# Methods and Means of Eddy Current Testing of Soldered Lap Joints of Electrical Machines
## 2 Materials and Methods
### Finite Element Model of ECP and TO Interaction
Measuring coils perceive the magnetic fields of excitation coils and eddy currents in the TO. When operating an ECP without active shielding, the magnetic field induction lines of the excitation coils penetrate the adjacent joints (Figure 6a). Once active shielding coils are installed, the direction of the magnetic field induction lines penetrating the adjacent joints is reversed (Figure 6b). This indicates the effect of the displacing field of shielding coils. Due to this, a counter magnetic field is created on the outside of the ECP element, distorting the excitation magnetic field, which provides a reduction in the influence of adjacent joints on the testing results. Serially connected excitation coils form a combined excitation magnetic field to induce eddy currents in the TO. In the gap between excitation coils and measuring coils, magnetic flux leakage is produced, which forms the tangential component of the eddy currents in the TO. This makes it possible to weaken the unchanging component of the magnetic field of excitation coils, increasing the relative amplitude of the signals produced by the eddy currents' magnetic field as a result. When excitation coils are located both below and above the TO and connected in-phase, a combined eddy current loop is formed, which crosses the plane of defect occurrence almost perpendicularly and homogeneously over its entire zone. The design and optimization of the FE model of ECP and TO interaction, as well as the process conducted to ensure precision in the calculations, are discussed in detail in a previous study [4]. As a result, a precise model was obtained that confirms the hypothesis of eddy currents passing perpendicular to the defect occurrence plane in a soldered lap joint when tested with a two-element tangential ECP (Figure 8). The signal of the ECP is the voltage \(U\) on the measuring coils of the ECP, and the test parameter is the amplitude of the voltage \(U\). Based on this model, the influence of the following parameters on the ECP signal was studied:  
1. Test parameters: 1. The size of the defect located along the side wall of the TO (Figure 3); 2. The size of the defect located along the end of the TO (Figure 3). Figure 8: Pattern of eddy current lines in the finite element model of the TO: 1—TO; 2—plane of soldered lap joint 3—excitation coils; 4—shielding coils; 5—eddy current lines. Figure 7: Model of ECP, TO, and adjacent solder joints: 1—TO; 2—upper element of ECP; 3—lower element of ECP; 4—upper solder joint; 5—lower solder joint. The value of the ECP signal deviation caused by the stray parameters is comparable to the value of the ECP signal range from the test parameter. The following means have been implemented to suppress the influence of stray parameters during the testing of soldered lap joints:  
1. The suppression of the influence of the TO material's electrical conductivity is provided by adjusting the measurement results based on the electrical conductivity of the upper and lower plates of the TO. The details of this procedure are given in the next section. 2. A reduction in the influence of the relative orientation of the ECP and TO is provided by probe design features based on calculations of the geometry and required stiffness for fixing the probe position relative to the TO [4]. The developed ECP (Figure 10a) consists of a fixed and a movable plank, inside of which coils for the upper and lower ECP elements are installed. The distance between the movable and fixed planks is secured by a clamp and an anti-backlash junction. The allowable change in distance between the planks is sufficient to compensate for the entire permitted range of thickness deviation for the lap joints. On the fixed plank, a block stop is installed, the position of which is secured by a clamp. This block stop ensures the consistent positioning of the ECP relative to the reference blocks during setup (Figure 10b) and relative to the TO's end during measurements [4]. Figure 10: Photographs of the developed ECP: (**a**)—ECP design; (**b**)—ECP installation on RB [4].