Copper Rotor Motors and Permanent Magnet Motors: An Efficiency Comparison
With both types of motors available today, it can be difficult for someone to make a purchasing decision.
For many years, industry claims have stated that the superior efficiency of Permanent Magnet Motors (PMM) would soon replace induction motors and would make them obsolete. Induction motors were labeled “old technology” that would soon be on the scrap heap of motor history. After industry papers were written and presentations were made on the impending demise of induction motors, today we see PMMs stand at less than 2% market share.
Permanent Magnet Motor technology is advanced, making the motors very efficient. The challenge comes when consumers are asked to pay a very high cost for this technology. This is largely due to the fact that the motors require an expensive variable speed drive to operate. When Copper Rotor Motors (CRM) hit the market in the early 2000’s, the efficiency bar was raised considerably for induction motors. Copper’s high conductivity makes these motors efficient and reliable. These motors do not require a drive in order to operate.
With both types of motors available today, it can be difficult for someone to make a purchasing decision. How can a professional know which is the better investment? Industrial facilities typically look for an efficiency payback of two years when purchasing a motor. But how does one know when a more expensive, higher-efficiency motor will offer that payback? With very limited research available on the comparative efficiencies of the CRM and PMM or of their overall financial payback, the Copper Development Association (CDA) decided to create an apples-to-apples comparison through a study titled Efficiency Comparison Between the Copper Rotor Motor and Permanent Magnet Motor. Its goal was to analyze the true cost, efficiency and overall payback of each motor to help the end-user make an informed purchasing decision.
CDA commissioned Advanced Energy Inc., an independent, third-party testing lab in Raleigh, NC, to conduct the testing. Under the guidance of Emmanuel Agamloh, Ph.D., P.E., Advanced Energy is an independent motor testing lab and highly respected in the industry. The lab conducts research and performs testing for the Department of Energy (DOE) and for motor manufacturers worldwide. In order to conduct the study, Advanced Energy purchased four CRMs: a 5 HP, 7.5 HP, 10 HP and 20 HP motor. They also purchased three PMMs at each of these horsepowers, for a total of 16 motors. The motors were purchased from either the manufacturer or a motor distributor. The suggested drive for each PMM was also purchased in the same manner. One of the 7.5 HP motors was loaned by the manufacturer and returned. During the testing, Advanced Energy was aware that the four CRMs were rated above NEMA premium efficiency. The PMMs were also rated above NEMA premium efficiency but no attempt was made to identify the most efficient commercially available PM motor.
Four stages of motor testing took place over a three-year period. All four phases provided a point-by-point comparison, due to the fact that both PMMs and CRMs were tested using the drive specified by that particular PMM manufacturer. However, it is important to note that applying a drive to an induction motor will decrease its efficiency. This was necessary in order to help ensure an accurate comparison. After each round, the data were extrapolated to determine the real-world comparing cost, efficiency, availability and simple payback in order to assist the end-user in making an informed purchasing decision.
During round one, a 7.5 HP CRM was tested against three randomly chosen 7.5 HP PMMs. During this phase, obtaining the PMMs became problematic. One motor had to be loaned and returned, one was immediately available, and one took 16 weeks for delivery, as it had to be manufactured. This extensive lead time could pose a challenge to an end-user. PMMs were also significantly more expensive than CRMs, considering the cost of both the motors themselves and the drives needed to operate them. The borrowed PMM had no cost, but the drive it required was $960 after taxes. The other two PMMs and drives used were purchased at a total cost of $1,773.35 and $2,039.99. The CRM used in this round was purchased for $604.05.
The tests were conducted following the design schematic in Figure 1. Payback was calculated based on the operating assumptions in Figure 2. In this round of testing, as shown in Figure 3, the CRM was found to be more efficient than two of the PMMs that were tested (PM-A and PM-C). With regards to the third motor (PM-B), the results were mixed, with the PMM showing a higher efficiency at only certain data points. When overall efficiency, motor cost, drive cost, annual KWh and annual operating cost were considered, the number of years it would take for the higher-cost PMM to pay back the owner’s original investment was as long as 58 years.
The second round of testing was conducted with three PMMs and one CRM at 5 HP, each using the same test set up and data points as round 1. The three PMMs and their drives, purchased at prices of $1,535.34, $2,716.30 and $1,965.73, were again significantly more expensive than the CRM, purchased at $426.84. Figure 4 shows the testing results and calculated payback for the PMMs. In this round, the efficiencies of the PMMs were consistently higher than the CRMs. However, considering the higher costs, the payback for this higher efficiency would not be gained for between five to nine years, depending on the motor that was tested. This may not be practical for all applications.
The 10 HP motors tested during round three showed similar results (Figure 5). PMMs were shown consistently to be more efficient than CRMs. According to Advanced Energy’s summary of the testing, “With costs for the PM motor ranging from 2.34 to 6.4 times the cost of the CRM and with payback ranging from 2.19 to 107 years, the question needs to be asked, at this point, does the difference in efficiency warrant this purchase?”
During the final round of testing, three PMMs and one CRM at 20 HP were compared. The cost of each motor and the payback conclusion can be seen in Figure 6. The data show that PM motors were again more efficient than the CRM, particularly PM-B, which showed high performance and an excellent payback time. There was insufficient information to calculate a payback for PM-C due to technical problems with the VFD. However, because the cost of that motor is 3.6 times the cost of the CRM, it would be difficult for a facility to justify any gain in efficiency. PM-A, which was 5.29 times more expensive than the CRM, had a payback of 27.42 years.
CDA’s study concluded that, aside from the 7.5 HP CRM, the PMMs tested demonstrated better efficiency than the CRMs, as expected. However, when cost and payback are factored in, as well as the considerably longer lead-time for the acquisition of PMMs, CRMs overall may offer a better value to facility managers. PMMs certainly have a place in the market for specific applications, but with the burden of the cost of rare earth material, the necessity of a variable speed drive and the higher acquisition time, they may become less attractive to the end user.
Considering the high cost of PMMs, which can be four to five times higher than copper rotor motors, the resulting payback from higher efficiency can take 9, 30 or even 100 years as demonstrated by the study. In some cases, CRMs were even shown to be more efficient, as well as more cost effective. Considering all of the factors, CRMs may offer a better value for purchasers and should be strongly considered when making a purchase. Using this information, the industry will be better equipped to make an informed purchasing decision when choosing a motor.