proposed slew-rate profiling approach for the overvoltage mitigation. The analysis and experimental results show that the motor overvoltage is fully mitigated when the output voltage rise/fall time is set as the cable antiresonance period, i.e., four times of the wave transmission time along the cable. Furthermore, the slew-rate profiling approach along with the ARCPI reduces the switching loss and improves the EMI performance at the high-frequency region, compared with the conventional hard-switching converter. Specifically, the maximum efficiency of the ARCPI is about 99%.

Impact of Parasitic and Load Current on the Attenuation of Motor Terminal Overvoltage in SiC-Based Drives

In SiC-based adjustable speed drives, the high voltage slew rate ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$dv\!/\!dt$</tex-math></inline-formula> ) of the switching transitions results in excessive overvoltage at the motor terminals due to the reflected voltages across the drive power cables. Besides the cable length, the switching rise/fall times of the voltage pulses are a key parameter to quantify the motor overvoltage in pulsewidth modulation (PWM) inverter-fed drives. These times are varying depending on the load current and parasitic elements of SiC <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">mosfet</small> s, that is, a standard two-level converter typically results in a nonuniform overvoltage envelop at the motor terminals. This article analyses the switching mechanism of the two-level converter considering the impact of SiC parasitic elements and load current showing how they affect the motor overvoltage in cable-fed drives. The analysis is then extended to the mitigation of the motor overvoltage using quasi-three-level (Q3L) modulation as a candidate filter-less approach with a T-type converter. The theoretical analysis is validated through experimental tests by using the Q3L T-type converter. The analysis and results show that the instantaneous load current value critically determines the peak motor overvoltage, while it allows either a full or partial overvoltage mitigation when the Q3L modulation is adopted.

Impact of High Switching Speed and High Switching Frequency of Wide-Bandgap Motor Drives on Electric Machines

The emergence of fast-switching wide-bandgap (GaN, SiC) power electronics devices has enabled motor drive systems to achieve high efficiency, power density, control bandwidth and high-level of integration. However, the fast-switching speed and high switching frequency result in an increased level of motor overvoltage at both motor terminals and stator neutral. This overvoltage increases the stress across the motor winding insulation and bearings. This paper investigates three types of motor overvoltages, i.e., differential mode (DM) (phase-to-phase) motor terminal overvoltage, common mode (CM) (phase-to-ground) motor terminal overvoltage and CM stator neutral overvoltage (motor neutral to ground). Both the high switching speed (high dv/dt) effect and the high switching frequency effect on the motor overvoltage are investigated in this paper, where the high switching frequency effect has not been fully addressed in existing literature. Significant overvoltage is observed when the switching frequency or its multiples coincide with the cable or motor anti-resonant frequency. The anti-resonant behavior in the cable and motor impedance has been used to identify the overvoltage oscillation frequency and to explain the overvoltage observations for the three types of motor overvoltages. The analysis has been tested using a 7.5kW motor setup with and without a four-core cable, driven by a SiC or a GaN three-phase inverter with a switching frequency up to 250kHz and switching speed up to 40kV/ μs. In addition, the motor bearing current, which is a main cause of bearing degradation, has also been tested and the increase of bearing current is observed due to the high frequency effect.

A Quasi-Three-Level PWM Scheme to Combat Motor Overvoltage in SiC-Based Single-Phase Drives

The emergence of fast switching wide-bandgap (WBG) power devices offers clear potential to implement higher power density and more efficient motor drives. However, the high voltage slew rate (dv/dt) of switching transients brought significant challenges that can hamper the wide adoption of WBG devices in motor drive applications. Specifically, the aggravated motor overvoltage oscillation, due to reflected voltage phenomenon under high dv/dt, is one of the most considerable challenges that degrade the motor lifetime. With filter networks acting as the mainstream mitigation method, the advantages of WBG-based motor drives are compromised due to additional size and power loss of the filters. This letter proposes a novel quasi-three-level pulsewidth modulation scheme as a software solution to eliminate motor overvoltage oscillations in cable-fed drives. The proposed scheme adopts a brief zero-voltage state, with a predetermined time, in the midway of each pole-to-pole voltage transition. This allows the voltage reflections along the cable to significantly discontinue after two propagation cycles, securing the motor operation at prescribed voltage levels. The proposed scheme is applicable to two-level voltage-source inverters (VSIs). In this letter, the scheme is presented on a single-phase two-level VSI motor drive, supported with theoretical and experimental proof of concept.

A Universal High-Frequency Induction Machine Model and Characterization Method for Arbitrary Stator Winding Connections

High-frequency modeling of induction machines plays an important role in investigating motor drive electromagnetic interference issues such as stator winding reflected-wave overvoltage and bearing discharging current. Characterization of high-frequency machine models requires measurements of machine's differential-mode (DM) and common-mode (CM) impedances up to tens of MHz. The machine's stator winding connections, e.g., single-, and series-, parallel-winding Y/Δ configurations, influence the measured DM and CM impedances and model parameters. In this paper, a universal high-frequency equivalent circuit model capable of representing induction machines with arbitrary stator winding connections is proposed. The new model features a simple structure with a straightforward characterization method. Specifically, only one stator winding configuration is required for impedance measurements to fully characterize the machine model for arbitrary stator winding connections. The proposed methodology is demonstrated using a 7.5 hp dual-voltage nine-terminal/lead induction machine and a drive system. The simulated DM and CM impedances as well as the motor overvoltages show excellent agreement with the experimental results. The proposed model and characterization method represent significant improvement in terms of accuracy, applicability/generality, and convenience compared to prior conventional models.

Inverter Side RL Filter Precise Design for Motor Overvoltage Mitigation in SiC-Based Drives

This paper investigates the use of resistor–inductor (RL) filters for motor overvoltage issues caused by cables connecting inverters to electric machines. Using existing filter design formulae and fixing the desired peak motor voltage, an RL filter is designed for a 15-kW industrial drive. This filter is used with a Silicon IGBT inverter and a matching between the design targets and the experimental results are found. Using the same filter with an SiC MOSFET inverter, a detrimental effect in overvoltage mitigation task is found. From simulations, it is noticed that combination of filter resistor parasitic inductance and high $dv/dt$ causes unpredicted additional high-frequency motor overvoltages. These phenomena require higher filter inductance values with respect to the standard one, thus previous filter design formulae cannot be used. New design formulae, valid when inverters with high $dv/dt$ are used, are proposed and experimentally validated. Interestingly, using the proposed formulae, it is discovered that the best filter resistance is not equal to the characteristic impedance of the cable. A design guideline that permits to optimally design the RL filter without simulation tool, considering the resistor parasitic inductance and the inverter $dv/dt$ is finally proposed.

Improved Sensorless Operation of a CSI-Based Induction Motor Drive: Long Feeder Case

Various applications, like in underground mines and oil and gas industries, require remote operation of vector-controlled medium-voltage variable speed drives via a long motor feeder. The use of voltage source inverters in such cases leads to motor overvoltage and harmonic quality problems. The current source inverter (CSI) is ideally matched to these applications because of its motor-friendly voltage output. Speed sensorless operation is mandatory due to the long motor feeder. Although the model reference adaptive system (MRAS) is a powerful and proven speed estimation tool, its implementation in long motor feeder drives faces many challenges. Among them, and addressed in this paper, are inherent dc offset in its stator model, the need for actual motor voltage and current values, and oscillations in the estimated speed due to errors in the motor current measurement signals. In this paper, a sensorless CSI vector-controlled drive, suitable for long motor feeder applications, is studied. Improved speed estimation is achieved by proposing 1) a modified dc-offset eliminator for an MRAS speed estimation and 2) a compensation technique for motor current's measurement errors. Intensive experimental results, for a low-voltage scaled model, along with simulations validate the effectiveness of the proposed technique.

Active $du/dt$—New Output-Filtering Approach for Inverter-Fed Electric Drives

Active <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">du</i> / <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dt</i> is a new output-filtering method to mitigate motor overvoltages. The inverter pulse pattern edges are broken down into narrower pulses, which control the filter <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LC</i> circuit. This results in an output voltage that does not have to exhibit the overshoot typically seen in common <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LC</i> circuits in output-filtering applications. Furthermore, the shape of the output-voltage edge has properties well suited for output-filtering applications. An appropriate filter rise time is selected according to the motor-cable length to eliminate the motor overvoltage. The basis of the active <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">du</i> / <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dt</i> method is discussed in brief. Considerations on the application of the active <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">du</i> / <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dt</i> filtering in electric drives are presented together with simulations and experimental data to verify the potential of the method.

High-Frequency Modeling of the Long-Cable-Fed Induction Motor Drive System Using TLM Approach for Predicting Overvoltage Transients

Induction motor drive systems fed with cables are widely used in many industrial applications. Accurate prediction of motor terminal overvoltage, caused by impedance mismatch between the long cable and the motor, plays an important role for motor dielectric insulation and optimal design of dv/dt filters. In this paper, a novel modeling methodology for the investigation of long-cable-fed induction motor drive overvoltage is proposed. An improved high-frequency motor equivalent circuit model is developed to represent the motor high-frequency behavior for the time- and frequency-domain analyses. The motor equivalent circuit parameters for the differential mode (DM) and common mode (CM) are extracted based on the measurements. A high-frequency cable model based on improved high-order multiple-π sections is proposed. The cable model parameters are identified from the DM impedances in open circuit (OC) and short circuit (SC). To obtain a computationally efficient solution that could potentially be integrated with the motor drive controller, the system equations are discretized and solved using transmission-line modeling (TLM) approach. The proposed methodology is verified on an experimental 2.2-kW ABB motor drive benchmark system. The motor overvoltage transients predicted by the proposed model is in excellent agreement with the experimental results and represents a significant improvement compared with the conventional models.

Common-Mode Voltage Reduction PWM Algorithm for AC Drives

The impact of common-mode voltage (CMV) generated by pulsewidth modulation (PWM) ac drives on motor bearings is well known. Several algorithms for CMV reduction have been proposed in the literature. While a few algorithms assume ideal switching and fall apart when nonidealities like inverter dead time are considered, some others are effective only in a limited operating range of the drive. In this paper, a previously proposed algorithm is modified for practical implementation to include compensation for dead-time and reflected-wave motor overvoltage stress while still producing output voltage waveforms with reduced common-mode content. Experimental results are provided to show the reduction in CMV over the entire operating range, with other performance attributes such as reflected-wave motor overvoltage that are identical to conventional space-vector PWM. The advantages of applying the algorithm to a fully regenerative ac drive are also demonstrated.

Mitigation of high d u /d t -originated motor overvoltages in multilevel inverter drives

An overvoltage at the motor terminals of a pulse-width-modulated voltage-fed drive can cause premature insulation failure in the windings of the motor. Pulsed inverter voltage and the impedance mismatch between the cable and the motor cause an oscillating overvoltage in the motor terminals. Several methods to reduce these overvoltages have been introduced, most of them being based on filtering with lossy passive components. Despite the recent interest in multilevel inverters, the overvoltage phenomenon in these applications has received only little attention. It is often assumed that in multilevel converters, oscillating overvoltages are relatively small, because a single voltage level constitutes only a part of the fundamental voltage. However, the voltage stress caused by high d u /d t -rating may induce partial discharges also in a medium-voltage machine. In this paper, overvoltages are shown to exist in multilevel inverter drives. The paper proposes a method to mitigate these overvoltages; the method uses two properly timed consecutive switchings to almost completely cancel out the voltage oscillations at the motor end of the cable. This cancellation is possible by using oscillations in opposite phases. The method is analytically derived, and its feasibility is tested with a 3 kV inverter prototype and two different motor cable lengths.

Reduction of Common Mode Currents Generated by Three-Level Inverters with Consideration of Motor Overvoltages

In adjustable speed drive applications, the switching of the inverter semiconductors generates common mode currents as well as harmful overvoltages on the motor terminals when long cables are used. Consequently, bulky and expensive input and output filters must be used. This work aims at reducing these disturbances from their origin by using a three-level neutral-point-clamped (NPC) inverter controlled with a new pulse-width-modulation (PWM) strategy. Whereas previous common mode noise-reducing strategies usually generate higher overvoltages than conventional ones, the proposed PWM is able to manage both problems thanks to its internal degrees of freedom.

Motor Overvoltage Limitation by Means of a New EMI-Reducing PWM Strategy for Three-Level Inverters

Pulsewidth-modulated (PWM) voltage-source inverters are known to provoke high-frequency disturbances in motor-drive applications, particularly when long cables are used. Indeed, the parasitic elements of the cable, along with steep voltage transitions due to semiconductor switchings, are responsible for the conducted electromagnetic emissions and high overvoltages on the motor terminals. Therefore, several specific PWM schemes have been proposed in order to reduce these phenomena without requiring large passive filters. However, strategies that reduce common-mode currents (mainly conducted electromagnetic emissions) tend to increase the overvoltages generated on the motor. This paper focuses on a new PWM strategy that reduces the common-mode currents generated by three-level inverters. It provides a control algorithm that makes sure that the motor overvoltages never exceed those of conventional strategies while improving the electromagnetic-compatibility performance of the drive.

Filter Networks for Long Cable Drives and Their Influence on Motor Voltage Distribution and Common-Mode Currents

Filter networks for long cable drives have been studied in the last decade to solve the overvoltage problem at the motor terminals. However, little research has been dedicated to analyze their influence on the voltage distribution in the motor stator winding and on the circulation of the common-mode currents. In this paper, the project of dv/dt filters through simulation and analysis from previous work is extended, including the motor voltage distribution and common-mode current analysis. Simulation and experimental results are presented, demonstrating the usefulness of the filter networks on the motor overvoltage mitigation. Three filter topologies will be discussed and compared here: RLC filter at the motor terminals, RLC filter at the inverter output and reactor filter, which is extensively used in the industry. Simulation and experimental results demonstrate that the RLC filter at the inverter output is the most interesting solution among the main types of passive filter networks.

A Novel Output Filter Topology to Reduce Motor Overvoltage

When an induction machine is supplied with a voltage-source pulsewidth-modulation (PWM) inverter, a ringing voltage overshoot appears at motor terminals due to steep PWM voltage pulse propagation along the motor cable. It has been reported that in particular situations, when very long motor cables are used and retrofit installations, the overvoltage could stress motors with poor insulation. This paper presents a novel inverter output filter topology consisting of an LC filter which has the star point of the common-mode capacitors clamped to the + and - dc-link rails by the means of two diodes. The proposed filter drastically reduces the voltage overshoot at the motor terminals, thus reducing motor insulation stress. This is a cheap alternative to more known industrial solutions.

High-frequency modeling for cable and induction motor overvoltage studies in long cable drives

High-frequency simulation models for power cables and motors are key tools to aid a better understanding of the overvoltage problem in pulsewidth modulation drives with long feeders. In this paper, frequency responses of the cable characteristic and the motor input impedances are obtained experimentally and suitable models are developed to match the experimental results. Several lumped segments incorporating a lossy representation of the line are used to model the cable. The cable and induction motor models may be implemented using a computational tool such as MATLAB, thereby providing a convenient method to analyze the overvoltage phenomena. Simulation and experimental results are presented for a typical 3-hp induction motor, showing the suitability of the developed simulation models. The most promising dv/dt filter networks are also investigated through simulation analysis, and a design approach based on a tradeoff between filter losses and motor peak voltage is proposed. Experimental results of an RC filter placed at the motor terminals demonstrate the validity of the simulation models.

Switching overvoltages in motor circuits

The switching of HV motor circuits in the megawatt range can cause large overvoltages on motor terminals. The amplitude of the resulting voltage spikes depends on breaker type and circuit characteristics. If the amplitude of switching overvoltages exceeds the basic insulation level of the motor, overvoltage protection circuitry should be installed to prevent a motor insulation failure. The decision whether overvoltage protection circuitry (surge pack) must be installed requires a prediction of the overvoltage level. This paper deals with switching overvoltages in motor circuits with a unit transformer between switchgear and motor. Overvoltages of motor circuits were simulated using specially developed simulation models. The results of three case studies revealed a large impact of cabling and transformer on motor overvoltages. In most of the cases, a unit transformer connected between switchgear and motor will provide an overvoltage protection to the motor. However, the transformer can also be a main cause of large overvoltages when connected to cables of specific length and capacitance. The paper provides some simple rules to recognize such critical configurations.

Transient overvoltages in electrical motors during sequential pole closure

This paper addresses the problem of transient overvoltages in electrical motors during sequential pole closure in industrial installations. A computer model that includes the circuit breaker, high-voltage cable and electrical motor is proposed. Multiconductor transmission line theory and modal analysis are applied to the components and the piecewise Fourier transform is used to simulate sequential pole closure. The mechanism of sequential pole closure is analyzed including parallel capacitors and series inductors. Critical times for switching transients are calculated. Finally, the effect of several system and machine parameters on transient overvoltages is investigated.