Voltage-Versus-Speed Characteristic of a Wind Turbine Generator

Exercise 1 Voltage-Versus-Speed Characteristic of a Wind Turbine Generator EXERCISE OBJECTIVE When you have completed this exercise, you will be familiar with the principle of electromagnetic induction. You will be able to describe the construction and operation of electric generators used in small-scale wind turbines. You will be able to explain how the rotor and stator of these generators produce an alternating (ac) voltage. You will know the effect that a change in the rotation speed of the generator has on the magnitude and frequency of the generated ac voltage. DISCUSSION OUTLINE The Discussion of this exercise covers the following points: Magnetic field Permanent magnets Electromagnetic induction Generators used in small-scale wind turbines Period and frequency of electrical waveforms Relationship between the rotation speed and the voltage induced by a wind turbine generator DISCUSSION Magnetic field A magnetic field is a space or region in which a force is exerted on moving electric charges. Figure 4 shows three different sources of magnetic field. Magnetic fields have two poles called north (N) and south (S). They are represented by lines of force. The Earth Permanent magnet Current in a conductor Figure 4.Sources of magnetic field. Festo Didactic 86353-7

Exercise 1 Voltage-Versus-Speed Characteristic of a Wind Turbine Generator Discussion Magnetic fields can be characterized by two quantities: the magnetic flux density,, and the magnetic field strength,. In the international system (S.I.) of units, the magnetic flux density is measured in teslas (T). One tesla (1 T) is equal to one volt-second per square meter (1 V s/m 2 ). the magnetic field strength is measured in amperes per meter (A/m). Permanent magnets A permanent magnet is a piece of iron or other metal surrounded by a persistent magnetic field, as Figure 5 shows. The north (N) and south (P) poles are points situated near the ends of the magnet where the magnetic attraction is greatest. If a permanent magnet is broken in half or into multiple pieces, all pieces will have a north and a south pole. North (N) pole Magnetic field Magnetic field South (S) pole Figure 5. A magnet has two poles called north (N) and south (S). The magnetic field of the magnet is represented by lines of force surrounding the magnet, as Figure 5 shows. The direction of the magnetic field is indicated by the line arrows: from north to south outside the magnet, and from south to north within the magnet. The intensity of the magnetic field is proportional to the number of lines of force passing through a given cross-sectional area. 8 Festo Didactic 86353-

Exercise 1 Voltage-Versus-Speed Characteristic of a Wind Turbine Generator Discussion Repulsion and attraction Like poles on magnets repel each other, while unlike poles attract each other, as Figure 6 shows. Repulsion: when a pole on a magnet is moved toward a similar pole on another magnet, the magnets repel each other, as Figure 6a shows. Attraction: when a pole on a magnet is moved toward an opposite pole on another magnet, the magnets attract each other, as Figure 6b shows. (a) Repulsion (b) Attraction Figure 6. Like poles repel each other, while opposite poles attract each other. Electromagnetic induction Electromagnetic induction is a fundamental rule of electricity discovered by Michael Faraday and, independently, by Joseph Henry around 183. This discovery led to the invention of electric generators (alternators), electric transformers, and induction motors. According to the principle of electromagnetic induction, whenever a conductor lies in a varying magnetic field, a voltage, or electromotive force (emf) is induced across the conductor. If the conductor is connected to an electrical circuit, a current will flow through the conductor. Festo Didactic 86353-9

Exercise 1 Voltage-Versus-Speed Characteristic of a Wind Turbine Generator Discussion Figure 7 shows an example of electromagnetic induction. A permanent magnet is moved from left to right near a fixed loop of electrical wire. As the magnet approaches the wire loop (Figure 7a), the magnetic field of the magnet starts to pass through the loop, and consequently, more and more lines of magnetic flux cut the wire loop. This causes the magnetic flux density in the loop to increase, and thus, voltage to be induced across the wire loop. The magnetic flux density reaches a maximum value when the magnet is aligned with the wire loop. At this moment, the voltage induced across the loop is zero since the magnetic flux density momentarily stops varying. When the magnet moves away from the wire loop, less and less lines of magnetic flux cut the wire loop. This causes the magnetic flux density in the loop to decrease, and thus, voltage to be induced again across the wire loop. Notice, however, that the polarity of the voltage induced when the magnetic flux density decreases is opposite to that of the voltage induced when the magnetic flux density increases. The polarity of the voltage initially induced across the wire loop depends on the direction of the magnetic field produced by the magnet, as is shown in Figure 7b. In this figure, it is the South pole of the magnet that passes close to the wire loop instead of the North pole as in Figure 7a. As the magnet in Figure 7b approaches the wire loop, the magnetic field of the magnet starts to pass through the loop, and consequently, more and more lines of magnetic flux cut the wire loop. This causes the magnetic flux density in the loop to increase, and thus, voltage to be induced across the wire loop. However, the polarity of the voltage initially induced across the wire loop is negative instead of being positive as in Figure 7a. The faster the magnet is moved, the faster the variation of the magnetic field passing through the loop and, therefore, the higher the voltage induced across the loop. 1 Festo Didactic 86353-

Exercise 1 Voltage-Versus-Speed Characteristic of a Wind Turbine Generator Discussion Induced voltage Magnetic field direction Wire loop Induced voltage Magnetic field direction Wire loop Magnet motion Magnet motion (a) The North pole of the magnet passes close to the loop (b) The South pole of the magnet passes close to the loop Magnetic flux density (T) Time Magnetic flux density (T) Time Induced voltage () Time Induced voltage () Time Figure 7. Electromagnetic induction: when a magnet is moved past a loop of wire, a voltage is induced across the loop as the loop is cut by the magnetic field of the magnet. The polarity of the voltage initially induced across the loop depends on the direction of the magnetic field produced by the magnet. Festo Didactic 86353-11

Exercise 1 Voltage-Versus-Speed Characteristic of a Wind Turbine Generator Discussion Generators used in small-scale wind turbines To convert the harnessed wind energy into electricity, wind turbines use electric generators operating on the principle of electromagnetic induction. In small-scale wind turbines, these generators often use permanent magnets that are rotated past conductive windings to produce electricity. Construction Figure 8 shows the construction of a simple generator used in a small-scale wind turbine. The generator rotor is the rotating part of the generator. It is made of nonconductive material. The rotor shaft is used for mechanical coupling of the generator to the bladed rotor of the wind turbine, usually via a gear box. Permanent magnets are mounted all around the rotor periphery. They are aligned with the axis of rotation, and disposed in sequence such that adjacent magnets alternate in polarity (north, south, north, etc.) The generator stator is the fixed part of the generator, in which the rotor turns. The stator consists of a ring-shaped metallic frame surrounding the rotor. Fixed loops of wire are mounted all around the stator such that they are cut by the magnetic fields of the rotor s permanent magnets upon rotation of the rotor. Each loop of wire is made of several turns of wire. The loops are generally interconnected in order to form three larger loops of wire called the generator windings. The number of magnets on the rotor and the number of wire loops on the stator vary according to the generator design. Wire loops mounted on stator Rotor Wire loops mounted on stator Figure 8. Construction of a simple electric generator. Operation When wind makes the bladed rotor of a small-scale wind turbine rotate, the rotor of the electric generator also starts to rotate. Consequently, the permanent magnets of the rotor pass close to each wire loop of the stator one after the 12 Festo Didactic 86353-

Exercise 1 Voltage-Versus-Speed Characteristic of a Wind Turbine Generator Discussion other. A varying magnetic field therefore passes through each wire loop on the stator. Since adjacent magnets alternate in polarity, the direction of the magnetic field passing through a loop reverses each time a new magnet passes close to the loop. As a result, the polarity of the voltage induced across each loop of wire varies between positive and negative. The resulting voltage induced in the generator is thus called alternating voltage, or alternating current (ac) voltage. Figure 9, for example, shows the polarity of the voltage induced across each wire loop when the rotor is at a given (initial) position, and the inversion of the voltage polarity after a rotation of 6. Stator wire loops (a) Initial rotor position (b) Rotor position after a rotation of 6 Figure 9. Operation of the electric generator in small-scale wind turbines. Figure 1 shows a typical waveform of voltage induced across the windings of a wind turbine generator. This voltage wave is sinusoidal: during each half of the sine wave, the voltage rises from zero to a maximum value and then returns to zero in a gradual, non-uniform way. Also, the polarity of the induced voltage reverses every half of the sine wave because of the alternation in the polarity of adjacent magnets on the generator rotor. Voltage (V) Time (s) Figure 1. Typical waveform of voltage induced across the windings of a wind turbine generator. Festo Didactic 86353-13

Exercise 1 Voltage-Versus-Speed Characteristic of a Wind Turbine Generator Discussion Period and frequency of electrical waveforms When an electrical waveform (voltage waveform, current waveform, etc.) repeats itself at equal intervals of time, the waveform is said to be periodic, or cyclical. Figure 11 shows an example of periodic waveform: the voltage induced in a wind turbine generator: The time interval during which the voltage is of positive () polarity, plus the subsequent time interval during which the voltage is of negative ( ) polarity form a complete cycle of the induced voltage waveform. The cycle repeats over and over at equal intervals of time, as long as the rotor of the generator stays in motion. 1-cycle duration (period Voltage (V) Frequency (Hz) = 1 / Period (s) Time (s) Figure 11. Periodic voltage waveform. The duration of one complete cycle is called the period. The period is symbolized by the capital Greek letter tau and is expressed in seconds (s). The reciprocal of the period, 1/, is called the frequency. The frequency is the number of times the waveform repeats every second. It is expressed in hertz (Hz). One cycle per second is equal to a frequency of 1 Hz. Relationship between the rotation speed and the voltage induced by a wind turbine generator The higher the rotation speed of a wind turbine generator, the faster the variation of the magnetic field passing through the wire loops and, therefore, the shorter the duration of the positive and negative intervals of the induced voltage waveform. Otherwise stated, the higher the rotation speed, the higher the induced voltage, and the higher the frequency (or the shorter the period ) of the induced voltage waveform. Figure 12 shows an example of the voltage induced in a wind turbine generator as a function of time, for three different rotation speeds,, and. 14 Festo Didactic 86353-

Exercise 1 Voltage-Versus-Speed Characteristic of a Wind Turbine Generator Discussion Voltage (V) Voltage (V) Voltage (V) (a) Rotation speed (b) Rotation speed 2 x speed Time (s) Time (s) Time (s) (c) Rotation speed 4 x speed Figure 12. As the rotation speed of a wind turbine generator increases, the magnitude and frequency of the induced voltage increase. Festo Didactic 86353-15

Exercise 1 Voltage-Versus-Speed Characteristic of a Wind Turbine Generator Procedure Outline PROCEDURE OUTLINE The Procedure is divided into the following sections: Construction of a wind turbine generator Wind-turbine generator voltage and frequency as a function of the rotation speed PROCEDURE High voltages are present in this laboratory exercise. Do not make or modify any banana jack connections with the power on unless otherwise specified. Construction of a wind turbine generator In this section, you will study the construction of a small-scale wind turbine generator. You will connect a dc voltmeter across one of the generator windings and observe what happens to the voltage developed across this winding when the rotor is slowly rotated manually. 1. Place the Wind Turbine Generator/Controller module on your work surface. Locate the rotor of the wind turbine generator. How many permanent magnets are mounted along the rotor s periphery? 2. If an extra rotor and a magnetic field strength indicator are available, observe that the magnets on the rotor s periphery are arranged such that adjacent magnets alternate in polarity. Is this what you observed? Yes No 3. Identify the stator of the generator. Notice the loops of wire mounted around the stator. Since the permanent magnets on the rotor produce magnetic fields, the wire loops at the stator are cut by the magnetic fields of the magnets. When the rotor of the generator rotates, the permanent magnets on this rotor also rotate, causing the intensity of the magnetic fields cutting the wire loops at the stator to vary. The wire loops at the stator are interconnected in order to form three larger loops referred to as the generator windings. The generator windings are connected to the terminals labeled Permanent Magnet Synchronous Generator in the upper left corner of the Wind Turbine Generator/Controller module s front panel. 16 Festo Didactic 86353-

Exercise 1 Voltage-Versus-Speed Characteristic of a Wind Turbine Generator Procedure 4. Connect a dc voltmeter across one of the generator windings. Make the rotor of the generator rotate slowly, and observe that the polarity of the voltage generated across the generator winding alternates between positive and negative, reflecting the changes in the direction of the magnetic field crossing the wire loops at the stator. Wind-turbine generator voltage and frequency as a function of the rotation speed In this section, you will use a prime mover to drive the wind turbine generator. You will vary the rotation speed of the prime mover by steps and measure the level and frequency of the ac voltage generated across one of the generator windings. 5. Refer to the Equipment Utilization Chart in Appendix A to obtain the list of equipment required to perform the rest of this exercise. Before installing the equipment in the Workstation, compare the pulley of the Four-Quadrant Dynamometer/Power Supply to that of the Wind Turbine Generator/Controller. Notice that the pulley of the Four-Quadrant Dynamometer/Power Supply has fewer teeth (24 teeth) than the pulley of the Wind Turbine Generator/Controller (32 teeth). Install the equipment required in the Workstation. Mechanically couple the Wind Turbine Generator/Controller to the Four-Quadrant Dynamometer/Power Supply. Before coupling rotating machines, make absolutely sure that power is turned off to prevent any machine from starting inadvertently. Set a multimeter to measure ac voltage and connect it across the terminals of one of the windings of the wind turbine generator, as Figure 13 shows. Appendix E of this manual shows in more detail the equipment and the connections that are required for the circuit diagram of Figure 13. Festo Didactic 86353-17

Exercise 1 Voltage-Versus-Speed Characteristic of a Wind Turbine Generator Procedure Wind Turbine Generator/ Controller (8216) Prime mover Rotor N 896 8216 Generator windings Figure 13. Setup used to plot curves of the wind turbine generator s voltage and frequency as a function of the rotation speed. 6. Set the main power switch of the Four-Quadrant Dynamometer/Power Supply to the O (off) position, then connect the Power Input to an ac power outlet. Set the Operating Mode switch of the Four-Quadrant Dynamometer/Power Supply to Dynamometer. This setting allows the Four-Quadrant Dynamometer/Power Supply to operate as a dynamometer or a prime mover, a brake, or both, depending on the selected function. Connect the Four-Quadrant Dynamometer/Power Supply to a USB port of the host computer. Turn the Four-Quadrant Dynamometer/Power Supply on by setting the main power switch to I (on). 7. Turn the host computer on, then start the LVDAC-EMS software. In the LVDAC-EMS Start-Up window, make sure that the Four-Quadrant Dynamometer/Power Supply is detected. Select the network voltage and frequency that correspond to the voltage and frequency of your local ac power network, then click the OK button to close the LVDAC-EMS Start-Up window. 8. In LVDAC-EMS, open the Four-Quadrant Dynamometer/Power Supply window: Set the Function parameter to CW Constant-Speed Prime Mover/Brake. This setting makes the Four-Quadrant Dynamometer/Power Supply operate as a clockwise prime mover/brake. 18 Festo Didactic 86353-

Exercise 1 Voltage-Versus-Speed Characteristic of a Wind Turbine Generator Procedure 9. In the Four-Quadrant Dynamometer/Power Supply window: Ensure the continuous refresh mode of the meters in enabled. The continuous refresh mode is enabled by clicking the Continuous Refresh button. Set the Pulley Ratio parameter to 24:32. This ratio corresponds to the number of teeth (24) on the pulley of the Four-Quadrant Dynamometer/Power Supply and the number of teeth on the pulley of the Wind Turbine Generator/Controller. 1. Make the wind turbine generator rotate at 1 r/min by making the following settings in the Four-Quadrant Dynamometer/Power Supply window: a Set the Speed parameter to 1 r/min. This sets the speed command to 1 r/min. Note that the speed command is the targeted speed at the shaft of the machine coupled to the prime mover, i.e., the speed of the wind turbine generator in the present case. The speed command can also be set by using the Speed control knob in the Four-Quadrant Dynamometer/Power Supply window. Start the prime mover by setting the Status parameter to Started or by clicking on the Start/Stop button. Observe that the prime mover starts to rotate, thereby driving the rotor of the wind turbine generator. Also observe that the Pulley Ratio parameter is now grayed out as it cannot be changed while the prime mover is rotating. Since the pulley ratio is set to 24:24, the Speed meter indicates the rotation speed of the wind turbine generator. Record this speed below. Rotation speed of the wind turbine generator = r/min Does the wind turbine generator speed correspond to the speed command set above? Yes No 11. Observe the rotation speed indicated on the display of the Four-Quadrant Dynamometer/Power Supply. It is the rotation speed of the prime mover. Record the prime mover speed below. Rotation speed of the prime mover r/min Is the rotation speed at the shaft of the prime mover higher than that at the shaft of the wind turbine generator? Why? Festo Didactic 86353-19

Exercise 1 Voltage-Versus-Speed Characteristic of a Wind Turbine Generator Procedure 12. In the Four-Quadrant Dynamometer/Power Supply window, stop the prime mover by clicking on the Start/Stop button or by setting the Status parameter to Stopped. 13. Make the rotation speed of the wind turbine generator vary from to 15 r/min in about eight steps distributed evenly by adjusting the Speed parameter. For each speed setting: Record the generator rotation speed in the first column of Table 1; Measure and record the generator voltage; Measure and record the generator frequency with the multimeter you are using, if this multimeter has a frequency meter function. Table 1. Wind-turbine generator voltage and frequency as a function of the generator rotation speed. Generator rotation speed (r/min) Generator voltage (V) Generator frequency (Hz) [optional] 15 14. In the Four-Quadrant Dynamometer/Power Supply window, stop the prime mover by clicking on the Start/Stop button or by setting the Status parameter to Stopped. 15. From the results recorded in Table 1, plot the curve of the wind-turbine generator voltage as a function of the generator rotation speed. According to the obtained curve, does the wind-turbine generator voltage vary in direct proportion to the generator rotation speed (i.e., the generator voltage doubles, triples, etc., when the rotation speed doubles, triples)? Yes No 2 Festo Didactic 86353-

Exercise 1 Voltage-Versus-Speed Characteristic of a Wind Turbine Generator Conclusion 16. If you recorded the frequency of the generator, plot the curve of the windturbine generator frequency as a function of the generator rotation speed. According to the obtained curve, does the wind-turbine generator frequency vary in direct proportion to the generator rotation speed (i.e., the generator frequency doubles, triples, etc., when the rotation speed doubles, triples)? Yes No 17. Close LVDAC-EMS and then turn off all equipment. Remove all leads and cables. CONCLUSION In this exercise, you studied electric generators used in small-scale wind turbines. You learned that these generators operate on the principle of electromagnetic induction. These generators consist of a rotating part called a rotor, on which permanent magnets are mounted. The rotor is surrounded by a fixed ring-shaped metallic frame called a stator, on which loops of wires (windings) are mounted. You learned that when the rotor turns, the permanent magnets of the rotor pass close to the loops of wire on the stator, causing an alternating (ac) voltage to be induced in the generator. You observed that when the rotation speed of the generator increases, the magnitude and frequency of the ac voltage induced in the generator increases. REVIEW QUESTIONS 1. What is a permanent magnet? Explain how the magnetic field around a permanent magnet is represented (direction and intensity). 2. According to the principle of electromagnetic induction, what happens when a conductor lies in a varying magnetic field? When a permanent magnet is moved close to a fixed loop of wire? Festo Didactic 86353-21

Exercise 1 Voltage-Versus-Speed Characteristic of a Wind Turbine Generator Review Questions 3. By referring to Figure 8, describe the construction of a simple electric generator coupled to the bladed rotor of a small-scale wind turbine. 4. By referring to Figure 9, describe the operation of the electric generator coupled to the bladed rotor of a small-scale wind turbine, when wind makes the bladed rotor of the turbine rotate. Explain how an alternating (ac) voltage is induced in the electric generator of this turbine. 5. What is the relationship between the rotation speed of a wind turbine generator and the ac voltage (magnitude and frequency) induced in this generator? Explain. 22 Festo Didactic 86353-