A common issue when surge testing an assembled motor is “rotor coupling” - a magnetic interaction between a squirrel cage rotor and the stator winding that can produce a dual trace of voltage as seen on the screen of a surge tester or an oscilloscope. Surge test.The surge test can detect turn-to-turn, coil-to-coil or phase-to-phase shorts. Per CSA C392 and ANSI/EASA AR100, the resistance unbalance limit for random windings should be 2% from the average, and 1% from the average for form coil windings. Lead-to-lead resistance test.By comparing the phases or circuits in the winding, the lead-to-lead resistance test can detect high resistance joints in winding and lead connections. 6.įigure 2: Insulation resistance test of motor stator windings. For those windings, the DAR is more useful, with a common selection of IR readings taken at 30 seconds and 60 seconds per IEC Std. It may not be meaningful for random-wound windings (coils made with round wire) because the winding absorption charging current decays within the first minute or so of applied voltage.

The PI test is most useful with stator form coil windings (coils made with rectangular or square wire). 6, the PI value would not be meaningful, and the PI test need not be performed. If the IR value is greater than 5,000 megohms, per IEEE Std. Windings with a lower PI value usually are unsuitable for service. The recommended minimum PI value for windings rated Class B (130° C) and higher is 2.0, and 1.5 for Class A (105° C) windings.

PI is an extension of the IR test and is calculated by dividing the IR reading at 10 minutes by megohm value at one minute. Table 1: Recommend minimum insulation resistance values at 40° C (all values in megohms) Courtesy: EASA Depending on operating conditions and availability of test equipment, offline testing and inspection also may include lead-to-lead resistance and surge tests, sampling lubricating oil for analysis and checking for soft-foot, output shaft runout and alignment of motor to driven equipment. Inspection and testingīesides visual inspection, offline condition assessment and diagnostic tests for 3-phase squirrel cage motors typically include insulation resistance (IR) and polarization index (PI) or dielectric absorption ratio (DAR) tests.

Most of these tests and inspections also apply to 3-phase wound rotor motors and induction and synchronous generators. The focus of this article is on diagnostic electrical testing of installed 3-phase squirrel cage motors, interpretation of results and key points of physical inspection. Besides routine maintenance, this requires that critical motors be inspected and tested regularly. Efficient, reliable operation of critical electric motors is top of mind for maintenance professionals tasked with keeping production at optimum levels while avoiding costly, unexpected shutdowns.

Motor winding resistance imbalance