Expert: Marc Ebelini - 2/2/2005

Question
Hi.
We repair (rewind) armatures and need to confirm that the replacement winding is exactly the same configuration as the original (sample or drawing). Is there a test method for doing this without doing performance testing? Can I energize 2 commutator bars and measure the field strength at a certain point?
Thanks.

Answer
Since this question was asked in the aviation section of, this web site I must assume the repaired part is for use in an aircraft. If so, all work must be accomplished in accordance with the specifications from the original parts manufacturer. You must also be a certified repair shop with certificated repair people. That said, I would say the manufacturer probably used an inductance reactance meter to determine the correct number of turns. This will also give assurance that the hysteresis of the magnetic field meets design criteria. Are you planning to use a gauss meter to measure the magnetic field? How will you determine where to measure the lines of flux and allowable strength? Obviously, if you do a performance test you will know immediately if what you are doing satisfies the specifications. You will still need to do some spot performance tests whenever you change material suppliers. If you are just supplying armatures, you can do that successfully once you have established quality control procedures. Years ago growlers were used to test armatures that gave a good, no good test, but it never meant the armature was working up to specifications. After passing the growler test many times the generator failed to deliver the rated output when tested for performance. So just measuring the magnetic field can be flawed. Since you never mentioned what the armatures were for I assumed you were writing about starters and generators? In generators, noise must be also taken into account. Noisy generators will jam electronics in aircraft. In alterntors hand selecting the diodes must also be done to reduce noise in addition to proper wiring. A working generator or alternator that is noisy can drive an avionics technician crazy.