Premium-efficiency motors replace old, poorly rewound standard motors, resulting in noteworthy savings while operating on a single shift.
Contrary to popular belief, two premium-efficiency (PE) motors are reducing electric operating costs by significant amounts while operating only one shift per day, instead of the usual two shifts or on a continuous basis. The two 460V motors, which are rated 75 hp and 40 hp respectively and drive conveyors, will cut electric costs by about $4353 per year. More importantly, they will have a payback of 1.1 and 1.7 years respectively.
Conventional wisdom as well as years of experience have shown that any continuous-duty motor application (or in some cases, a two-shift application) is an excellent candidate for a profitable retrofit with PE motors. In spite of this rule-of-thumb, however, engineers at a West Coast rock-mining plant spotted two likely candidates, each on single-shift duty at a conveyor. These old motors had been rewound several times; however, the rewinds had been done on a rush basis, with little thought given to how they were done. While auditing these motors during a maintenance check, it was discovered that they had not been rewound by an EASA (Electrical Apparatus Service Assn.) firm.
Watch out for rewound motors
Certain motor repair shops (non-EASA) have been known to deliver a rewound motor quickly by sacrificing efficiency for speed. Fast turnaround on a rewind often requires a higher temperature setting in the burnout oven, which is used to remove windings and insulation from the stator laminations. Overheating the laminations decreases the efficiency of the motor because electrical properties of the steel deteriorate from excessive heat. (See sidebar, on page 76).
With the above in mind, the two conveyor motors were immediately singled out for tests to check their efficiencies.
Field tests evaluate motors
To confirm suspicions of possible motor inefficiency, engineers suggested that field tests be run on these motors. This was because the printed or nameplate data could not be relied upon to provide an accurate comparison with PE replacement motors; the motors had been rewound, and their characteristics probably had changed. Thus, field testing was the best way to obtain the most accurate data possible.
The location for the tests was at the two side-by-side main-truss conveyors, which are 200 ft in length and pitched at an 18 [degrees] incline. The only difference between conveyors was that one carried a heavier payload of rock.
Each conveyor was powered by one of the rewound motors, which had to endure severe operating conditions, such as heavy dirt deposits on housings and an ambient temperature that could reach 110 [degrees] F in the summer.
The only objective way to compare the performance of the new motors to the old was to compare energy costs per ton. This was done through the use of digital scales on each conveyor. These scales are used for weighing the load running on the conveyors, since the conveyors' loads can vary significantly from hour to hour.
The existing driving motor at the heavier payload conveyor was the 75-hp motor, which was an open dripproof, T-frame type. The lighter payload conveyor was powered by the 40-hp motor, which was a TEFC, U-frame type.
Field tests for energy consumption on the existing motors were run; then, with these motors replaced, the same tests were run on new PE motors so that an accurate comparison of efficiency and operating costs between the old and PE motors could be evaluated.
Four tests were run. First, the original 75-hp motor was tested. The second test was on its replacement: a PE, TEFC, severe-duty motor of the same horsepower. The next two tests were the same but on 40-hp motors. Each motor was evaluated continuously for approximately five days. Running speed was a constant 400 ft per min, with the conveyors carrying about 500 tons of material per hour. The conveyor's digital scale recorded the weight, time, and speed of the material being moved.
A strip-chart instrument was used to record data and ran continuously during the test period, recording time, voltage, kVA, kW, and power factor (PF) of the conveyor motors before and after the retrofit. A demand-summary report was automatically printed at midnight each day, and indicated the following:
* Date and time of the four highest 15-min demand intervals of the day/month.
* Daily energy usage in kWh.
* Total test period energy use in kWh.
Current demand and PF were measured every 15 min. Finally, a manual check was done with handheld instruments twice a day to evaluate and confirm line voltage, current, kVA, kW, and PF.
After the tests were completed, the data yielded some surprising results.
A significant point was the low efficiency of the existing rewound motors. According to the U.S. Dept. of Energy's Average Standard Industry Motors table (published in 1980), the original 75-hp and 40-hp motors were supposed to have efficiencies of 90.8% and 89.4%, respectively. However, actual test results showed that the 75-hp motor was closer to 67% efficient, and the 40-hp motor was around 73% efficient. As a result, the existing rewound 75-hp motor used 42% more energy to do the same job than did the PE motor; the existing rewound 40-hp motor used 12% more energy than the PE motor.
As shown in Tables 1, 2, and 3 (on page 76), calculations based on $.0924/kWhr [TABULAR DATA FOR TABLE 1 OMITTED] [TABULAR DATA FOR TABLE 2 OMITTED] establishes an annual dollar savings of $3328 for the new 75-hp motor. A similar calculation for the new 40-hp motor shows a $1025 savings. After accounting for installation cost and salvage value, it was determined that the cost of replacing the 75-hp motor would be recovered in 1.1 years, while the 40-hp motor retrofit would pay for itself in 1.7 years. This was very encouraging, given that the calculations assumed a 40-hr work week operating 52 weeks a year. (Actual use varies between seven and 10 hrs a day. For calculations, 40 hrs per week were used as a conservative average.)
It's important to note that the test on the replacement motor (new PE motor) was run immediately after it was installed. This is important because new motors require a break-in period for the bearing grease to achieve its proper viscosity. Now that the bearing grease is broken in, actual energy savings should be slightly greater.
In the test summaries, the PF for the existing rewound motors tended to be relatively normal, ranging from 69% to 91% on the 75-hp model and 63% to 70% on the 40-hp model, depending upon loading.
The PF for the new 75-hp PE motor was found to be lower than expected, with a range between 30% and 63%. The range on the replacement 40-hp PE motor, however, improved over its predecessor, with a range between 81% and 92%.
Why the mixed performance? In the case of the 75-hp motor, the decrease in PF suggests that the motor is oversized. The original decision was to replace the existing motors with new motors of the same hp rating. With an input horsepower estimate of 25.6, and given the need in this application for greater start-up power to overcome the inertia of the loaded conveyor, a 50-hp PE motor was recommended to replace the 75-hp PE motor.
Installing a 50-hp motor would increase the savings, compared to the new 75-hp motor. At the same time, the initial cost of the 50-hp PE motor would be less than the new 75-hp PE motor. From this, you can infer that the payback on a new 50-hp motor would be significantly less than 1.1 years originally calculated.
The 40-hp motor, on the other hand, appeared to be properly sized. Its input low horsepower estimate, along with its improved PF, suggest that its hp rating is appropriate for the application.
Other retrofit benefits
Keep in mind that, with any retrofit, the running speed of a PE is greater than that of a standard-duty motor. For pump and fan applications, this means the new motor system would have to be adjusted to account for the faster speed.
In this conveyor application, however, the faster speed of the PE motor is a consideration only to the extent that the output of the conveyor will be increased slightly as a consequence of more stone being moved over time.
These new PE motors should also provide greater reliability in this application. With Class F insulation and a bearing system that's rated for 50,000 hrs in belted applications like these conveyors, the rock plant can expect trouble-free performance and long life from these motors.
The retrofit also provides an important maintenance benefit. Since the original standard motors were assumed to have damaged cores, it's reasonable to conclude that this condition inevitably would have lead to higher repair costs and, worse, some very expensive downtime. That's why looking at all motors in a selective retrofit scenario, regardless of the hours of daily service, is a valuable consideration.
RELATED ARTICLE: BE SURE AN EASA FIRM REWINDS YOUR MOTOR
The major potential problems associated with rewinding motors is that some less-than-conscientious motor repair shops do not use proper procedures during rewinding of the motor. The problem is that unreliable shops use excessively high burnout temperatures that damage the motor core.
Stator cores are comprised of laminations of electrical-grade steel. Made from flat-rolled steel, each manufacturer's lamination has a covering of insulation ranging from a surface-oxide film developed during steel processing to an inorganic coating applied to assure specific physical and electrical characteristics.
Some of the coatings used in older designs or standard-efficiency motors are more susceptible to deterioration at higher temperatures in burnout ovens.
The Electrical Apparatus Service Association (EASA), an organization dedicated to promoting quality electrical repair, stresses the need to maintain high efficiency in rewound motors. This need is given top priority as part of a comprehensive quality management system called EASA-Q.
As a result, shops that use the EASA-Q standards have the equipment and procedures to assure that efficiency-degrading temperatures that damage the motor cannot occur. Most EASA firms test the core laminations when the motor is received, during stages of processing, and after completion of rewind or repair to be certain that efficiency is maintained or, in some instances, improved.