Transformer Testing Using Handheld Turns Ratio and Winding Resistance Tester TWR-H.While this looks like an easy failure to capture, the motor was actually running the day before and would have been returned to operation had the testing not occurred. As shown on the screen capture from the PdMA, the test equipment is identifying the resistive imbalance, change in insulation resistance, and several other alarm issues.
This high resistance connection was found during a down day. In the example below, we were conducting testing as part of a program for a power generation station in North Carolina. Starting a regular, disciplined testing program with documented trends is a very effective deterrent to premature failure caused by high resistance connections. Our field teams use the PdMA analyzer as a basis of our Diagnostic Testing program. This sounds complicated, but PdMA analyzers make it relatively simple, and combine many other effective tests into the same piece of equipment. Therefore, it is more effective to conduct these tests periodically and examine trends. You can do the same in the field with a low ohm resistance meter.Īs a word of caution, a high resistance connection usually develops over time. Our shops conduct this test as part of acceptance testing on new windings (see Standard Electrical Testing). This would result in three measurements that should be very close (balanced) to each other. However, you have to go beyond basic insulation resistance testing to catch high resistance connections.Ī good method of finding high resistance connections is to measure the phase-to-phase resistance between each of the three windings (in a three-phase motor). This test is often misapplied, as it is easily misunderstood what minimum insulation requirements should be and that these readings must be temperature compensated. Many maintenance teams will occasionally test motor resistance to ground with a megohmmeter. Per the previously mentioned article by IEEE, “it is important to monitor and correct high-R connections for a reliable, efficient, and safe operation of the industrial facility”. How can you detect a high resistance connection? This motor is replaced many times and the failure cycle begins again.As these problems become more severe, motor temperatures increase and insulation damage begins.High resistance connections result in voltage imbalances will reduce the horsepower rating significantly.Many times a motor, although initially in perfect health, is installed into a faulty power circuit, with a high resistance connection for example.Noah went on to write in an article in Reliable Plant that:
The focus of today’s topic is the motor lead connections, which, done improperly, often lead to failure.Īccording to Noah Bethel of PdMA, a project conducted in 1994 on industrial power distribution systems “found that connectors and conductors were the source of 46 percent of the faults reducing motor efficiency”. You can find them at circuit breakers, motor starter terminals, disconnects, or anywhere on the power circuit where a connection is made. They are often the root cause of motor failures.Īccording to IEEE, “a high-resistance (R) connection in an induction-motor electrical circuit results in localized overheating and supply-voltage unbalance, which leads to a decreased efficiency and reliability and an increased fire hazard in the electrical distribution system and motor”. Loose, poor, heat, imbalance, and failure are a few often associated with high resistance connection, and for good reason.
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