Shaft Voltage Research Articles

Analysis of shaft voltage in a doubly-fed induction generator

Fast switching transients and common mode voltage generated by pulse width modulated voltage in high frequency applications may cause many unwanted problems such as shaft voltage and resultant bearing currents. The main objective of this research work is to analyse shaft voltage generation in a doubly-fed induction generator (DFIG) with a back to back converter. A detailed high frequency model of the proposed system has been developed based on capacitive couplings between different objects of the machine. The proposed model can be used for shaft voltage calculations and finding parameters which have key effect on shaft voltage and resultant bearing currents. A discussion about the presented technique for shaft voltage elimination in existing literature is also presented based on mathematical analysis and simulations.

Investigation of Shaft Voltage in Wind Turbine Systems with Induction Generators

This paper presents the analysis of shaft voltage in different configurations of a doubly fed induction generator (DFIG) and an induction generator (IG) with a back-to-back inverter in wind turbine applications. Detailed high frequency model of the proposed systems have been developed based on existing capacitive couplings in IG & DFIG structures and common mode voltage sources. In this research work, several arrangements of DFIG based wind energy conversion systems (WES) are investigated in case of shaft voltage calculation and its mitigation techniques. Placements of an LC line filter in different locations and its effects on shaft voltage elimination are studied via Mathematical analysis and simulations. A pulse width modulation (PWM) technique and a back-to-back inverter with a bidirectional buck converter have been presented to eliminate the shaft voltage in a DFIG wind turbine.

Design and Implementation of a Hybrid Output EMI Filter for High-Frequency Common-Mode Voltage Compensation in PWM Inverters

This paper presents a hybrid output electromagnetic interference (EMI) filter for common-mode voltage compensation in pulsewidth-modulation (PWM) inverters. Its structure is composed of a single-leg four-level active filter connected in series with a low-frequency blocking capacitor and a small passive LC filter that which enhances the suppression of the common-mode voltage in the high-frequency range. The series capacitor helps in avoiding the core saturation of the common-mode coupling transformer caused by the low-frequency components of the common-mode voltage. Detection of PWM voltages by high-speed optoisolators is done at the inverter's output to achieve better common-mode voltage detection and compensation characteristics. Analysis and design guidelines for the hybrid filter are also given to facilitate systematic design in the real implementation of the filter. Experimental results with a real drive system clearly illustrate a significant reduction of the common-mode voltage, the leakage current, the shaft voltage, and the bearing current.

Approaches to suppressing shaft voltage in brushless DC motor driven by PWM inverter

AbstractThis paper describes the approaches to suppressing the shaft voltage and bearing current by electrostatically shielding the stator end windings of the brushless DC motor driven by PWM inverter. At first, measured shaft voltage and bearing current are compared with those calculated waveforms to verify the common mode equivalent circuit of the brushless DC motor. Next, the relationship between shaft voltage and stator winding to rotor capacitance is calculated using the common mode equivalent circuit. Finally, the electrostatic shielding of the stator end windings is evaluated to reduce the shaft voltage by experiments and calculations. ©2008 Wiley Periodicals, Inc. Electr Eng Jpn, 166(4): 73–79, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20701

A passive EMI filter with access to the ungrounded motor neutral line—Its effect on attenuating bearing current

AbstractThis paper describes the effect of a passive EMI filter on preventing bearing current from flowing inside an inverter‐driven motor. Motor‐bearing damage is often caused by bearing currents resulting from the breakdown of grease films in the motor bearing. The high‐frequency common‐mode voltage generated by the PWM inverter induces a shaft voltage between the rotor and the frame. When the shaft voltage exceeds the breakdown voltage of the grease films, a destructive instantaneous discharge current with a peak value of about 1 A flows through the motor bearing. The passive EMI filter, which is unique in access to the motor neutral line, can reduce the shaft voltage as a result of eliminating the high‐frequency common‐mode voltage from the motor terminals. Hence, no breakdown occurs in the grease film, so that no bearing current flows. The viability and effectiveness of the passive EMI filter is verified by experimental results obtained from a 400‐V, 3.7‐kW laboratory system. © 2008 Wiley Periodicals, Inc. Electr Eng Jpn, 166(2): 78– 87, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20489

An Improved Active Common-Mode Voltage Compensation Device for Induction Motor Drives

This paper presents a new device for the compensation of common-mode (CM) disturbance in induction motor drives, based on the active cancellation approach. The detailed design and the experimental implementation issues of the new active compensation device for a 380-V/50-Hz pulsewidth-modulation (PWM) induction motor drive are discussed. Starting from the idea of the active common-noise canceller, the proposed active compensation device is suitably improved in order to overcome the limitations of similar previously proposed circuits. In fact, it can be successfully used within a drive system with a rated voltage of 380 V or higher by employing an improved active circuit with a dedicated dc power supply derived from the ac power supply line. In addition, the design follows the criteria of compactness and minimum cost. The performance of the realized active compensation device is verified through experimental measurements of the CM voltage, the CM current, and the motor shaft voltage. The effectiveness of the proposed solution is demonstrated by experimental results.

Five-Level Inverter Topology for Induction Motor Drives with Common-Mode Voltage Elimination in Complete Modulation Range

Common-mode voltage (CMV) generated by different topologies of the pulse width modulated (PWM) inverters causes shaft voltage, bearing current and ground leakage current in the induction motor (IM) drive system. Consequently the premature mechanical failure (fluting) of machine bearings and conducted electromagnetic interference (EMI) are observed in the multilevel PWM inverter fed IM drive systems. A five-level inverter topology with switching state combination selection strategy for PWM control is proposed for an IM drive for complete elimination of CMV in the entire operating range of the drive, including over-modulation. The proposed scheme is based on a dual five-level inverter fed open-end winding IM drive structure. Each individual five-level inverter of the proposed drive is formed by cascading a three-level neutral point clamped (NPC) inverter with two conventional two-level inverters. Hence, the proposed individual five-level inverter offers simple power-bus structure with less number of power diodes as compared to the conventional NPC five-level inverter. The proposed open-end winding IM drive structure requires nearly half the dc-link voltage and provides increased number of redundant switching state combinations as compared to a single five-level NPC inverter fed conventional IM drive. The proposed CMV elimination scheme is experimentally verified on a 1.5 kW open-end winding IM drive.

A passive EMI filter with access to the ungrounded motor neutral line: direct connection of a general‐purpose inverter to a three‐phase grounded voltage source

AbstractThis paper proposes a small‐sized passive EMI filter for the purpose of eliminating high‐frequency shaft voltage and ground leakage current from an AC motor. The motor is driven by a general‐purpose PWM inverter connected to a three‐phase grounded voltage source. The passive EMI filter requires access to the ungrounded neutral point of the motor. This unique circuit configuration makes the common‐mode inductor effective in reducing the high‐frequency common‐mode voltage generated by the PWM inverter with a carrier frequency of kHz. As a result, both high‐frequency shaft voltage and ground leakage current can be eliminated very efficiently. However, the common‐mode inductor may not play any role in reducing the low‐frequency common‐mode voltage generated by the diode rectifier, so that a low‐frequency component still remains in the shaft voltage. Such a low‐frequency shaft voltage may not produce any bad effect on motor bearings. The validity and effectiveness of the EMI filter are verified by experimental results obtained from a 200‐V 5‐kVA laboratory system. © 2007 Wiley Periodicals, Inc. Electr Eng Jpn, 159(2): 80–87, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20243

Approaches to Suppressing Shaft Voltage in Brushless DC Motor Driven by PWM Inverter

This paper describes the approaches to suppressing the shaft voltage and bearing current by electrostatic shielding the stator end windings of the brushless DC motor driven by PWM inverter. At first, measured shaft voltage and bearing current are compared with those calculated waveforms to verify the common mode equivalent circuit of the brushless DC motor. Next, relationship between shaft voltage and stator winding to rotor capacitance is calculated using the common mode equivalent circuit. Finally, the electrostatic shielding of the stator end windings is evaluated to reduce the shaft voltage by experiments and calculations.

Analysis of Shaft Voltage and Bearing Current in an Inverter-Fed Nongrounded Motor

This paper analyzes shaft voltage and bearing current in a nongrounded motor driven by a voltage-source PWM inverter. First, the shaft voltage of the nongrounded motor is measured. The experimental results indicate that a bearing current, which can not be explained by the conventional theory, exists, and that the bearing current may shorten the life of the motor bearing. To investigate the mechanism, some experiments and parameter measurements are done, and simulation models are built. As a result, it is shown that this issue is caused by unbalance in the inductance and stray capacitance of the motor windings and these resonant phenomena.

Operation on Unbalanced Voltage: One Motor's Experience and More

Derating a motor for operation on an unbalanced voltage system is still much of a black art although National Electrical Manufacturers Association and International Electrotechnical Commission have generic derating charts for this application. Numerous papers have been written in an attempt to provide a theory for this operating mode and the losses that may be generated. This paper will discuss the instrumentation and testing of a four-pole 1120-kW induction motor that was tested at three different levels of unbalanced voltage. Results will be presented on the stator and rotor heating, the speed-torque and locked rotor performance, as well as noise, vibration and shaft voltage. The problem has also been analyzed using time-stepping finite-element analysis

Integrated Filters and Their Combined Effects in Matrix Converter

A matrix converter has a topology that inherently exhibits sinusoidal input current waveforms and has small output voltage steps. This paper proposes to integrate the matrix converter with filters, which provide lower electromagnetic interference (EMI), lower common-mode current, lower shaft voltage, and sinusoidal output voltage waveforms. A salient aspect of the proposed integration is its ability to significantly reduce the value of the input EMI filter capacitors and still meet strict EMI regulations. This helps in reducing the ground leakage current, which in the past has been a serious limitation in applying EMI filters with low ground currents for human and cattle safety

A Passive EMI Filter for Eliminating Both Bearing Current and Ground Leakage Current From an Inverter-Driven Motor

This paper presents a practical approach to eliminating both bearing current and ground leakage current from an inverter-driven motor rated at 400 V and 3.7 kW. When the shaft voltage with respect to the motor frame exceeds the dielectric breakdown voltage of thin lubricating grease films in two metal bearings at the drive and non-drive ends, an electrical discharge machining (EDM) current flows through the bearings. A passive electromagnetic interference (EMI) filter can keep the shaft voltage in check, as a result of having eliminated high-frequency common-mode voltage from the motor terminals. Hence, no dielectric breakdown occurs in the grease films, so that no EDM current flows in the bearings. Experimental results verify the viability and effectiveness of the passive EMI filter designed in this paper

A Multi Conductor Transmission Line Model for the Evaluation of the Rotor Shaft Voltages in Adjustable Speed Drive Motors

Second University of Naples, ItalyAbstract—The use of switching devices, such as IGBTs, characterised by high switching frequencies and verylow switching times in new generation pulse width modulation (PWM) inverters has increased the efficiencyand performances of Adjustable Speed Drives (ASDs) for industrial and traction applications. However, suchsystems may be affected by disadvantages like over voltages at the motor terminals, when long cables are usedbetween the drive, and the generation of rotor shaft voltage, due to the capacitive couplings in the motor(between the windings and the rotor and between the rotor and the stator). The shaft voltage may cause thebreakdown of the lubricating film in the bearings. The resulting impulsive currents, by damaging the bearingelements shorten the component life, which in turn seriously affects the ASD reliability. For this reason, it is ofgreat importance to develop numerical models able to predict the shaft voltage so as to estimate the currentsflowing through the bearings. Several works, based on either concentrated or distributed circuit models, havebeen proposed for the evaluation of the shaft voltage magnitudes for several motors sizes. However, the resultsobtained by such approaches suffer from approximations and simplifications in the considered circuit model.Therefore, in the present paper, a numerical model able to accurately predict the shaft voltage in high powerinduction motor for traction applications fed by a PWM inverter is presented. The windings of the motorare modelled by a multi conductor transmission line (MTL), whereas the cables between the source and themotor are described by a single transmission line. The effect of wave propagation and reflection and of thefrequency-dependent distributed losses is considered by using a time-domain equivalent circuit to represent theMTL. A semi-analytical method, based on the perturbation theory of the spectrum of symmetric matrices, isadopted. The parameters of the MTL are obtained either analytically or numerically by using a commercialsoftware (Maxwellr by Ansoft). The effects of the rise time of the input voltage together with the length ofthe cables are considered.

モータ中性線を利用したパッシブEMIフィルタ<br>—ベアリング電流の抑制効果—

This paper describes the effect of a passive EMI filter on preventing bearing current from flowing inside an inverter-driven motor. Motor-bearing damage is often caused by bearing currents resulting from the breakdown of grease films in the motor bearing. The high-frequency common-mode voltage generated by the PWM inverter induces a shaft voltage between the rotor and the frame. When the shaft voltage exceeds the breakdown voltage of the grease films, a destructive instantaneous discharge current with a peak value of about 1 A flows through the motor bearing. The passive EMI filter, which is unique in access to the motor neutral line, can reduce the shaft voltage as a result of eliminating the high-frequency common-mode voltage from the motor terminals. Hence, no breakdown occurs in the grease film, so that no bearing current flows. The viability and effectiveness of the passive EMI filter is verified by experimental results obtained from a 400-V, 3.7-kW laboratory system. © 2008 Wiley Periodicals, Inc. Electr Eng Jpn, 166(2): 78– 87, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20489

Medium frequency shaft voltages in large frequency converter driven electrical machines

The operation of large electrical machines like turbo-generators with ratings of several hundred MVAs by static frequency converters can cause shaft voltages of several hundred volts. The shaft voltages consist of peaks with a time constant of some μs. They occur mainly during the rectifier switching at the grid-linked section of the converter. The voltages are the result of a coupled capacitive and inductive phenomenon. The root cause of these voltages is shown and a machine model of turbo generators for several 10 kHz is developed taking parasitic capacities and eddy currents into account. It is not sufficient to limit the model just to the generator. It is the combination of transformator, cables, frequency converter and generator, which allow for such high voltages. Comparisons with measurements on a 300 MVA generator are made to verify the model. A high level of agreement between measurement and calculation is reached for this recently discovered capacitive inductive shaft voltage phenomenon. Especially the high power class of the equipment involved reveals the underlying physics fairly clearly, but enforces a deep investigation and high predictability of the right counter measures in order to avoid substantial risk exposures.

Reduction of capacitive and induced shaft voltages in an induction motor drive using dual-bridge inverter approach

The dual-bridge inverter approach, developed to eliminate the common-mode voltage and the resulting bearing current generated by frequency converters, is investigated by means of experiments and simulations. The approach reduces the capacitive and induced shaft voltages by more than 80% in the examined 1.4MW induction motor drive. The remaining shaft voltage is found to be caused by the rectifiergenerated common-mode voltage and a time delay between the PWM pulses of the parallel inverter units.

Influence of voltage and current source inverters on low-power induction motors

Currently low- and medium-power induction motor drives used in industrial applications are based on voltage source inverters often with open-loop V/Hz control. The introduction of current source inverters into this area could lead to considerable marketing advantages. A comparison of the performances of a voltage-controlled current source inverter and a voltage source inverter, both feeding a low-power induction motor is presented with the aim of evaluating the potential benefits of using a current source inverter in a low-power induction motors. The experiments are designed to highlight harmonic losses, insulation stress, and common-mode phenomena. The losses in the induction motor as well as in both inverters are measured under several steady-state operating conditions. The voltage stress generated when the motor windings are switched on has been recorded for both drives. The common-mode voltage and current are also quantified and an analysis of the shaft voltage is presented. The experimental results show that the current source drive achieves a better performance in terms of the motor efficiency, insulation stress, common-mode voltage and shaft voltage.

An Approach to Eliminating High-Frequency Shaft Voltage and Ground Leakage Current From an Inverter-Driven Motor

This paper proposes a small-sized passive electromagnetic interference (EMI) filter for the purpose of eliminating high-frequency shaft voltage and ground leakage current from an ac motor driven by a voltage-source pulsewidth-modulation inverter. The filter requires access to the neutral point of the motor. A common-mode inductor is connected between the inverter and the motor. The neutral point of the motor is connected to the dc-bus midpoint via a resistor. The dc-bus midpoint is created by using two capacitors in series across the dc positive and negative buses. This unique circuit configuration makes the common-mode inductor effective in reducing the common-mode voltage appearing at the motor terminals. The validity and effectiveness of the EMI filter is verified by experimental results obtained from a 5-kVA laboratory system.

Early Warning of Developing Problems in Rotating Machinery as Provided by Monitoring Shaft Voltages and Grounding Currents

A unique and independent unit condition monitoring technology for rotating machinery, called Shaft Condition Monitoring (SCM) provides early warning of developing problems by monitoring and analyzing shaft voltage and grounding current profiles. The process begins with capturing and trending of critical information inherent in raw signals derived from reliable shaft-riding brushes. Through analysis of this critical information, problems in rotating machinery are detected at their inception or long before they are apparent on traditional instruments. With this data in hand, a proactive plan of monitoring and maintenance is facilitated, possibly averting the extreme costs of failure and forced shutdown.