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By Thomas A. Westerkamp
March 2006 -
Motors and drives are among the most important components of any facility's efforts to control energy use and ensure the smooth HVAC system operation. But organizations cannot achieve these goals without a comprehensive program of motor and drive inspection, troubleshooting and maintenance.
The U.S. Department of Energy (DOE) estimates that there are more than 12.4 million motors greater than 1 hp in service. Of these, nearly 3 million fail annually, resulting in 600,000 replacements. The remaining motors are repaired.
Motors consume more than 600 billion kW annually, so the percentage increase in energy efficiency through use of premium motors will have a huge impact on energy use. A closer look at motor and drive components that commonly cause problems, along with a discussion of problems technicians can look for, can help managers develop strategies that will benefit their organizations’ bottom lines.
The components of motors and drives that most commonly cause problems fall into three categories: switchgears, motors, and connected loads.
Switches tend to overheat due to the presence of dirt, abrasion, moisture and heat, and distribution wiring is affected by the same environmental stresses, as well as by motor loads. Motors also are subject to ambient conditions and loads.
Motor components that fail often include bearings, seals, contact, windings, commutators, brushes, rings, shafts, couplings and coupling inserts. Drive failures include gears, bearings, shaft seals, bushings, shafts, contaminated oil, belts, and pulleys.
The biggest enemies of motors are dirt, heat and moisture because they cause excessive vibration that often is the immediate cause of a motor mechanical failure.
A single-phase winding failure causes an opening in one phase supplying power to the motor. Causes of an opening include: damaged power distribution lines; open, oxidized or pitted contactors; and bad fuses or connections.
A motor’s insulation can fail due to ambient heat, abrasives or other airborne contamination, vibration, and a voltage surge. Motor nameplates show degrees temperature rise, which gives technicians an indication of the motor’s tolerance for heat.
Vibration can also generate heat and can be electrically caused by uneven voltage between phases resulting from unbalanced power-distribution-system loads, contacts with high internal resistance, and failing or loose terminal connections.
If all three phases are hot, it is probably due to an overload, where actual amps exceed full load amp capacity of the motor. Technicians can consult National Electrical Manufacturers Association (NEMA) standards for motors.
Under- or over-voltage can be beyond a motor's tolerance range. Problems also can arise from very high locked-rotor currents due to connected load failure or excessive starts or reversals.
The first step in scheduling maintenance tasks for motors and drives is to inventory equipment in the field and the applications for which they are used. Since the preventive maintenance (PM) and other routine maintenance vary by type of motor, managers will need to know how many of each type are in place.
The most common types include:
The types of starters and fused switches will vary by type and size of load. The process of identifying these components might be as simple as running a report from the CMMS equipment module, if one has been developed. Or can be as complicated as assigning technicians to go out and physically find each unit and record the information off the nameplate.
The maintenance manual for each specific type of motor is an excellent source for a description of inspection, testing and lube tasks, as well as their frequency. The vendor or manufacturer will gladly supply the manuals if they are not available in house.
A good first step in motor-inspection program is to inspect external area and surfaces for trouble signs, such as leaking grease or oil, dust, dirt or water accumulation on or near the unit, noise — which might indicate excessive vibration or slipping belts — and heat, which is the enemy of insulation.
Among the tests that technicians can use to determine motor and drive condition are: using voltmeters, meg-ohmmeters, and ammeters; vibration analysis; infrared imaging; and oil analysis.
Ammeters, or tong testers, measure actual current. Technicians can take the results of an ammeter test and compare them to nameplate full-load current rating, which shows balance between phases and overload conditions.
Vibration analysis show mils of vibration at selected rpms and indicates any wear or imbalance that might shorten motor life. Technicians can tell if the vibration is due to electrical or mechanical causes by shutting off the motor.
If the vibration stops while the motor is coasting, the cause is electrical, a result of current flowing across a rotating part, such as a bearing, generating an electrical field. If the vibration continues, the problem is mechanical — possibly bad bearings or a bent shaft.
Oil analysis can identify the conditions in a gearbox driven by the motor. Cuttings in the oil sample indicate wear in the gears that also can damage bearings. External checks of belt drives — a visual check for dirt and oil leaks and an audio check for slipping or vibration noise — will disclose need for repairs. Belts stretch over time, and if technicians do not adjust them, the bests will wear due to abrasion, which also can cause unnecessary wear on pulleys and sheaves.
What technicians find on the outside indicates whether the motor or drive needs further examination — disassembly and internal inspection inside — and even repair to be scheduled when the equipment is out of service.
Internal checks of switchgears should include visual checks of fuses and contacts inside switch boxes for looseness, pitting and oxidation. Using thermal-imaging equipment, technicians can make these checks without shutting down the equipment.
Thermal-imaging tests will identify hot spots, and technicians can schedule a shutdown to clean components with emery cloth and contact cleaner or to replace fuses and contacts, to eliminate the hot spots. This procedure also reduces loads so the equipment becomes more energy-efficient.
Internal visual and relay-test motor checks — actual disassembly and inspection — will identify dry or cracked insulation, dirty windings, and loose parts, such as brushes, bearings, coils, and commutators.
If PM testing and inspection indicate the need to replace a motor, selecting a premium-grade replacement is the best strategy because each such replacement upgrades energy efficiency and reduce the overall electrical load. If facility executives are to support maintenance and engineering efforts to develop a good motor maintenance plan, they must be aware of the results of these efforts. An accurate and complete equipment history will show the benefits to the organization, including higher reliability and lower energy cost.
Some managers started a motor and drive management program to prevent unexpected downtime. Instead, they ended up with that result, as well as energy savings as a bonus. They found that reduced service interruptions more than paid for the investment in premium motors, better PM coverage, and state-of-the-art testing equipment.
As a part of a motor management plan, it is essential to have all tasks — including switchgear, motor and drive testing and inspection — included in the department’s preventive maintenance (PM) program. These steps can ensure that all tasks are covered:
Managers who assign a time to each motor-inspection task also can use their plan to more accurately determine the number of maintenance hours planned annually and the level of staffing is required to carry out the plan effectively.
For example, assume that one test and inspection or replacement requires 1-1/2 hours and a technician performs the task once a month on 100 motors. In this case, the annual time required is 1,800 hours — 1.5 hours times 12 occurrences times 100 motors — which is a little less than one person working full time — 40 hours a week times 50 weeks a year equals 2,000 hours.
Comparing planned times to actual time spent also provides managers with a measure of performance. Comparing scheduled activity with completed activity gives managers an indication of schedule compliance — how many PM tasks are completed versus the number of tasks scheduled. This critical information enables managers to more effectively evaluate and improve the program, focus motor management training where it will do the most good, and aid supervisors to fully use and develop their crews.
— Thomas A. Westerkamp