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usa.siemens.com/ids What is drive system efficiency and why is it important? Problem description Drive system efficiency is the ratio of output power produced by the motor shaft to the input power of the drive system. A drive system includes a motor and a Variable Frequency Drive (VFD)*, but in addition it can also include an input filter, an external transformer, an output filter, and/or an especially long cable. To understand drive system efficiency, the power losses of individual components within the drive system must be independently understood. Power losses from motors The power losses from the motor are generated by electromagnetic properties and the material properties of the motor. In general, the power losses from the motor can be classified as the following different types of losses: 1. Stator Losses – Heating (I2R) losses are generated within stator winding proportional to its electrical resistance and square of load current. 2. Rotor Losses – Heating (I2R) losses are generated within rotor squirrel cage proportional to its electrical resistance and square of load current. 3. Core Losses – Iron magnetization losses are generated by the motor core as a function of frequency and induced eletromagnetic field strengh of the stator winding. 4. Stray Losses – High frequency harmonic losses are proportional to electromagnetic flux leakage occurring in motor air gap and winding fields. 5. Windage and Friction Losses – Mechanical losses are generated by rotor bearing frictions and shaft-mounted fan airflow power consumption. *Variable Frequency Drive (VFD) maybe used interchangeably with Adjustable Frequency Drive (AFD). Answers for industry. Power losses from VFDs The power losses from VFDs can be generated by power electronic devices (converters) that convert the input voltage and current to the desired variable output voltage current. Different topology of these devices in the VFD produces different properties, including output harmonic distortions. Converter Losses – Conduction loss in the switching devices, which is proportional to the current, and switching losses, which are proportional to the current and switching frequency, are generated by the circuit components of VFD output power converter. Some VFDs also have an integrated transformer in the design. This transformer contributes to the overall power loss of the VFD. Drive system efficiency The power losses of the individual components directly affect drive system efficiency. In general, efficiency is defined as the ratio of output power to input power of a system. In a lossless system, this ratio would equal 1.00 or 100%. However, in the real world, losses are ever-present. Input power can be seen as the output power plus the power losses of a system, which can also be shown as the product of the efficiencies of each component in the system, as shown in Figure 1. Because of the complexity of the drive system, evaluating drive system efficiency can be complicated. Consideration must be taken for the effects of the combination of the system components efficiencies on the entire system to determine the most optimized system for efficiency. Transformer losses – Core losses similar to those of the motor stator, eddy current losses, which are proportional to current and frequency squared, and heating losses (I2R) are generated by the winding and iron core of VFD input power transformer. It is important to note that although some VFDs can operate without a transformer, other VFDs may need to operate with an external nonintegrated transformer. However, when advertising the efficiency of their VFDs, the manufacturer may omit the transformer power losses because the transformer is external to the VFD. Therefore, although the VFD that requires the external transformer is reported as having a higher efficiency, the overall system efficiency may be lower when the external transformer efficiency is considered with the VFD. Losses from input/output filters Depending on the VFD design, the drive system may require an input filter to satisfy certain industry standard requirements, such as IEEE-519. Depending on the topology of the VFD, the drive system may need an output filter after the VFD to reduce output harmonics to protect standard motors or to satisfy customer requirements. A reduction in output harmonics may also decrease the stray losses of the motor, thereby making the motor more efficient. There are losses that are introduced by the addition of an input and/or output filter. Depending on the types of filters used, the causes for their losses are different. For example, passive filters may have heating and eddy current losses similar to transformer losses, while active filters may have electronic device losses similar to the converter losses of the VFD. When these filters are used, their losses must be taken into consideration when considering the efficiency of the drive system. Losses from long cable In some applications, the VFD is located far away from the motor because of specific application requirements or the harsh environment in which the motor is operating may damage the VFD. Therefore, a long cable may be used to connect the motor to the VFD. The losses in the long cable are mainly caused by heating losses (I2R), and these losses are proportional to the resistance of the cable, which is proportional to the length of the cable. 2 Figure 1 Drive system efficiency Problem example In a real-world based hypothetical scenario, an end-user purchases a motor and a VFD from two different manufacturers. The 5000 hp, 3600 rpm, 4000 volt motor outputs 3730 kilowatts of power (Pout) to drive its load. Under service factor load, the motor dissipates 190 kilowatts of heat (Plosses). Based on the equation from Figure 1, the input power (Pin) must be 3920 kilowatts which yields a rated efficiency of 0.95 or 95%. The VFD is a medium voltage drive that must output 3920 kilowatts of power (Pout) to drive the 5000 hp motor. At rated frequency and current the VFD dissipates 101 kilowatts of heat (Plosses). Based on the equation from Figure 3, the input power (Pin) must be 4000 kilowatts which yields a rated efficiency of 0.97.5 or 97.5%. So, when the motor and VFD are operating together one would expect for the system efficiency to be 0.93 or 93% given that the output power (Pout) is 3730 kilowatts and the input power (Pin) is 4000 kilowatts. However, the system efficiency under rated operation was reported to be 0.90 or 90%. Where did the additional losses come from? Because the motor and VFD were not necessarily designed for optimal operation with one another, there were additional core and stray losses induced in the motor from a large amount of harmonic distortion at the output of the VFD converter. Also, such reporting can vary with data coming from different industry equipment. Furthermore, the harmonic distortions may cause a voltage surge that would damage the insulations of the motor. In the addition, the input of the system does not satisfy IEEE-519 requirements. Although, the motor and VFD were able to operate together, the drive system efficiency deviations would incur additional energy costs over the life of the drive system, and the system cannot be install in all facilities that requires the system to satisfy IEEE-519. Since most of the losses show up as heat, additional cooling for some of the system components may be required. Problem solution Resolving this issue requires a sound technical understanding of the entire drive system. This can be an overwhelming task for even the most knowledgeable engineer. One method to address this issue would be to install a supplemental harmonic filter in the VFD cabinet if there is space available to filter the output harmonic distortions to the motor. If this is not feasible, a suitable after-market harmonic filtering system can be retrofit next to the VFD cabinet with special interfacing. Furthermore, an input filter will need to be installed to satisfy the IEEE-519 requirements. However, adding these two components into the drive system affects the overall system efficiency. The typical efficiency of input and output filters is 0.995 or 99.5%. Assuming to the output filter filters sufficient harmonic distortions so that the motor is at nominal efficiency, the system efficiency can be calculated using the equation in Figure 2 as follows: ηSystem 1=ηInput × ηVFD × ηOutput × ηMotor Filter Filter = 0.995 × 0.975 × 0.995 × 0.95 = 0.917 = 91.7% Integrated Drive System optimized solution The greatest advantage of Siemens Integrated Drive System is the knowledge, the expertise, and the ability to evaluate the efficiency of the entire drive system and optimize the system efficiency through design. In the problem example, the Integrated Drive System solution would be to use a SINAMICS GH180 VFD with an VFD efficiency of 97%. At first glance, it would appear that SINAMICS GH180 has a lower efficiency than the VFD in the Problem Example, and therefore the efficiency of the Integrated Drive System is lower than the Non-integrated Drive System in Problem Solution. However, the SINAMICS GH180 is designed such that it does not require an input filter to satisfy IEEE-519, and the output voltage can be operated with a standard motor without an output filter. Therefore, using the equation in Figure 2, the efficiency of the IDS system is calculated as follows: ηSystem IDS = ηVFD × ηMotor = 0.97 × 0.95 = 0.9215 = 92.15% Efficiency Gain = ηSystem IDS – ηSystem 1 = 0.45% The Integrated Drive System is about 0.45% more efficient than the Non-integrated Drive System composed of the input and output filters. We can calculate the costs saved by this efficiency savings with assumptions stated in scenarios shown in Figures 2 and 3. Drive system operating half of the time under continuous duty at 91.7% efficiency with an average energy cost of $0.09 per kWh would cost approximately $1,604,551 per year. 3730 kW 0.917 • 4383 hrs. • yr. $0.09 kWh = $1,604,551 yr. Figure 2 Yearly cost at 91.7% efficiency Drive system operating half of the time under continuous duty at 92.5% efficiency with an average energy cost of $0.09 per kWh would cost approximately $1,596,715 per year. 3730 kW 0.925 • 4383 hrs. • yr. $0.09 kWh = $1,596,715 yr. Figure 3 Yearly cost at 92.15% efficiency This comparison yields an average savings of $7,836 per year for the life of the system. Furthermore, greater efficiency can be obtained from the Integrated Drive System through design by either oversizing the transformer or designing the motorVFD system such that it uses the least amount of current to produce the required power to the customer. However, these methods may increase the initial capital cost of the system (increase in transformer cabinet size, increase in number of cells, etc.). Therefore, the tradeoffs between initial capital cost and the returns from efficiency savings will need to be evaluated before design changes are implemented. Besides the efficiency advantage, the Integrated Drive System also provides the following benefits to the customer versus Non-integrated Drive System: 1.Smaller foot-print, which can translate to construction cost savings for the customer. 2.Reduction in complexity, which mitigates risks that are caused as a result of a complex drive system. Through these benefits, the customer obtains the most production out of their investment in the drive system. Siemens, in turn, gains the business of the customer. Siemens Integrated Drive System is truly a win-win situation for both the customer and Siemens. 3 Get the most production out of your drive system By choosing a Siemens Integrated Drive System, you get the knowledge, the expertise, and the ability to evaluate the efficiency of the entire drive system and optimize the efficieny through design. Siemens Industry, Inc. 3333 Old Milton Parkway Alpharetta, GA 30005 1-800-241-4453 [email protected] usa.siemens.com/ids Results: • • • • • • Increased availability Higher revenue Reduction in complexity Lower risk Increased efficiency and enhanced reliability Lower total cost of ownership Subject to change without prior notice Order No.: DTAN-00020-0814 Printed in USA © 2014 Siemens Industry, Inc. The information provided in this flyer contains merely general descriptions or characteristics of performance which in case of actual use do not always apply as described or which may change as a result of further development of the products. An obligation to provide the respective characteristics shall only exist if expressly agreed in the terms of contract. All product designations may be trademarks or product names of Siemens AG or supplier companies whose use by third parties for their own purposes could violate the rights of the owners.