# Methods and Means of Eddy Current Testing of Soldered Lap Joints of Electrical Machines
### Probe Design  
Let us consider the applicability of various types of probes for eddy current testing. Absolute surface eddy current probes (ECPs) are easy to operate and used for one-sided access to the TO [11; 12]. When testing the soldering integrity of lap joints, low-frequency surface ECPs with a coil diameter equal to the thickness of the joint are used. Testing is performed by placing the ECP on the side surfaces of the lap joint, which ensures the sensitivity of the ECP to defects in lap joints. However, due to the limitation of the coil size and the high electrical conductivity of the TO material, defects are detected at a relatively shallow depth; therefore, a large untested zone remains in the center of the TO. When installing a surface ECP horizontally on a soldered lap joint parallel to the solder plane, the coils are positioned perpendicularly to the plane of defects. The respective paths of the induced eddy currents do not cross this plane; thus, surface ECPs are only limitedly applicable for the testing of soldered lap joints. Screening ECPs differ from surface ECPs due to coaxial positioning of excitation and measuring coils on different sides of TO. Screening ECPs applying the amplitude method demonstrate the ability to test the soldering quality of turbine generator stator cores. However, the reliability is low due to the requirement for accurate ECP positioning relative to the TO [13]. The transition from the amplitude method to the amplitude-phase method with the use of a screening ECP allowed the accuracy of measurements to be increased [14; 15; 16]. On this basis, a method was developed to assess the quality of soldered joints in electrical machine rods using calibration on two samples of tested joints with soldering integrity values of 0% and 100% [17]. To increase productivity and simplify the testing procedure, an ECP with reference coils was proposed, which implements the phase method while comparing the signals from the reference and measuring coils [18]. Thus, screening ECPs are used for two-sided access to the TO for defect detection at significantly greater depths in comparison with surface ECPs while providing an equal probability of detecting soldering defects throughout the entire volume of the TO. However, a significant disadvantage of screening ECPs is also their sensitivity only to defects in the plane perpendicular to the plane of ECP installation, which makes them inapplicable for the inspection of soldered lap joints. The eddy current probe DPS-6 (IMP UB RAS, Ekaterinburg, Russia) with a U-shaped probe is designed to detect soldering defects in winding bars of electrical machines [19]. This probe consists of a parametric coil, operating as both excitation and measuring coils, wound on a U-shaped core and composed of transformer steel plates. The developed device and methodology allow for the detection of soldering discontinuities in current-carrying rods. On this basis, a method of testing current-carrying bars in high-power superconducting magnets was later developed [20], including calibration on two soldering samples. The method made it possible to increase the reliability of soldering defect detection by reducing the influence of changes in external cross-sections on the testing results. A further increase in the reliability of testing was achieved by adding cutouts into the poles of the U-shaped core to create closed loops of eddy currents [21], which additionally made it possible to test the side walls of the TO. To solve the problem of the growing error in estimating the soldering quality of superconducting current-carrying electromagnet joints when the cross-sectional geometry of the tested joints changes, the authors developed a method of step-by-step eddy current probe testing using a U-shaped probe at two frequencies and three samples of the tested joint [22; 23]. In addition, the authors demonstrated the possibility of using the U-shaped core probe for the soldering testing of small-sized non-ferromagnetic joints for further development in this field [24].