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2004-01-3174 HIGH SPEED INDUCTION GENERATOR FOR APPLICATIONS IN AIRCRAFT POWER SYSTEMS Jay Vaidya, President, Electrodynamics Associates, Inc. Earl Gregory, Power Generation, Air Force Research Laboratory Copyright © 2004 SAE International ABSTRACT Electric generators have higher power density when the operating speed is increased. High speed electric generators direct coupled to gas turbines provide an ideal source of electric power for airborne applications because of reliable operation and high power density. To function reliably in the speed range of 60000 RPM to 120000 RPM, the rotor of the electric generator must be robust. Examples of the robust rotor technologies for the generator include: permanent magnet (PM), induction and switched reluctance. The objective of the present paper is to describe the current activities in the field of high speed induction generators and associated controllers. A range of induction generators and controllers rated from 5 kW to 200 kW operating at speeds to 62000 RPM is currently under development. The rotor is designed using high strength magnetic materials for the magnetic paths, and high strength alloys for the conductors that form the squirrel cage. Tests have been successfully conducted to demonstrate the high yield strength of the rotor construction materials. High power density of the induction generator designs is demonstrated by the electromagnetic weights of the induction generators: 5 kW weighs 1.7 Lbs., 30 kW weighs 5 Lbs., and 200 kW weighs 37 Lbs. in electromagnetics. The benefits of induction generator for a variety of airborne applications are: Simple, low cost, robust rotor construction; Electrical excitation allows de-excitation instantly in case of internal failure to prevent further damage; Output voltage can be well regulated dc or ac, under steady state and transient conditions; For very high voltage or for very low voltage applications, transformers can be used get the desired voltage output, which helps improve the controller power density; For generator control, the rotor speed information and not the rotor position information is needed, which simplifies the design of the sensor itself; Because no permanent magnets are required, the generator can function in hostile temperature environment. The 5 kW and 30 kW generators are designed for operation at 5000F for location inside the turbine. Three generators rated at 5 kW, 30 kW, and 200 kW are in various stages of fabrication and testing. Closed loop operation using DSP based controllers is implemented. Test data relating to the performance of the induction generators and controllers will be presented. The test conditions will include the steady state voltage regulation, transient voltage regulation, and generator efficiency analysis. INTRODUCTION The electric power generators convert mechanical energy from a rotating prime mover, into electrical energy. The approach for the energy conversion is based on the interaction between magnetic fields and electric currents. For airborne applications, power density in terms of power per unit weight is the essential figure of merit. In order to accomplish the high power density, operation of the generator at high speed is desirable. Typical speeds of airworthy turbines for power up to 200 kW are in the range of 60000 RPM to 120000 RPM. For operating in this speed range robust construction of the rotor of the electric generator is a must. Typical electrical insulators as well as active components do not perform reliably in this speed range. Therefore use of generator technologies that avoid insulated electrical windings, electrical connections, and rectifier diodes is desirable. The most commonly used wound rotor synchronous generator of a brushless design has two stages. These stages include the main and the exciter with a rectifier interface between the two. Thus this generator has two insulated winding, rectifiers, filter and several connections in the rotor. This approach is generally considered unreliable at high speeds of operation. Other technologies that are suitable for high-speed operation are: 1. Permanent Magnet 2. Induction 3. Switched Reluctance 4. Homopolar and Rice Of these several approaches, the objective of the present paper is to describe the induction generator technology and discuss some of the results of the work done by the authors in recent years in developing induction generators for airborne applications. OPERATING PRINCIPLES OF INDUCTION GENERATOR Induction motors are the most commonly used electric motors from household appliances to airborne and space power systems. The reason for this is that induction motors are robust, can be controlled easily, and cost less to produce compared to most other types of motors. Many of the same features of the induction motors apply to the induction generators as well. In construction, the induction generator is made in the same manner as an induction motor. It has two components: • A rotor having conducting bars located inside a slotted iron core, with the bars short circuited at two ends, and • A stator having a three-phase (or multiple phase) winding located inside the insulated slots of an iron core. Figure 1 shows a cross-sectional view of the induction generator. ARMATURE CORE a rotating magnetic field is created in the induction machine. This rotating magnetic field causes currents to flow in the short-circuited conductors of located in the rotor. The interaction between the magnetic field and the current in the rotor conductor causes production of a mechanical force or torque. This in turn causes the rotor to begin rotating. As the rotor rotates, the difference between the speed of rotation of the rotor (ωr) and the speed of rotation of the magnetic field (ωm) reduces. This difference is called the slip (s). As the slip approaches zero, the torque produced by the electromagnetic interaction reduces rapidly, until it reaches zero when the slip s is zero. Figure 2 shows torque speed characteristics of the induction machine as the speed rises from zero to ωm, and the slip falls from ωm to zero. Note that the rotational speed of the magnetic field is called the ‘synchronous speed”. And the rotor is said to rotate “asynchronously”. This range of operation is the “motoring” range when the electrical energy is converted into mechanical energy at the shaft of the induction machine. It should be noted that the torque production near the synchronous speed varies in a linear fashion as shown by the stable region indicated in Figure 2. If the induction machine is driven by a prime mover at a speed higher than the speed of the magnetic field (ωm), then the electromagnetic interaction begins to produce torque in the opposite direction. Or to view this more correctly, it absorbs the mechanical energy from the prime mover and produces electrical energy. This is what is called the “super synchronous” operation of the induction machine, or the generation mode. Figure 2: Induction Motor Torque v/s Speed ARMATURE WINDING 100 90 80 SQUIRREL CAGE ROTOR CORE Torque, % 70 60 50 40 30 20 10 0 0 10 20 30 40 50 60 70 80 90 100 Speed, % of Synchronous Speed Figure 1: Induction Generator Cross Section When a 3-phase ac electric power source is applied to the stator windings, It should be noted here that the useful range of operation of the induction generator is in the linear region marked as stable region in Figure 3. If the speed of operation exceeds the limit, then the output power starts falling rapidly. discussed here briefly prior to the discussion relating to the control of the induction generator. Figure 3: Induction Generator Torque v/s Speed Torque, % 0 -10 100 -20 -30 -40 -50 -60 -70 110 120 130 140 150 160 170 180 190 ISOLATED OPERATION The airborne power system must function independently and has only a small battery power source to draw from for initialization and emergency. It must function reliably to provide the power in a stable manner. 200 -80 -90 -100 Speed, % of Synchronous Speed Stable Region The torque-speed curves shown in Figure 2 and 3 are typical of the induction machines. These apply to specific designs at specific terminal voltages. The torque itself is a function of the voltage and varies as a square of the voltage under unsaturated magnetic conditions. Thus the power output is a function of the square of the voltage. As can be seen from Figure 3, it is a linear function of the slip in the stable region. In a typical generator application, the terminal voltage must be held nearly constant. Thus to vary the power output as dictated by the load requirements, the slip must be adjusted. Figure 4 shows how the change of slip at a given speed of the prime mover affects the change in the output power for a given terminal voltage. To change the slip, the electrical frequency of the voltage applied to the generator stator must be varied in relation to the rotational speed of the rotor. Figure 4: Torque Speed Curves with Varying Slip 0 -10 -8 -6 -4 -2 0 2 T o rq u e , % -20 -40 -60 -80 -100 CONSTANT VOLTAGE OPERATION For typical high voltage dc applications, the generator must provide a reasonably constant voltage under steady state load and overload conditions, with a permissible drop under external short circuit. SPECIAL APPLICATIONS: DIRECTED ENERGY In directed energy applications for high power microwave weapons, voltage step up using an ac 3-phase transformer is desirable. In certain weapon applications, the load appear more like a capacitor to be charged from say 10% of the nominal voltage to 100% of the voltage, with a constant current rather than constant voltage. ENGINE STARTING In most airborne applications, it is also desirable that the generator functions as a starter to bring the prime mover speed to the level when it produces the necessary torque. HIGH SPEED OPERATION Operating requirement of 60000 RPM to 120000 RPM requires a careful design of the rotor structure using high strength materials as well as containment design to limit the stresses in the rotating components. THERMAL Cooling of the stator and rotor is typically accomplished by airflow or by use of engine lubricant oil. In certain integrated starter generator applications, the generator is located on the same shaft as other turbine components and may be subjected to a high temperature environment of up to 5000F or more. Slip, % POWER CONDITIONING REQUIREMENTS FOR HIGH SPEED AIRBORNE GENERATOR The airborne electric power systems have peculiar and perhaps stringent requirements, which affect the design, operation, and control of the induction generator. These requirements are It is clear from the earlier discussion that the induction generator needs an ac source to magnetize the magnetic circuit with a certain frequency that can be adjusted. This is accomplished by use of a 3-phase inverter typically built using IGBT’s in the configuration shown in Figure 5. Hysteretic Control 1.4 Induction Machine Current + 0 0 10 20 30 40 50 60 70 80 90 Angle, Degree _ Figure 5: Three Phase Inverter Connected to Induction Machine The IGBT’s are switched to provide ac currents in the generator stator windings at the desired frequency. There are two techniques commonly used for this: i) Pulse width modulation (PWM) ii) Hysteretic current control In the PWM technique, the IGBT’s are switched at a fixed switching frequency and turned off when the desired level of current is reached. This is shown in Figure 6. Thus what is controlled is the “ON” time of the pulse. Pulse Width Modulation Figure 7 The control of the induction machine in the generator mode as well as in the start mode is accomplished by control of the current supplied to the 3 phases. The relationship between the torque at the shaft and the current can be visualized better in the 2-phase frame of reference, called the D-Q frame. From the fundamental electromagnetic relationships, it is known that the torque T is proportional to the product of the magnetizing current (ID) and the current IQ. In the non-salient machine such as the induction machine, the D-Q axes can be arbitrarily selected so that ID= IQ. Then we express the torque equation as: 2 τ rωs ) τ r 2ω s2 + 1 T = kT ( I DQ )( Where KT = Torque Constant, Nm/A2 IDQ = √2 ID, A 1 A m p litu d e τr = ωs = Rotor time constant, s slip frequency, radians/s For optimum torque, τr = 1 ωs , and 2 0 0 30 60 90 Angle, Degree Figure 6 In the hysteretic current control, the command current is compared to the actual current, and the switching of the IGBT’s is accomplished using a hysterisis band around the commanded current as shown in Figure 7. T = 12 kT I DQ ………………………………………(1) GENERATOR CONTROL Here we use the energy storage equation for the generator-load systems shown in Figure 8. 2 DC Bus 3 Phase Transistor Bridge Generator ( s + s0 ) 3 = s 3 + 3s0 s 2 + 3s0 s + s03 ki = ( s03τ em ) ηω (3s0 τ em ) − 2 /( RC ) 2 ,kp = ηω , kd = τ em (3s0 − 2 /( RC )} − 1 ηω R Here τem = electromechanical η = machine efficiency ω = shaft speed, rad/s C For constant voltage operation, the command voltage vc is held constant. For constant current charging of a capacitor, the command voltage is ramped up from the initial low voltage to the final peak voltage of the capacitor. Figure 8: Generator and Load Cv Wr = r 2 2 ………………………………………. (2) START MODE OPERATION In this case, the motor current in the DQ frame of reference is controlled and the output speed of the induction machine is compared to a commanded speed or a speed ramp. An IP control is used as shown in Figure 10. Where Wr = stored energy in the capacitor, J C = capacitance, F vr = voltage across the capacitor, V Equation (1) and (2) are used to develop, a PID control algorithm to control the square of the voltage (V2r) by comparing it to a command voltage (vc) squared. The result is a third order transfer function derived from Figure 9 for the control loop. RC / 2 RC ( + 1) 2s vr ωa _ T g /(τ rr s + 1) ηω τs + 1 + time constant, s _ ωr WL 1 / s ki + + W _ Wr ki 1 s + kp+kds Figure 9: Generator Control Loop kp + _ Figure 10: Motor Control Loop The transfer function and IP parameters are as follows: gk i / τ rr s 2 + s ( gk p + 1 ) / τ rr + gk i / τ rr Compare the denominator to The transfer function is: k iηω / τ em , where s 3 + as 2 + bs + c 1 + k pηωRC / 2 τ + RC / 2 + ηωkd RC / 2 a = em ,b = , c = kiηω / τ em τ em RC / 2 τ em RC / 2 Comparing the denominator with (s + s0 ) 2 ki = τ rr s0 g 2 ,k p = 2 s 0τ rr −1 g Where g = unit speed change per unit torque change, (rad/s)/Nm τrr = rotor time constant, s. _ SOFTWARE IMPLEMENTATION Implementation of the control algorithms is made in the software using MathWorks XPC modules or by using Texas Instruments DSP’s. HARDWARE IMPLEMENTATION Electrodynamics has implemented the design approach discussed here in the hardware for the 200 kW induction generator operating at speeds up to 62000 RPM. Similar approach is taken for the 5 kW and 30 kW induction generators, which are currently under development. Figure 11 shows the generator hardware Figure 13: Closed Loop Control of Voltage Figure 14 shows the test results of the current control in a closed loop system. The current command is shown by the blue line and the actual current is shown by the green line. As the command for the increase in current is applied, the actual current lags behind the command but reaches the desired current level in a few milliseconds. Figure 11: 200 kW Generator Figure 12 shows graphically the electromagnetic weights of the three generators to provide approximate scaling of the induction generators in 5 kW to 200 kW range. Weights of 60000 RPM Induction Generators 40 Figure 14: Closed Loop Current Control Weight, Lb. 35 30 25 Figure 15 shows the operation of the generator in the start mode. The generator speed follows the speed ramp command to reach the desired speed in 90 seconds as commanded. 20 15 10 5 0 0 50 100 150 200 Pow er, kW Figure 12 TEST RESULTS Figure 13 shows the test results of the closed loop control of voltage. The voltage at the dc bus is shown by the blue line and the current is shown by the green line. As the load is applied, the voltage dips somewhat but recovers in a few milliseconds to the original level. Figure 15: Start Mode Operation TECHNOLOGY BENEFITS Based on the experience in developing the induction generators and controllers in 5 kW to 200 kW range, we list below the benefits that the technology provides for a variety of airborne applications requiring DC bus voltage. ROBUST CONSTRUCTION The rotating component has no insulated windings or rectifiers, which are the weak members of electromagnetic guts. LOW COST Since no permanent magnets are used, the cost of materials and labor are reduced. FLEXIBLE CONTROL Either voltage or current can be regulated. Start mode motor operation and generator mode operation are accomplished easily with software based controls. HIGH VOLTAGE APPLICATIONS For applications such as high power microwave, the ac output of the induction generator is stepped up using a small transformer. The rectified dc voltage from the transformer is controlled by methods discussed earlier at the low voltage level. SPEED FEEDBACK Use of speed feedback rather than position feedback, simplifies the controller hardware and reduces the cost as well. HOSTILE THERMAL ENVIRONMENT Since no permanent magnets are used, operation in high temperature environment is feasible. INSTANT CONTROLLER RESPONSE In case of internal failure, the excitation and hence the production of power can be shut down within milliseconds, thus preventing further hazard to the equipment and the surroundings. CONCLUSION High-speed operation of the generators for airborne applications is desirable for reaching high power density goals. Induction generator technology offers an ideal approach to operation under hostile conditions of temperature and speed. It also provides the benefits of low cost robust construction with simple, flexible control. The approach taken by Electrodynamics Associates, Inc. in developing the induction generator technology is presented. Hardware and test data to support the capability claims for the technology are also presented. ACKNOWLEDGMENTS Major portion of the work described in this paper is supported by the funding from AFRL under contract number F33615-00C-2018. CONTACTS Jay Vaidya is the President and Principal Engineer at Electrodynamics Associates, Inc., 409 Eastbridge Drive, Oviedo, FL 32765. He specializes in design, development and manufacture of Electric Motors and Generators as well as Controllers using state of the art technologies including PM, synchronous, induction, and SR. He may be contacted at [email protected], or by phone at (407) 977 1825. Earl Gregory is with Air Force Research Laboratory, Power Generation Branch, Wright-Patterson AFB, OH 45433. He is the Program Manager for the AFRL contract cited above. He may be contacted at [email protected], or by phone at (937) 255 6205.