Ji n Yoo

Condition Monitoring of DC Link Electrolytic Capacitors in Adjustable Speed Drives

A new preventive maintenance strategy for the sensorless condition monitoring of inverter dc-link aluminum electrolytic capacitors based on equivalent series resistance (ESR) and capacitance C estimation is proposed. The main concept of the proposed method is to estimate the ESR and C of the capacitor, using the inverter whenever the motor is stopped. The parameters are estimated from the dc-link voltage and stator current measurements available in the inverter with the switched dc-link voltage applied to the motor stator winding. The temperature of the capacitor can also be estimated under thermal equilibrium based on the stator resistance estimate to take the influence of temperature variation into account. An experimental study on a 250-W permanent magnet synchronous motor performed under accelerated capacitor degradation is presented to verify the proposed technique. It is shown that the proposed method provides a reliable and sensitive indication of capacitor aging without additional hardware requirements for reliable, efficient, and safe operation of the inverter and driven process.

Detection and Classification of Stator Turn Faults and High Resistance Electrical Connections for Induction Machines

The goal of stator winding turn fault detection is to detect the fault at an early stage, and shut down the machine immediately to prevent catastrophic motor failure due to the large fault current. A number of turn fault detection techniques have been proposed; however, there is currently no method available for distinguishing turn faults from high-resistance(R) connections, which also result in 3 phase system asymmetry. It is important to distinguish the two faults since a high-R connection does not necessarily require immediate motor shutdown. In this paper, new sensorless on-line monitoring techniques for detecting and classifying stator turn faults and high-R electrical connections in induction machines based on the zero sequence voltage or negative sequence current measurements are proposed. An experimental study on a 10 hp induction motor performed under simulated turn faults and high-resistance circuit conditions verifies that the two faults can be reliably detected and classified. The proposed technique helps improve the reliability, efficiency, and safety of the motor system and industrial plant, and also allows maintenance to be performed in a more efficient manner since the course of action can be determined based on the type and severity of the fault.

Practical Implementation of PWM Synchronization and Phase-Shift Method for Cascaded H-Bridge Multilevel Inverters Based on a Standard Serial Communication Protocol

The use of the controller area network communication protocol for H-bridge multilevel (HBML) inverters, and localized pulsewidth modulation generation and automated voltage regulation is proposed in this paper. It is shown that the performance and reliability of conventional distributed controllers for HBML inverters can be improved with less communication hardware requirements, when using the proposed method. It is also shown that the expansion and modularization capability of the HBML inverter is improved since individual inverter modules operate more independently. The proposed hardware is described in detail and the validity of the proposed system is verified experimentally on a 6600-V 1400-kW induction motor drive.

Automated Monitoring of Magnet Quality for Permanent-Magnet Synchronous Motors at Standstill

Demagnetization of permanent magnets (PMs) used in synchronous motors can occur due to a combination of thermal, electrical, and/or environmental operating stresses. Since PM demagnetization results in degradation of motor performance, efficiency, and reliability, it is important to monitor the quality of PMs regularly. However, there are many limitations to the offline and online methods currently used for magnet quality assessment. In this paper, a new inverter-embedded technique for automated monitoring of magnet quality for permanent-magnet synchronous motors (PMSMs), which overcomes the limitations of existing techniques, is proposed. The main concept is to use the inverter to perform a standstill test whenever the motor is stopped to detect local or uniform PM demagnetization. The machine is excited with a pulsating field at different angular positions, and the change in the current peaks caused by the change in the degree of magnetic saturation due to demagnetization is observed. An experimental study on a 10-hp PMSM verifies that local and uniform PM demagnetizations can be detected with high sensitivity.

A New Strategy for Condition Monitoring of Adjustable Speed Induction Machine Drive Systems

A new strategy for monitoring the condition of adjustable speed induction machine drive systems is presented in this paper. The main concept of the new method is to use the inverter to perform off-line tests for quality assessment of the vulnerable components in the inverter, cable, and induction motor automatically whenever the motor is stopped. This new approach is suitable for monitoring system components that deteriorate gradually to failure, if the degradation is observable. Off-line test methods for monitoring the quality of the 1) dc-link aluminum electrolytic capacitor; 2) electrical connections; 3) cable and stator winding insulation; 4) stator core; and 5) rotor bar are presented along with a summary of the failure mechanism and existing test methods. It will be shown that the new strategy can be implemented with minimal hardware requirements, and that it has many advantages over existing off-line and on-line tests. An experimental study performed under simulated fault conditions for each component shows that the method provides a reliable and sensitive indication of drive system problems for reliable, efficient, and safe operation.

A design method of PI controller for an induction motor with parameter variation

This paper proposes a novel design method of speed and current controller for an induction motor with the variation of system parameters. Our goal is to establish a reliable and stable PI gain selecting procedure against the mechanical and the electrical parameter variation of an induction motor using Kharitonov robust stability theory. Although the system parameters are varied within some specific range, the proposed PI controller guarantees the robust stability and the performance for vector controlled induction motor. The performance of proposed design method is proved by digital simulation and experimentation with high performance DSP based induction motor driving system.

Screening of False Induction Motor Fault Alarms Produced by Axial Air Ducts Based on the Space-Harmonic-Induced Current Components

Motor current signature analysis (MCSA) based on the 50/60-Hz sidebands has become a common test in industry for monitoring the condition of the induction motor rotor cage. However, many cases of unnecessary motor inspection or outage due to false alarms produced by rotor axial duct interference have been reported. If the number of axial ducts and poles is identical, this can produce 50/60-Hz sideband frequency components in MCSA that overlap with that of rotor faults, resulting in false alarms. However, there currently is no practical test method available for distinguishing rotor faults and false indications other than testing the rotor offline or under the startup transient. In this paper, the feasibility of using the rotor fault frequency component produced by the space harmonic waves is evaluated as a solution for the first time. Since the fifth or seventh space harmonics have a spatial distribution of flux that does not penetrate in the rotor yoke to reach the axial ducts, they do not produce false alarms. The proposed method is verified on 6.6-kV motors misdiagnosed with broken bars via the 50/60-Hz sidebands of MCSA. It is shown that it provides reliable online indication of rotor faults independent of axial duct influence and can be used for screening out false alarms.

Current control method using a double band hysteresis

This paper presents a study on the double-band hysteresis current controller of the STATCOM (static synchronous compensator) used for reactive power compensation on a distribution network. Hysteresis controllers are intrinsically robust to system parameters, suitable for simple implementation and exhibit very high dynamic response. But in D-STATCOM, the current control using a conventional hysteresis controller has the disadvantage to be high switching frequency. In addition, the current error is not strictly limited. Therefore, in this paper, to reduce the switching frequency and current error, a double-band hysteresis current controller in stationary reference frame is proposed. The proposed double band hysteresis current control system was testified by the Borland C++ program and demonstrates the advantage of the proposed hysteresis current control method.

A novel hysteresis current controller to reduce the switching frequency and current error in D-STATCOM

This paper presents a study on a novel hysteresis current controller of a STATCOM (static synchronous compensator) used for reactive power compensation on a distribution network. Hysteresis controllers are intrinsically robust to system parameters, exhibit very high dynamic response and are suitable for simple implementation. But the current control using a conventional hysteresis controller in D-STATCOM has the disadvantage that high switching frequency may happen due to lack of coordination among individual hysteresis controllers of three phases. This will of course increase the switching loss. In addition, the current error is not strictly limited. So, in this paper to reduce the switching frequency and current error, a novel hysteresis current controller in synchronous rotating reference frame is proposed. The presented control system was tested with digital simulation in the Borland C++ program and demonstrates the advantage of the novel hysteresis current controller.

Robust speed control for torsional vibration suppression of rolling mill drive system

In this paper, a novel robust torsional vibration suppression control scheme is proposed for the rolling mill drive system. We propose the torsional vibration suppression controller that consists of reduced order state feedback controller and PI controller; the motor speed and the observed torsional torque are fed back. Using the estimated mechanical parameters by off-line RLS (Recursive Least Square) algorithms, we design the speed controller for torsional vibration suppression and its gains are determined using the Kharitonov robust control theory. The Kharitonov robust control theory can obtain the robust stability with a specific stability margin and a damping limit. Even if the parameters are varied within some specific limit, it guarantees high performance of vibration suppression. The effectiveness and usefulness of the proposed schemes are verified with the simulation and the experimental results on the fully digitalized 5.5 kW rolling mill drive system.