Squirrel cage rotors
For AC asynchronous motors, the rotor is characterized by the typical squirrel cage shape, with bars obtained directly by die-casting in the sheet metal stack.
The quality control of this type of rotors is carried out with manual or fully automatic testing stations (for in-line integration), whose purpose is to identify defects inside the die-cast bars of the cage such as interruptions in the material, porosities, defects in the connection to the end ring, geometric and shape errors.
The rotor under test is put in rotation. A special probe generates a magnetic field and acquires the signal generated by each bar when it is crossing the magnetic field.
The test can be performed immediately after the die-casting operation before assembly to the rotor shaft, in order to identify the coarsest defects. The repetition of the test after assembly to the rotor shaft and turning operation allows to detect the smallest defects inside the bars.
Permanent magnet rotors
In case of permanent magnet rotors for brushless motors, manual benches or automatic stations are proposed for performing Back-EMF tests.
The rotor under test is loaded between centers and brought into automatic rotation inside a sample stator, which is part of the testing station. The analysis of the voltage signals detected at the terminals of the stator windings allows not only to make evaluations on the geometry of the rotor and its magnetic field, but also to perform functional analyzes, in order to predict its behavior once mounted into the e-motor.
Should it be required to perform only measurements on the intensity and homogeneity of the generated magnetic field, stations are proposed for the dynamic measurement of the rotor, using suitable magnetic field sensors (e.g. Hall sensors) to inspect the part during rotation.
For wound rotors, whether they are intended for externally excited synchronous motors or for DC motors, it is usually required to carry out insulation tests on the windings.
In association with traditional insulation test methodologies, Insulation Resistance, Dielectric Strength (Hi-Pot), Surge, the Partial Discharge Test technique (PDIV) proposed by E.D.C. ensures the possibility of identifying even latent insulation defects, which could compromise the quality and reliability of the component over time.
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