When Maintaining Motors, Proactive Strategies Are Required.
Whether the subject is motors, a maintenance department or an entire organization, typical maintenance methods are categorized as reactive, preventive, predictive, reliability-centered and proactive.
Reactive maintenance allows a component or piece of equipment to run to failure, at which time the component or equipment is repaired or replaced. Preventive maintenance (PM) is performed at time-based intervals. Predictive maintenance (PdM) examines the condition of the equipment to determine the maintenance needs. Reliability-centered maintenance (RCM) considers equipment criticality to a process or facility to establish an impact-based priority.
According to the Federal Energy Management Program (FEMP) of the U.S. Department of Energy, proactive maintenance requires ongoing reviews of equipment history—including reactive maintenance, PM, PdM, corrective maintenance and repair records. Root-cause-analysis upon equipment failure is necessary to the success of proactive maintenance.
The best maintenance strategy depends on the situation. Even run-to-failure is a strategy. For example, an exhaust fan motor in a seldom-used washroom is not critical to most processes. However, a feed-water pump motor in a power plant is critical to the power generation process. Although it's nice to have a spare on hand, the strategy of allowing the washroom exhaust fan motor to run to failure is appropriate.
However, a plan should be in place for the maintenance of the feed-water pump motor – and all motors critical to plant operations.
Good Proactive Maintenance
Based on sound industry practices and experience, a good proactive maintenance strategy requires a system that captures repetitive failures so that appropriate corrective changes are made. This requires keeping good records of reactive maintenance, PM, PdM and corrective maintenance as well as root-cause-analysis of any maintenance performed.
These records should be reviewed at least annually, but semi-annually would be better. Include previous proactive maintenance in the review to evaluate the effectiveness of changes that were made, if any. If there are equipment failures and/or corrective maintenance performed, review specific equipment histories. Flag repetitive failures, and take steps to prevent the recurrence of the same types of failures.
Motor Maintenance Steps
For motors, PM requires following the manufacturers' minimum maintenance recommendations, and such maintenance can typically be scheduled during planned production downtime. PM tasks can include lubrication, visual inspection and parts cleaning.
PdM requires regularly taking non-intrusive measurements—such as vibration analysis, infrared (IR) analysis and insulation readings—and comparing the measurements so that equipment failure can be predicted. Predicting equipment failure allows maintenance and production departments to work together to schedule repairs.
Vibration analysis detects mechanical and electrical anomalies in motors and the rotating equipment that they drive. Vibration can occur due to improper alignment, improper mounting, contamination, bent shafts, a faulty motor or an unbalanced motor or pump. Bearing failures in either motors or pumps generally have specific vibration signatures. Pump cavitation has a distinct vibration signature as well.
IR analysis can detect electrical system overloading or faults with connections. It can also detect mechanical abnormalities, such as temperature differentials in the bearings or couplings. Temperature differences or elevated temperatures along pump seals or gaskets can indicate impending failures.
Motor Insulation Testing
Motor insulation testing, sometimes referred to as static testing, verifies many typical motor faults. Insulation testing helps verify whether there are high resistance connections within the motor winding, ground wall insulation condition and turn-to-turn insulation. The tests are typically performed by applying a high voltage/low current to individual motor windings to assess the overall condition and health of the insulation. Anomalies discovered in test results could potentially lead to catastrophic failure.
Other periodic insulation tests and measurements include on-line testing, sometimes referred to as dynamic testing, which looks at electrical system conditions. Power quality issues to the motor can greatly reduce insulation life. Typically, power quality issues to the motor will result in increased insulation temperature.
Some of the common components that are evaluated in this type testing are voltage/current unbalance, distortion, harmonics, voltage and current deviation from the motor nameplate, power factor and current signatures to evaluate rotor health. These electrical distribution system conditions typically originate outside the motors. However, they affect the operation and life cycle of motors significantly.
An electrical distribution system has an unbalanced voltage if the line-to-line voltage differs from the average. In the electrical industry, the maximum voltage unbalance for electric motors is generally considered to be 2 percent. Voltage unbalance can be caused by improper transformer setup, uneven single-phase loads, defective regulating equipment, unmatched conductor impedances, open connections or an actual unbalance from the utility. Voltage deviation and motor-voltage mismatch are also known as over- and under-voltage. The maximum allowable voltage deviation for electric motors is generally considered to be ±10 percent. Voltage deviations—over or under—can be due to improper motor selection, incorrect transformer setup or undersized conductors.
One of the leading causes of motor failure is bearings. Contamination and poor lubrication are the most common causes of bearing failure. Proper sealing and lubrication are the two most important actions for preventing bearing failure. Contamination can include metallic particles, dust, dirt and fluids such as water or chemicals. Bearing re-lubrication pushes old grease and embedded contaminants that may have sneaked past the seal out of the bearing. A bearing industry saying is that the best seal is moving grease.
Excessive loading or pre-assembly damage can cause failures as well. Excessive loading can be caused by misalignment, pump cavitation, excessive pump flow and exposure to temperatures outside bearing thermal limits. The shaft and housing could transfer excess heat to the bearing rings.
Typically, misalignment occurs when coupling the motor shaft to the pump shaft. Sometimes, a pump and motor combination can be in alignment when the equipment is not running but out of alignment at operating temperature.
If this is the case, operate the equipment until operating temperature is reached, and then shut it down. Perform the alignment while the pump and motor are still at, or close to, operating temperature.
Repair or Replace?
If a motor fails, the maintenance manager must decide whether the motor should be replaced or rewound. Many companies do not have a repair/replace policy. Waiting for a motor failure to occur to put this policy in place is probably not the best maintenance strategy.
The repair/replace decision depends on the particular motor, horsepower, efficiency, duty cycle and availability.
One rule-of-thumb commonly used in the industry is the 50-50 rule: if a motor is smaller than 50 horsepower or if its repair cost is more than 50 percent of the cost of a new motor, replace the motor instead of repairing it.
However, when purchasing a new motor, remember that its purchase price is only about 2 percent of its total life cost. The rest is operational utility cost. Based on this, it is usually economically justifiable to procure the most efficient motor as is practical for the application.
Based on sound industry practices and guidelines, every facility should have an established repair/replace policy. The motor population of the facility should be surveyed, tagging each with information about what to do if that motor fails. Motor spares should be included.
Old inefficient and unreliable motors that are critical to processes and operations should be replaced with more efficient and reliable motors.
Most motor manufacturers have online efficiency calculators that can help with the repair/replace decision. The MotorMaster+ program from the U.S. Department of Energy is useful as well.
Motors that will not be placed in service for six months or more from the date of shipment must meet certain storage requirements. Based on industry experience, the recommendations are:
- The storage location must be clean and dry.
- Storage temperatures must be between 50 and 120 degrees F.
- Relative humidity must not exceed 60 percent.
- If the motor has space heaters, they should be energized if storage conditions are predicted to reach dew point.
- Motors with anti-friction bearings must be lubricated when stored. Manually rotate an anti-friction ball and roller bearing motor shaft every quarter, and re-lubricate semiannually according to the manufacturer's instructions.
- Since sleeve bearing motors are drained prior to shipment, they must be refilled when stored.
- Manually rotate the shaft at least 15 revolutions each month.
Pumps & Systems, April 2011