Abstract:
The motor-generator circuitry of a flywheel energy conversion device can be utilized to preheat the rotor of the flywheel device. It may be desirable to preheat the rotor prior to normal operation because a rotor operating in cold temperature may be more susceptible to brittle fracture or other damage than a rotor operating at a specified operating temperature. The present invention may utilize the principle of induction heating to preheat the rotor. In preferred embodiments, high frequency current may be passed through armature windings of the motor-generating circuitry to induce surface currents into the periphery of the rotor. Heat may then be generated in portions of the rotor receiving the induced currents and then radiate from those portions to raise the rotor temperature to a desired level.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to flywheel energy conversion devices that include motor-generators and methods for heating rotors using the circuitry of the flywheel energy conversion devices. 
     Motor-generator systems typically use some type of rotor to incite motoring or energy generating action. For example, a rotor may be used in an automobile alternator to provide electrical power to the car&#39;s electrical system. Other applications may include using the rotor in a motoring function to drive, for example, a power drill. An example of a large scale application may involve rotating a rotor with a prime mover such as steam-driven turbine of a nuclear facility to generate electricity for a utility power grid. Yet another example may include rotors that are used in flywheel energy conversion devices such as that described in Clifton et al. U.S. Pat. 5,969,457 (&#39;457 patent), which is hereby incorporated by reference in its entirety. 
     Each of the rotors described above may exhibit material properties that require specific operating temperatures to operate safely. For example, rotors in flywheel devices may require an operating temperature of at least 15° C. before normal operation can commence. Therefore, to ensure safe operation of the rotor, the temperature of the rotor should be greater than a specified temperature to provide a substantial margin of safety against brittle fractures. (The specified temperature may be dictated by the material properties of the rotor itself, in that the impact energy of metallic materials is a non-linear function of temperature. It is also known that some material strength properties (e.g., impact energy and fracture toughness) are hampered when the temperature of the material is low. But the same material may exhibit resilient strength properties at higher temperatures.) If a rotor is forced to spin at a relatively high speed when its temperature is below a specified value, the stresses due to rotation may cause the rotor to shatter, crack, or experience brittle fracture. 
     Various preheating techniques have been applied to motor-generator applications to ensure that the rotor operates in a preferred operating temperature regime. For example, the process of preheating a rotor of a 500,000 VA generator is described as follows. In this example, steam may be used to transfer heat to the rotor and other associated components (e.g., turbine disks) by convective means. Initially, a limited quantity of steam may be introduced to gradually warm the rotor. Then, in controlled increments, greater quantities of steam may be applied to steadily raise the temperature of the rotor to a desired level. The heating process may progress over a period of several hours to several days, but is necessary to prevent potential damage that can be caused by changing temperature gradients that can inflict thermal stresses on the rotor. This heating process may be problematic because it is cumbersome, time consuming, and requires an external source of heat (e.g., steam) to raise the rotor temperature. Once the heating process is complete, however, the rotor is in condition to safely generate power. 
     Other known preheating methods have been used for preheating devices such as engines. For example, preheating an engine may involve installing heater coils strategically around vital parts of the engine. Power may be provided to heater coils so that they radiate heat to the engine via convective or conductive means. Eventually, this radiated heat may preheat the engine to a specified temperature prior to ignition. But this method requires the addition of external components (i.e., heater coils) to enable the preheating process. 
     In view of the foregoing, it is an object of this invention to preheat the rotor of a flywheel energy device prior to normal use. 
     It is a further object of this invention to preheat the rotor to obtain a substantially high margin of safety against brittle fracture or other damage. 
     It is also an object of this invention to use the circuitry of the flywheel energy device to preheat the rotor prior to normal use. 
     SUMMARY OF THE INVENTION 
     These and other objects of the present invention are accomplished in accordance with the principles of the invention by preheating the rotor of a flywheel energy conversion device prior to use. In preferred embodiments, the rotor may be preheated using the circuitry provided with the flywheel device. Such circuitry may include armature windings, field coil windings, electronics (including software), temperature sensors and other suitable features of the flywheel device. 
     The rotor is preferably preheated prior to use to provide a reasonable margin of safety that protects the rotor from incurring brittle fractures. Since brittle fractures are more likely to occur at lower temperatures, the present invention may implement several techniques to raise the rotor temperature to a safe operating level. These problems are particularly relevant when the temperature is below a specific transition temperature, which is dictated by the material properties of the rotor. This may be advantageous because it enables a flywheel device to operate in sub-zero arctic temperatures. 
     The present invention may utilize convective and radiative methods to heat the rotor. Convective methods are particularly useful if the rotor is not operating in vacuum conditions. An example of using this method may involve heating the field coils by passing current through them. As the current flows through the coils, they generate heat, and that heat may be transferred to the rotor via air, which serves as a heat transfer medium. After time, the convected heat may eventually raise the temperature of the rotor to the desired level. 
     In another embodiment, a method of radiative heating may be used. This technique utilizes high frequency magnetic induction to heat to the rotor. This may be advantageous because heat can be conveyed directly to the rotor without requiring any physical application of a device (e.g., heat iron) to the rotor. A further advantage of this technique is that the circuitry of the flywheel device can be used to induce the heat energy to the rotor. This eliminates the need for additional equipment to perform the heating process. In addition, heat may be imparted onto a rotor operating in a vacuum, an environment where convective methods cannot be employed. 
     Using the circuitry of the flywheel device, induction heating may be provided as follows: 1) provide a high frequency current (produced by flywheel device electronics) to the armature windings; 2) use the high frequency current to generates flux, which passes through the rotor; 3) induce currents in portions of the rotor where the flux passes through; and 4) generate heat from the induced current to raise the rotor temperature. 
     The induction heating process may be a single process or it may be separated into a preheating step and a settling step. During the preheating step, high frequency currents are continuously applied to the armature windings. It is during this step in which certain portions (e.g., toothed protrusions) of the rotor are constantly subjected to induced currents. Since these portions are constantly receiving induced currents, they may be heated to relatively high temperatures, whereas the rotor core, which does not have induced currents, may remain relatively cool. 
     Since heat may continue to flow from the inductively heated portions even after cessation of high frequency currents, it may be useful to transition from the preheating step to the settling step to avoid potentially overheating the flywheel device (e.g., the armature windings or rotor surface). The settling step may provide time for surface heat of the rotor to diffuse and fully penetrate the rotor such that the desired temperature is provided substantially throughout the rotor. 
     The present invention may also use different techniques to determine whether the rotor requires heating prior to use. Both indirect and direct rotor temperature measurements may be performed (either individually or in combination with each other). The indirect method may include monitoring the temperature of the stator (e.g., armature winding), outer casing, or some other portion of the flywheel device to obtain an approximate temperature of the rotor. If the monitored temperature is too low, then the electronics may determine how long to heat the rotor to obtain a desired temperature. If the induction heating technique is used, then the electronics may determine the preheating and settling time needed to safely heat the rotor. 
     If direct rotor temperature measurement is implemented, a device such as an infrared detector may be positioned within the flywheel to monitor rotor temperatures directly. The infrared detector may provide substantially accurate rotor temperatures, which may enable the electronics to more optimally heat the rotor by minimizing heating times and reducing risk of overheating the flywheel device. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects and advantages of the invention will be apparent upon consideration of the following detailed description, taken in conjunction with the accompanying drawings, in which like reference characters refer to like parts throughout, and in which: 
     FIG. 1 illustrates a cross-sectional view of an energy flywheel device constructed in accordance with the principles of the present invention; 
     FIG. 2 illustrates a top view of the rotor of the energy flywheel device of FIG. 1, taken from line  2 — 2  of FIG. 1; 
     FIG. 3 illustrates an enlarged version of a portion of FIG. 1 that shows flux lines and induced currents in accordance with the principles of the present invention, taken from circle  3  of FIG. 1; 
     FIG. 4 shows several steps illustrating various stages of heat distribution throughout the rotor in accordance with the principles of the present invention; 
     FIG. 5 illustrates a thermocouple device configured to monitor the temperature of the armature in accordance with the principles of the present invention; 
     FIG. 6 illustrates an infrared detector configured to monitor the temperature of the rotor in accordance with the principles of the present invention; and 
     FIG. 7 is a schematic block diagram of an uninterruptible power supply system in accordance with the principles of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention relates to preheating a rotor of a motor-generator device, and more particularly, to preheating the rotor of a flywheel energy conversion device. 
     Motor-generator devices are known and have been utilized in several different applications. For example, motor-generator devices are used in electrical generation facilities, automobiles (e.g., the alternator), uninterruptible power supply (“UPS”) systems, and other suitable applications. Motor-generator devices, in accordance with the principles of the present invention, are primarily described in context with UPS systems. For example, the &#39;457 patent describes a UPS system that implements a motor-generator device. 
     FIG. 1 illustrates a cross-sectional view of motor-generator device  100  which is in accordance with the principles of the present invention. Device  100  may include rotor  112  which has teeth  114  cut out of a portion of the outer circumference of rotor  112 . Teeth  114  are delineated by dashed lines  116  in FIG. 1, and may be more apparent from viewing FIG. 2, in which a top view of rotor  112 , taken along line  2 — 2  is shown. Rotor  112  is shown be to be encased by outer casing  118 , which is preferably constructed from a magnetically permeable and electrically conductive material such as steel. Outer casing  118  may be constructed such that air gaps  120  and  122  exist between rotor  112  and outer casing  118 . Also shown in device  100  are upper and lower field coils  130  and  132  respectively. Upper and lower field coils  130  and  132  may be wound in device  100  such that air gaps  134  and  136  exist between rotor  112  and field coils  130  and  132  respectively. 
     FIG. 1 also illustrates that several armature coils  144  are disposed around the periphery of rotor  112  such that air gap  146  is formed between armature coils  144  and rotor  112 . Armature coils  144  may be coupled to electronics (not shown in FIG. 1; see FIG. 3) that can provide power to (e.g., for motoring function) or receive power from armature coils  144  (e.g., for drawing energy from the rotating rotor). In the present invention, armature coils  144  may be used for conducting time-varying currents, that in turn, induce currents in rotor  112 , which cause the temperature of the rotor to rise. Persons skilled in the art will appreciate that the above discussion with respect to FIG. 1 is not intended to be an exhaustive description of various features that can be included in motor-generator device  100 . It does, however, discuss many of the various features pertinent to describing the present invention. The &#39;457 patent, for example, provides a substantial description of motor-generator device  100  and other similar motor-generator devices. 
     Using device  100  as described above, the present invention may be able to heat rotor  112  to a specified temperature prior to the occurrence of normal operations. Normal operation of device  100  may include spinning rotor  112  at a substantially high number of revolutions per minute in order to store enough energy in the spinning rotor to provide backup power for a sustained period of time. Heating rotor  112  may be accomplished by using one of several different techniques. Rotor  112  may be preheated by convection, electromagnetic radiation, or a combination of both. At this temperature, the rotor temperature may be within a reasonable margin of safety that guards against brittle fracture. 
     If a convection technique is used, and assuming that there is no vacuum applied to device  100 , then the air surrounding rotor  112  serves as the medium for heat transfer. Since air is the heat transfer medium, heat may be transferred to the rotor from other portions of device  100 . For example, current may be applied to field coils  130  and  132 , which are heated by the flowing current. When field coils  130  and  132  heat up, they may transfer heat to rotor  112  via the heat transfer medium. Another example may involve blowing hot air into outer casing  118  to heat rotor  112 . These different convective heating techniques may be able to heat rotor  112  to a specified temperature, but may require too much time to achieve the desired rotor temperature or additional costly equipment. 
     Various factors may contribute to lengthy rotor heat times using convective methods. One factor that may dictate the rate of heat transfer may be a convection heat transfer coefficient that exists at the surface of rotor  112 . Other factors that may affect the rate of heat transfer is the temperature of outer casing  118 , the ambient temperature external to device  100 , the surface area of rotor  112 , etc. The combination of these factors may result in a temperature versus time relationship that is exponential. That is for each temperature unit (e.g., one degree centigrade) increase, it takes exponentially more time for the temperature of rotor  112  to rise that additional unit. Therefore, the convection technique may require a substantial period of time to heat rotor  112  to a specified temperature. 
     The radiative technique, on the other hand, may raise the temperature of rotor  112  at a faster rate than the above-mentioned convection technique. Using this technique, heat is induced in portions of rotor  112  by magnetic induction heating and conducted throughout the medium of the rotor to increase the rotor temperature. This may be accomplished by passing high frequency currents through armature coils  144  to induce high surface currents in the periphery of rotor  112  via magnetic induction. These surface currents are primarily responsible for causing the rotor temperature to rise. (A more detailed description of the induction heating process is discussed below in conjunction with FIG. 3.) One particular advantage of this technique is that it can be performed in vacuum. In fact, the efficiency of the heating process may be enhanced if it is performed in vacuum because heat loss paths are substantially reduced. Therefore, this technique requires less time to achieve the specified rotor temperature. 
     Using this technique, the rate of heat transfer is approximately proportional to the level of current applied to armature coils  144 . Electronics (not shown) may be used to prevent excessive application of currents so that device  100  is not damaged by the heating current (e.g., by overheating armature coils  144  or rotor  112 ). Although the temperature versus time profile is exponential, this technique operates in the substantially linear portion of the temperature profile. This may allow the rotor to heat at a faster rate per unit of time than the convection technique. 
     FIG. 3 illustrates an enlarged view of the rotor and armature assembly encircled with dashed line “ 3 ” in FIG. 1, in accordance with the principles of the present invention. Though not shown in FIG. 1, electronics  360  is illustratively shown to be connected to armature  344  via lines  361  and  362  in FIG.  3 . Electronics  360  may include circuitry such as a DC pulse width modulation (“PWM”) converter that is capable of converting supply power to a high frequency PWM signal (e.g., 3 kHz signal). Using a three phase supply, the high frequency PWM signal may generate a relatively high speed rotating magnetic field that induces current in portions of rotor  312 . Persons skilled in the art will appreciate that other circuitry such as, for example, an AC cycloverter circuitry and a DC-AC inverter circuitry may be used in electronics  360 . 
     Current may be induced asynchronously in the rotor because the rotational speed of the magnetic field is rotating several times that of rotor  312  during induction heating. There may, however, be enough induced torque (reluctance torque) to cause rotor  312  to rotate up to as much as 100 RPM. In the event that rotor  312  does begin to spin too fast, electronics  360  may detect the potential over-speed condition and take appropriate action to reduce the rotor&#39;s speed. 
     Alternatively, the high frequency PWM signal may be applied to only one phase of the three phase armature winding. In this case, there will be no rotation of the magnetic field and substantially no possibility for rotor rotation. However, the magnetic field orientation may not be optimally disposed in proximity to the salient rotor poles to maximize inductive heat input for repeatable heating times. 
     When the high frequency PWM signal is applied to armature  344 , the time-varying current generates flux as illustrated by flux lines  370 . As the flux emanates from armature  344  it may pass through air gap  346  into rotor  312 . When the flux reaches rotor  312  it may induce current  380  on the surface of the rotor. Current  380  is sometimes referred to as eddy current. Typically, eddy currents are considered undesirable during normal operation of the motor/generator system because they represent heat loss and inefficiency. But in the present invention, such currents are purposefully induced to produce the desired effect—heating rotor  312 . As described above, a high concentration of current  380  is induced in the periphery of rotor  312 . In particular, the majority of current  380  may be induced in teeth  314  (delineated by dashed lines  316 ) and a smaller subset of current  380  may be induced in rotor portions located between teeth  314  (such portions are clearly shown in FIG.  2 ). Thus during the heating process, the portions of rotor  312  conducting the induced current may induce a relatively high temperature (e.g., 200° C.) that diffuses throughout the rotor. 
     FIG. 4 illustrates several steps of heat distribution throughout the rotor in accordance with the principles of the present invention. The process of heating the rotor may involve a preheating step and a settling step. Both steps are described below in conjunction with illustrative cross-sectional views and top views taken along line  4 — 4  of the cross-sectional views of the rotor. Step  410  illustrates the beginning of the preheating cycle by showing that heat  412  is concentrated primarily in the toothed portions of the rotor. Step  410  also shows heat  414  disposed along the peripheral edge of the rotor existing between the protrusions. Finally, step  410  also illustrates that there is substantially less heat  414  induced in the rotor than heat  412 . 
     Step  420  illustrates the diffusion of heat  422  as it is conducted away from constantly induced heat  412  and  414 . In this step, high frequency current is still being applied to the armature coils (not shown) while heat  412  and  414  is still being produced by the induced current. After a predetermined period of time or based on other calculations or based on direct or indirect measurement, the electronics (not shown in FIG. 3) may stop applying the high frequency current to end the preheating step and begin the settling step. Hence, when the high frequency currents cease being applied to the armature coils, heat is no longer induced in the rotor in this manner. 
     The settling step provides adequate time for the heat previously induced into the rotor during the preheating step to be relatively evenly distributed within the rotor. The settling step may also prevent the flywheel device (e.g., armature, rotor, outer casing, etc.) from overheating. For example, the induced heat at the rotor surface may cause the rotor surface temperature to rise at a rate appreciably faster than the center of the rotor where, upon rotation, the stresses and probability of brittle fracture would be the highest. By applying the heat in a controlled duty-cycle consisting of alternating between the on-state with heat generation, and the off-state with conductive diffusion (settling step), the time-average temperature difference between the rotor surface and the rotor center may be minimized thus reducing the risk of overheating the rotor surface. Similarly, conductive losses within the armature may cause the armature to heat faster than the center of the rotor and the settling step may be used to limit the temperature rise in the armature. 
     As shown in settling step  430 , heat  412  and  414  are no longer being induced in the rotor, but residual heat  432  existing in the rotor may continue to diffuse. Step  440  illustrates that heat  445  is now distributed substantially uniformly throughout the rotor. At this point the settling step is complete and the rotor may be in a condition suitable for normal usage. 
     The duration of time required for the preheating and settling steps may depend on a number of factors. Some factors may include, for example, initial temperature of the rotor, ambient temperature, construction of the rotor, whether a vacuum is in use, etc. Based on these and other factors, a software program may be able to determine how much time is required for the preheating and settling steps. Such a software program may be included as part of the electronics associated with the flywheel device. The versatile nature of software is advantageous for the present invention because it enables the device to safely heat the rotor regardless of the construction and material properties of the motor-generator device. 
     FIG. 5 illustrates thermocouple device  550  that can be used to provide temperature data to electronics  560  in accordance with the principles of the present invention. FIG. 5 also illustrates flywheel device  500  that includes rotor  512 , outer casing  518 , field coils  530  and armature  544 , all of which are constructed similar to motor-generator device  100  of FIG.  1 . In addition, device  500  may be constructed such that air gap  528  exists between rotor  512  and outer casing  518 , and airgap  546  exists between rotor  512  and armature  544 . Thermocouple device  550  may be coupled to armature  544  so that the temperature of armature  544  can be monitored directly. Persons skilled in the art will appreciate that other temperature sensing devices such as digital thermometers, resistance temperature detectors (RTDs), infrared sensors, and other suitable devices can be used to either directly measure or indirectly infer rotor temperature. 
     Using the above described configuration of FIG. 5, the timing of the heating process may be controlled by electronics  560  as follows. The software in electronics  560  may use the monitored armature temperature to determine whether rotor  512  requires heating before beginning normal operation. If heating is required, then the software may calculate the preheat and settle times based on the monitored armature temperature. For example, if the monitored temperature is −20° C., then electronics  560  may instruct the preheating step to last ten hours and then instruct the settling step to run for five hours. If the initial measured temperature is 0° C., then electronics  560  may, for example, instruct the preheat step to run for seven hours and permit the rotor to settle for three hours before enabling normal operation. The duration of preheat and settle time may be determined based on the initial temperature reading of armature  544  because other factors (e.g., temperature versus time profile) such as those described above are known or incorporated into the software. Therefore, it may not be necessary to measure the armature temperature after the heating cycle begins. 
     The temperature measured on armature  544  may not necessarily be the same temperature exhibited by rotor  512  because the thermal time constants of the respective devices may not be the same. For example, the thermal time constant of rotor  512  may be much slower than the thermal time constant of the stator (e.g., armature  544 ). Thus, prior to flywheel device activation, the temperature of rotor  512  may not be the same as armature  544 . Consequently, there may be instances when rotor  512  is much warmer than armature  544  and vice versa. This potential temperature difference, however, may not have an averse effect on the heating process because the typical operation of device  500  precludes such temperature differences from affecting the heating process. For example, during the shipment and/or installation of flywheel device  500 , a sufficient period of time probably elapses such that the temperature difference between rotor  512  and armature  544  is negligible. 
     Furthermore, electronics  560  may include a timer that keeps track of how long device  500  is shut down (e.g., for maintenance). If the timer indicates that device  500  has been shut down for at least a predetermined period of time before reactivation, then device  500  may initiate the heating cycle if the armature temperature is low enough to warrant such action. If device  500  is inactive for a relatively short period of time (e.g., less then the timer limit), however, electronics  560  may not initiate the heating cycle because rotor  512  may still be at or near enough to a specified temperature despite the monitored armature temperature. Persons skilled in the art will appreciate that other factors may be used to determine how long device  500  can be shut down and not require rotor heating upon activation. 
     Direct measurement techniques may also be implemented to determine the temperature of the rotor. FIG. 6 illustrates an infrared detector  670  that is configured to directly measure the temperature of rotor  612  in accordance with the principles of the present invention. Infrared detector  670  may be mounted on outer casing  618  as shown in FIG. 6 or in any other suitable position to monitor the rotor temperature. Direct rotor measurement may enable electronics  660  to make real-time adjustments to preheat and settle times. This may provide efficiency and minimize risk of potentially overheating rotor  612 . Persons skilled in the art will appreciate that other devices can used to provide direct measurement of rotor  612 . For example, a wireless thermocouple may be inserted into a hole bored into the rotor and transmit temperature data to electronics  660 . Also, a direct contact thermocouple may be temporarily positioned against the rotor during the preheat process and then removed prior to commencing operation of the device and rotation of the rotor. 
     In an alternative embodiment, the temperature sensors of FIGS. 5 and 6 may be used to provide real-time temperature measurement to the electronics during preheating. Real-time measurement may enable the electronics to control preheating without having to determine a preheating and a settling step. In yet another embodiment, rotor temperature measurements may be taken in intervals. 
     FIG. 7 illustrates a representative example of how the principles of the present invention may be applied to provide an uninterruptible power supply system  700 . System  700 , which typically receives power from a utility, and provides power to a load, includes a flywheel storage unit  730  that may be any suitable type of flywheel energy storage device. Flywheel storage unit  730  may include flywheel  735  and other circuitry (not shown). System  700  also has control circuitry  710 , which may provide various functions such as, for example, monitoring utility power provided to system  700 , monitoring the power provided to the load, and controlling flywheel storage unit  730 . Control circuitry  710  may be able to switch between a long term backup power system  750  and flywheel unit  730  whenever backup power is needed for a prolonged period of time. In addition, control circuitry  710  can control flywheel storage unit  730  such that unit  730  preheats flywheel  735  in accordance with the principles of the present invention. 
     Control circuitry  710  may include preheater circuitry  720 , which may be suitable for controlling the method of preheating flywheel  735 . Preheater circuitry  720  may have circuitry such as, for example, DC PWM converters, AC cycloconverters, and DC-AC inverters that provide high frequency current to flywheel storage unit  730 . As mentioned above, the high frequency currents cause flux to radiate into flywheel  735 , which induces current in the flywheel. The induced currents then generate heat in flywheel  735  to raise its temperature to a desired level. 
     Preheater circuitry  720  may also include software that takes part in controlling the preheating process. For example, before control circuitry  710  instructs flywheel storage unit  730  to operate, preheater circuitry  720  may take a reading from a temperature sensor to determine whether flywheel  735  requires preheating. If preheating is required, the software may calculate how long to induce current into the flywheel  735  based on a direct or inferred temperature reading of flywheel  735 . In addition, the software may also determine a quantity of current to be provided to the armatures of flywheel energy unit  730 . 
     Thus several techniques for preheating a rotor of a motor-generator system are provided. Persons skilled in the art will appreciate that the present invention can be practiced by other than the described embodiments, which are presented for purposes of illustration rather than of limitation, and the present invention is limited only by the claims that follow.