Up Time Magazine- April/May 2008 - (Page 48) Mechanical Tests All of the mechanical fits on the motor must be tested using calibrated outside and inside micrometers. The critical areas that effect efficiency include the bearing journals and housings. If the fits are too loose or tight, both the efficiency and the bearing life will be reduced. There are several ways to return bearing fits which include: • Peening: the practice of punching or marring mechanical fits to create a tighter fit. This practice is not recommended for repair as it is ‘uncontrolled.’ • Metalizing: consists of a one- or twopart spray process that requires metal to be removed first. This process is susceptible to separation from the material to which it is bonded in instances of non-symmetrical pressure or when the surfaces have not been properly prepared. This practice should not be considered when world-class energy efficient motor repair and reliability are being considered. • Welding: similar to metalizing. However, it creates a stronger metal-to-metal bond when properly applied. If a repair requires adding metal, this is the preferred method when compared to metalizing. However, significant experience is required in order to prevent stress fractures in the material. • Sleeving: the process of returning fits by machining and sleeving a motor shaft or housing. This is the recommended method of motor repair as it is more controlled. • Refabrication: while expensive, this method is the best for machining severely worn parts, shafts in particular. It is also highly recommended that motor bearings are replaced during each repair. They should also be replaced with the original class of bearing. Internal bearing fits and friction can have a large effect on motor efficiency. Fan replacement should also be considered when the original fan has been damaged. The replacement fan should be original equipment, as well. If a fan is replaced with a larger fan, or one with more fins, the motor efficiency will be reduced. If a fan is replaced by a smaller fan, or one with fewer fins, cooling will be reduced, resulting in a shorter motor life. Initial Winding Tests Upon receipt of a motor by an electric motor repair shop, certain tests should be performed, at a minimum. For the motors within the scope of this paper (220/440 VAC), the tests are normally less stringent than those performed on medium voltage, or formwound, motors. The first test is an insulation to ground test, which measures leakage to ground. For the lower voltage motors, 500 VDC is the acceptable limit, with a reading of 5 MegOhms as the absolute lowest reading. However, a reading below several hundred MegOhms should indicate some type of problem. A reading of zero indicates a direct short to ground. 48 april/may 2008 http://www.mikroninfrared.com http://www.mikroninfrared.com
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