Abstract: An electric coil to be tested for interturn and/or interlayer faults is coupled into a series resonant circuit. A measurement of the quality-factor (Q-factor) is carried out by measuring the overshoot voltage in circuit resonance. Through the selection of a suitable capacitance in the series resonant circuit, the resonant frequency can be placed in a range in which the quality-factor of the coil attains its maximum value. The coil to be tested is therefore a component of a series resonant circuit. The measuring system for determining the quality-factor of the coil is low capacitively coupled into the resonant circuit. Because the interturn resistance of the test coil must be measured in any event, since this value is also obtained during Q-factor measurement, the necessary operating and measuring periods can be optimized.

Abstract: A device having a first U-shaped core with an excitation winding and an additional U-shaped core with a measuring winding and a connected display device is configured such that an electric coil as a test piece winding can be wrapped around one of the arms of both U-shaped cores. The U-shaped cores and the yokes which close the cores consist of ferrite and have an air gap formed between the cores and the yokes. A voltage is induced in the winding coil by an excitation winding forming part of an exciting circuit. The stray field generated by a winding turn having an interturn fault, as a result of the short-circuit current flowing therein, induces a voltage in the measuring winding of the measuring curcuit. The measurement is performed at a variable frequency which results in a minimal voltage when the test piece is free from defects.