A compensated pulsed alternator is a generator capable of delivering high power energy pulses with current waveform flexibility. This versatile machine has applications in various fields where power density is at a premium. Recent advances in applying fiber/epoxy composites to rotating electrical machinery [l] have greatly enhanced the power density capabilities of this machine. A characteristic of these new machines is an absence of ferromagnetic material in the magnetic circuit, and they are therefore referred to as >air-core'' compulsators. This paper discusses the topological considerations and the capabilities of the family of machines called the air-core compulsators.Center for Electromechanic

Driga, M.D.

Pratap, S.B.

Spann, M.L

Walls, W.A

Texas ScholarWorks

AIR-CORE COMPENSATED PULSED ALTERNATORS Abstract Center for Electromechanics The University of Texas at Austin Balcones Research Center Building 133 Austin, TX 787584497 S. B. Pratap, M. L. Spann, W. A. Walls, and M. D. Driga A compensated pulsed alternator is a generator capable of delivering high power energy pulses with current waveform flexibility. This versatile machine has applications in various fields where power density is at a premium. Recent advances in applying fiber/epoxy composites to rotating electrical machinery [l] have greatly enhanced the power density capabilities of this machine. A characteristic of these new machines is an absence of ferromagnetic material in the magnetic circuit, and they are therefore referred to as "air-core'' compulsators. This paper discusses the topological considerations and the capabilities of the family of machines called the air-core compulsators. Jntroduction The compensated pulsed alternator (compulsator) was conceived at the Center for Electromechanics at The University of Texas at Austin in 1978. It was developed, then, as a power supply for laser flashlamps. The use of active compensation with the help of a compensating winding connected in-series with the armature winding enabled the machine to generate short pulse widths required by the flashlamps. The first prototype compulsator was iron cored. Since then various compulsators and rotary flux compressors have been designed, built, and tested for a variety of applications. At each stage the performance has been altered according to the application. However, the general trend has been to improve the power density. This trend is indicated in figure 1. Compensation techniques have also been developed to generate a broader range of pulse shapes.[2] These pulse shapes range from very peaky as required by flashlamps to flat-topped as required by electromagnetic launchers. Specific, tailored pulse shapes required by certain applications in countermeasures and for driving electrothermal guns are also possible. Figure 1 also indicates a large gain in power density obtained by going from iron-core to air-core machines. The Electromagnetic Gun Weapon System (EMGWS) compulsator to be commissioned in 1990 will have a power density of 1,700 kW/kg and the next generation compulsators are envisioned to have power densities in excess of 10,000 kW/kg. Most of these gains have been made possible by applying fiber/epoxy composite technology to rotating electrical machinery. Fiber/epoxy composites have greater strength and also modulus-to- density ratio. This combination allows them to be spun at high tip speeds. High rotational speeds have made it possible to alleviate some of the limitations due to the bond strength of epoxy and also increased the energy density of the machine. Simultaneously research has been conducted on increasing the bond strength of the epoxy from 24 MPa (3,500 psi) to 55 MPa (8,000 psi). In iron-core machines it is advantageous to utilize the magnetic circuit in order to reduce the excitation power requirements. One is therefore limited by the saturation field strength in iron. Air-core machines by their very nature require higher excitation currents. It is therefore not an advantage to limit the excitation field strength around 2 Teslas (T) as with iron-core machines. The limitation on excitation field strength is now determined by thermal considerations or mechanical strength considerations of the field coil conductor. These limitations are generally higher than the saturation field strength in steel. The thermal limit can be made a secondary consideration by pulse charging the field coil so that the field coil is on only for .a short time prior to and during the main discharge pulse. The exc tation field strength then is only limited by mechanical considerations. '"1 YEAR 1001.0174 Figure 1. Trend for the power density of the con-.pulsator All the factors discussed above result in an improved power and energy density. Higher power densities make the compulsator more amenable to field-portable and space-based applications. ToDoloPical Considerations Two basic configurations have been evaluated for the air-core compulsators: a) The drum type of machine and b) the 5isk type of machine. Figures 2 and 3 show two drum-type machines Tvith external and internal rotors respectively. Figure 4 shows al disk-type machine with a stationary field coil. The disk-type machine ccluld have a stationary armature and a rotating field coil. There are two types of energy storage densities to consider: a) the energy density of the rotor and b) the energy storage dmsity of the entire machine. The energy storage density of the rotor is @en by: e sdr =ql+($] where Vt = tip speed of the rotor ro = outer radius of the rotor ri = inner radius of the rotor 238 7 Cmpmrotlng Shield and Cuter field Coil ?jupport 3901.0284 Figure 2. Air-core compulsator with external rotor Figure 3. Air-core compulsator with internal rotor COMPENSATING SHIELD 7 c fSWFT 7 FIELDCOIL I I / I  I I 1- II I 1 I f ]  ARMATURE -/ WJNDJNG JOURNAL BEARJNGS Figure 4. Disk-type compulsator 1 STATOR STRUCTURE 3801 .om Based on the above equation it is clear that in order to maximize the energy storage density of the rotor it is necessary to maximize the inner radius given a fixed outer radius. The shell rotor of the extemal rotor machine therefore has the highest energy storage density. For a given tip speed the internal rotor and the disk-type rotor have the same energy storage density since the only difference in the geometries is the length-to-diameter ratio. Making a quantitative comparison of the energy storage density based on the entire machine mass is difficult because the mass of the stator also needs to be considered relative to the mass of the rotor. This may vary depending on the application. The overall energy storage density is given by: where - geometric mean of the outer rotor radius rg -and the outer machine radius pr = average density of the rotor pt = average density of the entire machine 1, = length of the rotor It = length of the machine With the drum-type machine the rotor winding needs to be supported by a high strength banding against centrifugal loads. This banding experiences a tangential stress due to centrifugal loading. With the external rotor the function of energy storage and banding are combined into one component since the armature winding spins at the inner surface of the rotor. With the internal rotor machine the armature winding spins near the outer surface of the rotor, thus requiring a banding solely to keep the armature flywheel interface under compression. The thickness of the banding in the internal rotor case is limited by other considerations such as coupling with the stator windings. This coupling is important from a standpoint of maximizing the peak power of the machine. The tip speeds possible with the external rotor machine are therefore higher than the internal rotor machine. Flywheels for applications other than electrical machiney have been built which can attain tip speeds in excess of 1,200 m/s. TWO internal rotor, air-core compulsators have been designed at tip speeds in the 50C-to W m / s  range, With external rotor machines and radially thin windings the tip speeds can be raised up to 1,200 m/s. The rotor windings in the disk type of machine are difficult to support. This difficulty stems primarily from the orientation of the windings. The windings instead of being laid axiaIIy as in the drum type of machine are now laid radially. A banding is no longer adequate to support these windings, since it would experience a large pressure corresponding to the radially thick winding. The rotor windings therefore have to be supported in shear along the entire radial length. The result is a machine with a lower tip speed, since the shear strength of an epoxy bond is relatively low. From a standpoint of energy and power density the external rotor machine has the greatest advantages. However from the standpoint of rotor dynamics it is the most difficult to maintain subcritical. The internal rotor and the disk type of machines are relatively easier to support dynamically. The composite materials used in the construction of the rotors of these air-core compulsators have a very high damping coefficient, greatly reducing the amplitude of vibration associated with passing through a critical speed. This makes shell-type rotors feasible. 239 The peak voltage generated per unit length of the machine is given by E = 3 X 8 ,  where v is the tangential velocity of the winding and B is the peak radial component of the excitation field. Table 1 gives a comparison of these values for iron-core and air-core compulsators. Table 1. Comparison of voltage capabilities RONCORE AIRCORE I TYPE I Velocity mk.1 Radial Field TI E Voltslm I 200.0 2.0 400.0 550.0 4.0 2200.0 I I This clearly shows an increase in generated voltage of over five times per unit length as compared to iron-core machines. Since the same generated voltage can be obtained with a shorter machine or fewer number of tums the machine inductance is also lower. This combination helps to enhance the peak current and therefore the power capability of the machine, especially while driving low impedance loads. For the same physical disposition of the windings the minimum inductance of an air-core and iron-core compulsator is about the same. However the maximum inductance is significantly different for the two cases, especially when considering the unsaturated inductance of the iron-core machine. The result is that the compression ratio obtained for identical winding geometries could be lower by an order of magnitude in the case of the air-core machines, compared to iron-core machines. The compression ratio is of importance to the extent that it affects the pulse width (full width half maximum). The minimum short circuit pulse width is given by: At = L c o s ( E )  sec - PWm 2 where wm = mechanical speed of the rotor CR = compression ratio p = number of pole pairs Figure 5 shows the change in pulse width with the tip speed for an air-core compulsator. Also shown on the same plot is the pulse width obtained from an identical iron-core machine at a tip speed of 150 m/s, which is the maximum tip speed possible for a laminated rotor. This plot shows that for the same geometry an air-core machine must have a tip speed in excess of 600 m/s in order to reduce the pulse width below the unsaturated iron case. This tip speed is certainly possible and is at the bottom range of the tip speed possible for air-core machines. The compression ratio of an air-core machine varies as (t/g+l), whereas the compression ratio of an iron-core machine varies as (t/g)*.[31 Where t is the pole pitch of the winding and g is the effective gap of the armature and compensating windings. Thus the air-core machines have a lower compression ratio but are less sensitive to the ratio of the pole pitch to effective gap. The absence of ferromagnetic material in the magnetic circuit contributes significantly to increase the power and energy densities of the compulsator. The absence of the ferromagnetic materiiil to channel the flux however, results in significant stray fields. This is rue of drum as well as disk-type machines. The presence of stray fields in compact, field-portable and space based applications is not desirable. Therefore some form of shielding is required to reduce the stray fields. The other option is to use a machine with a significantly higher nurrber of poles. Using a higher number of poles results in faster decay of th? far fields as illustrated in figure 6. 6 0 0  - - Iron core at 150mls 8 500 - %  Air core g 400 E 4 300 .- ln a' 1 0 0 -  0 C 200 400 600 800 1000 1200 Tip Speed of the air core machine-mk. Figure 5 .  Pulse width as a function of tip $#peed for air-core machines 5 4 3 2 9 w U 2 1 0 0.28 0.30 0.32 0.34 0.36 0.38 0.40 0.42 0.44 RADIUS (m) PEAK CURRENT DENSITY = 12.8 kNcm 1001.0088 Figure 6. Radial field distribution as a functicln of poles 240 Using a higher number of poles is also accompanied with its problems. Since the excitation fields decay faster with distance, a greater number of ampere-turns are required to obtain the same discharge performance. This is illustrated in figure 7, where the ordinate is normalized with respect to the ampere-turns required for a 2- pole machine. The result is a heavier machine or a machine with greater field coil losses. NO. OF POLES Figure 7. Current in the field coil for the same generator performance Another peculiarity of air-core machines is the presence of relatively strong, tangential excitation fields near the armature winding. These tangential fields interact with the axial currents and produce significant radial forces. These radial forces in combination with the discharge induced radial forces produce a non-uniform radial force distribution, which requires special attention in order to support. Excitation Sche me As discussed earlier the air-core machines have a greater excitation requirement compared to iron-core machines. This implies running the field coil conductor to higher current densities in order to maintain reasonably good coupling between excitation field and armature conductors. Superconducting, hyperconducting or cryogenically cooled coils could be used where the application so permits. An altemate scheme, where this is not feasible, is to pulse charge an ambient temperature field coil. This reduces the energy dissipation in the conductor thus limiting the temperature rise to a reasonable level. Pulse charging the field coil requires a considerable amount of power. A separate power supply to achieve this task would offset the benefits of the air-core machine. Self excitation of the field coil is therefore used. The armature winding itself is used to provide excitation power to the field coil if the characteristics are acceptable. A separate excitor winding could also be provided which has its characteristics optimized to charge the field coil. The output of the excitor winding is rectified through a full wave rectifier and the output of the rectifier is fed directly to the field coil. Phase control of the rectifiers is used to regulate the current and also to reclaim the magnetic energy from the field coil when the discharge pulse is completed. Several significant improvements have been accomplished in the area of power and energy density of air-core compulsators [4]. Efforts are ongoing to increase the performance of these machines beyond their present capabilities. Higher tip speeds are certainly possible. The dynamic behavior of these machines under supercritical operation needs to be investigated. It is necessary to determine the damping associated with the composite materials from a standpoint of the amplitude while transitioning through a critical speed and also in order to determine the onset of the instability associated with this damping. Higher field strengths can be achieved by applying superconducting or hyperconducting materials. If high temperature superconductors prove feasible it would have a significant impact on the design of these air-core machines. Research is ongoing to increase the bond strength in shear of the epoxy in the range of 83 MPa (12,000 psi). This would help in combination with higher tip speeds to reduce the pulse width and increase the power density significantly. Acknowledeemenb Funding for this work was provided by US. Army Armament Research, Development, and Engineering Center and the Defense Advanced Research Projects Agency. 111 121 [31 References W. A. Walls and S. M. Manifold, "Applications of Lightweight Composite Materials in Pulsed Rotating Electrical Generators," Proceedings of the 6th IEEE Pulsed Power Conference, June 29-July 11, 1987, Arlington, VA. S. B. Pratap, K. T. Hsieh, M. D. Driga, and W. F. Weldon, "Advanced Compulsators for Railguns," 4th Symposium on Electromagnetic Launch Technology, IEEE Transactions on Magnetics, January 1989, Volume 25, Number 1, April 12-14,1988, pp 454-459, Austin, TX. W. F. Weldon, W. L. Bird, M. D. Driga, K. M. Tolk, H. H. Woodson and H. G. Rylander, "Fundamental Limitations and Design Considerations for Compensated Pulsed Altemators," proceedings from the 2nd IEEE Pulsed Power Conference, June 12-14, 1979, Lubbock, TX. 141 W. F. Weldon, "Assessment of Potential Research Areas for Improving Rotating Electrical Machines for Pulsed Power Applications," report to the La Jolla Institute, La Jolla, CA, May 1979. 151 M. L. Spann, S.  B. Pratap, M. D. Werst, W. A. Walls, and W. G. Fulcher, "Compulsator Research at the University of Texas at Austin-An Overview," 4th Symposium on Electromagnetic Launch Technology, IEEE Transactions on Magnetics, January 1989, Volume 25, Number 1, April 12-14,1988, pp 529-537, Austin, TX. 24 1 

Air-Core Compensated Pulsed Alternators

A compensated pulsed alternator is a generator capable of delivering high power energy pulses with current waveform flexibility. This versatile machine has applications in various fields where power density is at a premium. Recent advances in applying fiberIepoxy composites to rotating electrical machinery (1) have greatly enhanced the power density capabilities of this machine. A characteristic of these new machines is an absence of ferromagnetic material in the magnetic circuit, and they are therefore referred to as >air-core> compulsators. This paper discusses the topological considerations and the capabilities of the family of machines called the air-core compulsators.Center for Electromechanic

Air-Core Compensated Pulsed Alternators

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