Plant Services - August 2007 - (Page 45) RELIABILITY Motors sample population of motors, fitting the data to an appropriate distribution, and analyzing the results, which reveals the probability of the motors having a particular lifetime. The sample of motors needs to be tested until each fails, and no one can predict how long that might take. These tests monopolize special laboratory facilities and equipment for the duration of the test program. The second approach specifies the desired motor reliability over a given life cycle and tests a statistically significant sample to demonstrate that the product meets the reliability goal. Figure 1 shows a typical graph of an analysis showing the number of samples to test as a function of test duration needed to demonstrate 95% reliability and 80% confidence level using a Weibull shape parameter of 1.5 with no failures allowed. This means that 95% of a given motor type will achieve a life of 2,500 hours. The life-hours vary from equipment usage in normal life because equipment used intermittently accumulates fewer hours. The data in the chart is from a real test program. You can either test a few motors over a long time or test a large number of samples for a relatively short time; both situations being expensive. A compromise can be found through accelerating the testing with a reasonable number of samples. Statistical tools analyze the data to determine reliability. Most of the failure patterns of electromechanical equipment can be represented by well-known distributions such as the normal (or Gaussian), exponential or Weibull. (The reliability information that results directly influences the OEM’s warranty policy.) A rig for testing motor systems for reliability is shown in Figure 2. These methods are practical only for single-phase motors and three-phase motors smaller than 5 hp. It’s estimated that about 60% of the U.S. integral horsepower motor population is rated between 1 hp and 5 hp and about 80% is rated between 1 hp and 20 hp. These tests aren’t feasible for larger motors. August 2007 When these fail, they’re repaired using best repair practices, and motor build inspection analysis helps improve operational reliability. Testing small motors Equipment driven by single-phase induction motors generally develops problems with the starting circuit components (start capacitor, centrifugal switch, starting winding), which are prone to premature failure. Figure 3 shows a 7.5 hp single-phase, doublecapacitor motor and a typical centrifugal switch, while Figure 4 shows a schematic of a typical capacitor-start, www.PLANTSERVICES.com 45 http://www.hillardcorp.com http://www.hillardcorp.com http://www.PLANTSERVICES.com
For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.