Efficiency does Matter
Why Monitor?
The concept of Predictive Maintenance has become a world-wide accepted practice and is being implemented and finely tuned by nearly every industry category.
Locating, defining and acting on potential problems before they become catastrophic is the main objective of a predictive maintenance program. Routine monitoring is not only the most effective method of locating potential problem areas, as an early diagnosis, but it has many other positive aspects. Often, upon a thorough inspection with state-of-the-art equipment, numerous issues, some minor but most unknown, each can be identified and rectified, therefore avoiding critical downtime and ensuring a more efficient operation. A series of minor issues can add up to a major problem.
Monitoring motor performance with properly trained technicians using modern equipment allows plant and facility managers to dictate their own down time, improve plant operations and quickly identify poorly performing equipment.
What to Monitor?
When establishing a new or enhancing an existing predictive maintenance program, defining which motors should be monitored must be the first consideration.
Criticality, the number of starts and stops, starting loads, ambient temperature, ease of testing, man power and availability of spares, are all concerns that must be considered. Every situation is different and each requires individual considerations, but the main objectives and basic plans will be similar. Motors that have a history of poor performance and those that have been problem motors should be monitored more often.
It is vital to the success of the predictive maintenance program that motor importance be defined, a routine schedule be established and followed and the indicated repairs and adjustments be made in timely manner.
How to Monitor
Predicting imminent motor failures requires knowledge, experience and as many “tools” as feasible. The more, properly used tools a technician has, the more likely he will be able to “predict” the health and longevity of his assets. Motor monitoring has become a vital tool with two facets that must be considered and fully utilized in order to obtain a successful diagnosis of the motor’s condition: offline testing and on-line monitoring.
The motor itself has numerous components including, copper winding wire, insulation systems, bearings, and other mechanical and electrical features that must be tested and trended. The insulation system consists of the very thin insulation manufactured onto the winding or magnet wire and the ground wall insulation that protects the magnet wire in the slots. Off-line testing equipment can effectively define the condition of these insulation systems and when properly trended will aid in “predicting” the probability of the motor’s ability to remain in service or if repairs and/or adjustments are indicated.
An effective off-line test should consist of winding resistance, meg-ohm, high potential, or step-voltage test, and surge tests. Winding resistance can locate shorted turns, open leads and phase imbalance issues. The meg-ohm test will identify grounded and contaminated windings. The high potential test looks for poor ground wall insulation and the surge test identifies turn to turn, or copper to copper weaknesses. It should be noted that the surge test is the only test that can identify weakness in the turn insulation long before it becomes a hard welded fault which will result in rapid winding failure.
On-line equipment has become the “tool of choice” for many maintenance personnel as it is safe, quick, non-intrusive and provides an enormous amount of information in one report. A motor is a part of a “total machine system” which comprises of three components: the incoming power quality, the motor itself and the load being driven by the motor.
On-line testers can locate both electrical problems and many mechanical issues that might otherwise go undiagnosed. Often motors fail and are repaired or replaced and returned to service without determining the “root cause” of the failure. On-line equipment can define subtle issues with power quality such as harmonics; low or high voltage and voltage unbalance situations. Rotor bar problems, bearing issues, misalignment and many other problem areas can also be identified. All of these issues can negatively affect the motors operation including its efficiency.
Understanding Efficiency
One major domain that is identified and tracked through dynamic electrical monitoring is efficiency. Efficiency is defined as the ratio between useful work performed and the energy expended in producing it. It is, simply put, the ratio of output power divided by the input power. Efficiency is usually defined in one of three ways, nominal efficiency, operating efficiency and minimum efficiency.
Nominal efficiency is that value which is assigned to a set or group of motors by the manufacturer and designated on the motor’s nameplate. Operating efficiency is the true efficiency of the motor as it is operating within its actual and normal environment. Minimum efficiency is the lowest efficiency value any motor within a “test sample” must maintain at full load. Modern test equipment will define the operating efficiency of the motor being tested.
To understand efficiency we must first understand how the values are derived and what the numbers mean to us. The Institute of Electronic and Electrical Engineering or IEEE defines how motor manufacturers must measure and assign efficiency values to motors. Publication IEEE 112B sets the standards and describes the method that must be followed when motors are produced.
Basically, motors are randomly selected off the production or assembly line and tested on a “dyno” within a completely controlled environment. The voltage is clean and perfectly balanced, the load is dynamically controlled and the test areas are separated from any possible vibration or sound interference.
A series of motors is measured and a mean average is determined. By definition, manufacturers are allowed a 20% window of the losses, which gets added to the mean average and all motors within that set are designated with that “Efficiency Rating”.
For example, consider a hypothetical situation where 100 motors are being tested in order to determine nominal efficiency. Let’s say that some will test as high as 95% and the lowest as low as 93% with a mean average of 94.5%. The mean average losses are 5.5% so the accepted 20% window would be 1.1%. This value gets added to the mean average of 94.5% which brings the “Nominal Efficiency” for this set of motors to 95.6% and that number gets “stamped” into each motor.
While this hypothetical case may be somewhat extreme it is not without possibility and indicates the confusion surrounding the idea of motor efficiency.
Why is Efficiency Important?
Why is motor efficiency so important to know? Worldwide, motors consume between 55% and 63% of all electrical energy being produced. Recent US Department of Energy reports show that energy costs could be cut by at least 18% by utilizing energy efficient motors. Premium efficiency motors have saved billions of dollars in energy costs each year and manufacturers are continually looking for ways to make motors more efficient.
How does efficiency affect the cost of plant operation? Consider a 100Hp motor/load that is operating 24 hours per day and 365 days per year where energy costs is $0.07 per kilowatt hour (kWh).
A motor operating at 85% efficiency will use 795,118 kilowatts in that year at a cost of $55,588.00. Improving to 90% efficient will lower the cost by $4,488.00 and reduce energy demand by 65,118kWh. A motor operating at 95% efficiency will use just 691,579kWh and cost $48,411.00, a tremendous savings, and that is just one motor.
Studies further confirm that efficient motors are more reliable with an extended useful life expectancy.
How can Efficiency Be Improved?
Motors are affected by numerous environmental, mechanical and electrical issues, many of which could be rectified or improved. Low or high voltage issues are generally correctible through transformer taps and harmonic issues can be mitigated with various reactors, shielded cables and isolation transformers. Misalignment, imbalance and other mechanical issues are generally easy fixes.
Low voltage levels cause motors to be slightly less efficient and may raise current levels beyond design. Motors “like” to operate around 90% to 95% and lower load levels tend to make them less efficient, especially smaller low voltage motors. Overloaded situations cause significant drops in efficiency along with numerous other issues including a significant rise in operating temperature.
Mechanical issues, such as misalignment, physical looseness and imbalance, not only adversely affect a motor’s performance and longevity but also its efficiency. Many times minor adjustments can add years to a motor’s life and result in big savings due to improved efficiency.
Additionally the costs of replacing poorly performing motors with Premium Efficiency motors will be quickly offset by the savings in energy costs.
Summary
Energy costs are a major portion of any plant or facility’s operating expenditures and motors consume the majority of those expenses.
Monitoring the motor’s performance and making necessary adjustments will improve reliability, extend the life of the motor and reduce the overall operating cost of the facility.
Providing well trained, proficient technicians with state-of-the-art equipment and with the necessary training, experience and time required to carry out a complete predictive maintenance program will also prove to be a great asset. Monitoring motors is a safe and sure method to determine motor condition and efficiency. Effective PdM programs are a cost savings mechanism.
A more logical approach to a refined PdM program may be to share monitoring and trending with an experienced partner. It is difficult and cost prohibitive for most companies to employ designated teams that have the experience, the time and the state-of-the-art test equipment to achieve a successful predictive strategy. Equipment costs, training and finding experienced technicians is often difficult and too expensive.
Finally, saving energy not only directly benefits the company but also correlates to a healthier environment!