MXPA01006541A - Prime mover for operating an electric motor - Google Patents

Abstract

A prime mover such as a microturbine generator is operated to generate dc power, and an inverter is controlled to convert the dc power to ac power. The ac power is supplied to an electric motor. The frequency of the ac power is ramped up during motor startup to reduce motor inrush current. The frequency or current of the ac power may be varied in response to process requirements during normal operation of the motor.

Description

PRIMARY MOVEMENT FOR OPERATING AN ELECTRIC MOTOR The present invention relates to electric motors. More specifically, the present invention relates to a method and apparatus for operating ac motors. Dna common oil refinery has a lot of pumps that are driven by electric motors. Electric power is usually distributed to electric motors by an energy grid. The energy grid, in turn, receives the electrical energy from a remote installation. Certain disadvantages are associated with the distribution of electrical energy to the motors by means of an electric grid as well. For example, the electric grid can be expensive to set up, especially for a large refinery. In addition, transmission losses can occur through the grid while the electrical energy is being distributed to the different motors. Transmission losses can also occur while the electrical energy is being transmitted to the electric grid from the remote installation. In addition, the distribution of electrical energy can be unreliable. There are also certain problems associated with electric motors. The charging conditions in the electric motor usually vary during normal operation. An electric motor that operates at a constant speed will operate efficiently under full load conditions, but will operate inefficiently under partial load conditions. Thus, inefficient operation due to operation with variable load may present a problem. , Another problem may occur during the start of the electric motor. During start-up, the motor receives an irruption of current. Usually, the current burst is 4 to 6 times the current received during the operation in the steady state. Consequently, the rated power or rated power of the motor is restricted between a quarter to a sixth of the maximum capacity of the power grid. The problems arising from the variable load conditions and the inrush current can be overcome by the use of a variable speed drive. The variable speed drive mechanism allows the electric motor to operate more efficiently under partial load conditions. The variable speed drive limits the current burst during start-up. However, variable speed drives are usually expensive. In addition, the variable frequency drive mechanisms have internal losses associated with their own operation. Thus, there is a need to limit the inrush current during start-up and increase the efficiency of the electric motor during normal operation without the use of a variable speed drive. There is also a need to increase energy savings and improve the reliability of electric power distribution to electric motors.

COMPENDIUM OF THE INVENTION A system according to the present invention includes an electric motor; a primary motor to generate electric power from; an inverter to convert energy into ac energy; and a controller or regulator for causing the inverter to at least vary the frequency or current of the ac power. The ac energy is supplied to the electric motor. The controller can cause the inverter to drive the electric motor at variable speed or the moment of force or torque during engine start and normal engine operation. Accordingly, it is possible to reduce the input current of the motor during motor start-up, and the efficiency of the motor during normal operation of the motor can be improved. In addition, it is possible to reduce the input current and the efficiency of the motor can be increased without the use of a traditional variable speed drive. It is possible to locate a primary motor such as a microturbine generator near the electric motor. The microturbine generator can distribute electrical energy to the electric motor without a power grid, thus increasing and saving energy and improving the reliability of the distribution of electrical energy.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a block diagram of a system according to the present invention. Figure 2 is a flow chart of an electric motor operating method, the method being carried out in accordance with the present invention. Figure 3 is an illustration of an ac power frequency profile fed to the electric motor during start-up, and Figure 4 is an illustration of an alternative embodiment of an inverter for the system.

DESCRIPTION OF THE PREFERRED EMBODIMENT Figure 1 shows a system 6 that includes an electric motor 8 and a primary motor for feeding electric power to the electric motor 8. The electric motor 8 can be part of a device such as a compressor, fan or bomb. In an oil refinery, for example, the electric motor 8 of a pump can be an ac induction motor having a rated power of 100 hp. In a preferred embodiment of the present invention, the primary motor includes a micro-turbine generator 10. The micro-turbine generator 10 includes a compressor 12, a turbine 14 and an integrated electric generator 16. The electric generator 16 is cantilevered from the compressor 12 The compressor 12, the turbine 14 and the electric generator 16 rotate by a single common shaft 18. Although in an alternative embodiment, the compressor 12, the turbine 14 and the electric generator 16 can be mounted on separate axes, the use of a only common shaft 18 allows compactness and reliability of the microturbine generator 10. The shaft 18 is supported by self-pressurized air bearings such as thin sheet metal bearings. Thin blade bearings eliminate the need for a separate bearing lubrication system and reduce the presence of maintenance service. The air entering an inlet of the compressor 12 is compressed. The compressed air exiting from an outlet of the compressor 12 circulates through air passages on the cold side 20 in a recuperator 22. Within the recuperator 22, the compressed air absorbs heat from the exhaust waste heat from the turbine. The heated, compressed air exiting the cold side of the recuperator 22 is fed to the combustor 24. By using the recuperator 22 to heat the compressed air the fuel consumption is reduced. The fuel is also fed to the combustor 24. It is possible to use gaseous or liquid fuel. In the gaseous fuel mode, it is possible to use any suitable gaseous fuel. Fuel options include diesel, torch gas, natural gas from the wellhead, waste hydrocarbon fuel streams, gasoline, naphtha, propane, JP-8, methane, natural gas and other synthetic gases. If gaseous fuel is chosen, the gaseous fuel - can be compressed by a fuel compressor 25 or regulator by a fuel regulator before entering the combustor 24. The use of the fuel compressor 25 is preferred if the gas pressure is too low , while the use of a regulator is preferred if the pressure is too high to match the necessary pressure. If the microturbine generator 10 is located on the site in an oil refinery or gas plant, the fuel of choice may be a stream outside of specifications that would otherwise be incinerated and discarded. If located in an oil well, the fuel of choice may be gas in solution or natural gas that could otherwise be vented or ignited. The ignition is a waste and usually does not give rise to the complete combustion of the gas, leading to an environmental risk, while a turbine can produce a minimum of harmful emissions while converting the gaseous energy into useful mechanical energy. A flow control valve 26 regulates the flow of fuel to the combustor 24. The fuel is injected into the combustor 24 by an injection nozzle 25. Within the combustor 24 the fuel and compressed air are mixed and burned by a lighter 27 in a exothermic reaction. The expanding gases, hot, resulting from the combustion in the combustor 24 are directed to an inlet nozzle 30 of the turbine 14. The intake nozzle 30 has a certain geometry. The expanding, hot gases resulting from the combustion are expanded through the turbine 14, thereby creating turbine energy. The turbine energy, in turn, drives the compressor 12 and the electric generator 16. The turbine exhaust gas circulates through side exhaust passages 32 in the recuperator 22. Inside the recuperator 22, the heat of the exhaust gas of the The turbine is transferred to the compressed air in the cold side air passages 20. In this way, some of the combustion heat is recovered and used to increase the temperature of the compressed air before combustion. After delivering part of its heat, the exhaust gas leaves the recuperator '22. It would be possible to add additional heat recovery stages in the power generating system 10. The generator 16 has a permanent magnet rotor 34 and stator windings 36. The rotor 34 is connected to the shaft 18. When the rotor 34 is rotated by the turbine energy generated by the rotary turbine 14, an alternating current is induced in the windings of the stator 36. The speed of the turbine 14 can vary from according to the external energy demands placed on the microturbine generator 10. Variations in the speed of the turbine will produce a variation in the frequency and energy generated by the electric generator 16.

Ordinarily, the turbine 14 will rotate the rotor 34 at speeds greater than 60,000 rpm. Therefore, the generator 16 will generate ac energy at frequencies above the common grid frequencies (e.g., 50 to 60 Hz). A rectifier 38 rectifies the high frequency output of the generator 16 to energy of, and the energy of is converted to grid frequency energy ac by an inverter 40. The ac energy produced by the inverter 40 is distributed directly by the electric motor 8. The transistors 42 of the inverter 40 are instructed to turn on and off and by this means convert the energy into ac energy. The control of the interruption or modulation frequency of the transistors 42 can control the frequency of the ac power. The control of the frequency of the ac energy, in turn, can control the speed of the electric motor 8. The control of the amplitude or depth of modulation controls the output voltage and hereby the current to the motor 8. The controller 46 generates switching instructions that cause inverter transistors 42 to modulate the power of. The controller 46 also controls the modulation frequency of the transistors 42 using, for example, a closed loop control including a speed controller and speed sensor. The speed detector generates a feedback signal indicating the speed of the motor. The speed controller compares a motor speed instruction with the measured motor speed and generates a switching or interruption frequency command that controls the frequency of the modulation. By properly instructing the transistors of the inverter 42 to gradually increase or increase the frequency of the ac power (and, therefore, the speed of the electric motor 8) during start-up, it is possible to reduce the input current to the electric motor 8. The speed at which the engine speed gradually increases (and, therefore, the rate at which the frequency increases gradually) can follow a predetermined profile. Thus, the controller 46 can use a predetermined velocity profile against time to generate the motor speed instruction. In the alternative, it is possible to measure the motor current (by means of a current detector 44, for example) and the controller 46 can gradually increase the speed instruction at a controlled speed so that the measured current of the motor does not exceed a limit. After the electric motor 8 has reached the normal operating conditions (e.g., full speed or full charge), the transistors of the inverter 42 can be instructed to vary the frequency or current of the ac power to track the charging conditions of the electric motor 8. For example, the power in CV of rupture or interruption in a pump varies as the hub of the speed. Control of the amplitude or depth of the modulation controls the amplitude of the ac energy. The control of the voltage applied to the motor in turn will control the current or moment of force of the motor. Hence, the reduction of the frequency or current of the ac power allows the electric motor 8 to operate with greater efficiency under partial load. The load of the motor can be measured directly by measuring the torque of the motor, or the load of the motor can be measured indirectly by measuring the current of the motor, which provides an indication of the motor torque. Whether the current or frequency varies will depend on certain requirements of the process 15 or system parameters. The "process requirements", when used herein, refer not only to the necessary operating conditions of an electric motor, but also to the desirable or convenient operating conditions. As an example of a process requirement, it may be necessary for a pump to pump liquid out of a tank and maintain a flow discharge rate constant regardless of the height of the liquid in the tank. The speed of the pump will remain constant since the flow is directly proportional to the speed. The controller 46 would use a flow rate transducer as a process variable. A reference point would be scaled as flow velocity but it would really be a speed reference point. The controller 46 would adjust the frequency of the ac power fed to the pump motor. When the tank is full, the suction pressure of the pump will be high, requiring the minimum amount of moment of force to maintain the flow. The strength moment requirement will increase as the level in the tank decreases, and will reach a maximum when the tank is almost empty. The inverter 40 will therefore send most of the current when the tank is almost empty and the minimum current when the tank is full. Thus, a variable speed constant velocity control scheme (moment of force) is preferred given the process requirement associated with pumping the liquid from a tank. On the other hand, if a constant differential is required through the pump, a constant force moment and a variable speed control scheme are preferred. The motor current remains constant and the motor speed varies to maintain the differential pressure. If the discharge pressure of the pump increases, the controller '46 increases the speed of the pump to maintain the differential pressure through the pump. The discharge pressure varies as the square of the speed and the control is carried out by increasing the frequency of the ac energy and maintaining a constant current. The controller 46 also controls the speed of the turbine by regulating the amount of fuel flowing to the combustor 24. The controller 46 uses detector signals generated by a group of detectors to determine the external demands placed on the microturbine generator 10 and then controls the fuel valve 26 accordingly. The group of detectors may include some temperature and pressure detectors for measuring different parameters of the microturbine generator 10. For example, the group of detectors may include an axis velocity detector and a turbine exit temperature detector. In addition, with reference to Figure 2, the operation of the electric motor 8 will now be described. The microturbine generator is started (block 100). It is possible to use a fuel, such as a waste stream. After the micro-turbine generator 10 has been started and when it is capable of generating electricity, the energy is fed to the inverter 40 (block 102). The frequency of the inverter is set to 0 (block 104), an output of the inverter 40 is connected to the motor 8 (block 106), and the inverter 40 is instructed to gradually increase the current to a normal operating value (block 108) . The inverter 40 is then instructed to gradually raise the frequency from an initial frequency such as 10 Hz to a desired frequency such as 60 Hz (block 110). An exemplary ramp is shown in Figure 3. When the frequency rises gradually, the speed of the electric motor 8 increases gradually as well. Thus, the input current is reduced. Once the electric motor 8 has reached normal operating conditions (e.g., a desired speed or a desired operable load), the inverter 40 is instructed to change the frequency or current in response to the process requirements (block 112). If energy demand is needed, the microturbine generator 10 is preferably coupled or articulated with other primary motors to drive the motor 8 (block 114). Also, if the micro-turbine generator 10 stops generating power, it is possible to provide backup power by the source 48, such as a local power installation or a backup generator (block 116). When back-up power is needed, an installation 50 switch is closed manually or automatically. The energy ac of the backup source 48 is rectified by the rectifier 38, is modulated by the inverter 40 under the control of the regulator 46 and is fed to the electric motor 8. The backup energy can also be supplied to the controller 46. The generator of microturbine 10 is "connect and use", requiring little more than a clean fuel supply, liquid or gas. This can be completely independent in an airtight box. The result is high energy density typified by low weight (approximately one-third the size of a comparable diesel generator) and a relatively small footprint (eg, approximately 3 feet by 5 feet by 6 feet in height). Thus, an invention is described which, without the use of a traditional variable speed drive, limits the input current to an electric motor 8 during start-up and increases the efficiency of the motor during normal operation of the motor 8. The elimination of the traditional variable speed drive mechanism offers benefits such as the reduction of the total cost of operation of the motor 8. A primary motor such as the micro-turbine generator 10 can be located close to the electric motor 8. The micro-turbine generator 10 can distribute electrical energy to the motor 8 electric without an energy grid, thereby increasing energy savings and improving the reliability in the distribution of electrical energy. The energy grid can be removed or used for backup power. The invention can provide power independent of the electric power of the public service. This capability is desirable in a process site that does not have access to public service energy. Thus, the invention can significantly reduce the capital cost of those facilities where the construction of a power line would be necessary to bring power from the public service to the process site. A separate microturbine generator 10 already packaged with a controller 46 does not need an additional controller to operate the electric motor 8. The controller 46 can operate "with variable flow and pressure", giving rise to a synergy in the use of the generator. of microturbine 10 in combination with the electric motor 8. The invention can reduce operating costs by using residual fuel sources to generate power, or by using commercial fuel to reduce electrical cost by suppressing spikes. A process plant will often have out-of-specification liquid or gaseous streams that are expensive to discard. The residual current will have to be compressed to be injected into a plant lighter. Thus, energy would be wasted. In addition, lighters are notoriously ineffective in converting waste streams to 100% carbon dioxide with low NOx emissions. Therefore, another practical use of the microturbine generator would be to use this waste energy stream to produce electrical energy. The result would be a lower capital cost to discard the waste stream, and a more environmentally friendly process since the turbine emissions are cleaner than the burner emissions. The present invention is not limited to the specific embodiments described above. For example, the primary engine is not limited to a micro-turbine generator 10. Other suitable primary engines include internal combustion engines such as those running on gasoline, diesel, natural gas, propane and other fuels.; fuel cells such as those using phosphoric acid, molten carbonate, proton exchange membranes and solid oxides; and Stirling engines, Brayton cycle engines, wind turbines and hydroelectric power sources. It is possible to use automatic interruption to allow a grid connection after that. the engine has reached a speed and full load. A plurality of primary motors can be "coupled" together to feed a dedicated electric motor. The coupling of the primary motors as a microturbine generator allows larger motors to be driven and controlled. It is possible to provide the system with utility power, and the inverter can be configured to automatically transfer the utility power to a process in the event that the primary motor fails. Such an inverter 240 is shown in Figure 4. The inverter 240 includes a power bus of 245, a bridge rectifier 241 for rectifying the ac power of the generator 16 and the placement of the rectified power on the bus of 245, and the transistors 42 to modulate the power on the 245 bus to produce ac power. The inverter 240 also includes a public service bridge with a 243 diode having an output equaling an output of the rectifier of the bridge 241. The diodes of the bridge 244 of the public service bridge 243 are powered by public service energy. If the utility voltage is slightly less than the bus voltage 245, the diodes 244 of the utility bridge 243 will have reverse polarity. Therefore, no energy will flow from these. However, if the generator 16 fails, the utility power will flow continuously to the bus 245, thereby taking delivery of the power requirements of the generator 16. Consequently, the reliability of the process increases providing power to the generator. backup in the event of failure of public service or primary motor power. Therefore, the invention is not limited to the specific embodiments described above, however, the present invention is considered in accordance with the following clauses.

Claims (25)

1. A system comprising: an electric motor a primary motor to generate electric power from; an inverter to convert energy into ac energy, ac energy being fed to the electric motor during motor operation; and a controller for controlling the inverter to vary at least the frequency or current of the ac power during the operation of the electric motor.

2. The system of claim 1, wherein the controller causes the inverter to increase the frequency of the ac power during motor start-up, thereby increasing the speed of the electric motor during start-up.

3. The system of claim 2, wherein the controller further causes the inverter to increase the current before increasing the frequency.

4. The system of claim 2, wherein the controller causes the inverter to increase the frequency according to a predetermined profile.

5. The system of claim 1, wherein the controller causes the inverter to vary the frequency of the ac power to track a process requirement.

6. The system of claim 1, wherein the controller causes the inverter to vary the current of the ac power to track a process requirement.

7. The system of claim 1 further comprises the means for providing backup power to the motor in the event that the primary motor fails to provide the power of.

8. The system of claim 1, wherein the primary motor includes a microturbine generator, the microturbine generator including a turbine for converting gaseous thermal energy into mechanical energy; an electric generator to convert the mechanical energy produced by the turbine into electrical energy; and a rectifier coupled to an output of the electrical generator, an output of the rectifier providing the electrical energy of.

9. The system of claim 8, wherein the controller also controls the miototurbine generator.

10. The system of claim 8, wherein the primary motor further includes a first rectifier having an input coupled to an output of the microturbine generator; and wherein the inverter includes a power bus, a ac ac converter coupled between the power bus and the electric motor, and a second rectifier having an input adapted to receive the utility power and an output coupled to the bus of, an output of the first rectifier also being coupled to the power bus of.

11. A microturbine energy generating system to operate an electric motor at variable speeds, the system comprises: a turbine to convert gaseous thermal energy into mechanical energy an electric generator to convert the mechanical energy produced by the turbine into electrical energy a rectifier that has an input coupled to an output of the electric generator an inverter having an input coupled to an output of the rectifier, the output of the inverter providing power ac to the electric motor, and a controller to cause the inverter to vary at least the frequency or current of Ac energy during engine operation.

12. The system of claim 11, wherein the controller causes the inverter to raise the frequency of the ac power during motor start-up.

13. The system of claim 12, wherein the controller further causes the inverter to raise the motor current before raising the frequency.

14. The system of claim 11, wherein the controller causes the inverter to vary the current in response to the requirements of the process during normal operation of the motor.

15. The system of claim 11, wherein the controller causes the inverter to vary the frequency in response to the requirements of the process during normal operation of the motor.

16. The system of claim 11, wherein the controller also controls the operation of the turbine.

17. The system of claim 11, wherein the inverter includes a power bus of, a ac ac converter coupled between • the power bus of and the electric motor, and a second rectifier having an input adapted to receive the power of the service public and an output coupled to the bus, an output of the first rectifier also being coupled to the power bus of.

18. A method for operating an electric motor, the method comprises the steps of: operating a primary motor close to the motor to generate electric power from using an inverter to convert the energy into energy ac to feed the energy ac directly to the motor; and controlling an inverter to vary at least the current for the frequency of the ac power during the operation of the motor.

19. The method of claim 18, wherein the frequency of the ac energy rises progressively during engine start-up.

20. The method of claim 19, wherein the current varies before the frequency rises progressively.

21. The method of claim 18, wherein the backup energy is converted to variable frequency ac power when the primary motor fails to generate power from.

22. The method of claim 18, wherein the frequency varies in response to the requirements of the process during normal operation of the engine.

23. The method of claim 18, wherein the current varies in response to the requirements of the process during normal operation of the engine.

24. The method of claim 18, further comprises the step of coupling other primary motors with the first motor.

25. The method of claim 18, wherein the primary motor is a microturbine generator, and wherein the microturbine generator operates using a residual current for fuel.