Patent Publication Number: US-2017370993-A1

Title: Controllable load systems and methods

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of priority from U.S. Provisional patent application No. 62/354368, filed Jun. 24, 2016, and entitled CONTROLLABLE LOAD SYSTEMS AND METHODS, which is incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to systems and methods for providing a controllable load, such as for use in testing electrical power devices. 
     BACKGROUND 
     Generators and other electrical devices need to be tested to ensure that they operate properly. In the case of generator, this is usually done by connecting a load bank to the generator to provide loading that simulates the actual load conditions. However, loading a generator with a traditional load bank typically generates a lot of heat and wasted energy. 
     SUMMARY 
     As one example, a system includes drive circuitry having outputs configured to provide drive current based on control parameters and having inputs configured to receive an output voltage of an electrical device. Simulation circuitry is configured to provide simulation signals based on the drive current and the output voltage. A controller sets the control parameters based on the simulation signals to control the drive circuitry to provide the drive current with an amplitude and phase to simulate a predetermined load condition for the electrical device. 
     As another example, a method includes receiving an output voltage supplied from an electrical device. The method also includes providing simulation signals based on the output voltage and drive current. The drive current is generated from the output voltage in response to control signals. The method also includes controlling the drive current based on the simulation signals to provide the drive current with a magnitude and phase to thereby simulate a predetermined load condition for the electrical device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing an example of a controllable load system. 
         FIG. 2  depicts an example of a controllable load system connected to apply an electrical load to a generator test stand system. 
         FIG. 3  depicts an example of simulator circuitry. 
         FIG. 4  depicts an example of a phase locked loop frequency synthesizer that can be utilized in the simulator circuitry of  FIG. 3 . 
         FIG. 5  depicts an example of a control loop that can be utilized for controlling drive circuitry of the controllable load system. 
         FIG. 6  depicts the controllable load system utilized in a first example regenerative configuration in which the electrical energy is regenerated to a power bus. 
         FIG. 7  depicts the controllable load system utilized in a second example regenerative configuration in which the electrical energy is regenerated to an electrical power grid. 
         FIG. 8  depicts the controllable load system utilized in a third example of regenerative configuration in which the electrical energy is supplied to a brake resistor. 
         FIG. 9  is a flow diagram depicting an example method for controlling a load system. 
     
    
    
     DETAILED DESCRIPTION 
     This disclosure provides controllable load systems and methods. These systems and methods provide electrical power loading of one or more devices under test according to a set of control parameters. The control parameters can set a target load condition that is to be applied to the electrical device under test. For example, the predetermined load condition may enable simulation of any combination of RLC (Resistance, Inductance and Capacitance) load. 
     As an example, a system includes drive circuitry having one or more outputs configured to provide drive current based on control parameters from a controller. Simulation circuitry is configured to provide simulation signals based on the drive current and a generator voltage produced by a generator (or other device under test). The controller sets the control parameters for the drive circuitry based on the simulation signals to control the drive circuitry to provide the drive current with an amplitude and phase to simulate a predetermined load condition for the generator. As mentioned, the load condition can be set (e.g., programmable in response to a user input) to establish the type and size of load being simulated. 
     In some examples, the systems and methods herein further may be implemented using COTS (Commercial Off-The-Shelf) AC motor drives as the loading device. The example systems and methods disclosed herein thus enable the drive to “think” it is connected to a motor and is controlling the motor, but in reality, provide the results of emulating a load bank to the generator. While this disclosure focuses on application of the systems and methods for testing generators, the concept is viable for a number of applications that employ a controlled electrical/electronic load having a predetermined load condition. 
     As a further example, the output from the drive circuitry may regenerate the electrical energy extracted from the generator directly to a power bus. In one example, a prime mover (motor) that is spinning the generator is connected to the power bus for immediate consumption of the regenerated energy by the prime mover. As another example, electrical energy extracted by the drive circuitry from the generator can be regenerated back to a Utility Grid. As yet another example, an all-in-one drive having an integrated controls and a brake resistor can be implemented efficiently using the controllable load system. For instance, the individual Resistive (R), Inductive (L) and Capacitive (C) load step elements can be replaced by a single braking resistor (BR) that is driven by the controllable load system. 
     As used herein and shown in various figures ( FIGS. 2-8 ), cross hashes are used to indicate that a given connection or bus may include any number of one or more transmission lines. That is, a given connection/bus with a cross hash may be a single line connection/bus, a double line connection/bus, a triple line connection/bus or have another number of transmission lines, which may depend on the particular application of the circuit and context in which it is being used. 
       FIG. 1  depicts an example of a controllable load system  10 . The controllable load system  10  is configured to simulate an adjustable load bank, such as can be utilized for testing an electrical device  12 . In this example, the output voltage V OUT  of electrical device  12  is connected to an input of the controllable load system  10 . In response to the output voltage V OUT , the controllable load system  10  provides a corresponding output, which may be can be supplied to and/or utilized by one or more associated electrical devices in a desired manner. 
     In some examples, the electrical device  12  is a generator and the controllable load system  10  is connected to the generator as part of a motor-generator test stand system. For instance, the generator provides a generator output voltage (V GEN ) according to the configuration and control commands of the generator  12 . The generator output voltage thus can supply power according to the capabilities and controls applied to operate the generator. While many examples herein are described in the context of using a generator as the electrical device  12  that is under test, other examples of the electrical device  12  include power amplifiers, inverters, batteries and battery chargers. 
     As an example, the controllable load system  10  includes simulation circuitry  14  that is configured to provide simulation signals generated based on the output voltage V OUT  and a drive current I D . The drive current I D  corresponds to current provided by drive circuitry  18 . For example, the simulation circuitry  14  provides the simulation signals as encoder output signals, such as simulating an incremental position of electromotive devices (e.g., motor and/or generator). In some examples, an encoder index may also be generated to simulate an absolute position, which may be used by the controller  16  to help synchronize the output voltage V OUT  and drive current I D  and/or set a desired phase offset. 
     For example, the drive circuitry  18  includes an arrangement of power switch devices, such as power metal oxide field effect transistors (MOSFETs), bipolar transistors, insulated gate bipolar transistors (IGBTs), thyristors or other switch devices. The power switch devices can be operated to implement an electrical power converter (AC power converter or DC power converter), such to provide DC-DC conversion, DC-AC conversion, AC-DC conversion or AC-AC conversion. The type and size of power switch devices and their operation may vary depending on application requirements. The drive circuitry  18  further may be implemented in a single-phase or a multi-phase configuration having one or more outputs, respectively, to provide the drive current I D  based on control parameters provided by the controller  16 . In some examples, the drive circuitry  18  provides the drive current as the output with the same number of phases as output of the electrical device  12   
     The input to the drive circuitry  18 , which corresponds to the drive current I D , is connected to the output of the electrical device, corresponding to V OUT , through a filter network  20 . The filter network  20  may include one or more inductors to provide filtering as well as facilitate energy transfer from the device  12  under test through the drive circuitry. In other examples, the filter network  20  may include both inductors and capacitors or inductors, capacitors and resistors. Various filter topologies may be utilized according to the configuration of the electrical device and the output voltage V OUT . The filter network filters the output voltage V OUT  to remove noise and reduce total harmonic distortion (THD), producing a filtered power signal corresponding to the drive current I D . In addition to filtering and energy transfer, inductance in the filter network may also enable regeneration of low voltage to higher voltage buses. Moreover, depending on the filter topology, the filter network  20  may (or may not) introduce a phase offset between V OUT  and I D . The filter network  20  may include one or more switch devices (e.g., contactors, relays or the like) to selectively configure the filter between the drive circuitry load system  10  and the output of the electrical device. For example, different filter components (inductors and/or capacitors) in the filter network  20  are selectively connected or removed in response to a command signal to achieve a desired filter response. 
     By way of example, the controller  16  utilizes the simulation signals from simulation circuitry  14  to control the drive circuitry  18  to provide the drive current with a desired magnitude and phase, such as corresponding to predetermined load condition for testing the electrical device  12 . For example, the controller  16  synchronizes the drive current I D  with the output voltage V OUT  during an initial phase, such as by switching in a pilot load during this initial phase in the absence of actual loading the electrical device. After synchronization, the pilot load may be removed (or adjusted), and the controller  16  employs the simulation signals to control the drive circuitry  18  to provide drive current I D  to set the magnitude and phase to implement the predetermined load condition. In other examples, the pilot load can be omitted if instantaneous, non-commanded load phases can be tolerated by the electrical device  12  that is being loaded (e.g., generator) momentarily or if phase control circuitry is fast enough as not cause damage to the electrical device under test. The predetermined load condition may be a fixed or variable over time, such as one or more testing intervals. Additionally, the controllable load system  10  can be configured according to a simulating setting (in response to a user input) with appropriate load parameters for loading the electrical device  12  in a desired manner. The predetermined load condition can be set in response to a user input (e.g., input commands to set KVA, PF, etc.), such as entered via a human-machine interface that is connected to the controllable load system (directly or via a network connection). 
     As mentioned, the simulation circuitry  14  can include encoder simulation circuitry to monitor each of the drive current I D  and the output voltage V OUT . The controller  16  can compare the encoder signals and determine a phase difference between the generator voltage V GEN  and the drive current I D . Based on the determine phase difference, the controller  16  sets the control parameters for operating the drive circuitry  18  to provide corresponding magnitude and phase for the drive current I D . The control parameters for the drive circuitry can depend on the type and quantity of predetermined load condition to which the load system  10  is to apply to the electrical device  12 . The predetermined load condition may be programmed as load simulation settings, which can be utilized by the controller to emulate any load bank condition that is applied to the electrical device  12 . For example, the predetermined load condition may be configured to set active power (Watts), reactive power (volt-ampere reactive (VAR)), complex power (VA), apparent power (magnitude of complex power) or a power factor (PF), corresponding to the ratio of active power to apparent power. The output of drive circuitry  18  thus can be connected to provide power to other circuitry or systems. 
     In some examples, such as disclosed herein, the controllable load system  10  can apply the predetermined load condition to the electrical device  12  while concurrently providing regenerative electrical energy to other circuitry or back to a power grid. Additionally or alternatively, the controllable load system  10  can apply the predetermined load condition to implement power factor correction. For example, the controllable load system  10  can be placed at or near the input power entrance to a building or other facility and apply loading to achieve total power factor correction for the building/facility. The controllable load system  10  disclosed herein can also more accurately implement for power factor correction since it is continuously variable/controllable, and does not require discrete component “steps” as in some existing approaches (e.g., capacitive load banks). Additionally, because of switch devices in the drive circuitry are independently controllable, the controllable load system  10  is able to “redirect” the energy between building facility inductances themselves—phase to phase energy transfer (Phase A inductance, Phase B inductance and Phase C inductance). As a result, the building inductances themselves can be used as the energy storage, which can eliminate (or at least significantly reduce) the need for other energy storage devices (e.g., capacitors) when used for power factor correction. 
     In some examples, such as where the electrical device  12  is a DC generator, the simulation circuitry  14  can be omitted from the system  10 . In this example, the controller  16  can control the drive circuitry to set current, such as by implementing a DC current injection mode for motor braking. For example, two phases can be connected to one terminal and the other phase connected to the other generator terminal. The filter  20  includes an inductor between drive circuitry  18  and generator  12 . The inductor provides filtering the switch action of the drive circuitry  18  as well as allows a lower generator voltage to be able to transfer energy to higher voltage DC bus (connected at the output of drive circuitry  18 ), utilizing inductance voltage kickback effect. 
       FIG. 2  depicts an example of a controllable load system  50  that is connected to apply a controllable load to a generator  52 . The controllable load system  50  may correspond to the load system  10  of  FIG. 1 . In this example, the generator  52  is coupled to a drive stand  54  that includes a motor  56 , which may be any prime mover. The drive stand  54  also may include a transmission, such as a gear box  58  that mechanically couples the motor  56  to drive (spin) the generator  52  under test. 
     The motor  56  drives the generator  52  in response to motor drive signals from the corresponding motor drive system  60 . For example, the drive system  60  can be connected to a power grid at  62  to receive input power, such as corresponding to single or three-phase AC power. For example, the drive system  60  includes a filter  64  that is connected to each power input for the drive system. The filter  64  can be an inductor-capacitor-inductor (LCL) filter, for example. Filtered power signals provided to an active front end (AFE)  65  that, in this example, converts the filtered AC power to corresponding DC power at a DC bus. The AFE  65  can also include filtering and other circuitry to mitigate THD and noise in the DC power bus. The AFE  65  thus provides DC power to power electronics, such as an inverter unit (INU)  66 . Other types of power converters may be used in other examples. 
     In this example, the INU  66  includes an arrangement of power switch devices configured to convert the DC power from the DC power bus. Each output of INU  66  is connected to provide corresponding AC drive current to the motor  56 . The drive system can also include corresponding motor control electronics (e.g., hardware and software)  67  to control the INU  66  to set the magnitude and phase of the motor drive current. For example, the motor control  67  may employ a motor (absolute or incremental) encoder to convert the motors mechanical position into corresponding electrical signals (code) representing the angular motor position. Various types of encoders may be used (e.g., optical, mechanical, magnetic and capacitance encoders). As disclosed herein, the drive current can be three phase current supplied to the corresponding motor inputs for driving the motor and, in turn, the generator  52  via the gear box  58 . 
     In response to driving the motor  56 , the generator  52  spins to supply output power to an output power bus. The generator thus provides a generator output voltage (V GEN ) and output current (I GEN ), which defines the output power. Generator control electronics  68  can be provided to control the power that is generated, such as by varying current supplied to generator field. The load system  50  is coupled to apply electrical loading to the output power bus. 
     As a further example, a filter network  70  can be connected between the output of the generator  52  and drive circuitry  74  of the load system  50 . The filter network  70  can correspond to the filter  20  of  FIG. 1 . The filter network  70  can apply filtering to the generator output voltage V GEN  and provide drive current I D  according to control parameters provided to the drive circuitry  74 . The filter network  70  includes an arrangement of filters  76 ,  78  and  80  electrically connected between the generator  52  and the drive circuitry  74 . Each filter  76 ,  78 ,  80  may include inductors and/or capacitors to provide corresponding filter functions. In examples, where a given filter  76 ,  78 ,  80  includes capacitive filtering, the phase of the drive current I D  will be different than the generator output voltage V GEN . In examples filters  76 ,  78  and  80  introduce such phase difference, the control circuitry  88  can compensate for this difference such that PF (magnitude and phase) at the generator  52  is as desired. 
     In some examples, the filter network  70  includes switch devices (SW) arranged to selectively connect or disconnect the filters  76 ,  78  and  80  into and out of the controllable load system  50 . For example, one or more switch devices SW can be connected to each output of the generator (e.g., in a three phase system) to selectively electrically connect a respective filter (or filters) in the electrical path between the output of the generator  52  and an input of the drive circuitry  74  of the load system  50 . This can be used to configure the filters  76 ,  78  and  80  to a desired filter topology, such as by balancing performance tradeoffs between THD %, cost and size for different expected loading conditions. In other examples, the filters may be configured according to application requirements and the switch devices omitted from the filter network  70 . The switch devices may be implemented as contactors or relays, for example. 
     In examples that include the switch devices SW, a load control circuit  72  can control the switch devices SW. For instance, the load control circuit  72  can activate and deactivate switch devices SW based on analysis of the generator (e.g., V GEN  and/or I GEN ) by analysis and measurement of generator operation, such as by analysis and measurement circuitry  73 . Additionally or alternatively, simulator circuitry  84  further can provide information to the load control  72  for controlling the switch devices SW. While the analysis and measurement circuitry is shown separate from the load control circuit  72 , in other examples, such circuitry may be combined. For instance, the functions of the load control  72 , the analysis and measurement circuitry  73  and control module  82  may be integrated into a single control system. Such control system may be implemented as one or more modules in the drive stand  54  or at another location to provide corresponding sensing and control functions. 
     As a further example, one of the filters  80  may be implemented as a pilot load filter. The pilot load filter  80  is electrically connected between the generator  52 , drive circuitry  74  and electrical ground via an arrangement of switch devices, such as shown in  FIG. 2 . Other configurations may be employed to utilize the filter  80  as a pilot load, such as prior to actual loading of the generator  52  by drive circuitry  74 . For example, the load control  72  controls the associated switch devices during an initial start-up phase to selectively connect the filter  80  between V GEN  and I D  as a pilot load having a predetermined impedance (inductance and/or capacitance). During this initial phase, while the pilot load filter  80  is connected to the generator voltage via associated switch devices SW, a control module  82  employs simulation circuitry  84  to synchronize the drive current I D  with the generator voltage V GEN . The pilot load filter  80  thus operates to reduce non-commanded, temporary difference in phase angle between output voltage V GEN  and current I D  in response to initial loading applied to the generator  52  by the controllable load  50 . Once synchronization is achieved, the pilot load  80  may be disconnected from the generator voltage via load control circuit  72  and/or used in conjunction with other filters  76  and/or  78 . Additionally, the control module  82  continues to maintain such synchronization and to control the drive circuitry  74  to provide any desired phase angle difference (or PF) during subsequent loading applied to the generator  52 . 
     Thus, at this stage, the load control circuit  72  can selectively activate and deactivate appropriate switch devices SW to provide a desired filter topology using corresponding filters,  76 ,  78  and  80  and the control module  82  controls the drive circuitry based on simulation signals from simulator circuitry to set the magnitude and/or phase of the drive current I D  according to any predetermined load condition. While the load control circuit  72  is shown as being separate from the control module  82  in the example of  FIG. 2 , it will be understood that the load control and control module may be integrated on a common printed circuit board (PCB) or otherwise be co-located within a housing to provide control system functionality corresponding to both the control module and load control circuitry consistent with this disclosure. 
     In this example, the simulation circuitry  84  includes a drive current encoder simulator circuit (ES 1 ) and a generator voltage encoder simulator circuit (ES 2 ). The drive current encoder simulator ES 1  is coupled to monitor the drive current to generate a set of encoder signals corresponding to the phase of the drive current I D . The generator voltage encoder simulator circuit ES 2  is coupled to monitor the generator voltage V GEN  and generates set of encoder signals corresponding to the phase of the generator output voltage V GEN . 
     Each of the encoder simulator circuits ES 1  and ES 2  is configured to provide information simulating an angular position according to a code (e.g., binary code, gray code or the like) based on the drive current I D  and the generator voltage V GEN . The simulated angular position information may be generated as a code simulating an incremental and/or absolute angular position. Each of the encoder simulator circuits ES 1  and ES 2  may provide respective simulation signals to the control module  82  via one or more corresponding interfaces. For example, the interfaces can correspond to I/O slots of the drive control module  82 . The control module  82  is connected to supply drive signals to the drive circuitry  74  based on the simulation signals and other control parameters, which may be set by a user. As a result, the drive current I D  is provided by drive circuitry  74  with a corresponding phase and magnitude to thereby emulate a predetermined electrical load condition that is being applied to the generator  52 . 
     As mentioned, for example, the magnitude and phase of the drive current I D  may be set by the control module automatically (e.g., at default or preprogrammed values) and/or set in response to a user input to provide the predetermined electrical load condition. For instance, a technician or administrator may employ a user interface to specify a desired KVA, KVAR, PF, or other desired load condition for the generator  52 , which can be stored in memory (not shown). The control module  82  may implement a control loop to determine corresponding control parameters to control the magnitude and phase of the drive current I D  to apply the specified load condition to the generator. The load condition applied to the generator by the system  50  may remain constant during one or more consecutive test intervals or the load condition may vary over one or more test intervals. 
     The phase angle offset between the generator output voltage V GEN  and generator output current I GEN  may vary depending on the topology of the filter network  70  that is being used. Accordingly, in some examples, the simulator circuitry  84  also includes another encoder simulator ES 3 . The encoder simulator ES 3  generates another set of encoder signals corresponding to generator output current I GEN . For example, the analysis and measurement circuitry control system and/or circuitry  73  compares the simulation signals from ES 2  and ES 3 , corresponding to the phase of V GEN  and I GEN , to determine if it is set to desired/commanded value or if further control action is required. As another example, other circuitry (e.g., separate circuitry or part of the control module  82  or generator control  68 ) may be configured compare generator voltage and current signals directly and provide phase commands to drive the generator via advance/retard (or other similar) type commands. It should be noted that if the filter network  70  includes only inductors (e.g., line inductances) and no capacitors, the phase of the drive current I D  will be same as the phase of generator current I GEN , such that the function of encoder simulator ES 3  may be omitted or deactivated. Moreover, in other examples, consistent with this disclosure, variations and modifications may be made to eliminate one or more of the three encoder simulators ES 1 , ES 2  and ES 3 . 
     In view of the foregoing example of  FIG. 2 , a basic approach is disclosed to use drive circuitry to emulate virtually any load bank for loading a generator or other source to with a predetermined load condition (e.g., desired KVA, KVAR, PF&#39;s, and the like). Various modifications to the drive circuitry may be implemented, for example, depending on the circumstances, such as programmability, budget considerations, etc. 
     As one example, when used in a drive stand (e.g., drive stand  54 ) already containing AC drives with a common DC bus link, the electrical energy extracted from the generator may be regenerated directly from the drive circuitry  74  to the common DC bus  86 , such as for immediate consumption by the prime mover/motor  56  that is spinning the generator  52 . As an alternative example, if the drive stand  54  does not already contain a common dc bus ac drive system, or is powered by an internal combustion engine or some other means, the energy extracted from the generator  52  can be regenerated back to the AC Utility Grid at  62 . As yet another alternative, use of an all-in-one drive with integrated Brake Chopper Unit (BCU) and Brake Resistor (BR) will not allow for regeneration of the extracted energy. However, the controllable load system can be used to make a lower cost implementation of this load control system, because the individual resistive, inductive and capacitive load step elements, which are typically used, can be replaced by a single braking resistor (BR) that is coupled to receive the driver current I D  from the drive circuitry  74 . Other uses of the controllable load system  50  may be implemented in other examples. 
       FIG. 3  depicts an example of an encoder simulator circuit  100 , such as can be employed to implement ES 1 , ES 2  or ES 3  in  FIG. 2 . The circuit  100  thus can receive a current or voltage, such as corresponding to a given phase of the drive current I D  or generator voltage V GEN  or generator current I GEN . A voltage limiting circuit  102  performs signal conditioning to normalize the input voltage or current to desired level for subsequent processing. Circuit  102  provides a voltage limited output to a low pass filter  104  to remove unwanted noise and high frequency components. A zero crossing detector  106  detects zero crossings of the filtered signal and provides a corresponding digital output representing each zero crossing instance. This may include crossings that occur on rising edge, a falling edge or both. 
     The detected zero crossings are provided as inputs to a phased-locked loop (PLL) frequency synthesizer  108 . The PLL frequency synthesizer  108  is used to generate a higher frequency output than that of the fundamental frequency of the current or voltage input. The PLL frequency synthesizer  108  generates an equivalent PPR (pulses per revolution) such as having a frequency that is at least twice that of the input signal. Using higher PPR enables more precise phase angle resolution and control. 
     The output of the zero crossing detector  106  can also be provided to digital logic component (e.g., an output latch, such as a D flip-flop)  110  to capture the output of the zero crossing detector. Thus, digital output circuit  110  provides quadrature outputs, demonstrated as “Z” and “not Z”, in response to the zero crossing output. For example the “Z” and “not Z” outputs correspond to an index (marker) associated with the encoder signals. 
     The output of the PLL frequency synthesizer  108  drives additional digital logic (e.g., D flip-flops)  112  and  114 . For example, the PLL output is provided to a clock input of latch  112  and to an inverter as to provide an inverted version of the PLL output to clock input of the latch  114 . Latch  112  thus provides corresponding “A” and “not A” encoder simulation signals and latch  114  provides corresponding “B” and “not B” encoder signals. 
       FIG. 4  depicts an example of a PLL frequency synthesizer that can be utilized as the PLL frequency synthesizer  108  of  FIG. 3 . For example, the synthesizer  108  can include a PLL  118  and a frequency divide-down counter  120 . 
     The phase lock loop  118  thus receives an output of zero-crossing detector  106  at a phase comparator  122 . An output of the frequency divide down counter  120  is supplied to another input of phase comparator  122  to provide a resulting comparison output, which is filtered by a low pass filter  124 . The low pass filter  124  removes unwanted noise and provides a filtered version based on the phase comparison. The filtered signal is amplified by an amplifier  126  having a predetermined gain. The amplified signal is supplied to an input of a voltage control oscillator (VCO)  128  to provide the corresponding periodic signal, such as having a 50 percent duty cycle and a frequency that is set according to the amplitude of the amplified signal provided by amplifier  126 . 
     In this way, the PLL synthesizer  108  generates a signal with an appropriate frequency to enable the encoder simulator circuit  100  to supply the set of signals “A”, “not A”, “B”, “not B”, corresponding to quadrature channels of an incremental encoder. The encoder simulator circuit  100  also may provide index signals, “Z” and “not Z”, corresponding to an index channel of encoder. As disclosed herein, each encoder simulator circuit thus provides simulated encoder signals indicative of the phase of the respective signals, including drive current I D  and the generator voltage V GEN  and, in some examples, generator current I GEN . The controller (control module) thus can evaluate the simulator signals to control the drive current I D  to implement the predetermined load condition. For example, the drive control loop of controller (control module) monitors the quadrature signal set (“A”, “not A”, “B”, “not B”), while an overlying process monitors the index signals (“Z” and “not Z”), to implement phase angle offsets, as desired, to provide the predetermined load condition. 
     As a further example, the encoder simulator ES 1 , connected to the drive current I D  of the drive circuitry, provides a set of encoder simulation signals to the controller (control module). The encoder simulations signals from ES 1 , indicate the equivalent motor slip is zero and all the drive current I D  thus can be represented as the real component (as No Load Amps (NLA) or magnetization current) with Iq component (Torque Producing Current) equal to zero. As such, the drive current I D  can be set to any desired current level. 
     As a further example,  FIG. 5  depicts an example of machine executable instructions (software) that can be executed by a controller (control module). As one example, the controller (control module) implements a tension control loop such as used for controlling slack in industrial processes. The encoder simulation signals (from ES 1  and ES 2 ) can be utilized by the tension control loop to provide respective slack-up and slack-out commands, which are used to set the relative offset in position between the drive current I D  and the generator voltage V GEN , such as corresponding to the phase angle of the load being applied to achieve a desired load condition (e.g., power factor). In other examples the controller (control module) may implement other types of phase control loops to control the phase angle between the drive current I D  and the generator voltage V GEN . 
     As shown in the example of  FIG. 5 , a difference between the encoder simulator outputs ES 1  and ES 2  can be determined and utilized to increment or decrement a differential counter  152 . The differential counter may be reset or locked on in response to a counter reset/hold input. A difference between the differential count output value and the phase angle offset input can be determined and supplied to an input of a tension proportional-integral-differential (PID) loop  154 . An output of the tension PID loop  154  can be provided as an input to a corresponding switch  156  for implementing process trim. The switch  156  can be enabled via a process trim enable input. In this case, the tension PID can be applied to a ramp signal, the result of which is supplied to an adder to combine with the ES 2  input. The difference between this “trimmed” command reference and actual ES 1  drive output can be computed by subtraction block  162  as a frequency error and supplied to input of speed PID  164 . The speed PID  164  can supply a corresponding input to a vector algorithm. The vector algorithm also receives a current magnitude input to, in turn, generate a corresponding drive control signal for the drive current I D . The drive circuitry in turn will supply drive current with a phase offset determined from on execution of the instructions  150  executed by the controller (control module). 
       FIGS. 6, 7 and 8  demonstrate some alternative examples of different types of controllable load systems that can be implemented. Each of these types of controllable load systems can be implemented to provide a predetermined load condition, such as disclosed herein with respect to  FIGS. 1-6 and 9 . Accordingly, reference may be made to other figures herein for additional information. 
       FIG. 6  depicts an example of a system  200  that includes a controllable load system  10 ,  50  connected to provide electrical energy regeneration from a generator  216  back to a common bus (DC link bus)  202  of a motor drive system  204 . The controllable load system  10 ,  50  may be configured to implement a DC load bank or an AC load bank that is applied to the generator  216  and regenerates to the bus  202 . As one example, the controllable load system  10 ,  50  is implemented as AC-DC converter. 
     In this example, the motor drive system  204  includes a filter (e.g., an L-C-L filter)  206  that is connected to a power grid  208 . The power grid  208  supplies single or multi-phase power to the drive system  204 . An Active Front End  210  includes a power converter that converts the power grid electrical power to a desired level of DC electrical power, corresponding to the common bus  202 . A drive circuit is controlled by a motor controller to supply electrical power (e.g., single or multi-phase current) from the common bus to drive a motor  214 . While not shown, the motor  214  may be connected to spin the generator  216  through a gear box or other form of mechanical coupling. In some examples, a filter can be connected between the output of drive circuitry of the controllable load system  10 ,  50  and the generator  216  to smooth out the instantaneous current and voltage exertions away from the ideal, desired sinusoidal output. Proper choice of inductor and capacitor filter elements in the filter can allow this current to approximate sinusoidal with low THD. Again, the inductive component of the filter allows a lower voltage generator  216  to supply power to a higher voltage common DC bus  202 . 
       FIG. 7  depicts an example of a system  220  that includes a controllable load system  10 ,  50  connected to provide electrical energy regeneration from a generator  224  back to a power grid  222  (e.g., an AC utility grid). The controllable load system  10 ,  50  may be configured to implement a DC load bank or an AC load bank that is applied to load the generator  224  and regenerates electrical energy back to the power grid  222 . In this example, a motor drive system  226  is connected to drive a motor  228  that is mechanically coupled to drive a generator  224 . 
     In this example, the motor drive system  226  includes a filter (e.g., an L-C-L filter)  230  that is connected to receive electrical energy from the power grid  222 . The power grid  222  supplies single or multi-phase power to the drive system  226 . An active front end  232  includes a power converter that converts the power grid power to a desired level of DC electrical power. A drive circuit  234  is controlled by a motor controller (not shown) to supply electrical power (e.g., single or multi-phase current) to drive the motor  228 . In some examples, a filter can be connected between the output of drive circuitry of the controllable load system  10 ,  50  and the generator  224  to smooth out the instantaneous current and voltage exertions away from the ideal, desired sinusoidal output. Proper choice of inductor and capacitor filter elements in the filter can allow this current to approximate sinusoidal with low THD. 
     In this example, the controllable load system  10 ,  50  supplies its output DC power to an Active Front End  240 . The AFE  240  includes a power converter that converts DC power to AC electrical power. The AFE  240  supplies the converted electrical power to a filter (e.g., an L-C-L filter)  242  that is connected to filter (remove switching frequency noise) from the regenerated electrical energy (power) that is supplied back to the power grid  222 . 
       FIG. 8  depicts an example of another system  250  that includes a controllable load system  10 ,  50  connected to regenerate electrical energy from a generator  252  back to a brake resistor  254 , such as to dissipate the energy as heat. The controllable load system  10 ,  50  may be configured to implement a DC load bank or an AC load bank that is applied to load the generator  252  and supply the electrical energy to brake resistor  254 . In this example, the controllable load system  10 ,  50  may also implement a brake chopper unit  256 . The brake chopper unit  256  can include one or more switching devices that are controlled to limit the voltage by switching the electrical power from the generator, as provided by the load system drive current, is diverted to the brake resistor  254 . While the electrical energy is not regenerated or recaptured, as in the examples of  FIGS. 6 and 7 , the topology of  FIG. 8  provides for a cost efficient solution and may afford improved performance over some existing approaches. This is because individual resistive, inductive and/or capacitive load elements, which are typically used, can be replaced by a single braking resistor  254 . 
     The system  250  also includes a motor drive system  258  that is connected to drive a motor  260  mechanically coupled to drive the generator  252 . The drive system  258  may be the same as in the examples of  FIGS. 6 and 7 . Briefly stated, the motor drive system  258  includes a filter (e.g., an L-C-L filter)  262  that is connected to receive electrical energy (single or multi-phase) from a power grid  264 . An active front end  266  includes a power converter that converts the filtered electrical power to a desired level and type (AC or DC) of electrical power. A drive circuit  268  is configured to supply electrical power (e.g., single or multi-phase current) to drive the motor  260 . 
     In view of the foregoing structural and functional features described above, an example method will be better appreciated with reference to  FIG. 9 . While, for purposes of simplicity of explanation, the method is shown and described as executing serially, it is to be understood and appreciated that the method is not limited by the illustrated order, as parts of the method could occur in different orders and/or concurrently from that shown and described herein. Such method can be executed by various circuit components and/or a control system executing machine readable instructions stored in memory, for example. 
       FIG. 9  is a flow diagram depicting an example method  300  for controlling a load system (e.g., system  10 ,  50 ). At  302 , the method includes receiving an output voltage supplied from an electrical device (e.g., electrical device  12 , generator  52 ). The output voltage may be a single phase or multi-phase voltage. 
     At  304 , simulation signals are provided (e.g., by simulation circuitry  14 ,  84 ,  100 ) based on the output voltage and drive current. The drive current is generated from the output voltage in response to control signal (e.g., from controller  16 ,  82 ). For example, the electrical device is a power generator and the simulation signals include a simulated drive current encoder signal based on the drive current and a simulated generator voltage encoder signal provided based on the generator voltage. The simulated drive current encoder signal and simulated generator voltage encoder signal may be analyzed to control the phase of the drive current. In an example, the drive current is generated by drive circuitry comprising a plurality of switch devices (e.g., power converter). 
     At  306 , the drive current is controlled (e.g., by controller  16 ,  82 ) based on the simulation signals to provide the drive current with an amplitude and phase to thereby simulate a predetermined load condition that is applied to the electrical device. As disclosed herein, the predetermined load condition may be set (e.g., to define an actual power, a reactive power, an apparent power and/or a power factor) in response to a user input. The controlling at  306  further may operate to synchronize the phase of the output voltage and drive current, initially, and once synchronized, impose a desired phase offset to simulate the desired load condition. 
     As disclosed herein, the method  300  may include filtering the output voltage, with filter circuitry (e.g., filter  20 ,  70 ), to provide a filtered power signal, and the drive current is generated from the filtered power signal. The filter may thus remove noise and reduce THD as disclosed herein. Additionally, drive current may be supplied to associated circuitry. For instance, while the phase of the phase of the drive current is set according to the predetermined load condition being simulated, a selected portion of the drive current (e.g., a selected percentage from 0%-100%) is diverted (e.g., by chopper unit  256 ) to a braking resistor (e.g.,  254 ) to dissipate corresponding electrical energy (see, e.g.,  FIG. 6 ). In another example, the method may include regenerating electrical power provided by a generator (e.g.,  12 ,  52 ) back to supply the regenerated electrical power to the motor (see, e.g.,  FIG. 7 ). In yet another example, the method may include regenerating electrical power provided by the generator back to supply the regenerated electrical power to an electrical power grid (see, e.g.,  FIG. 8 ). 
     In view of the foregoing, example systems and methods are disclosed to provide controllable loading for an electrical device under test. The approaches herein enable improved performance over existing approaches. This can be achieved for reduced initial investment compared to many existing load banks as well reduced costs during operation due to realized savings in electricity costs over time. Moreover, the controllable load systems and methods may be implemented in smaller spaces than traditional load banks. For the example of a typical aircraft generation system, a traditional load bank would fill a room, whereas a controllable load system configured according to this disclosure could be contained in a closet. 
     What have been described above are examples. It is, of course, not possible to describe every conceivable combination of structures, components, or methods, but one of ordinary skill in the art will recognize that many further combinations and permutations are possible. Accordingly, the invention is intended to embrace all such alterations, modifications, and variations that fall within the scope of this application, including the appended claims. Where the disclosure or claims recite “a,” “an,” “a first,” or “another” element, or the equivalent thereof, it should be interpreted to include one or more than one such element, neither requiring nor excluding two or more such elements. As used herein, the term “includes” means includes but not limited to, and the term “including” means including but not limited to. The term “based on” means based at least in part on.