Ring-to-ring fault detection of stator windings of three-phase squirrel cage induction motors considering the effect of magnetic saturation

Dissertation of Master of Electrical Engineering - Power

Abstract:

Monitoring the state of induction motors is an absolutely necessary and important technology for diagnosing various faults at the initial stage. which can prevent the spread of unexpected defects in the initial stages. Approximately 30-40% of induction motor defects are related to stator defects. In this thesis, a comprehensive review of various induction motor defects, their causes, and different methods of modeling these defects has been done. In the following, different indicators for diagnosing the ring to stator coil connection defect are introduced and have been investigated and studied from different aspects.

The main idea of ??this thesis is to simulate a defective induction motor with a ring to stator coil connection defect by considering the effect of magnetic saturation, and the simulation of a defective three-phase induction motor with a ring to stator coil connection defect has been done with and without considering the effect of magnetic saturation. Then, different indicators of this type of defect are extracted and compared with practical results in both linear and saturation conditions. Also, in this thesis, a new index with more desirable features has been introduced to identify the defect of the stator winding loop, and finally, the most desirable index has been introduced among the existing indices.

Keywords: Ring-to-ring defect of stator winding, induction motor, magnetic saturation, genetic algorithm, pendular oscillation phenomenon, stator inductance

Introduction

Induction motors have been widely used due to the simplicity and strength of the construction, cheapness, wide speed range and some other advantages. For this reason, it is very important to monitor the condition of these types of engines in order to identify errors in the early stages of their occurrence, especially in high powers. Therefore, detecting the ring connection error in the early times of its occurrence can include the following advantages:

Preventing major damage to the engine and its time-consuming and expensive repairs.

Preventing unexpected stoppage of the production line.

Reducing losses.

Achieving the above advantages requires timely knowledge of the severity and position (phase) of the motor ring connection error. This is usually possible by revealing some effects of the engine's behavior due to the occurrence of an error. The disadvantages of induction motors can be divided into three main groups: Mechanical, rotor and stator divided. Each of these errors are rooted in different factors and have different effects on engine performance. Even some errors may be the origin of other errors.

Mechanical errors are mainly caused by bearing errors [1 and 2]. Some factors that cause bearing errors are: Improper or insufficient lubrication, heavy radial and axial stresses due to shaft misalignment and assembly, adjustment or poor foundation. These factors accelerate the wear and erosion of bearings. Usually, bearing errors also result in rotor and stator misalignment errors. The intensification of the recent error can lead to the contact of the rotor and stator surfaces and create the disadvantages of the rotor and stator.

Breaking of the rotor bars, breaking of the end ring of the rotor and types of misalignment (static, dynamic and composite) are among the errors of the rotor [3]. The main reasons for these errors are as follows:

1. Thermal overload that can occur during acceleration, continuous operation or stopping the rotor.

2. Thermal imbalance or temperature difference in the rotor rods, which is caused by repeated startups, skin phenomenon, non-uniform heat transfer of the core and rotor rods, and some other factors.

3. Magnetic effects that lead to radial electrodynamic forces on the rods. These forces, which result from the interaction of the magnetic flux and the current of the rods, are proportional to the square of the current of the rods and cause the vibration and bending of the rods along the radial direction, and may eventually lead to the breaking of the rotor rods.

4. Inherent non-uniformity along the length of the air gap (inherent misalignment), which results from the non-ideality of the motor manufacturing and assembly technology, causes asymmetric magnetic tension on the adjacent surfaces of the rotor and stator. Because the rotor is subjected to larger traction forces on the side where the air gap is smaller. This causes the rotor to bend, aggravates the misalignment error, and finally leads to the collision of the rotor with the stator. As a result, the structure of the rotor and stator may be seriously damaged.

5.Increasing the stresses on the rotor bars due to permanent or fluctuating overload over time can lead to breaking the rotor bars. 

6. An increase in centrifugal forces due to an increase in the speed of the motor to more than the nominal speed can lead to tension in the end rings and break the connection between the rotor bars and the end rings.

The stator of the induction motors, like the bearings and the rotor, can fail under the influence of various factors [3]. Five types of faults have been reported for stator windings, all of which are rooted in the insulation failure of the windings [4] These faults are: 1. Loop-to-loop fault in a coil where two points of one or more loops of a coil are connected to each other (loop connection fault). 2. Coil-to-coil fault in a phase where a point of a coil is connected to a coil The point of the other coil of the winding of the same phase is connected.

3. A phase-to-phase fault in which a point of the coil of one phase is connected to a point of the coil of another phase.

4. An open circuit fault in which a phase or a part of a phase of the circuit is open due to a wire break.

5. A coil to ground fault in which A point of one phase coil is connected to the ground (body). Figure 1-1- Types of errors in the stator coil Figure (1-1) shows the types of stator coil errors. A faulty engine may have a combination of the above three faults. For example, in a motor, the motor axis may be bent, and this causes vibration and damage to the bearings, leading to contact between the rotor and the stator. As the engine continues to operate in this overheated state, the aluminum rotor bars may melt. The spread of molten aluminum on the stator winding results in a winding fault. About 75% of all failures of squirrel cage induction motors are related to stator and bearing faults, bearings failure (mechanical faults) 40-50%, stator insulation failure (stator fault) 30-40% and rotor rack failure (rotor fault) 5-10%. It has been reported [6]. If the progression of loop-to-loop faults is not prevented, the mentioned cases will lead to a phase-to-ground or phase-to-phase fault, which of course is a more likely phase-to-ground fault. The results of more comprehensive studies based on a statistical method for engines with different powers and speeds also confirm the aforementioned percentages [8, 7, 2]. Therefore, the errors of the stator winding account for a significant percentage of the total defects of the induction motor. Therefore, this dissertation focuses on the stator winding fault.

Stator winding fault types usually start with the short circuit of several loops adjacent to the phase winding (loop connection fault). In this way, the circulating current in the short circuits causes heat generation and temperature increase in the defective area of ??the coil, and with further destruction of the insulation in that place, it leads to more severe errors, i.e. coil-to-coil error, phase-to-phase error, or phase-to-ground error. . Although there is no empirical information on the time interval between the occurrence of the loop connection error and the insulation failure and the complete escalation of the error, it is certain that this process is not instantaneous and its speed depends on the severity of the error, i.e. the number of shorted connection loops. To detect the loop connection error, several indicators have been introduced so far and various methods have been presented to measure them and draw conclusions about the occurrence of the error, which are discussed below.

Causes of winding errors:

All types of stator winding errors are rooted in winding insulation failure. Various stresses in the structure of the motor and especially the stator may lead to the failure of the winding insulation and the occurrence of errors. These stresses can be categorized in the form of thermal, electrical, mechanical and environmental stresses [3 and 4]:

1-2-1- Thermal stresses

Thermal stresses arise due to the increase in the temperature of the coils and cause a decrease in the lifespan or destruction of the insulation of the coils. Standard tests show that for every 10 degrees Celsius increase in the coil temperature from the maximum allowed value, its insulation life is reduced by half [4]. The factors that increase the thermal stress are:

Excess voltage: increasing the voltage range increases the machine flux range, increases the core losses, increases the temperature and creates thermal stress.