Abstract:
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.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. provisional application Ser. No. 61/577,447, filed Dec. 19, 2011, the entire contents of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The subject matter disclosed herein relates to power converters and, more specifically, to improved control of and improved power conversion from polyphase alternating current (AC) machines during low speed operation. 
     In recent years, increased demands for energy and increased concerns about supplies of fossil fuels and their corresponding pollution have led to an increased interest in renewable energy sources. Two of the most common and best developed renewable energy sources are photovoltaic energy and wind energy. Other renewable energy sources may include fuel cells, hydroelectric energy, tidal energy, and biofuel or biomass generators. However, using renewable energy sources to generate electrical energy presents a new set of challenges. 
     Wind turbines, for example, provide a variable supply of energy. The supply is dependent on the amount of wind. Wind turbines are typically configured to generate AC energy and typically provide a multi-phase AC voltage at varying current levels. Due to the variable nature of the energy supplied, power converters are commonly inserted between the wind turbine and the utility gird or an electrical load, if operating independently of the utility grid. The power converters typically require that the wind turbine be rotating at a minimum speed, also known as a cut-in speed, such that it is generating a minimum level of power before the power converter begins operation. 
     However, wind turbines have substantial mass and require significant energy to accelerate from a stop to the cut-in speed such that the converter may begin to harvest energy. Further, some wind turbines may have an inertial “knee”, meaning they require a greater amount of energy to overcome, for example, static friction forces and begin rotation than the amount of energy required to continue rotation of the turbine. The inertial “knee” may, therefore, require a higher initial wind speed to begin operation of the wind turbine, but once the initial speed has been obtained, operation could continue at lower wind speeds. As a result, the wind turbine may have a range of wind speeds at which it may be capable of generating energy but the energy is lost if the wind turbine was not already rotating. Similarly, the inertia of a wind turbine may cause slow acceleration from a stop even if the wind is strong enough to accelerate the turbine to the cut-in speed. The slow acceleration may result in an undesirable amount of time to accelerate the wind turbine up to the cut-in speed. During the acceleration, the wind turbine is again failing to produce energy during periods at which the wind speed is sufficient for energy generation. 
     In order to obtain the highest potential energy generation from the wind turbine, it is desirable to have the wind turbine operating above the cut-in speed of the converter as often as possible. Thus, it would be desirable to provide a system that can overcome the inertial “knee” and/or help accelerate the wind turbine up to the cut-in speed. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The subject matter disclosed herein describes a system and method for controlling polyphase machines during low speed operation and, more specifically, a system and method for starting a polyphase AC machine having no position sensor coupled to the AC machine. 
     According to one aspect of the present invention, the invention provides 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. 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 also used for capturing the generated power. Under initial operation, a PWM modulation technique drives the machine. Periodically, the PWM modulation is stopped to read the electrical position of the generator. Other applications, which require similar starting requirements, such as fly wheels, may similarly apply the starting method. 
     According to one embodiment of the invention, a power conversion system includes a set of terminals configured to connect the power conversion system to a polyphase AC machine, a DC bus having a positive rail and a negative rail, a power converter connected between the set of terminals and the DC bus and configured for bidirectional power transfer between the set of terminals and the DC bus, a memory device configured to store a series of instructions, and a controller. The controller is configured to execute the series of instructions to execute a start up control module below a predefined speed, where the start up control module controls rotation of the AC machine, and to execute a current regulator above the predefined speed to transfer power generated by the AC machine to the DC bus. 
     According to another aspect of the invention, the power conversion system includes an output configured to be connected to a utility grid and an inverter module connected between the output and the DC bus. The inverter module is configured for bidirectional power transfer between the output and the DC bus, and the controller is configured to control the inverter module to maintain a desired DC voltage on the DC bus when the start up control module is executing. 
     According to still another aspect of the invention, the power conversion system may include an energy storage device and a second power converter configured to transfer energy between the energy storage device and the DC bus. The controller is configured to control the second power converter to maintain a desired DC voltage on the DC bus when the start up control module is executing. 
     According to yet another aspect of the invention, the start up control module may include a modulation module configured to convert a voltage on the DC bus to an AC voltage for the AC machine. The controller is configured to periodically disable the modulation module, and when the modulation module is disabled, the controller is configured to read a back-emf voltage present on the AC machine. 
     According to another embodiment of the invention, a power conversion system includes a first set of terminals configured to connect the power conversion system to a polyphase AC machine, a DC bus having a positive rail and a negative rail, a plurality of first switches configured to selectively connect the first set of terminals to the DC bus, a second set of terminals configured to connect to a utility grid, a plurality of second switches configured to selectively connect the DC bus to the second set of terminals, a memory device configured to store a series of instructions, and a controller. The controller is configured to execute the instructions in a first operating mode and in a second operating mode. During the first operating mode, the controller generates a gating signal for each of the first and second switches to accelerate the AC machine to a predefined speed, and during the second operating mode, the controller generates the gating signal for each of the first and second switches to transfer energy generated by the AC machine to the utility grid. During the first operating mode, the first switches may be controlled to provide a multi-phase AC voltage at the first set of terminals, where the multi-phase AC voltage has a variable magnitude and a variable frequency to control a speed of the AC machine. The second switches are controlled to transfer energy between the utility grid and the DC bus to maintain a substantially constant DC voltage on the DC bus. 
     According to another aspect of the invention, the power conversion system also includes a plurality of voltage sensors generating a signal corresponding to an amplitude of voltage present at the first set of terminals. The controller is further configured to receive each of the signals from the voltage sensors, and during the first operating mode, the controller periodically disables the first switches and reads each of the signals when the first switches are disabled. During the second operating mode, the controller continually controls the first set of switches and reads the signals in tandem with controlling the first set of switches. The controller may be further configured to determine a back-emf voltage present at the first set of terminals as a function of the signals read from the voltage sensors, and to determine an electrical angle of the voltage present at the first set of terminals as a function of the back-emf voltage. 
     According to another embodiment of the invention, a method of accelerating a polyphase AC machine for use in a wind turbine up to a predefined initial speed greater than a cut-in speed of the wind turbine is disclosed. The method includes the steps of controlling a power converter in a first operating mode to execute a modulation module to generate a voltage for the AC machine, where the voltage has a variable magnitude and a variable frequency to control a rotational speed of the AC machine. During the first operating mode, the method disables the modulation module at a periodic interval and determines a back-emf voltage present on the AC machine when the modulation module is disabled. The rotational speed of the AC machine is determined as a function of the back-emf voltage. The power converter is controlled in a second operating mode when the rotational speed is greater than the predefined initial speed, and during the second operating mode, the power converter transfers energy from the AC machine to a DC bus in the power converter. 
     According to another embodiment of the invention, a power conversion system for transferring energy generated from an AC generation source to a utility grid includes a set of terminals configured to connect the power conversion system to the AC generation source, a DC bus having a positive rail and a negative rail, a power converter connected between the set of terminals and the DC bus and configured for power transfer between the set of terminals and the DC bus, a memory device configured to store a series of instructions, and a controller. The controller is configured to execute the series of instructions to execute a modulation module above a predefined speed for continuous modulation of the power converter to transfer power generated by the AC generation source to the DC bus and to periodically insert a blanking time in coordination with the modulation module below the predefined speed for intermittent modulation of the power converter to transfer power generated by the AC generation source to the DC bus. 
     These and other objects, advantages, and features of the invention will become apparent to those skilled in the art from the detailed description and the accompanying drawings. It should be understood, however, that the detailed description and accompanying drawings, while indicating preferred embodiments of the present invention, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING(S) 
       Various exemplary embodiments of the subject matter disclosed herein are illustrated in the accompanying drawings in which like reference numerals represent like parts throughout, and in which: 
         FIG. 1  is a schematic representation of a converter according to one embodiment of the invention; 
         FIG. 2  is a schematic representation of an inverter according to one embodiment of the invention; 
         FIG. 3  is a graphical representation of the power generated by a wind turbine as functions of rotor speed and wind speed; 
         FIG. 4 . is a block diagram representation of one embodiment of the invention; 
         FIG. 5  is a graphical representation of a portion of one modulation period according to one embodiment of the invention; 
         FIG. 6  is a graphical representation of a three phase voltage present at the terminals of the converter of  FIG. 1  during operation under continuous pulse width modulation; 
         FIG. 7  is a graphical representation of a three phase voltage present at the terminals of the converter of  FIG. 1  during operation under pulse width modulation with a periodic blanking time; and 
         FIG. 8  is a graphical representation of one phase of the three phase voltage of  FIG. 7  over one period of the voltage. 
     
    
    
     In describing the preferred embodiments of the invention which are illustrated in the drawings, specific terminology will be resorted to for the sake of clarity. However, it is not intended that the invention be limited to the specific terms so selected and it is understood that each specific term includes all technical equivalents which operate in a similar manner to accomplish a similar purpose. For example, the word “connected,” “attached,” or terms similar thereto are often used. They are not limited to direct connection but include connection through other elements where such connection is recognized as being equivalent by those skilled in the art. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     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  FIG. 1 , 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.  FIG. 1  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  FIG. 4 , 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  FIG. 2 , 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.  FIG. 2  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  FIG. 4 ). 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  FIG. 3 , 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  FIG. 3  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  FIG. 5 , 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  FIG. 5 , 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  FIG. 4 , 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  FIG. 4 . 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  FIG. 6 , 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  FIGS. 7 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