An overview of motor protection considerations to improve system health
Electric motors are imperative to maintaining large operations — as well as everyday tasks — in modern-day life. Almost every industry, business or person utilizes electric motors in a variety of ways, every single day. From air conditioners and dishwashers to subway systems and automobiles, electric motors turn electricity into mechanical power that runs the world we live in.
When it comes to the water industry, electric motors serve a vital role in the operation of treatment plants and pumping stations. In treatment plants, motors are used to power pumps, blowers, mixers, centrifuges, chemical dosing systems and other equipment to move water and support the processes that prepare water for safe release into the environment or consumption by the public.
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Because the electric motors play such an important role in operating treatment facilities, it’s essential to provide the appropriate protect and assess overall mechanical health to maintain equipment reliability. Just like any other device that runs for long periods of time, the motor can wear down and break if continual upkeep isn’t provided, causing headaches for the water facility and its operators.
Two of the most important parts of an electric motor are the windings in the stator, and the bearings. Electric current is passed though the windings, creating a rotating magnetic field that accelerates the rotating portion of the machine — the rotor — and provides the power to drive processes. The rotor is suspended between a set of bearings that maintains motor alignment and counteracts forces from the connected equipment. For properly specified motors, wear in the windings typically occurs over time as insulation breaks down. Bearing life is also limited and it is standard maintenance practice to fix or replace bearings periodically.
Maintaining the health of the motor and minimizing unexpected downtime of a system or process requires properly protecting these two components from excessive stress and monitoring key indicators of performance over time to understand when action must be taken.
Motor Protection Considerations
The simplest and most common means of protecting electric motors is overcurrent protection. Overcurrent protection stops the motor whenever motor amperage ratings are exceeded for a period of time. Overcurrent is detected by bimetallic switches in motor starters, or more commonly today, by current sensors in variable frequency drives (VFDs), soft starters or smart motor overloads. These methods protect the motor from excess mechanical loading and reduce equipment damage if an electrical fault occurs as both instances increase current demand to the motor.
Though these methods provide adequate protection for most equipment, they offer little, if any, insight into potential maintenance issues over time. More advanced overcurrent protection monitors additional electrical parameters such as current imbalance, voltage, loss of motor phase and other parameters that improve protection provided by the motor controller.
Motor Temperature & Heat
Heat is a contributing factor to reduced motor life and an indication of excessive wear or potential failure. Motor windings and bearings are often outfitted with temperature switches, thermistors or resistance temperature detectors that are paired with control circuits to alarm or stop the motor if temperatures exceed allowable limits.
Motor temperature limits may increase beyond recommended limits due to high ambient temperatures, dirty motor housings and blocked cooling airways, or by imminent failures of bearings or windings. Temperature switches provide improved protection, while sensors enable trending over time to give maintenance staff insight into bearing wear or early warning of pending motor trips to allow for corrective action to be taken at planned times.
Motor protection is often supplemented by vibration sensing systems that detect bearing wear, rotating equipment wear and system misalignment. Basic systems provide switching or standard 4-20 mA analog feedback of vibrations from the motor bearings. More advanced systems process vibrational data to provide vibration magnitude at multiple frequencies. This helps differentiate bearing wear versus other vibrations in the system, giving more accurate feedback to maintenance staff than data provided by standard analog sensors.
Applications the Benefit Most From Motor Protection
Applications that benefit from increased motor protection include installations in locations with high temperatures, submersible pump or mixer applications, remote sites, mechanically demanding processes and large — greater than 150 horsepower (HP) — motors. Improved controller-based motor protection is becoming a standard feature from VFD or motor control center manufacturers. Additional switch-based protection is a low-cost step up from overload-only systems, while sensors require additional wiring, programming and reporting to enable maximum benefits. Advanced vibration sensing equipment is reserved mostly for greater than 500-HP motors driving difficult loads.
Another more cost-effective option to consider is setting up a motor maintenance program. With this program, maintenance staff create informational trends by periodically measuring and recording temperature and vibrational data with hand-held equipment at consistent intervals. Tabulating these trends can help better predict when the bearings, windings or other motor parts could experience heating or vibrational issues, serving as an early warning system for motor failure.
When conducting a maintenance program, it’s important to perform measurements under consistent process and ambient conditions and record these parameters to gain accurate context for each measurement.
Path to Better Motor Health
Finding the right protection method requires careful consideration of the equipment and loads. Applying improved protection on the most critical loads and assessing the benefits of increased motor protection is a good way to start. Maintenance programs can fill the gap for smaller, less critical equipment.
For owners and operators that are looking to make electrical systems upgrades or perform expansion work at their facilities, newer state-of-the-art motor controllers coupled with data collection systems should be strongly considered, as the total installed cost may be equivalent to standard offerings. Most packaged systems, such as blowers and pumps, have standard upgrade packages with improved sensing. Staff training and procedures are key to extracting the value of these systems.
Motors are essential to everything from large operations to everyday tasks. With a variety of protection options available to water facility operators, motor health maintenance should never have to fall by the wayside. It’s important to find a monitoring and protection solution that suits the facility and operations staff as well as one that can be maintained long term.