the no-load current drops greatly when the switch-off the no-load
transformer, and the magnetic field in the core disappear quickly, and the overhigh volatge produced in the coil and thus it maybe puncture the weak insulation place. A larger excitation flow will be produced when switch-on the no-load
transformer, and the coil maybe distorted or the insulation maybe damaged for the great mechanical force. And therefore too may times of switch-on or switch-off will affect the
transformer service life.