by Dan Jones, President, Incremotion Associates
This new technique promises to deliver both higher efficiency and better power factor for induction motors.
Induction motors are the workhorse of industry. They drive everything from industrial manufacturing processes to consumer equipment like washers and dryers, among others. And they’ve been around since Nikola Tesla applied for patents for the motors in the late 1880s.
One company, Revolution Motor Industries (RMI), has developed a new way to improve integral induction motor efficiency without adding more copper and magnetic iron or switching to permanent magnet rotors. They have developed a new winding technique to simultaneously raise motor efficiency and increase the power factor.
Today’s premium efficiency motor
The ac induction motor dominates the constant speed and variable speed markets. The premium efficiency three-phase induction motor has been undergoing improved efficiency ratings dictated by the U.S. Department of Energy (DOE). (One of the key factors in the drive toward better motor efficiency has been government mandates, specifically DOE directives for better efficiency from electric motors. [See sidebar.]) The recent regulations cover a number of different general purpose 2, 4, 6 and 8-pole induction motors. In Figure 1, Design A, B and C performance curves represent the majority of 1 to 500 hp general purpose ac induction motor torque-versus-speed performance curves.
For example, the 5-hp, 4-pole induction motor efficiencies at 100% load started at 83% in the 1980s, rose to 87.5% in the 1990s and is now at its current level of 89.5% for the 5-hp premium efficiency ac induction motor. A total of 1,262.8 kWh have been saved over a year’s time. If the power utility rate is $0.15/kWh, a savings of $189 is realized for the 5-hp induction motor. The dollar savings significantly increases for larger hp motors.
A significant increase in power efficiency with a 50-hp induction motor was also identified. The original 3-phase, 4-pole induction motor efficiency for a 50-hp motor started at 91.3% in the 1980s, remained at 91% in the 1990s and as a premium efficiency motor, is now set by the DOE at 93%. That is an increase of just 2% in efficiency levels. Once again, using 6,000 hours operating time in 1 year, the power savings increases to almost 2,500 kWh annually for an annual savings of about $400. While improvements in motor efficiency appear to be small, the savings in energy costs accumulate over time. In Europe, the equivalent motor efficiency levels are designated IE3. A number of motor manufacturers in the U.S., Europe, Brazil and Asia have developed motor products for the next higher level, the super premium or IE4 motor efficiency level.
Solving power factor without reducing efficiency
Power factor is a more complicated parameter. It is divided into two components: real power and apparent or reactive power (Figure 2). A perfect power factor is one or unity. Unfortunately, almost all electric motors have winding inductance, including induction motors. Each electricity user pays for power factor buried in their power utility bills. Induction motors possess lower power factors, usually above 0.94 at 100% rated load conditions. The power factor dives down in standard induction motors to lower power factor values as the motor load decreases to 25 and 50% load conditions.
The modified RMI induction motors maintain a higher power factor over all load conditions. The current induction motor requires an expensive variable speed drive (VSD) to improve the lighter load performance. The RMI upgrade motor can readily operate directly from electric grid power.
The RMI modified induction motor
RMI focuses on the induction motor’s three-phase stator winding. It covers the integral horsepower induction motors from 1 to 500 hp. It uses a dual winding concept with capacitors placed in series in the auxiliary winding that provides the high power factor values. Historically, other dual winding approaches have always maximized the induction motor’s power factor, but at the expense of the motor’s efficiency levels.
The RMI patent supplies a number of major improvements in both power factor and motor efficiency. The RMI engineers used transient FEA analysis tools to establish the many attributes of the motor. The unique winding distribution and careful sizing of capacitor values are key elements in the successful RMI design and implementation.
Testing the RMI motor
While modern FEA tools are quite accurate, the RMI design must be tested to prove and verify desired performance. RMI engineering went about proving their simulation performance by purchasing standard induction motors from authorized motor distributors. Over about 24 months, the 50- and 300-hp motors were obtained from a major motor supplier and were tested by an independent company, Advanced Energy (AE) Test Labs, located in Raleigh, N.C. They were on the AE Labs list (certified test labs) used by the DOE to establish the current motor efficiency standards. Motor performance characteristics were measured and recorded. The various motors were returned to RMI who replaced the original windings with a motor stator without a winding. Then they reinserted the new RMI dual winding stator with a capacitor per phase in the RMI auxiliary winding.
The DOE mandated test procedure requires full performance data points at 25, 50, 75, 100, 115, 125 and 150% of the rated torque and speed. These test points are specified by the IEEE 112-2004 standard for induction motors. The modified RMI motor test results were compared to the original motor test results. The power factor measurement displays a major improvement when compared with the original standard motor power factor. Only the stator winding was changed. RMI used all the other motor parts, just changing the stator winding.
The 300-hp induction motor was tested by AE Test Labs at various load conditions. At 100% load, the RMI outperforms the original 300-hp by 95.8% versus 95.3% motor efficiency (Figure 3). There is a much larger difference in power factor at 100% load conditions (0.98 versus 0.86) with RMI’s updated stator winding possessing the higher value. At 25% load, the RMI winding update has a 0.99 power factor when compared with the standard 300-hp value of 0.72 power factor.
The 2-hp induction motor was tested at Magtrol in Buffalo and again at a major motor supplier. Figure 4 summarizes the improved performance verified by using the RMI dual winding technology to simultaneously improve overall motor efficiency.
Evaluating costs to upgrade motor performance
Schultz Associates reviewed the various component costs of a representative 50 and 300 hp ac induction motor as purchased. It compared this standard motor cost with the updated costs of the same size ac induction motor employing the RMI dual winding technology plus capacitors installed in each of the three auxiliary windings. All other motor parts were equivalent. The fully burdened added costs for using the RMI technology in the standard general purpose 50 and 300 hp induction motors were determined to add about 8% to the 50-hp induction motor’s sale price and approximately 10% to the 300-hp induction motor’s sale price. The report included industry cost mark ups and margins to evaluate current industry prices and establish RMI’s adjusted prices.
Currently the DOE mandated premium efficiency level (IE3) covers general-purpose three-phase induction motors from 1 to 500 hp. The RMI technology can provide a cost-effective way to reach the super premium efficiency (IE4) level without using an expensive copper rotor.
The Schultz research report also computed the payback time cycle. Using a power utility rate of $0.14/kWh, the payback time period is six months for the RMI modified 50-hp induction motor and only three months for the RMI modified 300-hp induction motor. Testing of a 2-hp induction motor was undertaken to cover the entire range of EPact (Energy Policy Act) or premium efficiency induction motors from 1 to 500 hp.
In establishing the premium efficiency levels, the DOE extensively tested and simulated motor performance for 5, 30 and 75 hp general-purpose induction motors to cover the performance range from 1 to 500 hp. RMI chose a wider set of test motors at 2, 50 and 300 hp to evaluate the viability of their performance improvements in motor efficiency and power factor over a wide range of motor loads. A more cost-effective method is now available to increase both motor efficiency and power factor and at a lower cost.
Revolution Motor Industries