Industrial and manufacturing facilities rely on motors to drive their processes. They are included in preventive maintenance programs because of investments, their critical importance to operations, and the cost of production downtime.
Motors are less visible in commercial facilities, where they are typically located behind closed doors in equipment rooms and closets. Nonetheless, they are very important because they drive fans, pumps, chillers, compressors, and other mechanical equipment that directly impacts facility operation and the well-being of its occupants.
Induction motors transform electrical energy into mechanical energy by induction, just as energy is transferred from the primary to the secondary of a transformer. The only difference between an induction motor and a transformer is that the “secondary” of an induction motor is the rotor, which rotates.
The “primary” is the stator winding, and it produces the rotating magnetic field that drives the rotor by inducing current. Most induction motors used in commercial applications are squirrel cage induction motors, which consist of bars embedded in a solid rotor and connected electrically through solid end rings. Only wound-rotor induction motors actually have windings on the rotor, and only some have connections to the rotor windings with brushes and slip rings for speed or torque control.
Need for preventive maintenance
Preventive maintenance involves identifying potential problems and correcting them before the equipment fails. Due to their construction, induction motors are very reliable, so many customers do not have effective preventive maintenance programs.
This is especially true for smaller commercial facilities with limited maintenance staff who lack the time and understanding to perform routine motor inspections and maintenance. These customers typically do not believe that any type of preventive maintenance needs to be performed and that motors just run until they fail and then are either rebuilt or replaced.
Customers should be informed that unnecessary expensive repairs, premature motor rebuilds and replacements, equipment and system downtime, and disruption to facility and occupant operations can be avoided by establishing a simple cost-effective preventive maintenance program.
When do motors fail?
Motors tend to fail early in their service life due to manufacturing defects, damage before or during installation, improper installation, or misapplication. Similarly, motors have a high failure rate as they approach their rated life, which is typically determined by their insulation. In between these two endpoints, motors should experience low failure rates.
Motor preventive maintenance
Motor preventive maintenance, especially for small induction motors, is more than just inspecting and maintaining the motor itself. If left running under rated conditions, an induction motor will provide many years of trouble-free service and may even outlast the building.
However, induction motors don’t always operate in ideal conditions. Even in the best operating environments, they are occasionally subjected to adverse operating conditions due to spills, equipment failure, operator error, poor power quality, and other factors.
Motor failure is often symptomatic of external factors. A cost-effective preventive maintenance program for small induction motors should focus on external factors because service personnel can easily assess them and identify potential problems. A strategy for correcting the potential problem can be developed or additional tests can be performed on the motor to determine the gravity of the situation.
Preventive maintenance activities
Preventive maintenance activities should focus on inspection and limited testing. It should address the motor’s physical environment, its condition, and the load that it serves. The physical environment is especially important because it reflects on the motor’s insulation life, which in turn determines when the motor fails.
The ambient area temperature in which the motor is installed needs to be checked. Facilities change over time, and space may be reconfigured, ventilation may become restricted, and new motors and other heat-generating equipment may be installed near the motor.
Changes like these can result in an increase in ambient temperature, which can significantly reduce the motor’s insulation life and must be corrected. A quick check of the motor’s operating temperature can be made by using a contact thermometer or feeling the motor by hand.
Dirt and grease buildup on the motor or its windings can also result in insulation failure. Buildup on windings can reduce insulation levels and result in insulation failure that will require rewinding or replacement. Similarly, dirt and grease buildup affect the motor’s ability to transfer excess heat to its surroundings, which will damage winding insulation.
Excessive dirt and grease need to be removed from the motor housing and ventilating openings. If found in the windings, they should be removed in accordance with manufacturer recommendations.
The electrical operation of the motor and its mechanical output can quickly be checked by measuring the current drawn while the motor is operating under load by using an ammeter, which measures true root mean squared (TRMS) current. Under normal operation, the phase currents should be balanced, and the current
magnitude should be at or below the motor’s nameplate rating.
Unbalanced phase currents may indicate a problem in the motor’s windings, which can also be the electrical cause of excessive vibration, or slowness to reach starting rated speed. Similarly, excessively high currents can indicate winding problems in the motor, motor bearing problems, motor-load alignment, or a problem with the load itself. If any of these symptoms are encountered, additional investigation and testing needs to be performed to determine the root of the problem.
All rotating machinery vibrates when it operates, but excessive motor vibration can damage insulation and bearings. Electrical problems such as open circuits in the bars and end rings of a squirrel cage rotor can cause vibration.
More likely though, excessive mechanical vibration is caused by a mechanical problem in the motor such as a bearing failure, a failure to properly anchor the motor, misalignment between the motor and the driven load, or a mechanical problem in the load. The purpose of preventive maintenance in this case would be to note that there is excessive vibration by observing the operation of the motor, noting excessive noise, and/or simply placing a hand on the motor while in operation.
If excessive vibration is suspected, then additional testing is required. Chapter 25 of NFPA 70B entitled Recommended Practice for Electrical Equipment Maintenance provides detailed information on vibration analysis for rotating machinery.
Establishing an effective maintenance program
You must balance the costs of motor maintenance and failure. Maintenance takes time and is an expense for your customer. It would be impractical to perform the same tests, inspections, and maintenance on a 30 horsepower motor driving a fan as it would on a 300 horsepower motor driving a chiller. However, simple preventive maintenance activities are cost effective and can help reach their rated life by ensuring that these smaller motors are operating properly.
When establishing the preventive maintenance program, the motor’s size and cost should not be the only factors considered. The motor’s function and impact on the facility and its occupants should also be taken into account to determine the appropriate type of regular maintenance. In addition, any hazards that might result from a motor failure should also be factored into its maintenance program.
For example, a small motor driving an exhaust fan that removes fumes from a paint booth where artists work may require more attention than one that does not have the same safety implications for occupants.
If you already perform regular lighting maintenance for your customer, you could offer to perform routine motor maintenance. This should reduce the cost of performing the motor maintenance because service personnel are already on site. Similarly, other noncritical service work could be scheduled.
This type of preventive maintenance focuses on the customer’s smaller motors and involves mainly inspection and simple tests to identify potential problems that need to be corrected in order to avoid premature failure. It does not focus on large motors that require specific maintenance, testing procedures, and scheduled downtime and personnel that specialize in motor maintenance and repair.
This article is the result of ongoing research into the development of service contracting business by electrical contracting firms sponsored by the Electrical Contracting Foundation, Inc. The author would like to thank the foundation for its continuing support.
GLAVINICH is Chair and Associate Professor of Architectural Engineering at the University of Kansas. He can be reached at (785) 864-3435 or email@example.com.