On-Line Diagnostics of Large Induction Motors

The electric power system consists of static load, lump load, and motor. A large industry requires a large induction motor to run its production. However, the state of this induction motor harms the electric power system. The starting current on the motor is so high that it can be 5 to 10 times the nominal current. On the other hand, the motor absorbs much power from the electric power system and results in a voltage drop on the bus. This paper aims to improve the voltage drop on the bus and reduce the starting current of the induction motor. The analysis in this paper uses a transient curve of changes in voltage and starting current with time. Starting an induction motor with the Direct online (DOL) method as the basis for determining the voltage drop and the starting current of the induction motor. Furthermore, using the Capacitor Bank method, Fault Current Limiting (FCL), and Static Var Compensator (SVC) improves transients of voltage drop and motor starting current. All the results of these methods are compared to find out which method is the best.

This article presents a cost-effective variable frequency drive for starting and operating large three-phase squirrel cage induction motors. The proposed drive employs the constant V/F control scheme, which regulates the rotor frequency such that it is always kept below a predetermined value. This has the effect of drastically reducing the motor shaft torque oscillations and significantly limiting the maximum value of the motor line current. In addition to its simplicity and hence cost-effectiveness, the proposed scheme is shown to be capable of (i) reducing the shaft torque oscillations to approximately 22% of that of the direct on-line start of the large motor, and (ii) limiting the motor line current to approximately 36% of that of the direct on-line start value.

Starting an induction motor causes voltage sag in the industrial grid that may disturb the operation of grid equipment. Direct-on-line starting is the simplest and most cost-effective method for starting induction motors, but also the most problematic. Large industries often use internal power plants with synchronous generators and starting powerful motors may impact the generator operations. The synchronous generators could be operated with the automatic or manual mode of voltage control. As the operation experience proves, the generator voltage control mode has a significant impact on the transient behaviours in the industrial power grid when starting a large induction motor. This article presents a case study of the synchronous generator tripping within a true medium-voltage industrial grid during the direct-on-line starting of a large induction motor driving the feed water pump. An analysis of the generator protection logs after the tripping showed that the synchronous generator was controlled in manual mode and its protection relay settings were exceeded. The transients initiated by induction motor starts were studied for possible configurations and operating conditions of the grid using a model developed on the Matlab/Simulink Software platform. The simulations have shown that concern about starting large motors in industrial grids containing internal synchronous generators needs to be solved considering the grid configurations and the coordination of generator protection and control devices.

Large induction motors can have a high inrush and run-up current during starting, often up to ten times the rated current. In weak supplies, this could be a problem, causing system stability issues and the voltage to dip below acceptable levels. In islanded systems, the capacity could be pulled below its maximum. There are several different starting methods possible, but they are often only suitable for smaller machines. One method not investigated is the use of parallel capacitor compensation during the starting because large induction motors are very inductive during the starting sequence, so that supplying reactive power may be more effective than supplying energy. This paper first investigates several different induction motors with increasing size and assesses their compensation requirements in terms of the reactive power requirement during starting. It is shown that they do generally become more inductive as they increase in size. It is illustrated, using simulations, that using parallel capacitors during starting can significantly reduce the starting current. The concept is tested using a small experimental motor. Series inductors are added to the motor to make it more inductive and look more like a larger machine. The experimental results confirm that the method works. It is also found that the parallel capacitors require series filters to prevent harmonic current, and these can be realized using inductors in series with the compensation capacitors. Point-on switching using solid-state relays was tested, and these can reduce the transient switch-on current over the first voltage cycle.

Induction motors are the most common means of converting electrical power to mechanical power in the industry. Induction machines were typically considered robust machines; however, this perception began to change toward the end of the last decade as low-cost motors became available on the market. Nowadays the most widely used induction motor in the industry is a machine which works at the limits of its mechanical and physical properties. A good diagnosis system is mandatory in order to ensure proper behavior in operation. The history of fault diagnosis and protection is as outdated as the machines themselves. Initially, manufacturers and users of electrical machines used to rely on simple protection against, for instance, overcurrent, overvoltage and earth faults to ensure safe and reliable operation of the motor. However as the tasks performed by these machines became more complex, improvements were also sought in the field of fault diagnosis. It has now become essential to diagnose faults at their very inception, as unscheduled machine downtime can upset deadlines and cause significant financial losses. The major faults of electrical machines can be broadly classified as follows: Electrical faults (Singh et al., 2003): 1. Stator faults resulting in the opening or shorting of one or more stator windings; 2. Abnormal connection of the stator windings; Mechanical faults: 3. Broken rotor bars or rotor end-rings; 4. Static and/or dynamic air-gap irregularities; 5. Bent shaft (similar to dynamic eccentricity) which can result in frictions between the rotor and the stator, causing serious damage to the stator core and the windings; 6. Bearing and gearbox failures. However, as is introduced in the basic bibliography by Devaney (Devaney et al., 2004), the effect of bearing faults is, in most cases, similar to eccentricities and has the same effects on the motor. The operation during faults generates at least one of the following symptoms: 1. Unbalanced air-gap voltages and line currents 2. Increased torque pulsations 3. Decreased average torque 4. Increase in losses and decrease in efficiency 5. Excessive heating 6. Appearance of vibrations

This paper demonstrates the development of a flexible fault diagnosis methodology that can detect up to ten different faults in the induction motor (IM), simultaneously. The major IM electrical faults, such as the broken rotor bar (BRB), phase unbalance (PUF), and stator winding fault (SWF), and mechanical faults, such as bearing fault (BF), unbalanced rotor (UR), bowed rotor (BR), and misaligned rotor (MR), are considered with different fault severities for the diagnosis. The experiments are conducted with three varying loads and seven different speeds, and the frequency domain vibration and current data are acquired at a relatively low sampling rate of 1 kHz. Several statistical features are extracted and then the best feature-set is selected using the wrapper model. Thereafter, a data classification tool based on the support vector machine (SVM) is used for the fault characterization. Initially, a multi-fault diagnosis is performed by training and testing the SVM at the same operating conditions (i.e., load and speed). The performance of the classifier is found to be very good at all IM operating conditions. The main focus of this study lies in overcoming the fault diagnosis, where the data are unavailable at required operating conditions. This is accomplished by employing interpolation and extrapolation strategies for different loads and speeds. The proposed methodology not only solves practical problem of unavailability of data at different operating conditions but also shows good performance and takes low computation time, which are vital requirements of an online intelligent condition monitoring system.

There are applications where low current inrush and or high starting torque are specified for reducing disturbance on the electrical distribution network. There are also situations where it is required to successfully start large AC motors at reduced voltage conditions when only one substation transformer of a double-ended transformer is in service at petrochemical plants. The latter condition requires the motors to draw lower inrush current while producing high starting torque. These requirements can create unfavorable design characteristics where one or both requirements can not be accomplished using standard techniques at final design stage or at final motor test. The authors have accomplished designing and installing magnetic material retaining rings on Induction motors with significant improvement in inrush currents and starting torques. Inrush currents have been reduced from 600% to 508% and starting torques increased up to 14%. Several case studies of using magnetic retaining rings on large induction motors are described in this paper for machines ranging from 6,000 up to 14,000HP and testing as low as 380% inrush for machines with magnetic rings. The application of this concept is not limited to only AC induction motors. The same process can also be applied to synchronous motors. Very limited information is available on the benefits of applying magnetic retaining rings and what the consequences are for large AC motors. The application of magnetic retaining rings is a practical approach to solving inrush and torque problems at the manufacturing or design stage. The theory, test, concepts and consequences along with electrical and mechanical considerations are described in this paper.

There are applications where low-current inrush and/or high starting torque are specified for reducing disturbance on the electrical distribution network. There are also situations where it is required to successfully start large ac motors at reduced voltage conditions when only one substation transformer of a double-ended transformer is in service at petrochemical plants. The latter condition requires the motors to draw lower inrush current while producing high starting torque. These requirements can create unfavorable design characteristics where one or both requirements cannot be accomplished using standard techniques at the final design stage or motor test. The authors have accomplished the design and installation of magnetic material retaining rings on induction motors with significant improvement in inrush currents and starting torques. Inrush currents have been reduced from 600% to 508%, and starting torques have been increased up to 14%. Several case studies of using magnetic retaining rings on large induction motors are described in this paper for machines ranging from 6000 up to 14 000 hp and testing as low as 380% of inrush for machines with magnetic rings. The application of this concept is not limited to only ac induction motors. The same process can also be applied to synchronous motors. Very limited information is available on the benefits of applying magnetic retaining rings and what the consequences are for large ac motors. The application of magnetic retaining rings is a practical approach to solving inrush and torque problems at the manufacturing or design stage. The theory, test, concepts, and consequences along with electrical and mechanical considerations are described in this paper.

This paper presents a new control strategy for adaptive hysteresis current control based power quality studies for an induction motor to eliminate harmonics and to compensate the reactive power. The adaptive hysteresis band current controller changes the hysteresis bandwidth according to modulation frequency, supply voltage, dc capacitor voltage and slope of the reference compensator current wave. The large rating of three phase induction motor take very large low power factor currents while being started directly from a 3-phase supply. Normally soft-starts are used for avoiding this problem and to achieve smooth starting of large capacity induction motors. A three phase AC voltage controller is employed as a soft-start. But, this takes harmonic-rich current especially while operating at large firing angles. The adaptive hysteresis current control based shunt active filter has been inserted between the AC voltage controller and power supply to take care of the reactive power requirement of the motor, AC voltage controller and also to provide harmonic compensation. In shunt-active filter uses the Synchronous Reference Frame (SRF) theory to determine the suitable compensation currents. The simulation has been carried out through the MATLAB Simulink SimPowerSystems Toolbox and Hardware work has been done on a 22 kW induction motor with and without soft-start arrangement.

In this work we propose an original failure signature based on an improved combination of Hilbert and Park transforms. Starting from this combination we can release two failure signatures: Hilbert modulus current space vector (HMCSV) and Hilbert phase current space vector (HPCSV). These two signatures are subsequently analysed using the classical fast Fourier transform (FFT). The effects of HMCSV and HPCSV spectrums are described and the related frequencies determined. This analysis offers an easy interpretation to conclude on the induction motor condition and its voltage supply state. The proposed signature shows its effectiveness and its robustness in both electrical and mechanical fault detection. This approach was applied to a 1.1 kw induction motor under normal operation and with the following fault: voltage unbalanced, broken rotor bar, air-gap eccentricity and ball bearing defect.

Detection and identification of induction machine faults through the stator current signal using higher-order spectra analysis is presented. This research proposes two types of engine failures that are broken rotor bars, and rotor unbalance. Several conventional vibration and current analysis techniques exist by which certain faults in rotating machinery can be identified; however, they generally deal with a single fault only. Instead, in real induction machines, the case of multiple faults is common. When multiple faults exist, vibration and current are excited by several fault-related frequencies combined with each other, linearly or nonlinearly. The contribution of this paper is the development of a condition-monitoring strategy that can make accurate and reliable assessments of the presence of specific fault conditions in induction motors with single or multiple faults present.

On-line condition monitoring is widely used to detect failure mechanisms in large highvoltage three-phase induction motors. The high-voltage winding is installed in a complex, stator core structure which vibrates due to electromagnetic forces. Insulation degradation can be diagnosed via on-line monitoring of partial discharges but the fundamental cause is often mechanical damage due to stator vibration. This paper presents results of an investigation into the natural vibratory response of the complete stator assembly. Results from finite element models of complex stator assemblies, including the frames, are supported by experimental measurements. These include freely suspended stator cores as in previous studies, and also now introduce the much more realistic condition of the complete stator assembly securely bolted on to a solid baseplate. The knowledge gathered in this study will aid the application of a vibration monitoring strategy to detect stator core vibration which can lead to winding insulation damage.

Capacitor bank switching on medium voltage power systems has been in practice for nearly half a century. However, capacitor bank switching causes high inrush currents and voltage transients (as high as 3.0 p.u.) in the power distribution system potentially damaging electrical equipment, impacting the system reliability and power quality. Additionally, capacitor switching transients affect the life of the capacitor itself and can cause damage or sudden failure. Pipeline substations are often located in remote areas with insufficient SCC available on the distribution system to start a large medium voltage induction motor. In many such instances, the end user often ends up using a medium voltage soft-starter, a higher rated MVA transformer in conjunction with a reactor start, or a low inrush motor with less than ideal motor efficiency. This paper introduces a transient-free capacitor switching system for large motor starting on MV power distribution systems that can be used with a traditional capacitor bank system while restricting the voltage transient to less than 1.05 p.u. during capacitor switching. This solution eliminates the requirement of a MV soft starter or an auto-transformer motor start which are cost prohibitive. As a result, in lieu of a low inrush, low efficiency induction motor; a regular induction machine with higher efficiency can be applied.

An appraisal is made of the general problem of system-design synthesis by digital computers. Published literature on the design of electrical machines by computer is surveyed, and it is concluded that the majority of this work may be categorised as design analysis and partial design synthesis. The development of a pilot program for the design synthesis of large squirrel-cage induction motors is described, with emphasis on the methods adopted in the search for solutions. Particular attention is given to the development of special techniques dealing with the major discrete parameters, e.g. frame size and numbers of slots and conductors per slot. The final program comprises a marriage of methods of solution, including iterative techniques and preknowledge of the system functions, which was considered essential to achieve an economical convergence scheme. Results are presented, illustrating that the program produced satisfactory convergence to meet required performance specifications. A number of alternative solutions are obtained for each case of input data. There is considerable scope for the improvement and refinement of the program.

The main aim of this paper is to study effects of different voltage sags and their intensity on the behavior of induction motors using a simulation approach. For this purpose, necessary simulation was done in Matlab - Simulink first. Then, to ensure the reasonable accuracy of the simulation, some results of simulation were compared with the corresponding results measured in the laboratory. Then effects of intensity of symmetric voltage sag (three phase) and its starting point on the behavior of small and large induction motors was studied using simulation and subsequently, the effects of asymmetric voltage sags (single phase and two phase) has also been studied and finally the conclusion of the study were presented.