DESIGN AND DEVELOPMENT OF A TWO-MEGAWATT, HIGH SPEED PERMANENT MAGNET ALTERNATOR FOR SHIPBOARD APPLICATION

Co Huynh, Larry Hawkins, Ali Farahani, and Patrick McMullen Calnetix Inc. Cerritos, CA 90703

Abstract – Conventional gas turbine generator sets consist of a high speed turbine coupled to a low speed alternator through a speed reduction gearbox. This is required to maintain the alternator output frequency at 50 Hz or 60Hz, as output frequency is directly proportional to speed. Since power is also directly proportional to speed, the conventional system is bulky and possesses a very large footprint. The advent of solid-state inverters with their unique ability to efficiently and cost effectively change the alternator output frequency has made it possible to eliminate the need to link the alternator speed to the required 50/60 Hz output frequency. This output can be produced with a high-speed alternator, eliminating the need for a gearbox and greatly reducing the size, complexity, and weight of the machine by trading speed for torque. A direct drive system in which an alternator is coupled directly to a gas turbine is much more compact and highly efficient and requires much less maintenance. In this paper we will review the design and development of a high-speed permanent magnet alternator in an advanced cycle gas turbine system for shipboard applications. In addition to the alternator’s design features, we will discuss design considerations including electromagnetic design, thermal design and structural design of high speed electrical machines, and review the alternator development including risk mitigation.

INTRODUCTION The British Royal Navy has development program is to demonstrate the embarked on a development program to ACL-GTA system as a “diesel beating” evaluate advanced cycle low power gas solution, such that the ACL-GTA system turbine alternator (ACL-GTA) technology as should be able to compete in performance an alternative to the diesel generation (DG) and cost with DG systems while providing system for shipboard applications. The the added benefits mentioned above. ACL-GTA system is being considered to The ACL-GTA system shown in provide propulsion, ship’s service power at Figure 1 is an integrated unit with all major sea and in harbor, and independent components mounted on a common skid. emergency power generation. Target The skid is spring mounted to reduce the platforms include future surface combatants shock input to the system. The major and future carriers. ACL-GTA technology components include the gas turbine, the heat promises numerous advantages compared to recuperator, the high speed alternator (HSA), DG technology including smaller foot print, and the electronic unit (EU) for power environmentally compliant emissions, low conditioning. maintenance, and therefore reduced manning levels. The goal of this from the gas turbine at various speeds and ambient conditions up to rated power of 2MW. Table 1 summarizes the requirements for the HSA.

Figure 1 – ACL-GTA Configuration

The HSA couples directly to the gas turbine and operates at the same speed as the gas turbine. The use of a high-speed alternator Figure 2 – HSA Configuration instead of a conventional low speed synchronous machine operated through a The HSA is a four-pole, three-phase speed reduction gearbox is one of the unique machine. An integral cooling fan mounted features of this ACL-GTA system. By on the main rotor shaft provides cooling air trading speed for torque, the direct-drive for the rotor, and a liquid cooling loop in the turbo alternator technology provides a much housing cools the stator. The following more compact, highly efficient and low describes the machine’s unique design maintenance solution for the ACL-GTA features. system. This direct drive approach is further enhanced by the advent of low cost, high Table 1 – HSA Requirements power semiconductor switching devices. Max Output Power 2030 kW This paper will review the design Speed Range 19krpm to 22.5 krpm features of the two-megawatt, permanent Over Speed 27krpm magnet HSA, discuss design considerations Output Voltage 800Vdc @ EU output including electromagnetic design, thermal Duty Cycle Continuous design and structural design and review the Ambient Condition -20 to 55oC HSA development including risk mitigation. Available Cooling Water/glycol

DESCRIPTION AND UNIQUE DESIGN Graphite Composite Sleeve - The high speed FEATURES permanent magnet (PM) rotor utilizes a high The HSA, shown in Figure 2, is a strength graphite composite sleeve (GCS) permanent magnet synchronous machine for magnet retention. The use of graphite with surface mount magnets on the rotor. It composite material in high speed permanent couples directly to the gas turbine through a magnet machines provides some key flexible mechanical coupling. performance advantages. Compared to a The HSA functions as a motor to metallic sleeve, graphite composite, with its provide starting torque to the gas turbine and higher strength to weight ratio, is much as a generator producing electrical power as thinner, thus resulting in better machine the gas turbine operates at higher running magnetic performance and a much more speeds. In generation mode, the HSA is compact design. A GCS with its inherent designed to provide electrical output power “stranding” structure also has significantly compatible with mechanical power supplied lower rotor losses due to eddy current effects. The disadvantages of the GCS performance were part of the HSA design include lower temperature rating compared tradeoffs. Samarium cobalt was selected as to metal sleeve, negligible bending stiffness the magnet material for this application due as fibers are wound in hoop direction, and its higher temperature capability as well as very low thermal conductivity. Selection of better temperature stability and the PM rotor configuration for this, as well demagnetization characteristics when as any other high speed machine application, compared to lower cost neodymium iron is based on weighing these pros and cons to boron. Although samarium cobalt has lower determine the optimal approach for each tensile strength, this has little effect on the specific application. For this particular retaining sleeve design since good design application, the advantages of graphite practice for a robust high speed rotor is to composite sleeve more than offset its keep the magnets in compression under all disadvantages, as adequate air cooling is operating conditions. provided to cool the rotor, and rotor shaft carries adequate bending stiffness. DESIGN CONSIDERATIONS The design process of high-speed Integral Cooling Fan - The HSA design electrical machines involves continuous incorporates an integral cooling fan mounted iterations of electromagnetic, thermal, on the same shaft as the main rotor. The structural and rotordynamic design. As integral fan provides cooling air to cool the depicted in Figure 3, an optimum design rotor as well as buffer air for the shaft seals. requires the delicate balance of all three The fan eliminates the need to tap off design criteria. premium turbine compressor air from the system, thus optimizing system efficiency. The integral cooling fan is also much more compact when compared to an external fan as it eliminates the need for a separate support structure and drive mechanism. The primary drawback with an integral fan is the additional overhung load on the rotor that much be managed in machine’s rotordynamic performance. Figure 3 – HSA Design Criteria Split Air Cooling - One of the unique features of the air cooling scheme for the Electromagnetic Design – Electromagnetic HSA is the introduction of the cooling air to design is conducted using both closed form the middle of the stator stack. This mathematical model as well as finite innovative approach significantly reduces element (FE) method. The closed form pressure required from the fan, thus computer model, based on permanent resulting in a smaller fan design and higher magnet synchronous machine theory, fan efficiency. It also reduces the air provides a fast and accurate tradeoff study temperature gradient (inlet to outlet). of machine parameters such as the number of poles, stator configuration, material Samarium Cobalt Magnets – Consideration selection, rotor aspect ratio, etc. The FE of different magnet materials and their method is utilized in conjunction with the contribution to the overall system closed form method to provide design optimization and to assist in calculating magnetic flux leakage that is essential for accurate machine performance prediction. For a high speed design, the rotor aspect ratio (length over diameter) is one of the critical parameters and is often traded off in the EM design process. A low rotor aspect ratio has higher rotor stiffness, and is normally preferred for optimum rotordynamic performance. However, a low rotor aspect ratio requires a larger rotor Figure 5 – Thermal Considerations diameter, thus increasing the required magnet retaining sleeve thickness and often Since component operating temperatures are overall machine weight. In addition, larger a function of the machine losses as well as rotor diameter also leads to higher windage cooling parameters, worst case component losses, lowering machine efficiency. Figure temperatures occur at maximum ambient 4 shows this tradeoff of electromagnetic condition rather than rated load condition for weight and efficiency versus rotor aspect this particular design. Table 2 provides a ratio for this particular application. summary of HSA thermal performance.

410 98.5 Table 2 – HSA Thermal Performance 405 98.4 400 98.3 Critical Maximum Design 395 98.2 Component Operating Limit* for EM Weight 98.1 390 Efficiency 98 Temperature, Continuous 385 97.9 380 Operation

97.8 % Efficiency,

EM Weight, kG o o 375 97.7 Composite 158 C 177 C 370 97.6 Sleeve 365 97.5 o o 2.5 3 3.5 4 4.5 Magnet 158 C 230 C o o Rotor Aspect Ratio, L/D Winding 159 C 200 C * Limit considers ultimate strength of material and Figure 4 – EM Design Tradeoff built-in design margin based on experience

Thermal Design – The thermal design goal Structural Design – The structural design is to insure acceptable component operating involved both the rotor structural integrity as temperature under all system operating well as the rotordynamic performance. For conditions for optimum performance and rotor structural integrity, the main focus is high reliability. The critical areas for on the magnet retention. The finite element optimal thermal performance include the model of the rotor is shown in Figure 6. electrical insulation on the copper windings, This model was used to establish size and the composite sleeve and the permanent minimum required thickness of the rotor magnets on the rotor. Since the gas turbine sleeve. To ensure robust rotor design and output varies significantly with speed and reliability performance, the rotor analysis ambient conditions, the HSA thermal took into account all critical rotor analysis examines the machine thermal parameters such as: performance at various speeds, loads and ambient conditions, as shown in Figure 5.

Desired bearing support stiffness and damping characteristics were determined using a parametric study that examined both synchronous load and displacement response and shock response. The target values of stiffness of 30.7 MN/m (175,000 lb/in) and damping of 7,900 N-s/m (45 lb-s/in) are achieved in practice by using a beam spring type support and squeeze-film damper.

Rotordynamic Damped Nat Frequency Map 2MW Generator - 65mm ball bearings with fan Figure 6 – Rotor FE Model 40000

30000 ‰ Over-speed condition ‰ Operating temperature range 20000 ‰ Temperature distribution within the 10000 rotor (based on thermal analysis) Natural Frequency, cpm ‰ Manufacturing tolerances of all rotor 0 components 0. 5000. 10000. 15000. 20000. 25000. 30000. 35000. 40000. Rotor Speed, rpm Rotordynamic analysis is used to Figure 7 – Damped Natural Frequency Map determine the rotor natural frequencies and response to imbalance. An FEA model was Rotordynamic Bearing Load Plot built for the rotor and solved for free-free 2MW Generator - 65mm ball bearings with fan natural frequencies and mode shapes. A 1400 system dynamic model was constructed by 1200 coupling the rotor characteristics with 1000 Brg 2 800 Brg 1 bearing and housing characteristics using 600 modal superposition. Figure 7 shows the 400 damped natural frequency map with target 200 Bearing Load, Newtons Load, Bearing 0 bearing support characteristics. Both 0 5000 10000 15000 20000 25000 30000 35000 forward rotor rigid body modes are traversed Rotor Speed, rpm at low speed (below 10,000 rpm) and there Figure 8 – Synchronous Response to is substantial margin above the max normal Imbalance operating speed of 22,500 rpm to the first rotor bending mode. Bearing loads predicted by an unbalance response analysis from one DEVELOPMENT RISKS MITIGATION of several analyzed imbalance scenarios are To ensure a successful development shown in Figure 8. Typical synchronous program, major technical risks were bearing loads are under 400 N (90 lb) per identified early on in the program. Risk duplex bearing pair in the 19,000 – 22,500 mitigation includes analyses and testing of rpm operating range. This is with an scaled component models. imbalance level that is aged by a factor of 5 to account for changes to rotor balance state Rotor integrity – The two-megawatt PM during long-term operation. rotor design with an outer diameter of more than 200mm is the largest PM composite rotor design to date for operation in the magnetic loading within each magnet pole 20,000 rpm range. The rotor must withstand and temperature effects on demagnetization. over speed operation up to 27,000 rpm A small 4-pole PM alternator with similar through extreme temperature conditions rotor configuration and equivalent electrical without significant effects on dynamic loading was short circuit tested to measure balance. To mitigate program risk, a scaled magnet demagnetization. This testing was down rotor with the same diameter as the conducted with the unit stabilized at 180oC actual unit and one third of the length was inside a temperature chamber, as shown in fabricated in the early phase of the program. Figure 10, to simulate the high temperature The purposes for the scaled down rotor are operating condition. Measurement of no to demonstrate manufacturing feasibility and load voltage with the unit stabilized at room to verify rotor design integrity. The temperature before and after the short circuit completed scaled down rotor was mounted test indicated insignificant changes in rotor vertically in a spin pit as shown in Figure 9. magnetic flux. Subsequent testing demonstrated repeatable dynamic performance up to the over speed condition of 27,000 rpm and through extreme temperature conditions.

Figure 10 – Short Circuit Testing

Internal Faults - As with all other permanent magnet machines, concerns exist that internal faults may result in a safety hazard including fire since field excitation provided by the permanent magnets in the Figure 9 – Scaled Down Rotor Testing rotor is fixed and cannot be “turned off”.

Transient thermal analyses of various fault Magnet Demagnetization - One major scenarios were conducted to address these technical risk associated with permanent concerns and to ensure a fault tolerant safe magnet machines is magnet demagnetization design. Two fault scenarios were considered under high temperature and high electrical including a balanced three-phase short loading, such as a short circuit condition. circuit across zero impedance, and a line-to- Demagnetization will result in reduced field line short circuit with the remaining phase excitation from the magnets, leading to connected to the load. The transient analyses lower machine output power. Analysis can took into account the emergency shutdown be conducted to examine magnet loading sequence of the gas turbine system including under short circuit operation, however, the coasting duration. The results shown in testing is the best approach to verify Figure 11 and 12 for both scenarios demagnetization due to the wide variation of indicated that the HSA will not cause any safety hazard as component operating competing technologies. Key performance temperatures are still within acceptable advantages include: (1) highly efficient with limits based on manufacturers’ data and an operating efficiency of 98% at rated operating experience. condition, (2) small footprint for installation in a wide range of locations and applications, (3) high reliability and low maintenance, and (4) direct drive system providing simplified assembly and integration. In addition to ship board applications, this technology is also being evaluated for other commercial applications such as train, energy storage and turbo compressors.

REFERENCES Figure 11 – Balanced 3-Phase Short Circuit 1. Development of the Advanced Cycle

250.0 Low Power Gas Turbine Alternator, Ministry of Defense, British Royal Navy 200.0 Presentation at 2003 ASME Conference. 150.0 2. High Speed Generators for Microturbine

100.0 Applications, Energy-Tech Magazine, Stator Winding Stator Teeth Temperature, oC April 2003 Stator Core 50.0 Composite Sleeve Air Rotor Magnet 3. Propulsion Full Scale Development on Rotor Shaft 0.0 U.S. Navy Surface Ships, Naval 0 5 10 15 20 25 30 35 Time, Seconds Symposium on Electric Machines, October 1998 Figure 12 – Line-to-Line Short Circuit 4. Design and Construction of a 75KW FABRICATION AND TESTING Scale Model of A 19MW Propulsion SCHEDULE Motor Drive, Naval Symposium on Fabrication has been completed for Electric Machines, October 1998. the rotor assembly and all major HSA stator components. HSA integration and testing are scheduled to be completed in the first quarter of 2004. Performance mapping of the HSA will be conducted with the unit mounted in a back-to-back test configuration in which one unit will be operated as a motor driving an identical unit functioning as a generator, or load.

CONCLUSION The two-megawatt high speed generator offers significant cost and performance advantages compared to