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News Motors and Drives: Preparing for Failure

By James Piper, P.E. November 2016
The electric motor is one of the most important components in institutional and commercial facilities. It is also one of the most overlooked — until something goes wrong and a pump system goes offline, or the air conditioning system shuts down, or the facility’s fire-safety system fails to operate during an emergency.
Contributing to the problem is the experience that most maintenance and engineering managers have with motors. They are reliable. Most motors have a long, trouble-free service life while requiring a minimum of maintenance.
But while the risk of failure might be low, the consequences of a failure often are high. At best, a motor failure is an inconvenience to building occupants. At worst, it results in significant damage to building systems and disrupts operations for an extended period of time.
Limiting the damage and inconvenience a motor failure can cause requires several things. Technicians must understand which motors support which operations within a facility, they must understand the early signs of developing problems in a motor’s operation, and the department must have a plan of action should a particular motor fail — all before the failure occurs.

Taking stock
Any program that addresses motor reliability must begin with an understanding of installed motors and their functions. Depending on the size of the facility, managers might be responsible for the operation of hundreds of motors ranging in size from fractional horsepower units to 100 horsepower (hp) or more.
At a minimum, managers and technicians should know the function each motor serves, as well as its horsepower rating, efficiency rating, frame size, operating voltage, operating speed, and age. Without this information, technicians will be blindsided when a failure occurs and might be forced to install a motor with a higher horsepower rating than the motor that failed simply because it was readily available. The result is lower operating efficiency.
Technicians then should give each motor a rating based on the critical nature of the operation it supports. For example, a motor that drives the distribution of heating water in most cases has a higher rating than one that supplies domestic hot water to restrooms.
Similarly, if redundancy exists in a pumping system in which only one pump is needed, the motor drives for that system would be given a lower rating than if there were no redundancy.
It is also important that technicians develop a maintenance history for each motor in a facility. That history is a critical element for managers who must decide if repairing or replacing a failed motor is the most appropriate move.
Motors Rarely Fail Without Warning
Paying attention to the environment in which the motor operates will help technicians avoid a failure at the worst possible time. Technicians should consider the areas in which a facility’s motors operate. It is likely these areas share many of these common causes of problems:
Heat. This is by far the most common cause of early motor failure. One rule of thumb states that every increase of 10 degrees Centigrade in operating temperature for a motor’s windings reduces the motor’s operating life by 50 percent. Limiting temperature rise starts with making sure the motor is properly sized for the operation in terms of horsepower, starting current and rated torque. Technicians also must make certain the area has adequate ventilation.
Dirt. High levels of dirt and lint can gather in the motor’s cooling vanes and drive up its operating temperature. They then can travel into the motor, where they can abrade insulation, shafts, and bearings.
Moisture. Like dirt, moisture attacks a motor’s mechanical and electrical components. It corrodes bearings, rotors, stator laminations and couplings, and it penetrates the insulation of the motor’s windings. While moisture can be a problem for all motors, it is particularly damaging in applications where motors run infrequently, such as with fire pumps.
Technicians should visually inspect all motors regularly to make certain they are not operating in an environment that is too hot or where dirt and moisture are present. This action alone will significantly reduce the number of motor failures.
Vibration. Vibrations can originate within the motor itself or from its load. Failed motor mounts, poor alignment, unbalanced components, and even accumulated debris on moving parts can result in vibrations that can damage windings or cause bearings to fail.
Technicians should inspect all motors regularly for excessive vibration. Motor size and the critical nature of the load it is driving will determine the frequency of those inspections.
While technicians can perform routine inspections to identify issues with heat, dirt, moisture and vibration, additional non-visual tests might be beneficial, particularly for larger motors and those driving critical loads. Motors frequently give warnings of impending failure. Technicians can identify these warnings by conducting tests such as insulation resistance, polarized index, and DC high-potential testing. They will need to track test results over time to determine if deterioration is taking place and, if so, the rate of its progress. These steps will allow managers to plan for replacement before an outage occurs.
Energy-Efficient Motors Declared Mandatory Since 1997
A motor has failed, or technician testing has indicated it is deteriorating and needs to be replaced. Managers in this situation face two options. They can decide to have technicians repair the existing motor, or they can specify a new motor to replace it. In general, repairing it offers lower first costs while replacing it offers the opportunity to improve the application’s energy efficiency.
Most motors manufactured before 1992 are considered standard-efficiency motors. Since then, higher operating efficiencies have become mandatory, first in 1997 and again in 2010. Managers can recover the additional cost of higher-efficiency motors fairly quickly through energy savings, particularly if the motor is in continuous use. The U.S. Department of Energy and most motor manufacturers offer software managers can use to evaluate the savings produced by installing higher-efficiency motors for a range of applications and annual run times.
While energy efficiency is an important factor to consider when evaluating the repair-or-replace question, it is not the only one. Managers need to take into account additional factors that depend to a great extent on the application and vary among facilities.
The first step is to consider the application. Is the failed motor the correct motor for that application? Was it oversized or undersized? Has it been damaged in the past? What failed in the motor? Technicians can correct simple failures with such actions as bearing replacement or shaft straightening, which are relatively inexpensive. More extensive repairs, such as rewinding or replacing the rotor bars, are significantly more expensive and require careful evaluation.
In general, if the motor is a standard motor, meaning it is available off the shelf, and if it is less than 25 hp, the rule of thumb says it is more economical to repair failed bearings but not to perform a rewind on the motor. Bearing costs are low when compared to the cost of replacing a motor, but the cost of rewinding is not low.
If the motor is a standard motor and greater than 25 hp, the same rule of thumb applies to bearing replacement, though not to rewinding. For these motors, managers should get a quote for the cost of the rewind. The rule of thumb is if that rewind cost is 50 percent or less than the cost of a new, premium-efficiency motor, the rewind is the better option. Managers need to remember, though, that rewinding a motor results in a slight decrease in its operating efficiency.
Rules of thumb aside, downtime frequently drives the decision to replace rather than repair. In many cases, replacement motors are readily available, and technicians can install them in as little as a day. In other cases, the replacement motor might not be a standard motor and might require a fairly long lead time to order. The time required to repair a failed motor varies with the size of the motor and the workload in the repair shop. Managers must consider the length of time the application can be down before deciding on the most appropriate option for the facility.
Managers need to check with the local utility to see if rebates are available for improving the operating efficiency of motors in a given application. Combined with the improvement in energy efficiency, the rebate might be enough to tip the balance in favor of replacement.
Managers also need to consider the warranties offered for a motor repair compared with replacement. Warranties have value and are part of the evaluation process.

James Piper, P.E., is a national facilities consultant based in Bowie, Md. He has more than 30 years of experience with facilities maintenance and engineering issues