Maintenance and management strategies for better-performing electric motors

Motor maintenance insights

• A good motor management strategy must consider the motor life from beginning to end and having a consistent plan will reduce downtime and repair or replace costs.
• Motor maintenance involves many steps including inspection, installation, commissioning and more and the maintenance strategy must be consistently followed throughout to avoid potential issues.

Electric motors are the drivers for most equipment from factories to commercial building HVAC systems. Developing a strategy for the lifecycle of the motor makes good operational and business sense. A good motor management strategy must consider the motor life from receipt to repair to recycle. Planning for the motor’s life will pay dividends versus just letting it happen.

Motor inspection tips

When a motor is first received, it will need to be inspected for any damage that may have occurred in shipping. Remove any plastic covering used to protect the motor during shipment because this could cause condensation to build up while the motor is waiting to be installed. If the motor is not going to be installed right away, energize the motor space heaters, if installed, to keep condensation out of the motor windings, and fill oil reservoirs with oil to protect the bearings.

Every motor should come with an instruction manual with installation and maintenance instructions. The requirements and recommendations should be integrated into the facilities maintenance and management practices as much as is practical.

Motor storage best practices

If a motor is not going to be installed within a short time after receiving it, refer to the motor’s operations manual for the correct procedures for storing the motor. The procedures may vary due to the length of time a motor will be in storage. The storage area will need to be a space that will have a minimal amount of ambient vibration as this can damage the motor bearings. Periodic maintenance will need to be performed on the motor as well. This maintenance may consist of rotating the motor shaft, performing winding inspections such as megger checks, and oil or grease analysis.

Figure 1: 600hp 1800rpm Vertical Solid Shaft TEFC on a pipeline pump supporting the Canadian Oil Sands. Courtesy: Nidec Motor Corporation

Installation and commissioning of motors

When it is time to install the motor, couple of steps need to be completed. If the motor has been filled with oil, it will need to be drained before moving. Moving a motor full of oil could cause the oil to slosh around and wind up inside the motor. If the motor has been in storage for an extended period of time, it may need to go to a service shop to be inspected and brought back to “as new” condition.

A motor’s installation location is an important consideration. It is important to keep a motor clear of obstructions to not restrict ventilation. Unless otherwise specified on a nameplate, a motor is designed for operation in accordance with National Electrical Manufacturers Association (NEMA) MG1 “Usual Service Conditions,” which states an ambient temperature range of -15° to 40° C (5° to 104° F). Standard grease lubricated units are suitable for operation within this temperature range. Special lubricants may be required for ambient temperatures outside of this range.

Motors operating under rated load and allowable ambient conditions may feel hot when touched; this is normal and should not be cause for concern. When in doubt, measure frame surface temperature and confer with the motor manufacturer.

Enclosed motors often have condensation drain openings. Ensure drain openings are properly located and open (plugs removed) for the motor mounting position. Drain openings should be at the lowest point of end brackets, frame housing and terminal housing when the motor is installed.

Figure 2: Example of a 300hp 3600rpm TEFC methane compressor pulling methane out of dairy farms and compressing for a pipeline in California. Courtesy: Nidec Motor Corporation

Mount the motor on a firm, flat surface sufficiently rigid to prevent vibration. Drive belts and chains should be tensioned in accordance with supplier recommendations. Couplings should be properly aligned and balanced. For belt, chain and gear drive selection refer to the drive or equipment manufacturer. For application of drive equipment refer to applicable information in NEMA MG1.

Motor to equipment alignment is important to motor and equipment bearing life. Refer to the motor’s installation manual or driven equipment’s manual for the alignment specifications. For horizontal motors, check for soft foot conditions when performing motor alignment. When using shims use a size that will cover approximately 80% of the foot area. Use no more than 5 shims under each foot with no more than one shim being thinner than 0.003” and the sum of the thinnest three must be greater than 0.010.” Shaft alignments will vary depending on speed. Two pole motors (3600 RPM) will have the tightest alignment requirements.

For vertical motors, alignment will depend on what type of shaft the motor has. For vertical motors with hollow shafts, the pump shaft and coupling must be aligned within 0.003” and for vertical motors with a solid shaft, alignment must be within 0.002.”

Motor connections should be made by following instructions on the motor connection diagram. The direction of rotation should be determined before connecting to the driven equipment. Rotation may be reversed on three phase motors by interchanging any two-line connections. On single phase motors interchange leads per connection diagram on motor. The wiring of units, controls and grounding should be in accordance with local and National Electrical Code (NEC) requirements.

A key element of motor management is analyzing the health of the electric motor. Electrical and mechanical health can be measured over time based on resistance, amps and vibration. After initial installation it is good to perform baseline electrical and vibration checks. These initial values for resistance, amps and vibration are critical as a baseline to compare future checks to determine the health of a motor.

Equipment to perform a baseline measurement includes a “megger” to test resistance of the motor windings, an ammeter and a vibration measurement device. Periodic checks of the motor system using the equipment over time provides good insight into the health of the motor. Resistance and amps indicate how the insulation of the windings are holding up to the environment. Steady vibration values indicate the motor’s mechanical and some electrical elements are healthy. With this information, trended over time, maintenance, repair and replacement has a much better opportunity of being scheduled instead of occurring without warning.

Figure 3: Example of a 700hp 3600 rpm WP2 methane compressor pulling methane out of dairy farms and compressing for a pipeline in California. Courtesy: Nidec Motor Corporation

The importance of motor maintenance

Maintenance is key to keeping the motor healthy. Cleanliness and lubrication are key maintenance activities to be completed. At a basic level, an initial start to maintenance is a basic inspection of the motors at regular intervals. Motors should be kept clean and ventilation openings clear of dust, dirt or other debris.

One of the key factors in long electric motor life is ensuring it is fed by high quality power. Small imbalances in the voltage or frequency could cause major damage to the windings. It is important to regularly check the voltage and frequency of your plant’s power supply to make sure it falls within the motor manufacturer’s specifications. Running a motor with low quality or the wrong voltage or frequency can cause damage and shorten the motor’s life.

Winding resistance can be measured using a “megger” test. Completing this check on a periodic basis helps provide an understanding of how the insulation of the windings is holding up to the environment and allows the user to build a history of the motor. It also is beneficial to visually inspect the motor windings during a scheduled maintenance event for signs of damage, discoloration or dirt and debris.

Lubrication – grease and oil best practices

As a safety precaution, a motor should be disconnected from any power source before any maintenance or servicing. Periodic vibration or ultrasonic inspections can let you know if there are changes in your bearings and can let you plan for a bearing replacement instead of an emergency repair.

Bearings should be lubricated per the manufacturer’s operating instructions and any additional instructions found on a motor mounted plate. Lubrication is critical, but excessive grease may damage the motor. Following lubrication instructions, including process, periodicity and volume is important to maintaining the life of the motor.

Motors with grease lubrication are pre-lubricated at the factory and do not require initial lubrication. Relubricating interval depends upon speed, type of bearing and service. Refer to Table 1 for suggested regreasing intervals for some common size motor bearings. Harsh operating conditions such as high humidity or dirty environments may dictate more frequent lubrication.

Vertical high thrust motors are often used in pumping applications and require some special considerations to protect the bearings. For example, certain motor enclosures have screens that must be cleaned. Also, accessory parts for these motors such as lubrication and water-cooling systems for the high thrust bearings should be included in maintenance.

Efficient motor management will increase uptime and lower total cost of ownership but it must start with procurement and does not end until recycle. Courtesy: Nidec Motor Corporation

Motor equipment’s criticality to an operation

If the equipment the motor is driving is a critical component to plant operation, then decreased intervals with more check points should be implemented in the maintenance schedule.

Inspections should always include the cooling system, whether it is making sure the fins of a totally enclosed fan cooled (TEFC) motor are clear, the cooling fans of the motor are not damaged, or there are not outside factors increasing the ambient temperature. Always check to make sure the motor cooling system is operating correctly.

If air filters are installed on the motor, check them regularly to make sure they are not clogged and blocking airflow to the motor. Clogged air filters can cause a motor to overheat.

Figure 4: Example of a 30hp Vertical normal thrust in a wastewater treatment plant in California. Courtesy: Nidec Motor Corporation

Considerations for motor repair and replacement

A repair or replacement plan is needed for each motor. Of course, every good plan can change if conditions warrant it, but creating a plan will decrease the cost of keeping an operation running.

A motor plan should include several elements. First is the decision to repair or replace the motor. The larger the motor, the cost to purchase and maintain a spare or the likelihood a replacement motor would be in stock locally drives the decision toward repair rather than replacing it. Of course, if the motor’s criticality to the operation is very high, a spare may be the correct decision.

If repair is the intended solution, then a plan should be in place. The plan should include the shop or location for repair and, if possible, have motor winding information on file to speed up the repair process.

If replacement is the intended course, then a spare motor should be purchased or a replacement motor model should be identified including the manufacturer and the location and contact information for a local supplier. All of this planning speeds up the process of getting equipment back up and running. Creating a plan like this can seem daunting, but it can be completed over time.

Spare motor inventory can be a challenge for companies. With the average life of a motor at approximately 17 years, a spare could be a spare for a long time. That spare requires maintenance of its own as well as protection and it is an investment. Those factors add to the spares challenge.

To minimize spares a few processes and procedures can be implemented. The first step is standardizing motors being acquired across the facility. Having a consistent specification for multiple installations will allow for a single spare to be utilized in multiple applications. The second step is identifying reliable primary, secondary and even tertiary local motor distributors that can supply common replacement motors. With confidence in outside supply, spares can be focused on more unique installed motors. The third step is evaluating repair. In some cases, the spare investment may be too expensive or not practical. If that happens, have a repair plan in place with motor repair data ready to use. These three factors should assist in determining a reasonable spares plan.

A motor management plan should be holistic from receipt to recycle. Having procedures and proactive plans in place will lead to greater uptime and lower overall costs for the motors installed to run the facility.

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