# Methods and Means of Eddy Current Testing of Soldered Lap Joints of Electrical Machines
### Methods of Non-Destructive Testing of Soldered Joints  
Let us consider the applicability of known methods of non-destructive testing according to [5]. Visual inspection and penetrant inspection are currently used for assessing the quality of soldered lap joints; however, they are only applicable for finding surface defects and leave internal discontinuities undetected. Generally, the detection of unsoldered joints is currently performed using electrical, thermal, or thermography inspection. Since soldering defects in joints result in increasing the resistance of conductors, when an operating current is applied to the stator winding, local overheating develops in an unsoldered joint. The temperatures of unsoldered joints are generally 5 \({}^{\circ}\)C higher compared to non-defect joints, which can be detected using temperature-indicating paint. These procedures require the thorough preparation of TO surfaces and strict requirements for safety protection measures when operating with high currents. Furthermore, thermal inspection provides insufficient accuracy in testing due to the high thermal conductivity of copper [6; 7]. Magnetic particle and radiowave inspections are not applicable due to the nature of the TO material. Radiographic testing is technically applicable; however, it is impossible to place large-sized equipment directly around the TO. Ultrasonic testing methods have a high sensitivity and resolution but require the direct contact of measuring instruments with the TO and careful preparation of the TO's surface. Moreover, ultrasonic waves over 1 MHz significantly scatter when passing through the TO material, but lower frequencies have lower resolution; therefore, these methods are used limitedly. The eddy current methods [8] provide sufficient sensitivity when inspecting electrically conductive materials and do not require thorough preparation of the TO surface. Defects of products can also be detected under the insulation layer of connecting busbars. Their disadvantages are the strong dependence of eddy current device measurements on the electrical conductivity of the TO material and the requirement for adjustment and calibration samples to simulate 100% soldering of the TO and variations in TO defects for testing signal interpretation.  
Figure 1: Solder joint of copper busbars: (**a**) photograph of TO in stator winding; (**b**) drawing of TO [4].  
In a defect-free joint, the generator operating currents flow unobstructed through the entire volume of the joint, evenly distributing the current density over the entire cross-section of the lap joint [9; 10]. In the presence of defects, an area of disconnection in the current-carrying connection is formed; thus, the currents are redistributed and encircle the soldering defect, increasing the current density in the reduced cross-section of the conductor. This physical process is similar to the process of eddy current flow in defect-free and defective objects; therefore, the quality testing of current-carrying busbars in turbogenerators should be performed using eddy current methods of non-destructive testing.  
It is necessary to take into account an essential feature of soldered lap joints: the expected defects of soldered lap joints are located in the horizontal plane parallel to the plane of installation for the measuring probe on the tested joint.