Title of Invention

METHOD FOR PRODUCING CLEAN ENERGY FROM COAL

Abstract

A power control system (12) for an electric motor having at least one magnetic bearing has a DC/DC converter (18) supplied from a DC link bus (179) connected to a main power supply (14), the bus (17) supplying power for the electric motor and for a bearing actuator; the converter (18) provides a low voltage DC power supplies for a motor controller (23), a bearing controller (24) and a supervisory controller (26), the later monitoring the main power supply and communicating with the motor controller (23) and bearing controller (24) so as to cause the motor to operate as a generator in the event of a failure of the main power supply (14) to thereby supply power to the DC link bus (17) to maintain operation of the magnetic bearing. Circuit switching components are connected to the motor winding and selectively switched in a manner causing current generated in the motor winding to flow in one direction into the DC link bus (17) only while the winding voltage is greater than that of the DC link bus(17)

Full Text

A POWER CONTROL SYSTEM FOR AN ELECTRIC MOTOR AND A METHOD OF RUNNING DOWN A HIGH SPEED DC ELECTRIC MOTOR Field of the Invention This invention relates in general to a power control system for an electric motor and a method of running down a high speed DC electric motor, and in particular to a power supply for a motor using magnetic bearings. International Patent Application No. WO 98/33260 describes a high speed electric motor which is particularly suitable for use as a refrigeration compressor motor. Such a motor may be used in, for example, a compressor of the type described in Australian Patent No. 686174 and utilizing magnetic bearings for the suspension of rotating parts. Background of the Invention A known difficulty with the use of magnetic bearings is to supply power to the bearings during a system power failure, during which the power supply to the motor fails. While it is known to use auxiliary or back-up power supplies to the system, such as by way of batteries or the like, such auxiliary power supplies are relatively expensive and require additional switching controls to enable the auxiliary supply to take over when the main power supply fails. Further, the batteries have a limited life and generally must be replaced every two years. This adds substantial further costs to the system. It is therefore desirable to provide an improved power supply system for a high speed electric motor running on magnetic bearings which can maintain power to the bearings in the event of a main power supply failure. It is also desirable to provide a power supply system for continously supplying power to the magnetic bearings of a high speed electric motor following a failure of the main power supply until such time as the rotating parts are at rest. It is also desirable to provide an improved controller for a high speed electric motor which enables a spinning motor to spin down under controlled conditions. It is also desirable to provide a power supply controller for a high speed electric motor with magnetic bearings which is relatively inexpensive, which is efficient in its operation and which provides a substantially fail-safe power supply for its magnetic bearings. Summary of the invention In accordance with one aspect of the invention there is provided a power control system for an electric motor having at least one magnetic bearing, said system comprising a DC/DC converter supplied from a DC link buss connected to a main power supply, said buss supplying power for the electric motor and for a bearing actuator, said converter providing low voltage DC power supplies for a motor controller, a bearing controller and a supervisory controller, the supervisory controller monitoring the main power supply and communicating with the motor controller and bearing controller so as to cause the motor to operate as a generator in the event of a failure of the main power supply to thereby supply power to the DC link buss to maintain operation of the magnetic bearing characterised in that, circuit switching components are connected to the motor winding and selectively switched in a manner causing current generated in the motor winding to flow in one direction into said DC link buss only while the winding voltage is greater than that of the DC link buss. The invention is preferably adapted for use with a high speed electric motor, the rotor of which is supported solely by magnetic bearings. The invention may also be utilised for an electric motor having a combination of magnetic and gas bearings. Preferably, the DC link buss incorporates at least one capacitor, which, in normal use, is maintained in a charged condition by the main power supply. The capacitor is able to provide sufficient power in conjunction with that supplied by the motor running as a generator to run down the motor from full speed and maintain operation of the magnetic bearings during the run down period. Running the motor as a generator during the run down period extracts the kinetic energy stored in the motor and other rotating parts and constitutes an electric brake to quickly and safely stop the rotor rotation. During the run down period, power continues to be supplied from the motor, running as a generator, to the DC link buss which provides an uninterruptable power supply to the DC/DC converter and the several controllers as well as the magnetic bearing actuator for the full run down period. In accordance with another aspect of the invention there is provided a method of running down a high speed DC electric motor run on magnetic bearings in the event of a failure of the main power supply, said method including the steps of supplying the motor and the magnetic bearings from a high voltage DC buss connected to the main power supply, proving a DC/DC converter to supply low voltage DC power to a magnetic bearing controller and to a motor controller, using switching devices to control the motor operation, sensing a failure of said main power supply and providing a signal to the motor controller, characterized in that, said switching devices are selectively controlled on sensing said failure, to initially feed existing motor current to said buss, detecting when said buss voltage drops below a predetermined value and shorting said motor winding, and as soon as current flow in said winding commences, as a result of the short, removing said short whereby the winding voltage rises to above the buss voltage, feeding the resulting generated current back to said buss, and repeating said selective control as necessary until said motor is run down. According to one embodiment of the invention the power control system incorporates a plurality of switches operable to switch power between the two polarities of the DC buss and each end of the motor winding to switch the current flow through the winding. Each switch has a diode in parallel. When a mains power failure is sensed, such as by measuring a voltage drop across the buss, the switches are all switched to the "off' position and current existing in the motor coil is fed to the buss. As soon as the buss voltage drops again, the motor winding is shorted by closing the appropriate switches and then opened to let the current, generated by shorting the winding, be pumped back into the buss through the diodes. This control method uses the existing motor control IGBT switches to realise the generator function of the motor when the buss voltage falls. With this control system, there is no need to track the rotor position and control switching as a function of rotor position. The diodes enable the motor to act as a generator with all switches in the open position and the current being pumped into the buss to assist the installed capacitor for maintaining the buss voltage until the motor is run down. One embodiment of the invention will now be described with reference to the accompanying drawing wherein: Figure 1 is a block diagram of a power control system in accordance with the present invention; Figure 2 is a schematic circuit diagram illustrating the control switches for a single phase motor and Figure 3 is a flow chart of the control algorithm of the system shown in Figures 1 and 2. Description of one embodiment Refemng to the drawings and firstly Figure 1, the power control system 12 is connected to a three phase AC power supply 14 through a bridge rectifier 16. A DC link buss 17 supplies DC power to a DC/DC converter 18, magnetic bearing actuator 19 and electric motor power supply 21. A large capacitor 22 or capacitor bank is connected across the link buss 17 to provide a buffer of stored power for motor run down. In this embodiment, the DC link buss voltage is 1000V and the capacitor, or several capacitors, will have a capacity sufficient for a motor run down time of about 0.1 sec to about 1.5 sec, in the absence of any other power source. The DC/DC converter 18 provides low voltage DC power for a motor controller 23, a bearing controller 24 and a supervisory controller, which, in this embodiment, takes the form of a computer 26. The converter 18 also supplies low voltage power for various sensors associated with the magnetic bearings and for Insulated Gage Bipolar Transistors (IGBT) (not shown) which are used for control purposes in the magnetic bearing actuator 19 and the electric motor power supply 21. The magnetic bearing actuator 19 and associated IGBTs and the electric motor power supply 21 and its associated IGBTs are known in the art and will not be described in detail. Gate drive signals for the various IGBTs are generated by the bearing controller 24 and motor controller 23, respectively, to provide the desired operational parameters for the electric motor bearings. An AC power monitor 27 provides a signal to the supervisory computer 26 in the event of a failure of the main AC power supply 14. Alternatively, the supervisory computer 26 may monitor the buss voltage, through DC/DC converter 18, to detect a power failure which results in a voltage drop across the capacitor 22. On detection of a power failure, the motor controller 23 controls the IGBTs to feed 8. The method as claimed in claim 7, comprising the steps of connecting a capacitor across the DC bus and of providing an AC power monitor for the main power supply, said step of sensing a failure of the main power supply comprising one of: sensing a voltage drop across the capacitor and of: the AC power monitor emitting a power failure signal. 9. The method as claimed in claim 7, said step of detecting when the bus voltage drops below a predetermined value comprising measuring an indicator voltage by means of a voltage sensor. 10. The method as claimed in any one of claims 8 to 9, the switching devices comprising IGBT switches connected between each end of the motor winding and positive and negative side of the DC bus respectively, a diode being connected in parallel with each switch, the diodes enabling the motor to act as a generator and feed current into the DC bus to assist the capacitor in maintaining the bus voltage until the motor is run down. A power control system (12) for an electric motor having at least one magnetic bearing has a DC/DC converter (18) supplied from a DC link bus (179) connected to a main power supply (14), the bus (17) supplying power for the electric motor and for a bearing actuator; the converter (18) provides a low voltage DC power supplies for a motor controller (23), a bearing controller (24) and a supervisory controller (26), the later monitoring the main power supply and communicating with the motor controller (23) and bearing controller (24) so as to cause the motor to operate as a generator in the event of a failure of the main power supply (14) to thereby supply power to the DC link bus (17) to maintain operation of the magnetic bearing. Circuit switching components are connected to the motor winding and selectively switched in a manner causing current generated in the motor winding to flow in one direction into the DC link bus (17) only while the winding voltage is greater than that of the DC link bus(17)

Documents:

104-kolnp-2004-granted-abstract.pdf

104-kolnp-2004-granted-assignment.pdf

104-kolnp-2004-granted-claims.pdf

104-kolnp-2004-granted-correspondence.pdf

104-kolnp-2004-granted-description (complete).pdf

104-kolnp-2004-granted-drawings.pdf

104-kolnp-2004-granted-examination report.pdf

104-kolnp-2004-granted-form 1.pdf

104-kolnp-2004-granted-form 18.pdf

104-kolnp-2004-granted-form 2.pdf

104-kolnp-2004-granted-form 3.pdf

104-kolnp-2004-granted-form 5.pdf

104-kolnp-2004-granted-pa.pdf

104-kolnp-2004-granted-reply to examination report.pdf

104-kolnp-2004-granted-specification.pdf