U nderstanding Complete Tests

Transcription

1 Special Feature U nderstanding Complete Tests Performed On Induction Motors by William R. Finley, Mark M. Hodowanec, Khursheed S. Hussain, and John Larabee Siemens Energy & Automation, Inc. Introduction When new motors are purchased, complete tests can be conducted to verify performance and integrity. These tests are not standard and may add to the motor cost without adding significant value to the purchaser. The value of the tests depends on criticality of the application, the user s experiences, motor size, motor voltage, etc. There are many standards regarding testing of induction motors. Standards such as NEMA MG1, IEEE 112, IEC & -02, API 541, and IEEE 841 make recommendation as to what tests are required and how they should be performed. There are many different specified methods to performance test induction motors, all requiring that the motor be loaded (i.e., heat run). The different test methods do not necessarily produce the same results. Two common test methods are used today to load motors: coupled load test and dual frequency method. The coupled load test requires that the motor be coupled to a load machine and placed under rated load. The dual frequency test involves applying both 50- and 60-hertz power to the motor at the same time, simulating full-load heating. It is important the user understands the test employed and, if comparing motors from different vendors, that the motors need to be tested using the same test method in order to keep values comparable. A complete test uses a number of individual tests. These individual tests include: Locked-rotor test at a rated frequency Speed-torque curve No-load saturation curve Dual-frequency heat run or coupled heat run These four parts are detailed as follows. Locked-Rotor Test at Rated Frequency The locked-rotor test at rated frequency is used to determine the locked-rotor torque (LRT) and current (LRA). In order to determine the values at rated voltage, at least three test points of voltage versus current, watts, and sometimes torque are taken to as high a voltage as possible and then extrapolated to rated voltage on log-log graph paper to establish the desired values. Speed-Torque Curve On large motors it is difficult and costly to measure directly torque versus speed at rated voltage. In such cases the test is run first to determine the shape of the speed-torque curve. The curve is then calibrated utilizing the test results from the locked-rotor test to establish the actual speed-torque curve of the motor. A typical speed-torque trace is shown in Figure 1. Figure 1 Torque versus Speed Trace Summer

2 The speed-torque trace is normally determined utilizing a tachometer to measure motor speed as it accelerates to its no-load speed. The output of the tachometer is fed into a computer, where it is recorded as a function of time. The output is then differentiated with respect to time to arrive at the rate of change of speed versus time, which is the angular acceleration of the motor. This test is normally done at reduced voltages so as not to damage the test equipment and to get a good sampling. The resulting curve represents the shape of the speed-torque curve but does not yet establish absolute torque. The speed-torque trace is then calibrated using the locked-rotor torque value obtained in the locked-rotor test described above. By assigning this value to the curve at zero speed, a speed-torque curve is now accurately defined in absolute values at all speeds. No-Load Saturation Curve This test is performed to determine the windage, friction, and core losses in a motor. The saturation curve is taken with the motor running without any load. The test is usually performed after half an hour or more of the no-load run, to ensure the bearings have run in and input values have stabilized. At rated frequency, the line voltage on the motor is varied in steps from 125 percent of rated down to a value where further voltage reduction results in a disproportionate increase in the current. Voltage, current, power, and winding temperatures are recorded at each step. To segregate the losses, power input minus the stator I 2 R loss is plotted versus voltage, and the curve extended to zero voltage. Refer to Figure 2. The intercept on the power axis is the windage and friction loss. Core loss at rated voltage and frequency can then be obtained by subtracting the value of the windage and friction loss from the total loss from the curve at rated voltage point. Figure 2 Determination of Windage and Friction Losses Dual-Frequency Heat Run Dual-frequency heat run is a temperature test of an induction machine under simulated load conditions. The test involves using two separate sources of power with two separate frequencies: a primary source of rated frequency and a secondary source generally 10 hertz below the rated frequency. The two sources are set up to supply power simultaneously to the test machine by being connected either in series or superimposed by use of a series transformer. These are shown schematically in Figure 3 and Figure 4. Figure 3 Dual-Frequency Power Sources, 50 and 60 Hertz Connected in Series Figure 4 Dual-Frequency Power Sources, 50 Hertz Power superimposed on 60 Hertz by Use of a Coupling Transformer The frequency that the motor sees changes completely 10 times per second. This continuous change causes the revolving magnetic field inside the motor to change its synchronous speed between that of 50 hertz and 60 hertz. When the motor is under the influence of the 60-hertz supply, the motor accelerates towards the 60-hertz synchronous speed, drawing current in the process to achieve the acceleration and operating as a motor under high slip. However, because of its rotor inertia, it cannot reach that speed instantly. One tenth of a second later the motor sees 50-hertz power. The motor then decelerates towards the 50- hertz synchronous speed. The slip being negative, the motor now generates current and feeds it back to supply lines as an induction generator. With proper adjustments to input parameters, a steady operating 2 NETA WORLD

3 condition can be achieved wherein the motor sees rated root-mean-square (rms) voltage and rated rms line current. The wave shapes are not sinusoidal, but tests show that they produce similar heating in the motor. Table 1 lists comparative test results reported by various manufacturers. Rise by Source HP Volts Poles Hz Resist. Load Dual A B B A A C Table 1 Comparative Temperature Rise Between Dual Frequency and Load Test The rated condition is generally reached when the 50-hertz input voltage reaches 20 to 30 percent of the 60-hertz rated voltage as measured at V 1 and V 2, respectively, in Figure 3.. During the duration of the heat run, the terminal voltage and current of the motor are maintained at their rated 60-hertz values. Volt, ampere, and kilowatt readings at the motor terminals are recorded along with the motor temperatures. After the machine temperatures (as indicated by stator resistance temperature detectors or auxiliary thermocouples) have stabilized, the voltages of the auxiliary power and the prime power are reduced. After the motor is stopped and all breakers are opened and locked out, resistance is measured to evaluate temperature rise. During the heat run, the motor is being supplied from two power sources at different frequencies, and is subjected to the oscillatory torques associated with these frequencies. Consequently, the vibration will be abnormal during this condition and may not meet the normal limits of vibration. For this reason a no-load cold vibration is measured at rated voltage before the application of the auxiliary power. Then, at the end of the heat run after the temperatures on the machine have stabilized, the auxiliary power is removed, and the vibration at rated frequency and voltage is measured again to determine the vibration of the machine at normal running temperature. This is done without stopping the test motor, which allows the hot vibration to be recorded quickly since the machine especially an open machine cools down rapidly after the auxiliary power is removed. IEEE 112 recommends that the vibrations should be measured while the motor is within 25 percent of the normal operating temperature. Dual-frequency load testing is a cost-effective method for temperature testing of general purpose and vertical induction motors. The test setup is simple no test coupling, rigging, or alignment is required. It takes 50 to 60 percent less time to rig and test the motor than by the conventional coupled load method. Coupled Heat Run Coupled heat run is the direct-loading method for temperature testing of electric motors. The test machine is coupled to a dynamometer or a load machine. The load on the dynamometer or the load machine is increased until the test machine reaches rated load. In a coupled load test, the test machine is installed with a test coupling and then rigged and aligned to the load machine as shown. It is important that the test motor be set firmly on a stiff test bed. If it is raised on rails or blocks to match the shaft height of the load machine, the rails and blocks must be perfectly squared off and have adequate stiffness. Similarly, the test couplings, the center spool piece, the coupling on the load machine, and the load machine itself must be well balanced and aligned accurately to assure that no vibration is introduced as a result of inferior couplings or rigging. The setup on the test floor is normally temporary since motors of all sizes are tested in the same location. The hot vibration readings need to be taken while the motor is still hot from the loaded run but uncoupled to remove the effects of misalignment, etc. IEEE 112 recommends that the vibrations be measured while the motor is within 25 percent of the normal operating temperature. This is sometimes difficult to do for a coupled heat run test. In this case it can be necessary to stop the machine and uncouple it from the load, then start it up again to measure the vibration. This, of course, is not necessary if the vibration and vectorial change from cold to hot is good while coupled. Determine Performance Characteristics There are many different ways to determine the performance characteristics of an induction motor. These characteristics include efficiency, power factor, load current, and speed. In North America motors are tested in accordance with IEEE 112, although even within that there are still many different methods to use, including methods B, C, E, E1, F and F1. These methods do not require the coupled heat run method, but it is logical that, if a motor is coupled up for a method B or E test, it should be a coupled heat run. In addition, if a method F efficiency test is to be performed (where it is not necessary to couple up the motor), a dual-frequency heat run should be performed. Heat Summer

4 run and efficiency methods should never be implied or assumed. Performance will vary significantly depending on the method used. Efficiency will be accurate and higher if utilizing methods B or E, whereas method F will normally provide slightly less accurate and lower efficiency. Nevertheless, economics frequently outweigh the concern for accuracy, and thus, method F tests are commonly performed. In addition to IEEE 112 test methods, the IEC and JEC also have methods for testing induction motors. These test methods differ from one another in their details and arrive at different results. Cummins, Bowers, and Martiny in 1981 compared in detail these various methods for testing induction motor efficiency. The JEC and IEC methods tend to be less rigorous, and provide less accurate results when compared to IEEE methods, albeit they are less expensive to conduct. Table 2 illustrates the differences in results when the efficiency of a single machine was evaluated per various methods. IEEE-112 JEC-37 IEC 34-2 ANSI-C50-41 Stator I2R, kw Rotor I2R IEEE 122 Eff. Test Methods HP B C E E1 F F1 IEC (1) JEC <200 M M 1.6% M 1.6%.5% M M 1.25 M 1.2%.5% 0 >1500 M M.9% M.9%.5% 0 Table 3 Efficiency Different Test Method Conclusion No matter what tests are chosen, it is important to understand what information is being obtained from the tests specified. The benefit of having information obtained from rigorous tests must be compared against the additional testing cost. Critical applications that have historically been problematic may benefit from a complete test. Alternatively, noncritical, trouble-free applications would add unnecessary cost to the motor if the same tests were specified. By understanding the information delivered by the many different motor tests available, optimal test requirements can be specified. Core Loss kw Wind & Fr., kw Stray, kwq Total, kw Output, kw Input, kw Efficiency Table 2 Table of Stray versus Test Method M=Measured (1)- in this case stray is a percentage of the input, which makes the levels a little higher. A point of interest is the variation in stray load loss used in the different methods. Please see Table 3. In IEEE 112, one has the option either to test for the load loss (such as in method B or the Morgan test also known as reverse-rotation test in methods E and F), or use an assigned value to the load loss (such as in methods E1 and F1). Tested values of stray load loss provide the most accurate measurement of the efficiency as compared to using assigned values, but the cost could be prohibitive. References 1. Cummings, P. G., Bowers W. D., and Martiny, W. J., Induction Motor Efficiency Test Methods, IEEE Transactions On Industry Applications, Vol. IA-17, No. 3, May/June IEEE 112 Test Procedure for Polyphase Induction Motors and Generators, ANSI C Polyphase Induction Motors for Power Generating Stations. 4. NEMA Standards Publication No. MG (Rev. 1) Motors and Generators, IEEE Guide for Testing Turn-to-Turn Insulation on Form-Wound Stator Coils for Alternating-Current Rotating Electric Machines. 6. Finley, W. R., Hodowanec, M. M., Holter, W. G., An Analytical Approach to Solving Motor Vibration Problems, IEEE Transactions, Vol. 36, No. 5, September/October William R. Finley received his BS in Electrical Engineering from the University of Cincinnati. Present responsibilities for Siemens Energy and Automation include being the operations manager for the NEMA product out of Little Rock, Arkansas, and manager of engineering for the same NEMA product where he is responsible for design, development, and quality assurance out of Norwood, Ohio. He is a Senior Member of IEEE and has previ- 4 NETA WORLD

5 ously published over 12 technical papers, which resulted in one first place, two second place, and one honorable mention award. Most of the papers were included in the IEEE transactions. He is currently active in over ten NEMA and IEC working groups and subcommittees. He is Chairman of NEMA s Large Machine Group and International Standardization Group. Mark M. Hodowanec received BS and MS degrees in Mechanical Engineering from the University of Akron, Ohio. Currently, he is the manager of mechanical engineering for ANEMA induction motors built in the US at Siemens Energy and Automation, Inc., Cincinnati. For the past ten years he has worked in a variety of engineering positions including design, product development, order processing, shop testing, and field support. He is currently active on various NEMA, IEEE, and API working groups. In addition to his ANEMA motor experience, Mr. Hodowanec has worked on a wide assortment of induction motors such as NEMA, submersible, and MSHA motors. He is the author of numerous published technical articles. Khursheed S. Hussain received his BS from University of Poona, India, and his MS in Electric Power Engineering from Rensselar Polytechnic Institute, Troy, NY. Currently, he is the principal product engineer for ANEMA induction motors built in the US at Siemens Energy and Automation, Inc., Cincinnati. He has over thirty-five years of engineering and project management experience in motors and generators for industrial, nuclear, and government applications, including ten years in application, design, and development of ship service generators for the US Navy. He is an IEEE member, and member of the working group on IEEE Std. 112, Standard Test Procedures for Induction Motors and Generators. John A. Larabee received his BS in Electrical Engineering from Florida International University, Miami. Currently, he is manager of product engineering and testing for Siemens Energy and Automation, Inc., Cincinnati. He has a background of design engineering, process engineering, and information technology within Siemens. Summer