Another factor related to voltage is unbalanced voltage. There is no rule-of-thumb to estimate whether overvoltage will increase or decrease motor current, and likewise with undervoltage. Too high a voltage can increase heating of the motor’s magnetic core, while too low a voltage can reduce its torque capability (see Table 2). 60034-1 (10% for limited duration and frequency of occurrence). Line-to-line voltages test.Line-to-line voltages should be within 10% of the motor’s rated voltage, according to NEMA Std. When taking measurements, unless the ammeter can measure momentary inrush (peak) current, it will only indicate the steady-state, locked-rotor current. For a motor with a higher than typical locked-rotor current, it can be high enough to trip circuit breakers. Consequently, it could be as much as 22 (2.8 x 8) times the full-load current. MG 1, the inrush current can be 1.8 to 2.8 times the locked-rotor current, which is typically six to eight times the full-load current. 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 3). On large motors or those powered by variable-frequency drives (VFDs), it also is important to check for shaft currents. If the motor can operate safely, these may include measuring the starting (inrush) current, line-to-line voltages and voltage unbalance. Online (running) tests vary by machine type (e.g., squirrel cage induction, synchronous, wound rotor). A more rigorous yet simpler criteria is to limit runout to no more than 0.001 inch (0.025 millimeter) for 2-pole motors, 0.002 inch (0.051 millimeter) for 4-pole motors and 0.003 inch (0.076 millimeter) for motors with six or more poles. MG 1) allows up to 0.003-inch (0.08 millimeter) total indicated runout (TIR) for shaft diameters of 1.625 inch to 6.500 inches (41 to 165 millimeters). Shaft runout test.Mechanical tests include the output shaft runout test, which uses a dial indicator to measure shaft displacement at the end of the shaft (if possible) or adjacent to the coupling during one revolution. Perform the surge test only if the winding has an acceptable IR value and, if applicable, an acceptable PI value. Turning the rotor a few mechanical degrees will merge the traces, unless the winding has a fault or other defect (e.g., unbalanced winding circuits). 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. IR readings are temperature sensitive, so to be meaningful they should be corrected to the standard temperature of 40° C (see Table 1).įigure 2: Insulation resistance test of motor stator windings. It consists of applying the test voltage and measuring the winding’s resistance to ground after one minute. Insulation resistance tests.The IR test is well a defined method of evaluating the ground insulation of all types of motor windings (see Figure 2). If necessary, schedule nonessential maintenance or repairs for the next regular shutdown. Record all damage and defects, remove debris and contamination and perform any maintenance or repairs that need immediate attention. 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.
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