PM and PdM for electric motors

Build the right balance of predictive and preventive tactics to extend long-term operating service life.

By Tom Bishop, P.E., EASA

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We often hear the terms preventive maintenance (PM) and predictive maintenance (PdM) of electric motors, but far less often do we give consideration to the tasks associated with these methods of maintaining motor operation and extending operating service life. This article addresses some of the more common activities associated with PM and PdM, with the focus on three-phase squirrel cage motors.

Preventive maintenance activities

Preventive maintenance primarily consists of face value “snap-shot” assessments of motor health taken at planned and fixed intervals, and then making judgments on machinery condition usually based only on a single set of readings and observations. That is, a test or inspection is performed and evaluated based solely on its outcome.

If the assessment is obvious or easily compared to acceptance parameters, the action to be taken, if any, is clear. For example, an insulation resistance (IR) test is regularly performed on a winding using a megohmmeter, and the winding condition is assessed based on the megohm value obtained in the test; a stator winding insulation resistance value of zero megohms indicates a probable need to rewind the motor.

Electrical tests commonly used in a preventive maintenance program include insulation resistance, polarization index (PI) and measurement of the current and voltages of each phase. Less common electrical tests include measuring motor electrical circuit parameters (i.e., resistance, inductance, and capacitance), and motor current signature analysis (MCSA).

The insulation resistance (IR) test (see Figure 1) is an offline test that is typically performed using a megohmmeter. The test is applied between winding and the frame (ground) for 1 minute at a voltage level in accordance with ANSI/EASAAR100. If the phases of the winding can be isolated, such as with a wye-delta connection, each phase should be tested separately. AR100, and the source standard for insulation resistance testing (IEEE 43) define minimum acceptable IR values.

The polarization index (PI) test is an extension of the offline IR test. The IR test voltage remains applied for 10 minutes, and the ratio of the 10 minute to 1 minute insulation resistance is obtained. This ratio is termed the polarization index, and is a useful test for form coil AC windings. For random windings, which have a different insulation resistance versus time characteristic, the PI result is rarely, if ever, meaningful; thus the IR test value should be used to assess random winding ground insulation. An advantage of the PI versus IR test is that the PI test assessment is not affected by winding temperature as long as the winding temperature does not change during the 10 minute test period.

The less common motor circuit electrical parameter tests require more sophisticated test equipment than the megohmmeter. A digital low-resistance ohmmeter (DLRO) is typically needed to measure winding resistance. Likewise, specialized test equipment is needed for inductance and capacitance measurements. Resistance tests of pairs of three phase winding leads can determine high-resistance connections, and possibly winding misconnections. The measurements of inductance and capacitance between pairs of winding leads can be compared to confirm balance.

Motor current signature analysis (MCSA) detects rotor currents induced into the stator windings. A current transformer on the motor line provides a current signal that is displayed by a spectrum analyzer. Rotor cage faults usually produce sidebands associated with the pole pass frequency. Because slip is required for this test to be useful, it is generally accepted that the motor should be operating at greater than half of rated load. Cracks in rotor bars can create sidebands of frequencies that are multiples of the operating speed. Open or loose rotor bars have been known to cause line frequency sidebands around rotor bar pass frequency, and multiples of rotor bar pass frequency. The rotor bar pass frequency is equal to the number of rotor bars times the operating speed.

Mechanical PM activities include lubrication, lubricant level checks, and lubricant analysis. Grease lubricated bearings require periodic relubrication via grease fittings, making certain that the relief plug is left open long enough for older grease to be purged from the lubricant chamber.

Visual inspection is another important mechanical activity. The motor should be inspected for missing, broken, or damaged parts; indication of lubricant leakage (see Figure 2), and for evidence of contaminants and blockages of the airflow paths. During PM or PdM services minor adjustments can be made, such as to belt tension or loose bolts; and dirt or contaminant accumulation removed from accessible locations. Thermal scanning using an infrared thermometer or camera can be used to check for over-temperature, or increasing temperature conditions. Similarly, ultrasonic testing can be used to detect bearing condition and adequacy of the lubricant fill.

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