Pinjia Zhang

A Survey of Condition Monitoring and Protection Methods for Medium-Voltage Induction Motors

Medium voltage induction motors are widely used in industry and are essential to industrial processes. The breakdown of these medium voltage motors not only leads to high repair expenses, but also causes extraordinary financial losses due to unexpected downtime. To provide reliable condition monitoring and protection for medium voltage motors, this paper presents a comprehensive survey of the existing condition monitoring and protection methods in the following five areas: thermal protection and temperature estimation, stator insulation monitoring and fault detection, bearing fault detection, broken rotor bar/end-ring detection, and air-gap eccentricity detection. For each category, the related features of medium voltage motors are discussed; the effectiveness of the existing methods are discussed in terms of their robustness, accuracy and implementation complexity; recommendations for the future research in these areas are also presented.

An Impedance Identification Approach to Sensitive Detection and Location of Stator Turn-to-Turn Faults in a Closed-Loop Multiple-Motor Drive

A single closed-loop inverter drive with multiple motors connected to it is a type of drive topology commonly used in steel processing industry, electric railway systems, and electric vehicles. However, condition monitoring for this type of drive configuration remains largely unexplored. This paper proposes an impedance identification approach to detect and locate the stator turn-to-turn fault in a multiple-motor drive system. Sensitive and fast fault detection is achieved by utilizing the characteristics of current regulators in the motor controller. Experimental results show that the proposed method can reliably detect and locate the stator turn fault on two shaft-coupled 5-hp induction machines under different operating conditions and fault levels with no need of any machine parameters. Although originally developed for multiple-motor drives, the detection scheme can also be directly applied to most of the conventional closed-loop induction motor drives.

A survey of condition monitoring and protection methods for medium voltage induction motors

Medium voltage induction motors are widely used in industry and are essential to industrial processes. The breakdown of these medium voltage motors not only leads to high repair expenses, but also causes extraordinary financial losses due to unexpected downtime. To provide reliable condition monitoring and protection for medium voltage motors, this paper presents a comprehensive survey of the existing condition monitoring and protection methods in the following five areas: thermal protection and temperature estimation, stator insulation monitoring and fault detection, bearing fault detection, broken rotor bar/end-ring detection, and air-gap eccentricity detection. For each category, the related features of medium voltage motors are discussed; the effectiveness of the existing methods are discussed in terms of their robustness, accuracy and implementation complexity; recommendations for the future research in these areas are also presented.

A Remote and Sensorless Stator Winding Resistance Estimation Method for Thermal Protection of Soft-Starter-Connected Induction Machines

This paper proposes a remote and sensorless stator winding resistance estimation method for thermal protection of soft-starter-connected induction motors. By changing the gate drive signals of the thyristors in the soft starter, a small adjustable dc bias can be intermittently injected to the motor for the estimation of the stator winding resistance. Based on online and continuous monitoring of the stator winding resistance, the stator winding temperature can be monitored using only motor voltage and current. In addition, the torque pulsation caused by the injected dc bias is analyzed. It can also be controlled under an acceptable level by adjusting the level of the injected dc signal. The influence of cable resistance is also studied, and a compensation method is proposed. The proposed method has been verified by experimental results from two induction motors. The proposed stator resistance estimation method can provide remote, sensorless, and accurate thermal protection for soft-starter-connected induction motors.

Magnetic Effects of DC Signal Injection on Induction Motors for Thermal Evaluation of Stator Windings

Active thermal protection techniques have been proposed for the thermal protection of induction motors. By injecting dc signals, the dc model of the induction motors can be used to estimate the stator winding resistance and, thus, the average stator winding temperature. The injected dc signals may lead to unbalanced magnetic saturation, which may degrade a motors performance and may increase heat dissipation in the motor. This paper presents a detailed analysis of the magnetic saturation caused by dc signal injection, with its effects discussed. It is shown that the injected dc signal induces a varying magnetic saturation in an induction motor during each cycle. The effects of the magnetic saturation on the motor currents, the motors thermal behavior, and the estimation of the stator winding resistance and temperature are analyzed. Such analysis is illustrated from both the finite-element simulation and experimental results. It is shown that the magnetic saturation does not affect the performance of the stator resistance and temperature estimation, which proves the feasibility of these active thermal protection techniques under different load conditions. In addition, it is shown that the injected dc signals have a little impact on the thermal performance of induction motors, which proves that the dc signal injection can be widely used for thermal protection without causing much thermal stress in induction motors.

A Nonintrusive Thermal Monitoring Method for Induction Motors Fed by Closed-Loop Inverter Drives

Closed-loop induction motor drives have wide applications in electric traction system and industrial processes. Accurate thermal monitoring not only protects the induction machines from overheating, but also boosts the usage and performance of the overall drive system. State-of-the-art thermal monitoring schemes for induction machines utilize thermal models, which are inaccurate for drive-fed machines and are not adaptive to varying cooling conditions. This paper proposes a nonintrusive thermal monitoring scheme for induction motors fed by closed-loop inverter drives. By applying offsets to the current controllers in the motor drive, dc currents are injected into the stator windings of the machine. The accurate estimation of the injected dc voltage is achieved by carefully analyzing and compensating for inverter nonidealities. The stator winding temperature can thus be monitored based on the estimated dc stator resistance. The method is nonintrusive because it eliminates the thermocouples, requires no hardware change to the existing motor drive, and has minimal impacts on the normal operation of the motor. The proposed method is practically implemented in a programmable inverter drive and is validated both by simulation and real-time experimental results.

A DC Signal Injection-Based Thermal Protection Scheme for Soft-Starter-Connected Induction Motors

This paper proposes a remote and sensorless thermal protection scheme for soft-starter-connected induction motors. A dc signal injection-based method is used to estimate the stator winding resistance and, thus, the stator winding temperature. Based on continuous monitoring of the stator winding temperature, a thermal condition monitoring and protection scheme is proposed using only the motors input voltages, currents, motor nameplate information, and the ambient temperature. An adaptive Kalman filter is designed to reduce the stator winding temperature estimation error. In addition, the influence of the cable resistance was investigated, and a compensation method is suggested to improve the accuracy of the estimated temperature. The requirements of the data acquisition system, such as sampling frequency and analog-to-digital conversion resolution, and their affects on the accuracy of the estimated temperature are also discussed in detail. The proposed thermal monitoring scheme has been validated from the experimental results of a 7.5-hp TEFC induction motor under various load conditions. The importance of this new thermal monitoring scheme lies in its remote and sensorless nature.

An Active Stator Temperature Estimation Technique for Thermal Protection of Inverter-Fed Induction Motors With Considerations of Impaired Cooling Detection

Thermal protection is one of the most important aspects of any motor control system. This paper proposes a stator winding temperature estimation method for the thermal protection of inverter-fed electric motors. By modifying the space vector pulse width modulation (SVPWM) in an open-loop motor drive, a dc voltage can be intermittently injected into the motor. The stator temperature can be estimated by measuring only the dc component of the phase current under both constant and variable load conditions. The evaluation of the resultant torque pulsation and the compensation for serial resistances are also discussed. The proposed stator temperature estimation method is validated from experimental results under variable load conditions and both healthy and impaired cooling conditions. The error in the stator temperature estimation is within 8°C under different operating conditions. The significance of this method lies in its non-intrusive nature: only current sensors are required for implementation; the normal operation of the motor is not interrupted.

A Transfer-Function-Based Thermal Model Reduction Study for Induction Machine Thermal Overload Protective Relays

High-order induction machine thermal models have been widely studied for the analysis of the thermal behavior of induction machines. However, the real-time implementation of such models for the online thermal protection of induction machines is very difficult due to the lack of accurate information on the machines. This paper proposes a novel simplified thermal model of induction machines for thermal overload relays. Instead of using a lumped thermal network, a transfer-function-based approach is proposed for reducing the order of induction machine thermal models. The proposed thermal model requires significantly fewer thermal parameters for the accurate modeling of the thermal behavior of the machine. The proposed thermal model is validated via experimental results on a 7.5-hp open-drip-proof induction machine under various load conditions. The major features of the proposed thermal model are the following: 1) high accuracy of the stator winding temperature estimation, with a root-mean-square error within 3°C; 2) low computational requirement, which reduces the cost of thermal overload relays; and 3) easy implementation since only current sensors are required.

Active stator winding thermal protection for AC motors

AC motors are the main workhorses in process industries. Their malfunction may not only lead to repair or replacement of the motor, but also cause significant financial losses due to unexpected process downtime. Reliable thermal protection of ac motors is crucial for reducing the motor failure rate and prolonging a motors lifetime. In this paper, conventional thermal protection devices and state-of-the-art thermal protection techniques are reviewed with their advantages and limitations discussed. Since 2001, the research team at the authors company and at the Georgia Institute of Technology has been cooperatively developing a series of active motor thermal protection methods, as a low-cost alternative approach to the traditional passive and sensor-based methods. These active thermal protection methods monitor the average stator temperature via stator resistance estimation based on the dc equivalent model of ac motors, using only motor stator voltage and current measurements. In this paper, an overview of the active thermal protection techniques for line-started, soft-starter-connected and inverter-fed ac motors is presented, with their advantages and drawbacks discussed. The active thermal protection techniques are capable of providing accurate, and non-invasive thermal protection of ac motors.