# Methods and Means of Eddy Current Testing of Soldered Lap Joints of Electrical Machines
## 2 Materials and Methods
### Development of Means of Metrological Assurance for Measuring the Soldering Integrity  
The design documentation for copper busbars states the use of copper grade M1 (Cu-ETP) [43] and does not specify the use and testing of electrical grade copper, for which electrical conductivity is standardized. Taking into account the quality of incoming products, an inadmissible variation in the electrical conductivity of copper busbars in a single electrical machine is expected. At the same time, as previously stated, electrical conductivity (relative or specific) is a significant stray parameter when testing is procured using the developed ECP. Therefore, the testing procedure requires the adjustment of probe measurements using a correction factor based on the value of the electrical conductivity of the TO. In order to compensate for the variation in electrical conductivity, a methodology for suppressing the influence of electrical conductivity on the measurement results was developed. Electrical conductivity measurement in the process of measuring the soldering integrity was performed using a conductivity meter, \"Konstanta K6\", with a PF-IE-6e-Cu probe (LLC \"Konstanta\", Saint-Petersburg, Russia), providing a measurement range of 25 to 59 MS/m and a standard permissible error limit of 3%. In order to verify performance and calibrate the conductivity meter, a set of SO-220 electrical conductivity calibration blocks, (LLC \"Konstanta\", Saint-Petersburg, Russia) were used. A set includes 3 cylindrical copper group samples with high long-term characteristic stability, with nominal electrical conductivity values of 42, 50, and 58 MS/m. Metrological assurance in measuring the soldering integrity of soldered lap joints of copper busbars in electrical machines is represented by sets of reference blocks (RBs) (Figure 11). One set includes a monolithic block to simulate 100% soldering integrity and a composite block to simulate 0% soldering integrity, composed of two parts, glued together through a dielectric gasket. RBs are manufactured specifically for each type of busbar for the tested electrical machine from electrical copper, the conductivity of which was tested. The developed RBs were used to study the correlation between the instrument readings and the different conductivity values of the upper and lower copper busbars in a solder joint with 0% and 100% soldering integrity values. In addition, the instrument readings were obtained from copper busbars, whose plates had different conductivity values (Figure 12). Figure 11: Reference blocks of soldered lap joints: 1—RB of 100% soldering integrity; 2—RB of 0% soldering integrity. Based on these correlations, a formula was derived for correcting ECP measurement results, taking into account the deviation in the conductivity of solder joints:  
\[S=(k-\sigma_{\text{TO}})+\sqrt{\sqrt{\frac{\sigma_{0}+\sigma_{100}}{\sigma_{ \text{TO}}}}*\frac{\sigma_{TO}}{k}}+Rez, \tag{1}\]  
\[k=\frac{\sigma_{100}*Rez+\sigma_{0}*(100-Rez)}{100} \tag{2}\]  
where  
\(S\)--TO soldering integrity, %;  
\(\sigma_{0}\)--electrical conductivity of the 0% soldering reference block, MS/m;  
\(\sigma_{100}\)--electrical conductivity of the 100% soldering reference block, MS/m;  
\(\sigma_{\text{TO}}\)--arithmetic mean value of conductivity of plates of the solder joint, MS/m;  
\(Rez\)--instrument reading, %. Thus, the main principles behind the methodology for suppressing the influence of electrical conductivity on the results from measuring the soldering integrity of soldered lap joints of copper busbars in the stator windings of electrical machines are as follows:  
1. Testing conditions: Testing is carried out at least 3 h after soldering the copper busbars of the turbine generator. The RBs should be kept together with the TO for at least 1 h.
2. Before soldering testing, the conductivity meter is calibrated with conductivity calibration blocks. 3. The conductivity values of the 0% and 100% RBs are measured. 4.