Dissertation of the Master's course in the field of electricity (M. Sc)

Strength: power

Abstract:

In this thesis, electrical and mechanical defects in electric machines have been investigated and the factors that cause them and the methods of fixing these defects have been stated. After that, with the help of the winding function method of the simulation machine and the intended error, i.e. the stator wiring error, it has been applied and the results have been investigated. The main parameter that we have used to diagnose the fault in this thesis is the three-phase current of the stator in healthy and faulty state under different loads. Then the effect of the stator and rotor grooves on the induction machine refner with the inverse air gap function is used in its true form. Then, how to apply the effect of stator and rotor grooves in the winding function method is explained and the method of calculating the internal and mutual inductance matrices of the stator and rotor and the inductance derivative matrices with the presence of stator and rotor grooves in four cases are investigated. Then, the application of the stator wiring error with the effect of the stator and rotor grooves is investigated and finally the results are compared.

According to the mentioned materials, it is concluded that with the timely diagnosis of each of the primary defects in the induction machine, it is possible to prevent the occurrence of secondary accidents that lead to heavy damages. In this regard, it has been tried to analyze, investigate and diagnose one of these errors, the stator wiring error of a squirrel cage induction motor, to take an effective step in the implementation of a predictive maintenance system and to prevent heavy damages to industries and national resources by using condition monitoring systems on such machines. Introduction: Electric motors play an important role in the effective start-up of machines and industrial processes. Especially squirrel cage induction motors, which are known as the workhorses of the industry. Therefore, diagnosing the errors of these engines can have many economic benefits. Among other things, it makes the management of industrial plants easier, raises the level of reliability of the system, lowers the cost of maintenance and reduces the cost-to-profit ratio significantly. Bonnett and Souk up have proposed five failure modes for stator failures of three-phase squirrel cage induction motors, which are: ring-to-ring, coil-to-coil, phase disconnection, phase-to-phase, and coil-to-ground[1]. For squirrel cage motors, stator winding failures and bearings are considered as total failures, and also most induction motor stator winding failures result from ring-to-ring insulation breakdown.[2] Some researchers have divided motor failures as follows: failure of balls (bearings) 40-50%, failure of stator insulation 30-40% and failure of rotor rack 5-10% [3] which if ring-to-ring failure is not prevented, it leads to phase-to-ground or phase-to-phase fault, which is more severe phase-to-ground fault. In the articles [4] [5] the theory of winding function and its application in the transient analysis of induction motors under fault are described. This theory has been used in stator ring-to-ring error modeling. In addition to the above methods, the stator error of the induction motor can be studied with the help of spatial vectors [6]. Introduction: [7] The failures of a squirrel cage motor can be divided into two categories, electrical and mechanical. Each of these failures is caused by several factors and stresses. These stresses are generally in the form of thermal, magnetic, dynamic, mechanical or environmental stresses that cause failure in different parts of the machine such as the axis, bearings, stator windings, rotor core sheets and stator and rotor rack. Most of these failures are caused by not using the right machine in the desired working conditions, lack of coordination between the designer and the user, and improper use of the machine. In this part, an attempt has been made to first examine the types of stresses on the machine, the factors that cause them and their effects.

Before examining the types of stresses on the induction machine, the following should be considered:

1- By specifying the working conditions of the machine, it is possible to predict the thermal, mechanical and dynamic stresses and choose the suitable machine for those conditions. For example, the working cycle of the machine and its type of load, the number of times it is turned off and on, and the time interval between them, are factors that will have a direct effect on the emergence of stresses on the machine.

2- The condition of the machine power supply network in terms of voltage drop in permanent mode and starting conditions and the amount of harmonics of the network will be effective in the emergence of the type of stress and as a result of failure in the machine.

1-2- Investigating the types of stresses applied to the induction machine:

1-2-1- Stresses effective in stator failure: [1.7]

A Thermal stresses: These types of stresses can be caused by factors Know the following:

? Start-up cycle: Heat increase in induction motors occurs mostly during start-up and stop. During starting, a motor draws five to eight times the rated current from the mains to run under full load conditions. Therefore, if the number of starts of a motor increases in a short period of time, the temperature of the winding will increase rapidly, while an induction motor has a limit for heating, and if this limit is not considered, the readiness of the motor to cause an error will increase. The stresses that arise due to the sudden stop of the motor are far more effective than other stresses.

? Thermal overload: due to voltage changes and also unbalanced voltages, the temperature of the winding increases.

According to an empirical rule, every 1-3% unbalanced phase voltage, the temperature of the phase winding increases by 25% with its maximum current.

? Thermal wear and tear: according to the law with 10?c increase in the temperature of the stator winding, its insulation life is halved. Therefore, the typical effect of thermal wear is the vulnerability of the insulation system. b. Stresses caused by the poor quality of the work environment: The factors that cause these stresses are as follows:

Moisture

Chemical

Scratch (abrasion) [1]

Small foreign particles

c. Mechanical stresses: The factors that cause these stresses are as follows are:

? Rotor strikes: the impact of the rotor on the stator causes the stator sheets to destroy the insulation of the coil, and if this contact continues, the result is that the coil in the stator groove will ground very soon, and this is due to the excessive heat produced at the contact point. ). This force has its maximum value during start-up and causes the coils to vibrate with twice the frequency of the grid and move them in both radial and tangential directions.

1-2-2- Stresses effective in rotor failure:

A. Thermal stresses: The factors that cause this type of stress in the rotor are as follows:

? Non-uniform distribution of heat: This problem often happens when starting the engine, but lack of The uniformity of the rotor material due to the manufacturing process may also cause this. Continuous start-ups and skin effect increase the possibility of thermal stresses in the rotor bars.

? Sparking of the rotor: In manufactured rotors, many factors cause sparking in the rotor, some of which do not cause problems for the rotor (non-destructive sparking) and others cause errors (destructive sparking). Non-destructive sparks occur during normal operation [2] of the engine and mostly during startup.

? Hot spots and excessive losses: Several factors may cause more losses and create hot spots. Contamination of the rotor sheets or the presence of stains on them, unusual connection of the rotor rods to its body, variable distance between the rods and the rotor plate, etc. can occur during the engine manufacturing stage. Of course, motor manufacturers use special tests such as ultrasonics to reduce these effects.

b Magnetic stresses: Various factors cause these stresses on the rotor, such as the asymmetry of the air gap and the sharpness of the grooves, which these factors and their effects are examined below:

? Electromagnetic noises: the asymmetry of the air gap, in addition to creating an asymmetric magnetic field, creates a mixture of harmonics. in the current of the stator and consequently in the current of the rotor. The mutual effects of current harmonics cause noise or vibration in the motor. These forces often arise from the inhomogeneity of the air gap

? Unbalanced magnetic pull: Unbalanced magnetic pull causes the rotor shaft to bend and collide with the stator winding. In practice, the rotors are not completely in the center of the air gap.