Resistance and continuity
Commutator bar-to-bar resistance testing passes a small direct current through the armature winding and measures the voltage drop between adjacent commutator bars, all the way around. Each reading reflects one coil and its connections, so an open coil, a shorted turn or a poor riser joint shows up as a reading that differs from its neighbors.
Commutator bar-to-bar resistance testing is a check of the armature winding of a direct-current motor or generator. The armature winding is a closed loop of coils, and each coil is connected to a pair of commutator bars, so the resistance between two adjacent bars reflects the coil or coils joined across them, together with the solder or weld joints that tie the coil ends to the bar risers. A low-resistance ohmmeter or a voltage-drop arrangement with a stable direct current source is connected across successive pairs of bars, and the reading is taken at each position until the whole commutator has been surveyed. The result is a set of very small resistance values that can be compared with each other. The test is used on DC motors and generators in industrial drives, cranes, elevators, steel and paper mills, excavators, traction and other service where commutator machines remain in use. The published acceptance standard lists it as an optional test for rotating machinery with a commutator, and it is also a favorite of repair shops after a rewind or a failure.
The armature of a commutator machine is one of its most heavily stressed parts. Heat, vibration, centrifugal force and arcing at the brushes can crack riser joints, open a coil, or short a few turns together. Such faults are not always visible from outside. A high-resistance joint heats under load and may throw solder, a shorted coil circulates a large local current and overheats, and an open coil causes severe sparking at the brushes and burns the bars that it feeds. Machines with these faults often still run, but with poor performance, uneven commutation, rising brush wear, excess heat and noise, and then fail suddenly in service. Measuring bar-to-bar resistance finds these problems on a shop bench or during a planned outage, before they turn into a burned commutator or a destroyed armature. It is useful on a new or rewound armature to prove the workmanship, on a machine suspected of commutation problems, and as part of routine maintenance for critical DC drives, in addition to insulation and winding checks that are made on the same armature.
Between adjacent bars the winding presents a small resistance made up of the coil wire, the joints at the risers and the bars themselves. When the armature is sound and well made, each coil is nearly identical, so the resistance between any pair of adjacent bars is nearly identical, and a plot of the readings around the commutator is a flat line with only small scatter. Every kind of fault disturbs that pattern in a characteristic way. A poor or cracked riser joint adds resistance and raises the reading at the affected bar. An open coil raises the reading dramatically because current has to find a path the long way around the winding. A shorted turn or a bar-to-bar short lowers the reading, because part of the coil has been bypassed. A different winding type, for example a lap and a wave winding, changes the number of parallel paths and therefore the absolute values, which is why the comparison is among bars of the same machine. The measured value depends on the current used and on the probes making firm contact, and the voltage-drop method keeps the result independent of lead resistance.
The machine is de-energized, isolated, locked out and verified dead under the owner's procedures, and the armature is accessible, either in the machine with the brushes lifted so that they do not short bars together, or on the bench. The brushes are raised and secured because a brush resting on the commutator would create a parallel path and spoil the reading. The commutator surface is cleaned so that the probes touch bare copper, and any film or carbon dust is removed. The test current from the instrument is passed through the armature, either by connecting the current leads to two points on the commutator, or by a pair of probes that sit on adjacent bars while the same pair senses the voltage, depending on the instrument. A reading is taken between each pair of adjacent bars in sequence, and the position is marked so that no bar is missed. The shaft is rotated by hand or on rollers to bring each position within reach, and the same probe spacing and pressure are used throughout. Where the winding has equalizers or parallel paths, the manufacturer's guidance for the machine determines whether adjacent bars or bars at a set spacing are measured. A repeat of any reading that looks odd confirms it before conclusions are drawn. The ambient and winding temperature are noted at the time.
The record shows the reading at each bar pair in sequence around the commutator, the type and size of the machine and its winding, the number of bars, the instrument and its settings, the calibration status of the instrument, the temperature of the armature and of the surroundings, and the date. It also notes the as-found condition of the commutator surface, for example clean, glazed, burned or with raised bars, whether the brushes were lifted, and any positions that were remeasured or showed unusual results. A map of the readings around the commutator makes it easy to see the pattern, and it gives later tests something to be compared with.
This page gives no pass or fail limit, because the meaning of a reading depends on the machine. The judgment is by comparison among the bars of the same armature: the readings should be close to each other all the way around, and a bar pair that stands apart from its neighbors, whether higher or lower, is the one to investigate. A higher reading points toward a poor joint or an open coil, and a lower one toward a short. The size of the allowed scatter between readings is set in the published criteria and by the manufacturer's data, and results are also compared with earlier readings on the same armature or with the readings of a sister armature of the same design. The measurement is named as an optional test for DC rotating machinery in ANSI/NETA ATS-2025; consult your own licensed copy for how it is defined and judged, and use the machine manufacturer's data for the values that apply to a given winding.
Acceptance use gives the armature a clean bill of health on delivery or after a rewind, and the pattern recorded then becomes the baseline. A maintenance use is usually prompted by a symptom, such as sparking, uneven brush wear, a hot spot, a burned bar or loss of performance, or by a scheduled overhaul. The winding type changes how the readings should look. Lap windings have many parallel paths and low, uniform readings that expose a single bad coil clearly. Wave windings have fewer parallel paths and the bars to be compared may be separated by a defined pitch. Machines with equalizer connections or special compensating windings need the manufacturer's guidance. Large machines are best done with the armature out of the frame, small ones can be done in place. The test is complementary to insulation resistance, to winding resistance of the field circuits and to a visual and mechanical inspection of the commutator, since each finds a different kind of fault.
The equipment classes and standard sections that call for this test, and the UltraDb forms that carry it.
A reading higher than its neighbors suggests extra resistance in the path, most often a cracked or poorly soldered riser joint, or an open coil. Open coils usually give a dramatically higher value, while a poor joint gives a smaller rise. The reading is repeated and the bar and riser are inspected.
A reading lower than its neighbors points to part of the coil being bypassed, such as a shorted turn or a short between bars caused by carbon, copper dust or damaged insulation. A short circulates heavy local current and overheats, so it deserves attention even when the machine still runs.
A brush touching the commutator forms a parallel path across the bars and changes the reading. Lifting and securing the brushes removes that path, so the measurement reflects only the armature winding and its riser joints.
No. Winding resistance measures a whole circuit, while bar-to-bar resistance looks at each coil position around the commutator and so finds localized faults that would hardly move the overall value. The two tests complement each other, along with insulation resistance.
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