The paper analyses a kind of technology and products of ideal and normal non-contact electromagnetism tap changer, poses the calculating formula of circulating current. It is easy to control with the project of normal tap changer technology, but in this case the transition resistances and reactors need to be used, and the circulating current will appear when the tap position is changed. It will lead to electricity dropping, so the performance is not good. There is the accurate test technology of current zero and voltage zero in the project of tap changer technology based on intelligent module MCT801A, the high power current limiting resistances or reactors will not be used, the current can be continuous without impact, over-electricity or electricity dropping phenomena when the tap position is changed, the waveform of computer simulation and the waveform measured exactly of voltage and current are provided by the paper when the tap position is changed ideally, It will have positive significance on On-Load Tap Changer (OLTC) study of the Smart Grid.