the various features and advantageous details of the subject matter disclosed herein are explained more fully with reference to the non - limiting embodiments described in detail in the following description . turning initially to fig1 , an exemplary power converter 10 incorporating one embodiment of the present invention is illustrated . the power converter 10 is configured for bidirectional power transfer between an alternator 6 connected to the power converter 10 and a dc bus 12 present in the power converter 10 . the power converter 10 includes three input terminals , t 1 - t 3 , configured to be connected to the alternator 6 . in one operating mode , each of the input terminals , t 1 - t 3 , of the illustrated embodiment is configured to transfer power from the alternator 6 to the dc bus 12 of the power converter 10 . the alternator 6 may be driven by an external source , such as the wind , and generate , for example , three phase alternating current ( ac ) voltage , v 1 - v 3 , each phase connected to one of the input terminals . an input filter 28 is connected in series with each of the terminals , t 1 - t 3 . in another operating mode , the power converter 10 may be configured to convert the dc voltage present on the dc bus to a variable amplitude and variable frequency ac voltage on the terminals , t 1 - t 3 , controlling rotation of the alternator 6 . when the alternator 6 is generating power , the power converter 10 receives the multiphase ac input voltage , v 1 - v 3 , at the terminals , t 1 - t 3 , and outputs a desired dc voltage , vdc , present on a dc bus 12 using switching devices , 20 and 21 . the dc bus 12 includes a positive rail 14 and a negative rail 16 which are made available at outputs , + vdc and − vdc . as is understood in the art , the positive rail 14 and the negative rail 16 may conduct any suitable dc voltage potential with respect to a common or neutral voltage and are not limited to a positive or a negative dc voltage potential . further , either of the positive rail 14 or the negative rail 16 may be connected to a neutral voltage potential . the positive rail 14 typically conducts a dc voltage having a greater potential than the negative rail 16 . the switching devices , 20 and 21 , are typically solid - state power devices . fig1 shows the switching devices , 20 and 21 , as bipolar junction transistors ( bjts ); however , it is contemplated that any suitable switching device according to the application requirements may be used , including , but not limited to , insulated gate bipolar transistors ( igbt ), field effect transistors ( fet ), silicon controlled rectifiers ( scr ), thyristors such as integrated gate - commutated thyristors ( igct ) or gate turn - off thyristors ( gto ), or other controlled devices . a diode 22 is connected in parallel to each of the switching devices , 20 and 21 , for reverse conduction across the switching device , 20 and 21 , as required when the switching device , 20 and 21 , is turned off . this diode 22 may also be a part of the semiconductor switch . for each phase of the input , a positive switch , 20 , is connected between the input terminal , t 1 - t 3 , and the positive rail 14 of the dc bus 12 , and a negative switch , 21 , is connected between the input terminal , t 1 - t 3 , and the negative rail 16 of the dc bus 12 . each of the positive switching devices 20 are controlled by a positive gating signal 24 and each of the negative switching devices 21 are controlled by a negative gating signal 25 . each of the positive and negative gating signals , 24 or 25 , is enabled or disabled to selectively permit conduction through the positive or negative switching devices , 20 or 21 respectively . a capacitance 50 is connected between the positive rail 14 and the negative rail 16 of the dc bus 12 . the capacitance 50 may be a single capacitor or any number of capacitors connected in series or parallel according to the system requirements . the capacitance 50 is configured to reduce the magnitude of ripple voltage resulting from the voltage conversion between the input voltage and the dc bus 12 . a controller 40 executes a series of stored instructions to generate the gating signals , 24 and 25 . the controller 40 receives feedback signals from sensors corresponding to the amplitude of the voltage and / or current at various points throughout the power converter 10 . the locations are dependent on the specific control routines being executed within the controller 40 . for example , input sensors , 26 a - 26 c , may provide an amplitude of the voltage present at each input terminal , t 1 - t 3 . optionally , an input sensor , 26 a - 26 c , may be operatively connected to provide an amplitude of the current conducted at each input terminal , t 1 - t 3 . similarly a current and / or a voltage sensor , 28 and 30 , may be operatively connected to the positive rail 14 and the negative rail 16 , respectively , of the dc bus 12 . the controller 40 interfaces with a memory device 42 to retrieve the stored instructions and with a communication port 44 to communicate with external devices . the controller 40 is configured to execute the stored instructions to control the power converter 10 as described herein . referring next to fig4 , an exemplary power conversion system includes a first power converter 10 and a second power converter , also referred to herein as an inverter , 60 connected by a dc bus 12 . optionally , an energy storage device 18 may be connected between the positive rail 14 and the negative rail 16 of the dc bus 12 . the alternator 6 , such as the generator of a wind turbine , supplies power to the power converter 10 , which is converted to a dc voltage on the dc bus 12 , and the inverter 60 , in turn , supplies power to an electrical load 4 or to a utility grid ( not shown ) from the dc bus 12 . the storage device 18 may also include a dc to dc converter to convert the dc voltage present on the dc bus 12 to a suitable dc voltage level according to requirements of the storage device . the storage device may be , for example , a lead - acid battery , a lithium ion battery , a zinc - bromide battery , a flow battery , or any other suitable energy storage device . the dc to dc converter operates to transfer energy between the dc bus 12 and the storage device 18 according to the application requirements . referring now to fig2 , an exemplary inverter 60 is connected to the dc bus 12 . the inverter 60 may be configured for bidirectional power transfer between the dc bus 12 and the utility grid . in one operating mode , the inverter 60 converts the dc voltage from the dc bus 12 to an ac voltage suitable to be supplied , for example , to the utility grid or an electrical load , such as a motor . in another operating mode , the inverter 60 may be configured to regulate the dc voltage present on the dc bus 12 by regulating current between the utility grid and the dc bus 12 . control of the inverter 60 in either operating mode is performed using switching devices 70 which selectively connect either the positive rail 14 or the negative rail 16 to one of the phases of the output 62 . the switching devices 70 are typically solid - state power devices . fig2 illustrates the switching devices 70 as bipolar junction transistors ( bjts ); however , it is contemplated that any suitable switching device according to the application requirements may be used , including , but not limited to , insulated gate bipolar transistors ( igbt ), field effect transistors ( fet ), silicon controlled rectifiers ( scr ), thyristors such as integrated gate - commutated thyristors ( igct ) or gate turn - off thyristors ( gto ), or other controlled devices . a diode 72 is connected in parallel to each of the switching devices 70 for reverse conduction across the switching device as required when the switching device 70 is turned off . this diode 72 may also be a part of the semiconductor switch . each switching device 70 is controlled by a gating signal 74 . the gating signal 74 is enabled or disabled to selectively permit conduction through the switching device 70 . a controller 90 executes a series of stored instructions to generate the gating signals 74 . the controller 90 receives feedback signals from sensors corresponding to the amplitude of the voltage and / or current at various points throughout the inverter 60 . the locations are dependent on the specific control routines being executed within the controller 90 . for example , sensors , 76 a - 76 c , may provide an amplitude of the voltage present at each phase of the output 62 . optionally , the sensors , 76 a - 76 c , may be operatively connected to provide an amplitude of the current conducted at each phase of the output 62 . similarly a current and / or a voltage sensor , 78 and 80 , may be operatively connected to the positive rail 12 and the negative rail 16 , respectively , of the dc bus 12 . the controller 90 interfaces with a memory device 92 to retrieve the stored instructions and with a communication port 94 to communicate with external devices . according to one embodiment of the invention , the first power converter 10 and the second power converter 60 are separate modules having separate controllers 40 , 90 and memory devices 42 , 92 configured to control operation of the respective power converter . optionally , a single controller and memory device may be configured to control operation of both power converters . in operation , the power conversion system is configured to increase the availability of a wind turbine to generate energy . if the wind turbine is configured with an inertial “ knee ”, as previously discussed , the power conversion system is configured to first accelerate the alternator 6 to an initial speed sufficient to begin operation and then to begin transferring the power generated by the alternator 6 to the utility grid . optionally , the wind turbine may include an anemometer providing a signal corresponding to the wind speed to the controller 40 . the controller 40 may operate to accelerate the alternator 6 when the wind speed is greater than the cut - in speed required by the power converter 10 but less than the initial speed required by the wind turbine to begin rotation of the alternator 6 . even if the wind turbine does not have the inertial “ knee ”, the power conversion system may be configured to accelerate the alternator 6 up to the cut - in speed to reduce the amount of time required to begin operation of the wind turbine . under either operating condition , the power conversion system is configured to operate in a first mode to control the rotational speed of the alternator 6 and in a second operating mode to convert the power supplied from the alternator 6 to the dc bus 12 of the power converter 10 . subsequent energy storage devices 18 or inverter modules 60 may be connected to the dc bus 12 either to store the power generated by the energy source or to deliver the power generated by the energy source to the utility gird , respectively , when the first power converter 10 is configured to transfer power from the source 6 to the dc bus 12 . ( see also fig4 ). the energy storage device 18 may include a dc - to - dc converter to control power between the dc bus 12 and the energy storage device 18 . alternately , the dc - to - dc converter and / or the inverter module 60 may be configured to regulate the voltage present on the dc bus 12 when the power converter 10 is configured to control rotation of the alternator 6 . in either operating mode , the controller 40 , 90 of each power converter 10 , 60 executes one or more control modules which generate gating signals 24 , 25 , or 74 to selectively connect the switches 20 , 21 , or 70 , respectively , between the dc bus 12 and either the input terminals , t 1 - t 3 , or the output 62 according to the desired form of power conversion . according to one embodiment of the invention , a wind turbine may include blades that rotate a low speed drive shaft as a function of the speed of the wind . the low speed drive shaft is input to a gearbox , which , in turn , rotates a high speed drive shaft output as a function of its gearing . the high speed drive shaft rotates the rotor portion of the alternator 6 , generating ac voltages , v 1 - v 3 , on the stator . referring next to fig3 , a graph 100 illustrates the relationship between power generated by the alternator 6 as a function of the rotor speed for an exemplary wind turbine operating under varying wind speeds . the speed of the turbine blades may be controlled , for example , by varying the pitch of the blades . thus , for a constant wind speed , the speed of rotation of the low speed drive shaft and , consequently , the speed of rotation of the rotor in the alternator 6 can be varied . however , the potential exists that the pitch of the blades may not be adjustable at a fast enough rate to respond to varying wind conditions . in addition to , or in lieu of , pitch control , the power converter 10 may help regulate the speed of the alternator 6 by regulating current drawn from the alternator 6 such that a variable braking force is applied to the alternator 6 . the electronic control of the current may , therefore , compensate for variations in the wind speed to maintain operation at the maximum power point . as further illustrated in fig3 by the dashed line 101 , operation of an alternator 6 may follow a squared power rule , where the power produced by the turbine increases as the square of the wind speed . for each wind speed , the controller 40 is configured to operate at a maximum power point ( mpp ), such that the maximum power that may be generated by the alternator at that wind speed is transferred to the dc bus 12 . tracking these maximum power points at the various wind speeds results in the exponential , squared power curve 101 until rated power production occurs . at that point , the controller 40 is configured to limit power production to the rated value to prevent damage to the alternator 6 or to the components of the power converter 10 . the controller 40 may be configured to execute control routines both to control the pitch of the blades and to control the current conducted between the alternator 6 and the dc bus 12 . optionally , separate controllers 40 may be used , each executing one of the control modules . in order to regulate the current drawn from the alternator 6 during normal operating conditions , the controller 40 may implement a synchronous current regulator as is known in the art . a synchronous current regulator receives a current reference and using measured current signals determines a current error value . the synchronous current regulator then determines a desired controlled current to compensate for the current error value . the controller 40 then determines appropriate gating signals , 24 and 25 , to selectively connect each phase of the input terminals , t 1 - t 3 , to the dc bus 12 to produce the desired controlled current between the alternator 6 and the dc bus 12 . because the alternator 6 generates ac power , the controller 40 also requires knowledge of the electrical angle of the ac voltages present at the input terminals , t 1 - t 3 . when operating above a minimum speed , the controller 40 may determine the electrical angle by detecting the back - emf present at the alternator 6 . as the speed of rotation of the alternator increases , the amplitude of the back - emf similarly increases . however , the back - emf is a function of the alternator parameters as well as a function of the rotor speed . thus , the minimum speed at which the back - emf may be detected is a function of the application . however , the amplitude of the back - emf may typically be reliably detected between about 5 % and about 10 % of the rated speed of the alternator 6 . referring next to fig5 , the synchronous current regulator uses the desired controlled current value and the detected electrical angle of the alternator 6 to generate a voltage reference signal 154 and to generate gating signals 24 , 25 . in fig5 , generation of gating signals 24 , 25 for a segment of one cycle for one phase of the ac voltage according to an exemplary sine - triangle pwm modulation technique 150 is illustrated . in the sine - triangle pwm modulation technique 150 , a triangular waveform 152 is compared to the voltage reference 154 to generate the gating signals , 24 and 25 . one period of the triangular waveform 152 is defined by the switching period 156 of the pwm routine . during the switching period 156 , if the voltage reference 154 is greater than the triangular waveform 152 , the positive gating signal 24 is set high while the negative gating signal 25 is set low . if the voltage reference 154 is less than the triangular waveform 152 , the positive gating signal 24 is set low while the negative gating signal 25 is set high . it is contemplated that other modulation techniques , as would be known to one skilled in the art , may also be used to generate the output voltage , such as space - vector or multi - level switching . further , the modulation techniques may be implemented by comparing analog signals , as shown in fig4 , digital signals , such as a register being incremented up and down , or a combination thereof . the controller 40 is configured to operate in the two operating modes discussed above , namely a motoring and a generating operating mode for the alternator 6 . thus , it may be desirable to provide a start up control module in the controller 40 . the start up control module controls the power converter section 10 as an inverter , treating the alternator 6 as a motor , to accelerate the wind turbine up to an initial speed . once at the initial speed , the controller 40 can again control the power converter section 10 as a converter and begin transferring power generated by the alternator 6 to the dc bus 12 . the alternator 6 of a wind turbine typically does not include an encoder or resolver to provide a feedback signal corresponding to the angular position of the alternator 6 . thus , when the controller 40 operates in the motoring operating mode , an open - loop motor control technique must be employed . as an ac machine is rotated , a back - emf is established . the magnitude of the back - emf waveform is a function of the speed of rotation of the alternator 6 . as the speed of rotation increases , the amplitude of the back - emf generated similarly increases . using known techniques , such as a phase - locked loop , the controller 40 may periodically sample the back - emf of one or more of the phases to determine the electrical angle of the alternator 6 . knowledge of the electrical angle is necessary during motoring to provide smooth control of the alternator 6 and during generating to regulate the power transferred from the alternator 6 to the dc bus 12 . controlling the alternator 6 in the motoring mode requires the controller 40 to generate gating signals 24 and 25 to control the positive and negative switches , 20 and 21 respectively , of the power converter 10 according to a modulation technique . because a wind turbine is typically connected to a utility grid , the system includes both a power converter 10 and an inverter 60 as shown in fig4 . however , when the alternator 6 is operated in motoring mode , the power converter 10 is temporarily controlled as an inverter to transfer power from the dc bus 12 to the alternator 6 . similarly , the inverter 60 is temporarily operated as a converter to transfer power from the utility grid to the dc bus 12 . optionally , energy may be transferred from an energy storage device 18 connected to the dc bus 12 via a dc - to - dc converter for use in driving the alternator 6 as a motor . thus , either the utility grid or an energy storage device 18 provides the power necessary to drive the alternator 6 as a motor . modulation techniques control the positive switches 20 and the negative switches 21 to alternately connect each of the terminals , t 1 - t 3 , between either the positive or negative rail , 14 and 16 , of the dc bus 12 . by controlling the direction of the current flow , the controller 40 causes the alternator 6 to operate either in a motoring or a generating operating mode . referring next to fig6 , the resulting modulated waveforms from alternately connecting each of the terminals , t 1 - t 3 , between either the positive or negative rail , 14 and 16 , of the dc bus 12 is illustrated . during low speed operation , the amplitude of the modulated waveforms is much greater than the amplitude of the back - emf generated by the alternator 6 and introduces significant noise or uncertainty in attempting to read the value of the back - emf . referring next to fig7 and 8 , the controller 40 executes to control the alternator 6 during motoring operation by introducing a short interval , or blanking time 120 , during which the modulation is stopped . during the blanking time 120 , the controller 40 may read the back - emf voltage without interference from the modulated voltage . the blanking time 120 is short enough such that the inertia of the alternator 6 and the blades of the wind turbine keep the alternator 6 rotating with little or no slowing of the alternator 6 . according to one embodiment of the invention , the blanking time is between 1 - 3 msec and repeated at periodic intervals spaced between 5 - 20 msec apart . according to a preferred embodiment , the blanking time is about 2 msec and repeated at about 10 msec intervals . during periods of modulation , the power applied to the alternator 6 causes the alternator 6 to accelerate . as the speed of the alternator 6 increases , the amplitude of the back - emf increases . at some point , typically about 5 - 10 % of rated speed , the magnitude of the back - emf is large enough that it may be read during continuous modulation . thus , the controller 40 controls the alternator 6 from a stop and at low speeds using the blanking time until the alternator 6 reaches a speed at which the back - emf may be continuously monitored . at this speed the controller 40 stops using the blanking time and continuously modulates the voltage to the alternator 6 . when the alternator 6 has reached the desired cut - in speed , the controller 40 switches from the motoring operating mode to the generating operating mode . consequently , the power converter 10 ceases operation as an inverter and resumes operation as a converter , namely transferring power from the alternator 6 to the dc bus 12 . similarly , the inverter 60 ceases operating as a converter and again operates as an inverter to transfer power from the dc bus 12 to the utility grid . it is further contemplated that use of the blanking time to read back - emf may be used to extend the range of operation as an alternator during low - speed operation . as previously discussed , knowledge of the electrical angle of the ac power produced by the ac alternator 6 is required for the synchronous current regulator to control power transfer from the alternator 6 to the dc bus 12 . as the speed of the rotor slows , the magnitude of the back - emf decreases until the amplitude becomes too low to accurately detect during continuous modulation . introduction of a blanking time , as described above , allows the power converter 10 to temporarily discontinue modulation and read the back - emf . the electrical angle of the back - emf is determined and corresponding adjustments made to the angle used by the controller 40 to perform modulation . modulation of the switches , 20 and 21 , is resumed at the modified angle to transfer power from the alternator 6 to the dc bus 12 . it should be understood that the invention is not limited in its application to the details of construction and arrangements of the components set forth herein . the invention is capable of other embodiments and of being practiced or carried out in various ways . variations and modifications of the foregoing are within the scope of the present invention . it also being understood that the invention disclosed and defined herein extends to all alternative combinations of two or more of the individual features mentioned or evident from the text and / or drawings . all of these different combinations constitute various alternative aspects of the present invention . the embodiments described herein explain the best modes known for practicing the invention and will enable others skilled in the art to utilize the invention
a method of starting a wind turbine generator of any type of polyphase ac machine , including , but not limited to , brushless dc or permanent magnet machines is disclosed . the machine starts from a dead stop or from low speed operation and is accelerated to the cut in speed for power production . the start - up is realized utilizing the common set of electrical conductors and the power converter also used for capturing the generated power . under initial operation , the power converter executes a pwm modulation technique to drive the machine . periodically , the pwm modulation is stopped to read the electrical position of the generator .