7120/E Electrical Test for Wound Armatures

The ROTORSCOPE can detect the following faults:

  • detection and location of short-circuits within the coils or among several coils or in the commutator;
  • detection of interruption of the coil or break in the coil-commutator connection; 
  • inversion in the connection or one or more coils;
  • insulation between commutator and ground;
  • errors in the relative position between commutator and slots; 
  • non-uniformity between laminations of the commutator;
  • non-uniformity between the number of turns in the various coils.

 The test set can also locate eccentricity of the yoke with respect to the motor shaft and, in unwound rotors, short-circuits between the lamination of the yoke caused by burrs due to poor shearing. The test set can therefore identify the presence of faults in the winding and permits the easy and exact location of the fault.

The magnetic test can identify short-circuits. The electrical test can identify inversion and interruption connections.

The test set operates based on the electrical and magnetic effects caused by the circulation of induced currents into the winding of a rotor, which is made to turn at a constant speed, within a magnetic field produced bay permanent magnet. The result of the tests, i.e. the e.m.f. induced into a fixed probe (magnetic probe) and the voltages derived across two adjacent seg¬ments of the commutator through a moving probe (electrical probe) are transferred to an oscilloscope for easy interpretation.

The relationship between the turning rotor under teat and the oscilloscope display is ensured by a synchronization system obtained by using a photo-electrical element along the horizontal trace of the oscilloscope. The development of the trace corresponds to little more than a complete turn, in order to allow for accurate observations. The electrical effect taken from the various segments of the commutator or the e.m.f. induced by the currents circulating into the coils are successively displayed.

The inducing magnetic field is produced by a permanent magnet, which acts on one slot at a time. The direction of the magnetic field is such that the turns of a moving coil cut the lines of force normally. The induced currents, which are proportional to the field intensity (distance of the magnet from the rotor), on the speed of rotation (constant) and on the rotor characteristic (winding resistance and number or turns), produce magnetic fields, the symmetry of which enable a clear observation or the turns in the short-circuited condition.

A fixed probe, subjected to the magnetic fields produced by the armature under test, provides the signals, which after being amplified, are applied to the oscilloscope tube. The induced e.m.f. cause voltages to appear between the commutator laminations and the amplitude, shape and recurrence or these voltages permit the location or faults in the rotor winding. The correctness or the winding can be observed by taking (into a low ohmic load) the voltage between two successive segments through a moving probe fitted with two brushed wiping the commutator.