TOTAL COST OF OWNERSHIP
                          A Financial Approach to the Process Industry



                        ...
decision! This paper examines the actual          While a pulley system may have better
system efficiencies for each of th...
Manufacturers’ literature lists Harmonic
                                                Filter inefficiencies that range ...
voltage.        Since motor manufacturers          So, anything that can be done to reduce the
typically use insulation ra...
The basic principle of operation of an Eddy
Current Drive is that an armature, typically a
steel drum that may have anothe...
overall the technology is the same as it was       found in magnetic drives, in a Hydrokinetic
in the 1930’s.             ...
relative motion between the parts creates an     speeds from 90% - 100% of rated motor
interwoven, eddy current field that...
Total System Energy Efficiency Comparison of Various Technologies


   Technology           Stand-Alone         Total Syst...
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  1. 1. TOTAL COST OF OWNERSHIP A Financial Approach to the Process Industry TCO-Energy / October 1, 2006 A Complete System Approach to Energy Efficiency Abstract: Manufacturers of electric motor Variable Speed Drive (VSD) technologies advertise the energy efficiency of their VSD’s as stand-alone devices. While these advertised efficiencies appear, on the surface, to be impressive, the end-user needs to consider the energy efficiency of the entire VSD system when selecting which technology to purchase and implement in their process. Ancillary equipment as well as mechanical losses typically results in realized efficiencies significantly lower than the published VSD numbers. A notable exception to this rule is found in VSD’s based on permanent magnet technology. System Energy Efficiency… The methods available to adjust the speed What’s the truth? of a driven system are numerous. Pulley Systems, Variable Frequency Drives, Eddy It is an accepted fact that in variable load Current Drives, Fluid Drives, and Permanent process applications, the use of speed Magnet Adjustable Speed Drives top the list control on motor driven systems offers of options. But which one is the best significant energy savings. A quick look at option? the Affinity Laws that govern centrifugal pump applications helps to explain and When making this decision it is important to confirm this fact. In summary, the Affinity look at the Total Cost of Ownership Laws are: associated with each of these options. Total Cost of Ownership (TCO) analysis has to 1. Flow - Q1/Q2 = N1/N2 include all costs that go along with each option. Initial purchase price typically is only 2. Head - P1/P2 = (N1/N2)2 10 to 25% of TCO. Drive system energy efficiency, non-energy system operating 3. Horsepower - HP1/HP2 = (N1/N2)3 costs (such as long term maintenance requirements), drive system life, and the Where Q = Flow, N = Speed, P = Pressure, and cost of process downtime all need to play a HP = Power. part in the purchase decision. Some of these, such as initial price, are easy to get a What the Affinity Laws tell us is that flow handle on. Drive system energy efficiency changes are proportional to changes in however, tends to be a little more difficult to pump speed but that the power required to determine. drive the flow is proportional to the cube of the pump speed. So, a 50% reduction in Most manufacturers of adjustable speed flow requires a 50% reduction in pump systems focus the customer on the savings speed. However, the power required to realized by taking advantage of the Affinity produce this lower flow will only be 12.5% of Laws. While this is important, it is also the original level! critical to determine the true efficiency of the system. A system that provides its user with It is this relationship between flow, speed, $100 of up front savings but that costs an and power that makes adjustable speed additional $150 to run because of energy control such an attractive option in the efficiency losses is not a good business process world.
  2. 2. decision! This paper examines the actual While a pulley system may have better system efficiencies for each of the above system energy efficiency than a mentioned adjustable speed options. restrictive flow control device (such as a valve or damper), a pulley system will Pulley Systems: generally have the worst overall system energy efficiency compared to all of the Pulley systems are the oldest of the VSD other VSD technologies. technologies having been in use even before the introduction of motors to industry. Variable Frequency Drives: There are several hundred suppliers in North America and the technology has seen Variable Frequency Drives (VFD’s) were very little innovation in the past few introduced to industry in the 1980’s. Major decades. suppliers in North America include: Rockwell Automation; ABB; General Electric (GE); In a pulley system, speed of the driven load and Siemens. The VFD technology has is adjusted by moving a drive belt to a seen significant amounts of innovation different size pulley or by using an generally focused on getting more power out adjustable pulley, or sheave. If the drive of a smaller package. The adoption rate of pulley is kept at a constant diameter then a VFD’s by industry in the 1990’s was quite doubling of the driven pulley diameter will high. This adoption rate has slowed result in a halving of the speed. Unless a dramatically in recent years as more users sheave is being used, this type of system become aware of the problems associated requires that the process be stopped in with VFD’s. The biggest issues identified by order to change the speed and provides a users are: limited set of speed choices to the operator. • Reliability of drives This is not an appropriate choice for speed • Need to replace or upgrade drives control in a true process control loop. every 4 – 7 years However, in a system requiring a one-time, • Harmful impacts to other equipment permanent adjustment this seems, on the on same electrical service. surface, to be a fairly viable option. Adjustable pulley drive systems, or sheaves, operate using the same principles and function by moving the sides of an adjustable drive pulley towards or away from each other. This changes the effective pitch diameter of the pulley, and therefore, the output speed. While capable of continuously modulating speeds in response to a process loop, these systems tend to be hard on belts and produce relatively high mechanical losses. Unfortunately, the losses associated with these types of systems can be fairly Figure 1: Low Horsepower VFD’s substantial. In a pulley driven system there are efficiency losses due to friction between Due to their apparent low initial cost, Many the belt and the pulley, non-symmetrical industrial users think of VFD’s as the de- loading of the motor bearings, and non- facto standard for process speed control. symmetrical forces on the load bearings. This technology has a long history and the Recent experience at a Wastewater market price continues to decline. These Treatment plant in the U.S. demonstrated factors make the VFD appear, on the energy efficiency losses of 16% - 25% due surface, to be a good value. However, to belt slippage, friction, and bearing wear in VFD’s also carry with them several issues a pulley system. that negatively affect not only the efficiency of the process they are used in, but also the reliability of those processes.
  3. 3. Manufacturers’ literature lists Harmonic Filter inefficiencies that range from 1% to 3%. Also, depending on the quality of power available to the VFD, power-conditioning equipment may be required. This equipment is another source of lost efficiency in the range of 1% - 2% and it is critical that the process owner fully understand what this impact will be. VFD’s are essentially small, specialized Figure 2: High Horsepower VFD’s computers and are subject to the same environmental restrictions as a computer. VFD’s control the speed of a motor by VFD’s should be installed in locations where changing the input AC voltage into DC the maximum ambient temperature does not voltage, and then “chopping” the DC voltage exceed 104° F, (40° C). If temperatures into the desired frequency. Modern VFD’s higher than the VFD’s recommended levels utilize microprocessors to control the are experienced, then de-rating of the VFD frequency supplied to the motor in response will most likely occur. Because of this, to the process loop’s feedback signal with cooling systems are often required for VFD’s the most common type of VFD being the used on larger motors. Cooling systems, pulse width modulating design. This whether water-cooling or air-conditioning, process, however, has been directly tied to use energy to operate which is energy not system harmonics, increased motor heating, being used to produce goods. (A rule of damage to motor bearings (fluting), and thumb found in the literature is that distortion of the voltage on the balance of approximately 1 kWh of cooling system the grid. In addition, VFD’s often require energy is required to remove 3 kW of ancillary equipment (air conditioning and generated heat.) Depending on the size of transformers, for example) that also the VFD, the wasted energy required to negatively impact the energy efficiency of operate the necessary cooling system can the VFD installation. be as much as 10%. Typically, VFD manufacturers will advertise Additionally, when sizing air conditioning for efficiencies in the range of 96% - 99%. It is VFD’s installed in locations above 3000 ft important to note, however, that these (1000 m) above sea level, it is important to efficiencies are for the VFD alone and do not remember that cooling capacity must be de- take into account losses associated with the rated as a result of the reduced capacity of inefficiencies listed above. the air to convectively remove heat from the system. A common rule of thumb is a 2% As a variable frequency drive operates, it de-rating for every 1000 ft above the 3300 ft introduces harmonic currents into the driven altitude level. motor’s windings. These harmonics flow in an opposite direction to the desired current In VFD installations that involve long wire and effectively reduce the capacity of the lengths, it is often recommended that LC windings to transfer usable power. Energy Filters be installed in the motor leads. (“L” losses due to harmonics are typically seen stands for inductance and “C” stands as heat losses. The effect of these for capacitance. Sized properly, this harmonic currents can be up to an increase combination of inductance and capacitance of 5 – 10% in motor heating and, therefore a will filter out harmful resonant frequencies.) equivalent reduction in system efficiency It is a well established fact that VFD’s that due to these energy losses. Harmonic Filters have long lead distances between the motor can be used to minimize the impact of these and the drive unit generate reflected currents on the system’s efficiency but these “standing waves” in the leads that can result filters aren’t without an efficiency cost. in output pulsations of up to 3 times line
  4. 4. voltage. Since motor manufacturers So, anything that can be done to reduce the typically use insulation rated for twice line Reactive Power component of electrical voltage, the effect of these pulsations is that systems will result in increases in cost the motor’s insulation begins to degrade efficiencies for that system’s user. leading to shorts in the motor windings. “Inverter Duty” motors address this issue by The VFD energy efficiency losses due to using higher voltage insulation in their ancillary equipment such as filters, construction. While this is an effective power conditioning and cooling solution, the cost of these motors can be equipment, and transformers can be significantly higher than regular duty motors. significant. Therefore, despite advertised The option of using LC Filters, therefore, is efficiencies by manufacturers of 96 to widely used in industrial applications using 99% for the VFD, true VFD system energy VFD’s. While these filters are effective at efficiency could be below 75%! minimizing the voltage surges being seen by the motor, they do reduce the system’s Eddy Current Drives: efficiency. Typical inefficiencies for LC Filters are in the range of 3%. Eddy Current Drives were first introduced in the 1930’s for use on railcars. Their use Industrial customers who have medium and quickly spread throughout industry such that high voltage applications in their facility also the leading supplier, Eaton, was selling over face another efficiency drain when they $100 million per year of these drives during utilize transformers to drop the line voltage the 1970’s. Since that time, there has been down to a level that can be handled by very little product innovation in this category standard VFD’s. Medium and high voltage of VSD’s and relative market share has VFD’s are much more costly than standard fallen. Two of the largest suppliers in North VFD’s. A quick study of transformers listed America are TECO/Westinghouse and in various manufacturers’ literature shows Dynamatic. inefficiencies ranging from 1% - 4%. In addition to the direct losses of efficiency Eddy Current Drives use an electromagnetic caused by the ancillary equipment typically coupling between the motor and the load required by VFD installations, another, often that it is driving. The motor runs at its rated disregarded, concern is the effect on the speed at all times while the speed of the user’s power factor. load is adjusted. There are two basic configurations of Eddy Current Drives currently on the market – foot-mounted Apparent Power (Figure 3) and shaft-mounted (Figure 4). (KVA) Reactive Power (KVAR) O Actual Power (KW) Power Actual Power = = Cosine O Factor Apparent Power Figure 3: Foot Mounted Eddy Current Drive In the equations above, Actual Power is the electrical power actually used to do work in a system and Reactive Power is the electrical power that is absorbed/wasted as a result of any non-work producing loads within the same system. Apparent Power is the trigonometric addition of Actual and Reactive Power. Unfortunately, the power company charges customers for Apparent Power. Figure 4: Shaft Mounted Eddy Current Drive
  5. 5. The basic principle of operation of an Eddy Current Drive is that an armature, typically a steel drum that may have another conductive material as a lining, is attached to the drive motor. This armature assembly turns at the nameplate speed of the motor at all times. Attached to the load shaft is a multi-pole electromagnet. A variable DC current is supplied to the electromagnet. As the current to the electromagnet is increased, the magnetic field increases. As the magnetic field moves relative to the armature, eddy currents are created, magnetically coupling the two elements and transferring torque from the motor to the load. Alignment between the armature and the electromagnet is maintained with supporting bearings within the drive unit (Figure 5). Figure 6: Comparison of VSD Efficiencies Despite the data showing that the stand- alone Eddy Current Drive has approximately 6% worse energy efficiency than a stand-alone VFD, true Eddy Current Drive system energy efficiency will often be better than found with a VFD system due to VFD ancillary equipment inefficiencies. Figure 5: Eddy Current Drive Schematic Fluid Drives: According to published data, Eddy Current The principle of using fluid to transmit power Drives will provide increased efficiency when was introduced in 1905 by the Vulcan compared to restrictive flow control. Again, Engineering Company and was originally this is because Eddy Current Drives follow applied to driving a low-speed ship’s the principles of the Affinity Laws. However, propeller with a high-speed steam turbine. because the Eddy Current Drive technology The advantage of this new Fluid Drive was uses electrical power to energize the that it allowed for speed reduction without electromagnets, there is a loss of efficiency the use of complicated and temperamental when compared to stand-alone VFD’s. gear systems. In 1930 the first variable Several industry publications present this speed Fluid Drive was installed in England graph showing an energy efficiency with the first U.S. installation following difference between VFD’s and Eddy Current shortly in 1932. At the time of introduction, Drives of about 6% (Figure 6). Fluid Drives offered the benefits of smooth and continuous control of speed using a Generally, Eddy Current Drives require constant speed motor. Compared to the water cooling above 200 HP. This water options available at the time (gears and cooling requirement is a direct reduction in pulley systems), the Fluid Drive was an total system energy efficiency. Also, shaft attractive technology. mounted Eddy Current Drives typically are used in combination with belts and pulleys. While the technology has continued to be Belts and pulleys will reduce overall system available to this time, the operating efficiency. However, the inefficiencies due principles and designs of Fluid Drives have to ancillary equipment with Eddy Current remained rather stagnant. The largest Fluid Drives are generally much less than the Drive suppliers today, (Falk and Voith), have inefficiencies associated with VFD’s. introduced small changes to their drives, but
  6. 6. overall the technology is the same as it was found in magnetic drives, in a Hydrokinetic in the 1930’s. Drive it results in a temperature rise within the drive’s oil. As the oil temperature The category of Fluid Drives consists of increases, its viscosity and ability to transmit Hydroviscous (HV) Drives and Hydrokinetic torque decreases. Because of this, oil (HK) Drives. Fluid Drives use a hydraulic cooling heat exchangers are typically fluid to transmit torque between an impeller required in a Hydrokinetic drive. The pump on the motor side of the drive and a rotor on used to circulate the oil through the heat the adjustable-speed load shaft. exchanger is driven through a power take-off Hydroviscous Drives transmit motor torque from the motor shaft. though a thin film of oil that is captured between discs attached to the motor shaft Fluid Drives are advertised as one of the and corresponding discs that are attached to most energy efficient VSD options the load shaft. The amount of torque that is available to industrial users. However, transferred between the two sets of discs is despite manufacturers’ claims, Fluid determined by varying the amount of Drives (both HK and HV) are actually less pressure being used to press the two sets of efficient than Eddy Current Drives due to discs together. A separate hydraulic the losses described above. cylinder is used to provide this pressure. Permanent Magnet Adjustable Hydroviscous Drives offer the benefit of Speed Drives being mechanically “locked” together at 100% speed. This theoretically provides for The introduction of Permanent Magnet complete transfer of torque from the motor Adjustable Speed Drives (PMASD’s) by to the load. Of course, any misalignments MagnaDrive Corporation, a U.S. company between the motor and load shafts will result based in Bellevue, Washington, has given in a constant flexing of the drive the process industry another option to components. Energy spent in this way is consider when reviewing adjustable speed energy that is not being used to drive the applications. The company’s technology process and is a loss of efficiency. has one of the fastest adoption rates for a new introduction in its targeted industries Hydrokinetic Drives operate in a manner with over 5,000 installations currently similar to the automatic transmission in an operating. automobile. While there is no physical connection between the impeller and the The benefits of MagnaDrive’s technology rotor, there are supporting bearings within are evidenced by the company’s close the drive to maintain the proper clearances relationship with the US government. Early and alignments. During operation of a funding for MagnaDrive was supplied by a Hydrokinetic Drive, there is a small amount grant from the US Department of Energy of slip between the impeller and the rotor. (DOE). Also, DOE testing demonstrated Additionally, as the hydraulic fluid moves that MagnaDrive’s products reduce energy within the drive housing, there will be usage by up to 70%. The DOE operates frictional losses between the oil and the several MagnaDrive units in mission critical walls of the housing. Industry data shows applications at nuclear facilities. The US that these losses can be as much as 5% - Navy and US Air Force also utilize 7% when compared to Eddy Current Drives. MagnaDrive technology. The Navy operates One feature of a Hydrokinetic Drive that is well over 1,000 units across ten different found in manufactures’ literature is its ability ship classes in its fleet. to accept some degree of misalignment between the motor and load shafts. It is PMASD‘s consist of two primary important to note that, in order to allow for components. The first component, a set of this misalignment, there are clearances copper conductor plates, is connected to the between the impeller and the rotor which motor shaft; the second component of the may cause inefficiencies within the drive. permanent magnet drive is a rigid assembly While the small amount of slip in a of permanent, rare earth magnets which is Hydrokinetic Drive is consistent with what is connected to the load. During operation,
  7. 7. relative motion between the parts creates an speeds from 90% - 100% of rated motor interwoven, eddy current field that transmits speed PMASD’s are about 5% more efficient torque across the air gap. To adjust the than VFD’s. From 80% - 90% of rated motor speed of the driven load, the amount of speed the energy efficiency of the two torque transmitted from the motor to its load technologies are the same. Below 80% of is controlled by changing the distance rated motor speed, VFD’s are approximately between the conductor plates and the 5% more efficient than PMASD’s. Because magnet assembly. During operation and more energy is used at the higher load throughout the entire speed range, no speeds, the average efficiency of a PMASD physical connection exists between the is approximately 3% - 4% higher than an motor and the load (Figure 7). This equivalent stand-alone VFD. When total “disconnected connection” has system energy efficiency is looked at, the demonstrated vibration reductions up to PMASD is clearly the better alternative. 85%. In comparison to stand-alone Eddy Current Drives, stand-alone PMASD’s have higher energy efficiency because they utilize permanent magnets while the Eddy Current drive must energize its electromagnets from an external power source. This advantage for PMASD’s can only grow when looking at total system energy efficiency due to Eddy Current drive system losses in pulleys and water-cooling. Figure 7: Operation of Permanent Magnet ASD Generally, Fluid Drives have much lower PMASD’s can be used on any centrifugal total system energy efficiencies than load application with the current technology PMASD’s. The advantage of PMASD’s able to handle up to 4,000 HP loads. becomes even more obvious when non- Because the speed of the load is changed energy operating lifecycle costs are while allowing the motor to continuously considered. operate at its nameplate speed, motor The potential value of permanent magnet heating and the resulting inefficiencies are technology is clear. In site evaluations, not an issue. Also, because the permanent the permanent magnet coupling concept magnet drives are mechanical devices, they for speed control has been proven do not introduce harmonics into the user’s across all industries. In comparison to power grid. Filters, transformers, and air available baseline energy information, conditioning systems are not necessary. the use of a permanent magnet drive can result in significant energy savings. The eddy currents being produced in a Additionally, the disconnected nature of PMASD are created, as noted above, by these drives also provides the industrial relative motion, called “slip”, between the user with significant savings in conductor rotors and the magnet rotors. In a maintenance and operational costs. typical PMASD, this slip results in an approximate speed loss of 1% - 2% at full References: speed. The only ancillary equipment 1. AC Drive Worldwide Outlook. ARC Market Study. inefficiencies are due to an actuator (used to 2. Coyote Electronics, Inc. Fort Worth, TX. vary the size of the air gap), and the need 3. DSI – Dynamatic, Inc., Sturtevant, WI. for water cooling above 600 HP. 4. Emerson Electric, St. Louis, MO. 5. Frost & Sullivan. N Amer & Euro VSD 2005 Report. 6. Hydraulic Institute, Variable Speed Pumping, 2005. Studies conducted by the DOE show that 7. Jones, Garr. Pumping Station Design, Butterworth- PMASD’s save 60% - 70% on energy costs rd Heinemann, 3 ed., Woburn, MA, 2005. versus restrictive flow control methods. 8. Kuphaldt, Tony (2003). URL: www.allaboutcircuits.com. Also, looking only at stand-alone efficiencies 9. MagnaDrive Corp., Bellevue, WA. of the VFD (ie: not taking into account 10. US DOE. Industrial Electric Motor Market Study. ancillary equipment losses) at operating 11. Voith Turbo GmbH & Co., Crailsheim, Germany.
  8. 8. Total System Energy Efficiency Comparison of Various Technologies Technology Stand-Alone Total System Description of Energy Efficiency Energy Efficiency Potential Losses Restrictive Flow Highest energy cost – does 30% - 40% 30% - 40% Control not follow Affinity Laws Energy savings due to Affinity Laws. Energy losses Pulley Systems 85% - 90% 75% - 85% from belt slip, friction, and bearing wear Energy savings due to Affinity Laws. Losses from VFD’s 96% - 99% 75% - 90% ancillary equipment and vibration from misalignment Energy savings due to Affinity Laws. Losses due to Eddy Current Drives 90% - 93% 80% - 90% electromagnet energizing, pulleys, and water cooling above 200 HP Energy savings due to Affinity Laws. Losses due to Fluid Drives 83% - 87% 80% - 85% fluid viscosity changes, oil circulation pump, and vibration Energy savings due to Affinity Laws. Losses due to PMASD’s 97% - 99% 94% - 97% actuator and water cooling above 600 HP

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