Motor Condition Monitoring: Efficiency does Matter
30 December 2008
The concept of Predictive Maintenance has now become accepted practice, worldwide. Locating, defining, and acting on potential problems before they become catastrophic is the main objective of a predictive maintenance programme.
Before and after
Techniques include thermography, vibration analysis and motor monitoring. Routine monitoring is not only the most effective method of determining an early diagnosis, but has many other positive aspects.
Monitoring motor performance using modern equipment allows plant and facility managers to dictate their own down time, improve plant operation and quickly identify poorly performing equipment.
Photo caption: Preventing critical pump failure - a case study
Here the Baker Explorer tester was used to trend the likely time taken for a sewage works pump to become clogged with rubbish. Clogging had previously caused many pumps on this large wastewater site to run overload and fail. Energy consumption was also reduced through condition monitoring and a planned cleaning programme in advance of critical failure.
WHICH MOTORS TO MONITOR?
It is important to establish which motors are to be monitored. Criticality, starts and stops, ease of testing and availability of spares are all concerns that must be considered. But essentially, motors that have a history of poor performance and problems should be monitored more often.
It is vital to the success of the predictive maintenance programme that motor importance be defined, a routine schedule be established and followed and the indicated repairs and adjustments made in timely manner.
HOW TO MONITOR
Motor monitoring has become a vital tool with two facets that must be considered in order to obtain a successful diagnosis of a motor’s condition. These are ‘offline’ testing and ‘on-line’ testing.
A motor has numerous components including, copper windings, insulation systems, bearings, that must be tested and trended. The insulation consists of the very thin protection to the winding or magnet wire plus the ground wall insulation that covers the magnet wire in the slots. Off-line testing equipment can effectively define the condition of this insulation and when properly trended aids in “predicting” the probability of the motor’s future.
An effective off-line test should consist of winding resistance, meg-ohm, high potential and surge tests. Winding resistance can locate shorted turns, open leads and phase unbalance problems. The meg-ohm test will identify grounded and contaminated windings. The high potential test looks for poor ground wall insulation and the surge test locates weakness within the copper wire itself.
On-line equipment has become the “tool of choice” for many maintenance personnel. It is safe, quick and 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, 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. Motors often fail and are repaired or replaced and returned to service without critically, the “root cause” of the failure being determined. On-line equipment can define subtle power quality issues such as harmonics; low or high voltage and voltage inbalance. 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.
One major domain that is identified and tracked through motor monitoring is efficiency.
Efficiency is the ratio between useful work performed and the energy expended in producing it. Put simply it is 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 normally operating. Minimum efficiency is the lowest efficiency value any motor within a “test sample” must maintain. 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 they mean. 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 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 get 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% which 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?
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 utilising energy efficient motors.
How does efficiency affect the cost of plant operation? Consider a 75 kW motor/load that is operating 24 hours per day and 365 days per year with energy costs at $0.05 per kilowatt hour (kWh).
A motor operating at 85% efficiency will use 768,819 kilowatts in that year at a cost of $38,441.00. Improving to 90% efficient will lower the cost by $2136.00 and reduce energy demand by 47,512kWh. A motor operating at 95% efficiency will use just 687,891kWh and cost $34,395.00, a savings of over $4,000.00.
HOW CAN EFFICIENCY BE IMPROVED?
Motors are affected by countless environmental, mechanical and electrical issues that 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.
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. 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.
Energy costs are a major portion of any plant or facility’s operating expenditures and motors consume a very large part 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 time required to carry out a complete predictive maintenance programme will also prove to be a great asset. Monitoring motors is a safe and sure method to determine motor condition and efficiency.
Finally, energy saving not only directly benefits the company but also correlates into helping the environment!
Timothy M. Thomas,
Senior Applications Engineer
Baker Instruments, an SKF Company.
On behalf of Whitelegg Machines Ltd
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