Tom Bishop, P.E.
EASA Senior Technical Support Specialist

This article addresses electrical testing and inspection of installed 3-phase squirrel cage motors. The main purposes of testing installed electric motors are condition assessment for continued service or to diagnose suspected faults. The emphasis here will be on diagnostic electrical testing and interpretation, as well as physical inspection key points. Note: Most of the tests and inspections described in this article can also be performed on 3-phase wound rotor motors, and induction and synchronous generators.

Testing of motors
Routine tests to perform for condition assessment or diagnostic testing of any type of motor include insulation resistance (IR), polarization index (PI) or dielectric absorption ratio (DAR), and visual inspection. These tests are all performed with the motor offline. Online tests will vary depending on the type of machine, e.g., squirrel cage induction, synchronous, or wound rotor.

The IR test (see Figure 1) is well defined and applicable to all types of windings. The IR reading at 1 minute should be corrected to 40°C to assess the condition of the winding ground insulation. If the winding is random wound (round wire), the PI value may not be meaningful because of the winding absorption charging current decaying within the first minute or so of applied voltage. For that reason, the DAR is more useful, with a common selection of IR readings taken at 30 seconds and 60 seconds per IEC 60034-27-4. The PI ratio is determined over a longer period by dividing the megohm value at 10 minutes by the value at 1 minute. The PI is most useful with stator form coil windings, i.e., coils made with rectangular or square wire. If the IR value is greater than 5000 megohms, per IEEE 43 and IEC 60034-27-4, the PI value would not be meaningful and the PI test need not be performed. 

The scope of the visual inspection will vary with motor enclosure. If there is no access to the motor interior, e.g., removable covers, the inspection will be limited to external surfaces. In that case, items to check include fan covers, cooling fans, terminal box, evidence of cracks in the frame or feet, and broken or missing hardware. The output shaft and coupling or other shaft-mounted components should be inspected for evidence of wear or cracks. If the motor enclosure allows access to the interior, a visual inspection can be made of the windings and other accessible internal components, and the air gap between rotor and stator can be checked. In addition to visual inspection of the interior, a borescope and/or mirrors may be used to probe further into interior components. 

Testing of a 3-phase squirrel cage induction motor
If the motor can be operated, routine tests during running, i.e., online, include measuring current for each of the 3 phases, and line-to-line voltages. Depending on operating conditions, and availability of test equipment, offline tests include lead to lead resistance, hipot, surge test, output shaft runout, alignment of motor to driven equipment, soft-foot check, and lubricating oil sampling for analysis.

The lead to lead resistance test can detect high resistance joints in winding and lead connections, and the surge test can detect intra-winding faults. CSA C392 recommends a limit of 2% and IEC 60034-23 recommends a limit of 3% from the average for random windings, and both standards recommend a limit of 1% from the average for form coil windings. Further, the hipot test should only be performed if the winding has an acceptable IR value, and also if applicable an acceptable PI value. The hipot test should be performed only if the end user is willing to accept the risk of a failure during the test.

The surge test can detect turn-to-turn, coil-to-coil, or phase-to-phase shorts. A common issue when surge testing an assembled motor is “rotor coupling.” That is the magnetic interaction between a squirrel cage rotor and the stator winding. During the surge test this can cause a dual trace to be observed. Turning the rotor a few mechanical degrees will allow the traces to merge if the winding does not have a fault or other defect, e.g., unbalanced winding circuits. The surge test should only be performed if the winding has an acceptable IR value, and also, if applicable, an acceptable PI value.

Mechanical tests include the output shaft run out test, which is performed using a dial indicator. The indicator is positioned at the outer surface of the end of the shaft, if possible, or at the shaft adjacent to the coupling and the shaft is rotated. NEMA standard MG1 (hereafter MG1) allows up to 0.003” (0.08 mm) total indicated runout (TIR) for shafts over 1.625” to 6.500” (41 to 165 mm). A more rigorous yet simpler criteria is to limit runout to no more than 0.001” (0.025 mm) for 2-pole motors, 0.002” (0.051 mm) for 4-pole motors, and 0.003” (0.076 mm) for motors with 6 or more poles.

Strictly speaking, inrush is the asymmetrical DC offset that occurs in the first cycle, or few cycles, after an AC motor is energized (see Figure 2). MG1 states that the inrush current can be 1.8 to 2.8 times the locked rotor current, which is typically 6 to 8 times the full load current. The inrush current may thus be up to 22 (2.8 x 8) times the full load current. The inrush current of a motor that has higher than typical locked rotor current can be high enough to cause circuit breaker tripping. Note: Unless the ammeter used to measure the starting current has the capability to measure the momentary inrush (peak) current, it will only indicate the steady-state locked rotor current.

The line-to-line voltages, per MG1, should be within 10% of the rating on the motor; per IEC 60034-1, it should be within 5% of the rating on the motor (within 10% for a limited duration and frequency of occurrence). Too high a voltage can increase heating of the magnetic core of the motor, while too low a voltage can reduce the torque capability of the motor (see Figure 3). There is no “rule of thumb” to estimate whether overvoltage will increase or decrease motor current, and likewise with undervoltage.

Another factor related to voltage is unbalanced voltage. MG1 states that the motor should be de-rated if voltage unbalance exceeds 1% (see Figure 4). This requirement is often confused with the tolerance for voltage variation. Utilities frequently use a 3% voltage unbalance limit for the power they supply. According to the MG1 standard, the horsepower derating for a 3% unbalance is about 12%. Derating is not often practical, thus end users may be forced to operate motors with unbalanced voltages. The effect on a motor is reduced output torque and higher current. The higher current is especially significant because the current deviates more than voltage. MG1 further states that current unbalance with load can be expected to be 6-10 times the percent voltage unbalance. Applying this rule to the 3% voltage unbalance, the current unbalance could be 18-30%. 

Heating is a function of the power loss in a winding; that is, the current squared times the resistance (I²R). With 3% voltage unbalance, the highest current “leg” of the winding may have about 18% more heating due to the associated current unbalance. The additional heating is estimated by taking it as twice the unbalance voltage squared, i.e., 2 x 32 = 18% in this case. A thermal image scan of the windings, if accessible, can be used to check the actual temperatures resulting from an unbalanced voltage and current condition.

Infrared thermographic scanning of the motor exterior can reveal abnormal heating of parts of the motor (see Figure 5). There are no specific temperature standards for the outer surface of electric motors, i.e., the “skin”. Comparing the surface temperatures of identical rated motors with the same or similar load conditions may reveal abnormal heating.

If the interior of the motor is accessible, visual inspection for defects or damage, such as to winding coil bracing (see Figure 6), can be made. Mirrors on extension rods can be used to further probe the interior. Borescopes can probe even further than mirrors and may allow inspection of the rotor interior, and between the stator core and frame. Inspection of the rotor interior may reveal debris or other contamination, loose fit of the rotor core to shaft, or possibly cracked welds. Likewise, inspection between stator and frame may reveal debris or other contamination, cracked welds and possibly blocked ventilation ducts.

Large motors and motors supplied by variable frequency drives (VFDs) should be checked for the possibility of shaft currents (see Figure 7), even if none are suspected. Large motors may have shaft currents induced due to dissymmetry in the magnetic circuit, e.g., due to segmented laminations. The shaft voltage measurement methods described in the next paragraph can be used to check for the probability of damaging bearing currents in a motor while it is in the factory or service center. However, for other causes of bearing currents, such as from VFDs, field testing is necessary. VFDs may cause capacitive coupling to link the rotor and stator, causing current to pass through the bearings. The bearings will fail prematurely due to degradation from the consequential circulating “shaft” current. Measuring the current directly is not practical, as that would require a current transformer wrapped around the shaft inside the motor, i.e., between bearings. The alternative is to measure the voltage from frame to shaft to determine if the voltage magnitude is great enough to indicate the possibility of damaging shaft currents. 

One method for measuring shaft voltage is to attach a true RMS voltmeter lead to the frame and the other lead to the shaft by some type of brush-like device. The shaft connector can be a fine copper wire, such as a brush shunt. The copper wire drags the shaft and senses voltage. Use of the meter lead to directly sense voltage is not recommended because it is typically not a continuous contact. If the sensed voltage is greater than 100 millivolts AC for rolling bearings or over 200 millivolts AC for sleeve bearings, damaging shaft currents are probably present. Another criterion is given in MG1, stating that a shaft voltage in excess of 300 millivolts AC measured from the shaft at the opposite drive end to the shaft at the drive end indicates damaging shaft currents may exist.

Field testing and inspection of motors is an important part of maintaining these essential and often critical machines. Taking time to learn about the proper tests and procedures, and how to apply them, will allow you to help your customers improve reliability and reduce costs.

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Categories: AC motors, Alternators/generators, Testing, Motor testing

Tags: Motor testing Generators Field service

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