Bar to Bar Test
A Bar-to-Bar test gives an indication of the comparative value of resistance that exists between electrical circuits in an armature. This is done by testing the resistance between the commutator bars of the armature.
Held together by insulated rings a commutator consists of insulated segments assembled in a tight cylinder. Carbon and Graphite brushes transfer electric current to the armature which can create carbon dust, a conductive contaminant that can penetrate into crevices, cracks and inlet of the armature. Due to constant rubbing copper from the commutator adds to the contamination process especially if wrong brushes are used or improperly installed. A lack of pre-emptive maintenance can also lead to accumulation of contamination and possible failure. Windings can be shorted if the contaminants enter the cracked insulation between the commutator bars.
Using standard INTERSEL Bar-to-Bar testing procedure, the MCE (Motor Circuit Evaluation) Bar-to-Bar test machine provides accurate data that is stored digitally on a computer and graphed for analysis.
Some key information required for a good test is knowing whether the armature is lap wound or wave wound, the number of poles in the motor and the number of commutator bars and whether the armature is wound with equalizers.
To check if the armature windings are shorted, the first test is the “180° Resistance” test. Measure the resistance with the ohmmeter (Ohms) and then measure the resistance from two commutator bars 180° across from each other.
The next test is the “Bar to Bar Resistance” test. In this test, we check each coil in the motor armature. The value we get is based on the design of the motor, which includes the wires per loop, the number of turns per coil, and the wire gauge. Both tests, the 180° Resistance test, and the Bar to Bar resistance test, the needed result is that each measurement should be about the same.
The last armature windings test is measuring the resistance of every commutator bar to the iron armature stack. The armature shaft can be used for the measurement if the motor is directly pressed onto the armature shaft.