Up Time Magazine- April/May 2008 - (Page 49) In many cases, a motor repair shop will test the phase-to-phase resistance of the electric motor with a milli-Ohmmeter, or Wheatstone bridge, then attempt to operate the electric motor before disassembly (assuming the motor passes the incoming tests). This is done to indicate what types of defects are within the motor. For electrical testing, the phase current is taken at full voltage, no load, and both noted for later use and compared to ensure that one phase is not drawing more current than the others. If the motor passes these tests, it is disassembled and cleaned using solvent, hot soap and water, steam, or some other accepted method. If the stator has been cleaned with soap and water, it must be dried before further testing in an oven set for a temperature of around 196°F (90°C). If damage occurs to the insulation as a result of cleaning, or if the insulation appears to have minor defects, it may be dipped and baked using a Class F, or better, insulating varnish. VAC = 0.65 * (2Em + 1,000V) VDC = 0.65 * (2Em + 1,000V) * 1.7 Where: Em is the nameplate voltage value Equation 3 - Test Voltage Hi-Pot seconds; and, 1.72 times for 1.2 microseconds. In this test, the wire insulation between conductors is being evaluated for weakness. Older standards recommended the use of surge values at those shown in Equation 3. There are no reasons why non-destructive and low voltage winding analysis tests above and beyond these, may not be performed. In the next issue, we will cover coil removal practices, stator winding, post winding tests, varnish insulation and final testing. Howard W Penrose, Ph.D., CMRP is the President of SUCCESS by DESIGN Reliability Services and a member of the National Writers Union (UAW Local 1981). SBD provides training and facilitation for the application of motor management programs for clients that include General Motors and US Steel. For more information, contact SUCCESS by DESIGN at 800 392-9025 ext 203 (USA Only) or 860 577-8537 ext 203, via email at info@ motordoc.net or our website http://www. motordoc.net. Once cleaned, the windings should have an AC or DC high potential (Hi-Pot) test performed at a voltage figured in Equation 3. The AC Hi-Pot is a pass/fail test, because if it arcs to ground, the insulation will be damaged beyond repair. The DC Hi-Pot is more forgiving, especially if the leakage can be monitored. Any sudden increase indicates that the insulation has failed. If it is below the calculated voltage when it fails, then the winding should be rewound. If the motor completes this test successfully, it should be subject to a surge comparison test. The new voltage value limits for this test are covered under the IEEE Std P1068 (under development) and are: 1.21 times the motor nameplate voltage for an instrument rise-time of 0.1 microseconds; 1.4 times for 0.5 micro- www.uptimemagazine.com 49 http://www.motordoc.net http://www.motordoc.net http://www.vibinst.org http://www.uptimemagazine.com
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