Electricity: motive power systems – With flywheel or massive rotary member
Reexamination Certificate
2001-06-21
2002-11-26
Ro, Bentsu (Department: 2937)
Electricity: motive power systems
With flywheel or massive rotary member
C318S150000, C307S068000, C322S004000
Reexamination Certificate
active
06486627
ABSTRACT:
This invention pertains to a flywheel uninterruptible power source and more particularly to a flywheel energy charging system that provides increased life, reliability and efficiency over previous designs. The charging system also includes a safety mechanism that prevents operation of the flywheel when the vacuum surrounding the flywheel is degraded.
BACKGROUND OF THE INVENTION
Flywheel uninterruptible power supplies have emerged as an alternative to electrochemical batteries for prevention of power interruptions to critical loads. Electrochemical batteries used in these applications, in particular, valve regulated lead acid batteries, have many undesirable traits. The life of batteries is short, typically between 1 to 7 years depending on the environment and use. They require periodic maintenance and inspection, are subject to thermal degradation and can fail unpredictably. Lead acid batteries and other types as well are also environmentally noxious. However, lead acid batteries are relatively inexpensive. Flywheel systems show promise to eliminate the disadvantages of batteries with the expectation of achieving 20 year lives with minimal or no maintenance, temperature insensitivity, previously unachievable reliability while being environmentally benign.
A flywheel uninterruptible power source is shown in FIG.
1
. The power source
10
includes a high-speed flywheel
12
, in which energy is stored in the form of rotating inertia. Flywheels can be either constructed of metal or of composite materials. The flywheel is supported for rotation using upper and lower bearings
14
and
15
. The flywheel can be supported on mechanical bearings, magnetic bearings or a combination. An attached motor/generator
16
is used to accelerate and decelerate the flywheel
12
for storing or retrieving energy. Many designs of motor/generators exist and can be employed. Motor/generators can also be made as separate components. To reduce the losses from aerodynamic drag, the interior
13
of the housing
11
surrounding the flywheel
12
is maintained at a low pressure, or for slower flywheels it can be filled with a gas of small molecule size such as helium. The flywheel uninterruptible power supply
10
includes electrical connection for operation and conversion of power. Utility power
21
is input to the input conversion
20
and power is supplied to a critical load
19
through output conversion
18
. A system control
17
provides control for the system
10
. The system control
17
controls the operation of the flywheel
12
by limiting currents
22
, controlling speed
23
and also by monitoring parameters through diagnostics
24
.
Regardless of the physical design employed, the operating life of the power source and its components is preferably maximized in order to offset the higher initial cost of the flywheel system over batteries by actually becoming cheaper when considered over the life of the power source. One element of flywheel uninterruptible power supplies that deserves particular attention is the power system electronics. Designing electronics for an operating life that is preferably greater than 10-20 years without failures is challenging.
A power system configuration of previous flywheel uninterruptible power supplies is shown in FIG.
2
. The power system
30
takes in utility power
31
and supplies protected direct current power at the output
32
. For many telecommunications systems such as telephone and wireless, the output voltage
32
required is −48 volts or 24 volts. For other applications, such as high power ride-through for data centers or critical manufacturing, the input and output voltages are higher. The input voltage
31
is rectified to a DC bus
34
using a rectifier
33
which can be either controlled or uncontrolled. The DC bus
34
supplies power to a PWM (pulse width modulated) inverter
36
also known as a servo amplifier. The servo amplifier
36
converts the DC current in the bus
34
to synchronous alternating current
37
that provides power to the flywheel motor/generator to accelerate the flywheel to normal operating speed. When the utility power is operating normally, the DC voltage in the bus
34
is converted to the output voltage
32
using a DC-DC converter
35
.
During an interruption in the utility power
31
, energy from the rotating flywheel supplies power to the output
32
by providing power to the DC bus. The inverter
36
provides power to the DC bus instantly and automatically when the utility power is discontinued typically by the antiparallel diodes included with the H-bridge, not shown, inside the inverter
36
or through use of a paralleled separate rectifier, not shown. Power automatically flows back and is rectified to the DC bus whenever the generator voltage is greater than the DC bus
34
. As the flywheel speed slows, the voltage to the DC bus drops. The output DC-DC converter
35
maintains the constant output voltage
32
during discharging of the flywheel.
The charging of the flywheel uninterruptible power source
30
is regulated through use of the PWM inverter
36
. The PWM inverter uses high frequency (−20 kHz) switching that chops the DC bus voltage
34
into varying width pulses that are combined to provide regulation that results in current control and speed control for the flywheel. The high frequency switching yields in several trillion cycles on the semiconductor switches in the inverter over a 20 year system life. The high number of switching cycles of the direct current stresses the inverter and could potentially result premature failures of the flywheel power source. The diagnostics of the flywheel system can also potentially have components with operating lives of concern. This is especially true if vacuum monitoring gauges are used. Vacuum gauges such as ion and thermocouple gauges are unlikely to last for the life of the power system and also are expensive
SUMMARY OF THE INVENTION
The invention is a flywheel uninterruptible power source having a charging system that provides increased life and reliability compared to previous systems. The charging system works by replacing the current regulation and/or speed control normally accomplished by a high frequency pulse width modulated inverter with a very low frequency, line commutated converter that regulates by switching the alternating current from the utility power. Pulse width modulation inverters or motor drives for high speed motors have typically 3 phase designs that invert a DC input voltage from the supply bus to a high frequency synchronous alternating current that drives the motor. In most applications, the voltage of the DC bus is fixed because it is typically the output of a fixed DC power supply.
The speed control and current control for the motor are achieved by using high frequency (~20 kHz) pulse width modulation inside the motor drive. The very high frequency is required so that the speed of the motor can be held nearly constant and free of pulsations. In contrast, the invention takes into account the very unusual application of flywheel systems. An energy storage flywheel is a unique application for a high-speed motor. The high-speed motor is coupled to a very large rotational inertia for the sole purpose of storing energy. Such flywheels can take 8 hours or more to accelerate from stopped to 30,000 rpm. Because of the large inertia coupled to the high-speed motor, speed fluxuations and pulsations are not an issue. Regardless of any torque pulsations, the only goal is to store energy in the rotating flywheel and if the energy is added in pulses, it makes no difference because the energy is being added. Therefore, the disclosed invention makes use of the uniquely stable rotational speed of energy storage flywheels by reducing the speed regulation frequency to conventional line current frequencies, frequencies under 200 Hz and 60 Hz in particular.
By reducing the switching frequency, the switching losses are reduced and the life and reliability of the switches are greatly extended. The switching losses are linearly proportional to
Indigo Energy, Inc.
Neary J. Michael
Ro Bentsu
LandOfFree
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