Realize Payback From Motor Insulation Tests
Maintenance Technology | June 15, 2017
Insulation problems are to blame for a high percentage of motor failures and associated unplanned costs.
According to SKF (skf.com, Gothenburg, Sweden, and Lansdale, PA), 40% of failures in electric motors are caused by bearing problems. Another 40%, however (a percentage that’s even higher in motors operating above 4 kV), are caused by insulation problems associated with coil windings or loose connections. Unfortunately, predictive-maintenance techniques to detect insulation weakness typically aren’t employed as much as those used to keep tabs on bearing health, i.e., vibration analysis and infrared thermography. (Ground-wall-insulation “megger” testing is common in plants, but, as SKF explained, it’s not a complete test.)
The problems caused by insulation weakness, including catastrophic motor failures and, in some cases, fires, can be just as serious as those caused by worn bearings or overheating. For this reason, it is important for personnel to have a way of assessing insulation integrity and be able to take timely action.
There are two types of insulation in an electric motor. Groundwall insulation is found between the motor stator and the electrical windings. The insulation strength of new groundwall insulation is very high, often 40 times operating voltage. Winding insulation is the thin insulation on the wires used in the motor windings. The insulation strength of new winding insulation is about 15 times operating voltage.
Most motor insulation failures start as winding insulation failures since that insulation strength is vastly weaker. When a winding insulation failure occurs, the motor can fail quickly, often becoming so hot as to also damage the groundwall insulation, causing it to fail.
Automated motor-insulation testing, using a device such as the SKF Baker AWA-IV, has been shown to make insulation testing easy and remove operator error and inconsistency.
Static insulation testing is done with the motor disconnected from the power supply, and typically performed from the motor control cabinet (MCC). Testing from the MCC also allows detection of electrical faults outside the motor itself, such as in junction boxes or feeder cables. Motors also can be electrically tested in situ through dynamic monitoring, which can reveal problems in the wider power-machine-load system. Typical insulation testing includes:
• Coil resistance tests
• Meg-ohm test
• Polarization Index (PI) test
• DC step-voltage test
• Hipot test
• Surge test.
While the first five tests assess the health of a motor’s groundwall insulation, it’s important to keep in mind that a unit’s winding insulation is more prone to failure. The last procedure on the list, the surge test, focuses on winding insulation.
All of the six listed insulation tests produce clear, unambiguous results that require little interpretation. Those results can also be trended over time. This allows operators or maintenance managers to assess the progress of a potential condition over time. For instance, increasing non-linearity in step voltage could suggest weakening of the groundwall insulation.
Motor-insulation testing can have an enormous impact on a plant’s bottom line. Consider these real-world examples:
Case #1 (static testing): Technicians with a leading pulp and paper company began using SKF Baker AWA-IV testers to identify problems in about 800 motor systems. Among the many problems this testing found—and solved—were:
• blown holes in insulating boots that covered cable lugs in junction boxes (identified through step-voltage testing)
• a bad lug connection in a motor junction box (found after a failed resistance test)
• a stator coil turn-to-turn short on a booster fan (identified by surge testing)
• a cable shorted to ground in starter, and a pinhole in the cable (found after failed surge and step-voltage tests).
In all, the company reportedly was able to reduce its annual motor costs by nearly a third through the use of the insulation testing rigs.
Case #2 (static testing): A steel mill in Australia had a 6.6-kV pump motor that required maintenance–including a rewind. The motor was rewound at a motor shop, then transported 500 mi. (aprox. 800 km) to a second facility for vacuum-pressure impregnation (VPI). A subsequent surge test, however, indicated that insulation strength was still not right. The motor was then put onto a test stand and run. Motor currents and vibration tests were acceptable so, since the mill was in a hurry to resume production, the motor was put back into service. Three days later, the unit failed catastrophically and ignited a fire. The cause was traced to the windings, which had probably been damaged when the motor was transported to or from the VPI facility. The surge test picked up on the problem, but had been discounted.
This failure showed that tests for insulation strength are real, and to be ignored at a motor user’s peril. If vibration monitoring indicates that a bearing is starting to fail, it is replaced. The same should happen with insulation.
Case #3 (dynamic testing): U.S. utility company Pacific Gas & Electric was facing a potential $23,000 bill to replace a motor on a 125-hp screen-refuse pump, which was overheating and drawing excessive current. Rather than simply replace the motor, the company looked into the reason for the high current, as there were no signs of bearing problems, current imbalance, excessive harmonics, or rotor-bar problems.
Dynamic testing with SKF Baker Exp3000 technology revealed that the load was running higher than the motor’s rated value. Looking back through the maintenance history, the personnel found that a 15.75-in. impeller on the pump had been replaced with a 17-in. impeller. Once the correct-sized impeller was installed, the current returned to normal values.
This testing helped prevent a costly mistake, given the fact that the oversized pump impeller would also have overloaded a new replacement motor.
Case #4 (dynamic testing): On-line testing also prevented a huge loss at a Progress Energy power plant in the United States. Technicians were investigating why one of three submerged circulating water pumps was requesting less input power and, as a result, running faster. An SKF Exp3000 captured the torque signature of all three motors, giving a snapshot of the load demands of each unit. The pump in question had a torque of about 75% of a healthy pump. The torque band was also too wide, and varied dramatically. A diver sent to examine the underwater pump discovered that its end bell had fallen off.
The unit was quickly repaired, which helped to maintain output when one of the other pumps failed soon afterward. The company estimated that it would have lost $3.5 million in revenue if output levels had fallen.
These examples (and many others) show that a simple, inexpensive insulation-testing regime can generate significant benefits for an operation. One example also demonstrates that ignoring insulation-testing results can cost an operation dearly. MT
For more information visit skf.com.