Evaluating Direct-Current Motors Using Motor Circuit Analysis Basics

Electrical Reliability, Insulation Systems, Motor Diagnostics, Motor Management, Reliability, Uncategorized

Howard W Penrose, Ph.D., CMRP

MotorDoc LLC, www.motordoc.com

The term MCA (Motor Circuit Analysis) is derived from a test method that provides information on the basic components of an alternate-current (AC) or direct-current (DC) electric motor. These basic components include: resistance, measured in Ohms; impedance, measured in Ohms; inductance, measured in Henries; the induction winding phase angle, measured in degrees; and, insulation resistance, measured in Meg-Ohms, capacitance, and dissipation factor. The instrument that will be referred to in this article provides these readings by generating a low voltage, true sine-wave, alternating current (impedance, inductance, phase angle), signal at frequencies from 100 to 800 hertz, a low-voltage DC signal for resistance, and 500 or 1,000 volts DC for the insulation resistance test. In addition, a special test called I/F is performed in which the applied frequency is doubled and a ratio results from the change in the winding impedance. This test is introduced to identify early winding shorts that may exist in the winding. Using the applied data, the condition of a DC motor winding can be evaluated through coil comparisons, comparisons to known readings, or by trending changes to the windings over a period of time.

COMMON DC MOTOR FAULTS IDENTIFIED

Here are four of the most common DC motor electrical faults that result from issues specific to DC motor design as a result of temperature, friction and internal contaminants such as carbon or graphite.

  • WINDING CONTAMINATION: One of the most common causes of winding faults in a DC motor is from winding contamination from carbon or graphite (carbon) dust from the brushes. The fine powder permeates all of the stationary and rotating windings and will create a path between conductors or between conductors to ground. Carbon is often trapped and problems aggravated further through cleaning and maintenance practices when the carbon is blown with compressed air or the armature is cleaned and baked. In either case, the carbon may become tightly packed in corners, usually just behind the commutator. This will end as a ground fault or shorted turns right at the commutator connection.
  • COOLING-RELATED FAULTS: Another common fault, that is often not considered, is cooling of the DC machine. This may occur because cooling passages are blocked, the armature is turned too slow with no additional cooling, or from dirty filters (the most common cooling-related fault). Temperature is the greatest enemy of electrical equipment—particularly the insulation system—of which the life will be reduced by half for every 10 degrees centigrade increase in temperature (accepted rule of thumb). As the insulation weakens, its reliability decreases until winding faults between turns occur. In addition to the insulation system degrading, brushes also degrade faster, causing increased wear on the commutator and additional carbon contamination of the windings.
  • FAULTS WHEN FIELDS ARE ENERGIZED: Another fault that is related to heat is generated from practices that have the fields energized with the armature at rest (de-energized). This is a common mode of operation that requires a separate blower to provide cooling to the motor that normally has filters that must be kept clean. This type of fault normally results in shorted shunt coils, reducing the motor’s ability to produce torque and may end with the dangerous condition of armature overspeed if not maintained properly.
  • COMMUTATOR FAULTS: The commutator also provides opportunities for faults, as well as an indicator of motor operation and condition. A properly operating DC motor will have a fine glaze of carbon on the commutator with the bars looking uniform. Burned commutator bars, streaked glazing, heavy carbon, or overheated commutator conditions indicate potential problems that should be addressed.

FIXING THE FAULTS

General electrical testing of direct-current electric motors is made much easier with new techniques available with static motor circuit analysis. For the first time, early turn faults can be detected in series, shunt and armature windings before they take equipment out of operation. Predictive maintenance tests can be performed from the drive with troubleshooting tests being performed at the motor. In general, the following tests are relatively quick, requiring less than five minutes per motor for predictive maintenance testing, with additional time required for troubleshooting. Overall, MCA testing dramatically improves DC motor testing over the traditional methods of continuity tests.

  • ARMATURE TESTING: DC armatures are the most time consuming but easiest component to test. There are three basic methods that will be introduced: trending; assembled; and, disassembled. In the case of trending, all measurements are used, however, in the case of assembled and disassembled testing a bar-to-bar impedance measurement will be used. Impedance is viewed because the armature is an AC component and simple resistance measurements may miss some faults including shorts and grounds.
    • When testing an assembled DC motor armature, the best method is to perform what’s commonly known as a bar-to-bar test using the motor brushes. In the case of a DC motor that has two brushes, none of the brushes needs to be raised, in the case of a DC motor that has four or more sets of brushes, all but two sets 90 degrees from each other need to be raised, which takes them out of the testing circuit. Make sure that good contact is maintained on the commutator by ensuring that 90%+ of the brush is in contact with the commutator bars and that the commutator bars are clean. If they are not clean, polish the armature gently, using an approved method, before testing. If the commutator is badly worn, it will need to be disassembled and the commutator “turned and undercut,” in which case a disassembled bar-to-bar test would be appropriate. Once set, mark the position of one bar on the commutator, then bring the bar to a position where it is just under the leading edge of one of the brushes. In the assembled test, you will probably be covering at least one and a half bars with the brush. Perform an impedance test, mark down the reading, and move the armature so that the leading edge of the brush is over the next commutator bar. Take the next impedance reading and continue until each bar has been tested. A good result will show a consistent pattern, while an inconsistent pattern will identify a poor armature.
    • Disassembled bar-to-bar testing is similar to assembled testing, other than the armature is out of the frame and the tester has full access to the commutator. In this case, the tester will use an armature fixture or test leads to connect from bar to bar. The spacing between each impedance reading should be constant and about 90 to 180 degrees from each other. The first bar should be marked and testing should continue until one leg of the testing fixture or test lead has made it 360 degrees around the commutator. Mark the impedance for each bar-to-bar test then look to ensure that there was a consistent pattern.
  • SERIES MOTOR TESTING: Series electric motors are very challenging to troubleshoot as they do not provide sets of fields to compare to. Readings may be taken from S1 to S2 and A1 to A2 then trended over time or compared to other similar machines.
    • When trending the readings over time, simple resistance readings must be corrected for temperature, usually relative to 25 degrees Celsius. Impedance and Inductance normally has limited change due to temperature while the phase angle and I/F readings will remain constant, regardless of temperature. Variations in the I/F and phase angle will indicate shorted turns, while changes in Impedance and Inductance will normally indicate dirty windings.
    • Comparing like motors will require additional information. The operator will have to ensure that the motor is of the same manufacturer and design, as well as speed, power, among others. The “model” motor must be new or rebuilt to original manufacturer’s specifications. When performing comparative readings, the testing temperature should be similar from motor to motor, however, the I/F and phase angle readings can be directly compared. These readings should not change more than +/- 2 points for I/F and +/- 1 degree for phase angle.
    • A common error when series field windings are rebuilt, although less common that shunt coils, is an incorrect replacement of wire size, which will impact the ability of the motor to generate torque.
  • SHUNT MOTOR TESTING: Dual-voltage shunt motors provide the ability to compare two sets of windings while single-voltage motors will have the same test procedure as testing series motor windings, using F1 to F2 as opposed to S1 to S2. With dual voltage, the shunt windings are labeled F1 to F2 and F3 to F4 allowing the analyst to test and compare these two sets of windings.
    • When testing and troubleshooting the readings over time, simple resistance readings must be corrected for temperature, usually relative to 25oC. Impedance and Inductance will change more than a series wound motor because of the higher simple resistance of the circuit. The phase angle and I/F will remain constant, within 1 to 2 points, regardless of temperature. Variations in the I/F and phase angle will indicate shorted turns, while changes in Impedance and Inductance will normally indicate dirty windings. Comparisons between F1 to F2 and F3 to F4 should be less than 3% in resistance, inductance and impedance and no more than 1 point different in I/F or phase angle.
    • Similar motors can be tested and compared the same as series wound motors. When possible, the motors should be tested when trending readings are at the same temperature as the previous tests. For instance, motors should be tested after they had a chance for the temperature to stabilize or prior to starting.
  • COMPOUND MOTOR TESTING: In place testing, trending and troubleshooting is much simpler with a compound motor. Single-voltage compound motors are normally labeled A1 to A2, S1 to S2 and F1 to F2, dual-voltage compound motors are normally labeled A1 to A2, S1 to S2, F1 to F2 and F3 to F4. A key additional point to a compound wound motor is that the series winding is normally wound on top of the shunt winding, allowing for possible faults between these two windings.
    • Trending a compound motor, the tests are normally taken from the DC drive terminals. Standard MCA tests using the ALL-TEST involve low-voltage, higher-frequency signals that will not harm the output electronics of the equipment, reducing the need to disconnect the leads from the drive while testing. However, if the analyst wishes to check insulation resistance between the series and shunt windings, the leads must be disconnected from the drive. When trending from the DC drive, test A1 to S2 and the two field leads then perform a 500 Volt insulation resistance test between the S2 and F1 leads and compare to previous tests or similar motors, in either case, insulation resistance readings should remain above 100 Meg-Ohms. The ALL-TEST unit allows the analyzer to immediately compare the past to present readings as a quick check allowing the analyzer to make a quick decision to test the windings further. As mentioned in series and shunt motor testing techniques, the I/F and phase angle readings should not change more than 1 point between tests, over time, the series and field windings will vary dramatically from each other, however.
    • Troubleshooting compound motors should be performed at the motor, itself. Disconnect all motor leads and separate them. Test the series and field windings as outlined in the series and shunt winding instructions, then perform an insulation resistance test between the series and shunt windings, the insulation resistance should be greater than 100 Meg-Ohms.

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