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EPRI Proprietary Licensed Material many modern flywheels the same rotating machine serves both functions. The machine is called a motor alternator or motor generator and consists of a wound- or permanentmagnet rotor, usually revolving within a stator containing electrical winding through which charge (or discharge) current flows. Note that this machine, along with any power electronics, limits the power rating of the flywheel system. And in some practical systems the generator for discharging the wheel is higher power than the recharging motor. Thus at full power charging the wheel will require more time than discharging. The starter motor and alternator or generator are connected to the flywheel via the same steel shaft and may be either a single machine or two different machines. In both cases the rotor becomes part of the flywheel mass. When separate, the starter motor is typically a simple induction motor that is able to produce starting torque. When combined in one synchronous motor/alternator, with either permanent magnet or wound rotor, electronics are required to spin up the flywheel. In this configuration the power electronics are also used to convert the variable output frequency to a constant 60-Hz frequency. Figure 7 shows both arrangements. Figure 7 Two Possible Flywheel Charging Configurations: Induction Motor Starter And Power Electronics For Starting And Frequency Control. (Courtesy Of Piller Premium Power System) Further integrating is obtained in some models where the magnets in the machine rotor are embedded within the flywheel rotor itself, which rotates around a stator containing the electrical windings. This arrangement usually improves the energy and power density of the system, but challenges the cooling, by combining the two components into one. Power Electronics and Electro-Mechanical Couplings Most flywheel energy systems have some form of power electronics that convert and regulate the power output from the flywheel. As the motor-generator or alternator draws on mechanical energy in the rotor, the rotor slows, changing the frequency of the acelectrical output. These power electronics, in the form of rectifiers and inverters, compete with electromechanical methods, such as the eddy-current clutch and induction coupling, to convert the changing flywheel speed to dc or to constant frequency ac power. Flywheels Page 8
EPRI Proprietary Licensed Material Power electronics have won this competition in all the high-speed wheels on the market. However several high-power, low-speed systems use electromechanical couplings to isolate the shaft and effectively couple an accelerating and decelerating flywheel with a constant-speed generator. Whether electronic or mechanical the main function of these devices is to allow energy to be taken from the wheel before its frequency and power output drop below usable levels. In fact the low-end (i.e., end-of-discharge) cutout speed at which the flywheel is considered discharged is primarily dependent on the current carrying capability of the electronics (or electromechanical coupling) and the size of the load. For example, most flywheels have output current proportional to load and inversely proportional to speed. This means a lighter load can go to a lower speed before the system cuts out on maximum current. The flywheel system can actually deliver 1.5 to 2 times more energy at light load than high load as can be seen in performance curves. While for different reasons, in this feature the flywheel is similar to a conventional leadacid battery. When power electronics are used the variable frequency, ac output of the flywheel alternator is simply rectified providing a dc voltage and current. From this point a power electronic inverter may be used to recreate ac at the desired waveform, frequency and voltage. So the function of the power electronics is to couple the fixed-frequency ac electrical grid with the variable-speed flywheel and also to invert, regulate, and provide wave shaping for the ac electrical output of the system. By reversing the process the power electronics are also able to draw power from the ac line connection, and drive the flywheel motor to spin up and recharge the wheel. In some cases a power electronics system recharges the flywheel and can use energy drawn from the electric grid. The most common power electronic systems use two matching bi-directional or 4-quadrant converters to carry out all of the functions described here. Controls and Instrumentation Flywheel systems require some controls and instrumentation to operate properly. Instrumentation is used to monitor critical variables such as rotor speed, temperature, and alignment. Rotor speed and alignment are also often controlled variables, through active feedback loops. The latter is especially important for systems with magnetic bearings, and most magnetic systems have complex controls to reduce precession and other potentially negative effects on the rotor. In many systems, other instrumentation is used to monitor performance or design parameters related to failure modes. In some composite flywheel systems, for example, instrumentation is used to measure deformation of the rotor over time, alerting operators when the rotor shape indicates possible failure in the future. System Packaging Nearly all-modern flywheel systems have some type of containment for safety and performance enhancement purposes. This is usually a thick steel vessel surrounding the rotor, motor-generator, and other rotational components of the flywheel. In the event of catastrophic failure, the containment vessel would stop or slow parts and fragments, preventing injury to bystanders and damage to surrounding equipment. Flywheels Page 9
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