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Identification
and Testing of DC Armatures (Continued) |
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| Analyzing results of test procedures Indications that the winding is free of open and shorted coils: Similar voltage drop readings between adjacent segments of simplex lap and wave windings Similar voltage drop readings between odd-numbered segments of duplex double reentrant lap and wave windings Similar voltage drop readings between even-numbered segments of duplex double reentrant lap and wave windings Indication that the winding is free of grounded coils: Zero voltage drop reading between commutator bars and armature shaft regardless of winding Indications that the winding contains shorted coils: Wave windings: P/2 zeros or very low readings for each shorted coil Indications that the winding contains open coils: Wave windings: P/2 high or off scale readings preceded and followed by several zero readings for each open coil Indications that the winding has grounded coils: Wave windings: There will be a series of readings that decrease to zero as bars are tested. These readings will increase and change in polarity as additional bars are tested. There will be as many indications of this type as number of main poles. Isolating ground in lap and wave windings Shift the polarity of the potential applied to the commutator bars. Retest for grounded coils as previously described. Record the readings obtained. Compare the first set of readings to the second set. One set of readings between the same bars and shaft will remain the same. This is the actual ground. The other set of readings will have shifted from the original bars to another set of bars. This is the phantom ground. Actions required for armatures failing the bar-to-bar test Examine accessible windings to locate the cause of the open or short. Repair or reinsulate the faulty winding. Examine accessible windings to locate the cause of the grounded winding. Clean, repair, and reinsulate. Deliver the armature to the shop for repair or rewind if unable to accomplish repairs inplace. Ohmmeter method of armature identification The ohmmeter method should be the logical choice between the two methods if a low-reading ohmmeter is available. The ohmmeter method is accomplished in the same manner as the milliammeter method. The important difference between the two methods, besides the simplicity of the ohmmeter method, is that the readings obtained will be exactly opposite. Growler method of testing armatures for shorts, opens, and grounds The growler method can only be used on armatures that have been removed from the motor. This method is not as reliable as the bar-to-bar test but can be used if time and equipment constraints are factors. An advantage of the growler test method is that it eliminates the need for a separate source of test voltage, as in the millivoltmeter method. Types of growlers
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| • External:
Also called a vee growler because of the V shape of the laminations where the
armature is placed during test. External growlers range in size from the bench type for smaller armatures to large roll-around types that can accommodate large armatures. Handheld growlers are available which can be rotated around the armature laminations to test the windings. Some external growlers will have a built-in ac millivoltmeter and test leads.
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Click on image to
enlarge. External Growler |
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| Internal: An
internal growler is a completely portable growler that is normally used to test ac stators
but can also be used to test dc armatures. A separate ac millivoltmeter must be used with
an internal growler during testing procedures. Some internal growlers will have a built-in thin piece of metal that will vibrate when a shorted coil is found. This design feature eliminates the need for a separate hacksaw blade or piece of thin metal.
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Click on image to
enlarge. Internal Growler |
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Preparation for growler testing  |
This lesson first discusses the shop- or bench-type external growler, then
the internal growler. The procedures are basically the same, regardless of the type used.