Maintenance For Refrigeration Motors
Thanks to Tropical Storm Stan, RETA members were deprived of golf, deep-sea fishing, and a few other excursions in this touristy area. Rain poured inside at least one area hotel's lobby, and the hotel shuttles themselves were working nonstop. But the training sessions would have been well attended anyway; that's what RETA is all about. Anyway, the Molly Hatchet concert was indoors.
One of the technical presenters was Randy Naugle of RAM Industries Inc., Leesport, Pa. He covered motor construction, failures, and a preventive maintenance program.
WHAT'S INSIDE A MOTOR?Naugle started by explaining what makes up an electric motor that a refrigeration technician would likely see. Naugle pointed out a motor's five main parts:
1. The stator, a stationary winding inside the motor.
2. The rotor, a rotating winding.
3. Bearings: ball, roller, or babbitt (sleeve).
4. The frame, essentially the machined stator housing.
5. And end brackets for the center bearings and rotor.
"Everything goes together and rotates," Naugle said. "That's about as basic a description as you can get.
"Stator laminations," he continued, "are approximately 0.0185 inches thick and pressed together to make the stator core."
It's important that they are made of a good grade of steel. Stator windings can either be form wound or mush wound, depending on the motor's horsepower and voltage. Form wound windings are insulated copper. This manufacturer's mush wound windings have varnish film insulation on variable frequency drive (VFD) rated wire.
"It withstands high voltage spikes," Naugle said.
Cast iron frames, he continued, are machined to fit the stator and end brackets. "The feet are machined flat to within 0.003 inches."
Rotors can either be fabricated copper bar or cast aluminum. The copper bars are brazed to the end ring to complete the electrical circuit, he explained.
It's also critical that bearing housings are machined to close tolerance, he said.
"RAM uses SKF open ball bearings. With open bearings and the RAM end bracket design, the grease flows through the bearings from the top of the inside bearing cap." The grease is pushed into the bearing, with the grease relief at the bottom of the bearing bracket.
WHEN MOTORS FAILCommon motor failures, Naugle said, are electrical problems, thermal degradation, insulation contamination, and mechanical failure. Sometimes multiple failures can occur, he said. "Sometimes one looks like another."
According to Naugle, a single-phased winding failure results when one phase of the power supply to the motor is open. It is usually caused by a blown fuse, an open contactor, a broken power line, or a bad connection.
Insulation failures typically are caused by contaminants, abrasion, vibration, or voltage surge.
Thermal deterioration of insulation in one phase of the stator winding can result from unequal voltage between phases, Naugle said. Unequal voltages are usually caused by unbalanced loads on the power source, a poor connection at the motor terminal, or a high resistance contact (weak spring). "A 1 percent voltage unbalance may result in a 6 percent to 10 percent current unbalance," said Naugle.
Thermal deterioration of the insulation in all phases of the stator winding is typically caused when load demands exceed the motor rating. Undervoltage and overvoltage (exceeding the National Electrical Manufacturers Association's (NEMA) standards) will result in the same type of insulation deterioration, he added.
Severe thermal deterioration of the insulation in all phases is normally caused by high currents in the stator winding due to a locked rotor condition, said Naugle. It may also occur as a result of excessive starts or reversals.
"Insulation failures like this usually are caused by voltage surges," he said. "Voltage surges are often the result of switching power circuits, lightning strikes, capacitor discharges, and solid-state power devices."
Thermal degradation, he continued, may be caused by excessive temperature inside a motor. "Motor insulation has different classifications," Naugle said. "Insulation is rated for hot spot temperatures. Normally motors don't run nearly that hot inside."
To find the source of the heat, check the ambient temperature in the room. "A high [room] temperature doesn't cool the motor well." Every 10°C rise in winding temperature reduces the life of the insulation by 50 percent, Naugle said. That means every 10° rise above design temperature will reduce insulation life 50 percent.
Airflow is essential to keeping a motor cool. "Contamination of the motor winding or cooling fins will reduce airflow," he said. This will increase temperature on the motor and reduce the motor's life.
"You can put stator [resistance temperature detectors] RTDs inside large motors to monitor the temperature," he said.
According to Naugle, almost 90 percent of motor failures are mechanical. Common mechanical failures include bearing defects, worn bearing fitting (shaft or housings), winding movement, rotor bar fatigue (can be detected through vibration analysis or current spectrum), misalignment, and imbalance.
"You can have electrical failure due to mechanical failure," he pointed out.
For example, a motor problem may at first appear to be a defective bearing. The motor was put on a VFD, and didn't run long before the bearing got marks (fluting) in it. "That's an electrical problem there," said Naugle.
MAINTENANCEPeriodic maintenance needs to be done to keep motors and equipment running, Naugle emphasized. Grease bearings with the appropriate lubricant and use the proper amount.
"More bearings are ruined by overgreasing than undergreasing," he said. Greasing intervals are determined by bearing size, speed of the unit, and housing. Change the filters. Keep the unit clean. Monitor the unit's temperature with RTDs for the stator and bearings.
Space heaters should be installed if motors are used outside and are to be de-energized for extended periods of time. Keep the motor above dew point temperature.
Predictive maintenance tests can be performed to find defects before they cause catastrophic failure. These tests include vibration analysis, infrared thermographs, insulation testing, and oil analysis.
Vibration analysis, Naugle said, can detect bearing defect frequencies, imbalance, mechanical looseness, misalignment, soft foot, and electrical defects.
Insulation systems can be tested using a multimeter, low-resistance ohmmeter, megohmmeter, surge test, or high potential (hi-pot) test.
"Megging your motors is a good idea," he continued. A megohmmeter can tell a technician whether the motor is grounded and the value of current leakage. It can also perform a polarization index (PI) test or dielectric absorption test. It cannot determine copper-to-copper failures, he said, adding, "80 percent of electrical failures start as copper-to-copper failures."
Surge testing determines imminent copper-to-copper faults. "Hi-potting only determines insulation integrity to ground," Naugle said. "Surge testing raises the voltage potential between turns, which identifies insulation integrity between turns."
If it helps prevent a failure, particularly for customers with sensitive loads (that includes most refrigeration applications), it's a good thing.
Publication date: 01/23/2006