Advanced Control of a Compensator Motor Driving a Variable Speed Diesel Generator with Rotating Stator

energies

Article Advanced Control of a Compensator Motor Driving a Variable Speed Diesel Generator with Rotating Stator

Mohammadjavad Mobarra 1 , Bruno Tremblay 2, Miloud Rezkallah 3 and Adrian Ilinca 1,*

1 Wind Energy Research Laboratory (WERL), Université du Québec à Rimouski, Rimouski, QC G5L 3A1, Canada; [email protected] 2 Department of Computer Science and Engineering, Université du Québec à Rimouski, Rimouski, QC G5L 3A1, Canada; [email protected] 3 Electrical Engineering Department, Ecole de Technologie Superieure, Montréal, QC H3C 1K3, Canada; [email protected] * Correspondence: [email protected]; Tel.: +1-418-723-1986 (ext. 1460)

Received: 4 April 2020; Accepted: 22 April 2020; Published: 2 May 2020

Abstract: Variable speed generators can improve overall genset performance by allowing the diesel engine to reduce its speed at lower loads. In this project, a variable speed diesel generator (VSDG) uses a rotating stator driven by a compensator motor. At lower loads, the stator turns in the opposite direction of the rotor, a process that can be used for purposes like maintaining a fixed relative speed between the two components of a generator. This allows the diesel engine to turn at a lower speed (same as the rotor) and to increase its efficiency. The present research addresses the control of the compensator motor driving the generator’s stator using a variable-frequency drive that adapts the speed to its optimal value according to the load. The performance of the proposed control strategy was tested using a Freescale microcontroller card programmed in C-code to determine the appropriate voltage for the variable-frequency drive. The control algorithm uses a real-time application implemented on an FDRM-KL25Z signal processor board. The control performance of a 2 kW asynchronous motor (LabVolt EMS 8503-00/208 V/3 ϕ/60(50) Hz) was demonstrated experimentally at different operating conditions.

Keywords: variable frequency drive; motor speed control algorithm; digital signal processor (DSP); variable speed diesel generator; rotating stator concept

1. Introduction Nowadays, several electric production applications such as hybrid standalone or grid-connected applications are still using diesel generators (DGs). Despite the huge growth of renewable energy contributions in electric production, DG performance optimization is critical due to its important role in electric energy production. This research focuses on the optimization of a variable speed diesel generator (VSDG) based on a new rotating stator technology for an electric generator. The stator rotation and synchronization with the rotor is controlled by a compensator variable speed AC motor. This section addresses the state-of-the-art of existing technologies related to the project.

1.1. Variable Speed Diesel Generator Conventional diesel generators (DGs) generally operate at fixed speeds, 1800 or 1500 rpm, to produce 60 or 50 Hz of current, respectively. During operation, the electric load varies and affects the operation regime of the DG; for lower loads, while maintaining the fixed rotational speed, genset efficiency drops, and specific fuel consumption (the fuel quantity required to produce a given amount of energy) and greenhouse gas (GHG) emissions increase [1]. The reason is that at lower loads, the

Energies 2020, 13, 2224; doi:10.3390/en13092224 www.mdpi.com/journal/energies Energies 2020, 13, 2224 2 of 15 efficiency of the diesel engine decreases as it operates at the constant speed required by the electrical generator to maintain power quality [2]. In this paper, we address the control of a variable speed generator that allows for the operation of the genset at maximum efficiency by adapting the diesel engine speed to the load. For higher loads close to the nominal regime, the genset operates at its nominal speed, either 1800 or 1500 rpm to produce 60 or 50 Hz of current, respectively. With a variable speed generator, when the load decreases, one can reduce the diesel engine speed such as to operate at optimal efficiency. In this research, we address the constraints to increase diesel engine efficiency and maintain power quality to reduce the complexity and the costs of the entire genset. Several solutions are available in the literature [3,4] to adapt diesel engine (DE) speed to electric load variations in order to avoid unnecessary torque production. In [5], a variable-ratio transmission allows for the maintenance of a constant generator speed, while engine speed slows down at lower loads. According to the authors, continuous variable transmission (CVT) simultaneously offers variable engine speeds adapted to the load with constant mechanical torque for the power generator. This CVT consists of two toroidal traction discs connected together with actuated rollers. This shiftless transmission allows the input to be variable within its operation range [6]. One way to drive a DE at variable speed is to use power converters. They consist of different power devices such as power switches, inductors, and capacitors. Meanwhile, system efficiency augmentation, unwilling noise reduction, and precise output control are different parameters that need to be considered. Therefore, researchers employ sophisticated control schemes using dq-projection to control the performance of power devices to have ideal sinusoidal voltage and current [7]. Power converters connected with energy storage systems can also produce a constant voltage and frequency at the output point during different load conditions. Fault protection, protective relays, and high standard electric production are some advantages of using power converters [8]. However, genset performance is always limited by the power converter, as engine speed cannot decrease or increase beyond the power converter capacity. In this paper, we explore a new VSDG technology that uses a rotating stator driven by a compensator motor. The diesel engine shaft is solidly fixed to the generator rotor and turns at the same speed. To allow for diesel engine speed reduction at lower loads, the stator turns in the opposite direction of the rotor to maintain a fixed relative speed between the two components of the generator. As such, it is possible to maintain a constant current, voltage, and frequency and to lower diesel engine speed (same as the rotor) to increase its efficiency. A variable speed AC motor, fed directly from the generator, drives the stator at the required synchronized speed with the rotor. The control of this compensator motor driving the stator uses a variable-frequency drive that adapts the speed to its optimal value according to the load. In the following sections, we present the detailed description of this variable speed diesel generator; here, we review the literature regarding the variable speed AC motor.

1.2. Variable Speed AC Motor More than 60% of electric load consumption belongs to AC motors in industrial and residential sectors [9]. Several control methods are available to adjust AC motor speed to meet different application conditions [9]. Vector control is an integrated control method for AC motor speed based on a dynamic model. Vectorcontrol integrates torque, magnetic flux, and rotor position elements. The implementation of these space vector methods, which requires precise and fast mathematical calculations, increased significantly with the development of more powerful microcontrollers in the past few decades, as detailed in [10,11]. Vector control techniques are complex and expensive, but they can accurately control the speed of motors in large intervals from zero to more than the rated speed. Generally, proportional integral and derivative (PID) regulators in inner and outer control loops simultaneously achieve a variable voltage and variable frequency regulation [12]. This strategy improves motor response time and decreases unreliable performance. The flux production of armature and stator magnetic field are of main interest. The magnitude of the rotating magnetic field is proportional to the Energies 2020, 13, 2224 3 of 15

fluctuation of both voltage and frequency. Therefore, the value of the rotating magnetic field keeps constant if the change of both voltage and frequency magnitude keep constant. As such, machine speed varies its speed without any disturbance [13,14]. For instance, in [15], a closed-loop space vector control method employed a hysteresis, current controlled, voltage source inverter. The author proposed an integral plus proportional controller, based on a sinusoidal pulse-width modulation (SPWM) inverter, as an effective method to achieve a fast dynamic response and linear performance of the AC motor. Another important parameter in motor speed control is magnetic torque. Commonly, the treatment of the motor-phase current is an effective way to proportionally change motor speed and electric torque [16]. Finally, significant improvements to control the speed of AC motors are using vector control methods [17]. Power converters facilitate control within a wide range of speeds and induction torques, specific to different applications. Industrial automation can rely on electric drives (EDs), leading to a higher productivity and better efficiency. However, the fast growth and wide variety of EDs in industrial applications may impair power quality and induce harmonic pollution into the power system.

2. Variable Speed Diesel Generator (VSDG) with Rotating Stator In this research, we adjusted the diesel engine speed to the load using a new generator concept based on a rotating stator. Unlike in other generators, here, the stator rotation is driven by a “compensator” AC motor in such a way to maintain a fixed relative speed between the stator and the rotor [18]. Therefore, the generator speed is no longer limited to the rotor speed. Subsequently, synchronous speed is achieved using rotor speed, stator speed, or a combination of both. Rotating-stator technology reduces the need for power electronics to maintain a constant voltage and frequency. As such, the generator maintains its synchronous speed using stator rotation, while the diesel engine can adapt its speed to operate at the maximum efficiency dictated by the operation conditions [19]. Based on the explanation above, the synchronous generator (SG) speed is the sum or difference between rotor and stator speed, respectively, if they turn in opposite or same direction:

n = nRotor n synch (Total) ± Stator Consequently, at lower loads, the diesel engine slows down to avoid unnecessary torque production, and, simultaneously, the stator turns at the required speed to meet power quality standards. Figure1 illustrates the general operation of the proposed VSDG using a controlled compensator motor to drive the stator. The compensator motor automatically starts once the grid consumption drops under a threshold value. Based on the real-time load variation, a speed control algorithm was developed for the induction motor (compensator motor). The compensator motor (CM) is connected to the stator of the synchronous generator (SG) using a small gearbox and is powered by the SG itself. The rotating stator technology improves the performance and efficiency of VSDG applications and reduces the role of power converters. The variable-speed operation reduces fuel consumption when the system operates under a low electric load regime [20]. No power converter is used on the SG’s end, as the variable speed is achieved by controlling the stator speed. This technology reduces the risk of power capacitor shortage in the power converter and improves system flexibility because the DE speed is no longer limited by the power converter capacity. However, adding a compensator motor increases the mechanical complexity and maintenance process of the machine [21]. The main objective of this research was to improve the efficiency of the VSDG at low load regime. For instance, if, at a given load, the optimal efficiency of the diesel engine is at 1500 rpm, it means that the generator rotor should also turn at 1500 rpm. Consequently, the CM turns the stator at 300 rpm in the opposite direction of the rotor. Synchronous generator speed reaches 1800 rpm while the diesel engine speed stays at 1500 rpm, as required by its optimal operation. Energies 2020, 13, x FOR PEER REVIEW 3 of 15 inverter. The author proposed an integral plus proportional controller, based on a sinusoidal pulse- width modulation (SPWM) inverter, as an effective method to achieve a fast dynamic response and linear performance of the AC motor. Another important parameter in motor speed control is magnetic torque. Commonly, the treatment of the motor-phase current is an effective way to proportionally change motor speed and electric torque [16]. Finally, significant improvements to control the speed of AC motors are using vector control methods [17]. Power converters facilitate control within a wide range of speeds and induction torques, specific to different applications. Industrial automation can rely on electric drives (EDs), leading to a higher productivity and better efficiency. However, the fast growth and wide variety of EDs in industrial applications may impair power quality and induce harmonic pollution into the power system.

2. Variable Speed Diesel Generator (VSDG) with Rotating Stator In this research, we adjusted the diesel engine speed to the load using a new generator concept based on a rotating stator. Unlike in other generators, here, the stator rotation is driven by a “compensator” AC motor in such a way to maintain a fixed relative speed between the stator and the rotor [18]. Therefore, the generator speed is no longer limited to the rotor speed. Subsequently, synchronous speed is achieved using rotor speed, stator speed, or a combination of both. Rotating- stator technology reduces the need for power electronics to maintain a constant voltage and frequency. As such, the generator maintains its synchronous speed using stator rotation, while the diesel engine can adapt its speed to operate at the maximum efficiency dictated by the operation conditions [19]. Based on the explanation above, the synchronous generator (SG) speed is the sum or difference between rotor and stator speed, respectively, if they turn in opposite or same direction:

()= ±

Consequently, at lower loads, the diesel engine slows down to avoid unnecessary torque production, and, simultaneously, the stator turns at the required speed to meet power quality standards. Figure 1 illustrates the general operation of the proposed VSDG using a controlled compensator motor to drive the stator. The compensator motor automatically starts once the grid consumption drops under a threshold value. Based on the real-time load variation, a speed control algorithm was developed for the induction motor (compensator motor). The compensator motor (CM) is connected to the stator of the synchronous generator (SG) using a small gearbox and is powered by the SG itself. The rotating stator technology improves the performance and efficiency of VSDG applications and reduces the role of power converters. The variable-speed operation reduces fuel consumption when the system operates under a low electric load regime [20]. No power converter is used on the SG’s end, as the variable speed is achieved by controlling the stator speed. This technology reduces the risk of power capacitor shortage in the power converter and improves system flexibility because the DE speed is no longer limited by the power converter capacity. However, adding a compensator motor increases the mechanical complexity and maintenance Energies 2020, 13, 2224 4 of 15 process of the machine [21].

Energies 2020, 13, x FOR PEER REVIEW 4 of 15

The main objective of this research was to improve the efficiency of the VSDG at low load regime. For instance, if, at a given load, the optimal efficiency of the diesel engine is at 1500 rpm, it means that the generator rotor should also turn at 1500 rpm. Consequently, the CM turns the stator at 300 rpm in the opposite direction of the rotor. Synchronous generator speed reaches 1800 rpm while the diesel engine speed stays at 1500 rpm, as required by its optimal operation.

3. Architecture of the VSDG with Rotating Stator Figure 1. Configuration of the variable speed diesel generator (VSDG). Figure 2Figure illustrates 1. Configuration the architecture of the ofvariable the VSDG speed withdiesel a generator rotating (VSDG)stator and. compensator motor. 3. ArchitectureThis new technology of the VSDG increases with Rotating the overall Stator performance of the VSDG by allowing the DE to operate at maximum efficiency speed. In a conventional genset, the engine and generator speed are fixed. Figure2 illustrates the architecture of the VSDG with a rotating stator and compensator motor. Moreover, an SG needs to rotate at its synchronous speed to produce the required power quality when This new technology increases the overall performance of the VSDG by allowing the DE to operate it operates at either a low or full load. The engine shaft coupled with the generator’s rotor rotates at the at maximum efficiency speed. In a conventional genset, the engine and generator speed are fixed. SG’s nominal speed at all regimes. Therefore, an oscillation of the DE speed affects power quality. Load Moreover, an SG needs to rotate at its synchronous speed to produce the required power quality when oscillation is another factor that reduces system efficiency in fixed gensets. These phenomena affect it operates at either a low or full load. The engine shaft coupled with the generator’s rotor rotates at power quality and increase DE fuel consumption. In addition, the DE is not able to follow these the SG’s nominal speed at all regimes. Therefore, an oscillation of the DE speed affects power quality. oscillations, and its poor performance is not independent of the fuel governor device [22]. Load oscillation is another factor that reduces system efficiency in fixed gensets. These phenomena This VSDG technology uses several rows of bearings installed between the outer frame of the affect power quality and increase DE fuel consumption. In addition, the DE is not able to follow these stator and the generator in order to allow for the independent rotation of the stator. These bearings oscillations, and its poor performance is not independent of the fuel governor device [22]. have minimum friction with the generator housing. The stator frame and the rotor shaft are also This VSDG technology uses several rows of bearings installed between the outer frame of the mounted in the generator housing on both sides with the use of robust bearings. This modification stator and the generator in order to allow for the independent rotation of the stator. These bearings allows for both parts of the generator, the rotor and stator, to rotate around the same axis. It is have minimum friction with the generator housing. The stator frame and the rotor shaft are also important to mention that the stator windings, core, and connections remain the same as those of a mounted in the generator housing on both sides with the use of robust bearings. This modification conventional generator [23]. allows for both parts of the generator, the rotor and stator, to rotate around the same axis. It is important to mention that the stator windings, core, and connections remain the same as those of a conventional generator [23].

Figure 2. Architecture of the VSDG with rotating stator.

A pulley is coupled to theFigure stator 2. Architecture frame and mountedof the VSDG on with the rotating generator stator. casing. By applying mechanical force on the pulley itself, the stator starts to rotate at the desired speed and direction. A pulley is coupled to the stator frame and mounted on the generator casing. By applying A small-size compensator motor is ﬁxed above the generator casing and is connected to the stator mechanical force on the pulley itself, the stator starts to rotate at the desired speed and direction. A small-size compensator motor is fixed above the generator casing and is connected to the stator pulley with a timing belt. A small mechanical gearbox is installed on the SG’s housing and fixed on the CM shaft. The reason for having this gearbox is to easily connect the CM shaft with the timing belt and to adjust the stator speed to the desired value. In the patent describing the technology, the speed of CM shaft is converted to 250 rpm using the mechanical gearbox. However, depending on the optimum

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pulley with a timing belt. A small mechanical gearbox is installed on the SG’s housing and fixed on the CM shaft. The reason for having this gearbox is to easily connect the CM shaft with the timing belt and Energiesto adjust 2020, 13 the, x statorFOR PEER speed REVIEW to the desired value. In the patent describing the technology, the speed5 of 15 of CM shaft is converted to 250 rpm using the mechanical gearbox. However, depending on the optimum speedspeed efficiency efficiency of of the the DE, DE, the the mechanical mechanical gearbox gearbox may may be be adjusted adjusted [24]. [24]. CM electricity isis suppliedsupplied by bythe the gensetgenset itselfitself andand isis ableable toto runrun atat eithereither 5050 oror 6060 Hz.Hz. DuringDuring lowlow load,load, thethe DEDE speedspeed decreasesdecreases to to save fuel, and the the CM CM begins begins to to rotate rotate the the stator stator at at the the appropriate appropriate speed speed and and direction direction to tokeep keep the the generatorgenerator at at synchronous synchronous speed. ForFor instance,instance, if if engine engine speed speed falls falls to 1500 to rpm1500 forrpm effi ciencyfor efficiency purposes, purposes,the CM the adjusts CM statoradjusts speed stator at speed 300 rpm at 300 in rpm the oppositein the opposite direction direct of theion rotor.of the Therotor. result The result is that is the thatgenerator the generator keeps itskeeps speed its atspeed 1800 rpmat 1800 to supportrpm to electricsupport demand, electric demand, whereas thewhereas DE speed the adjustsDE speed at its adjustsmost at effi itscient most point. efficient point. TheThe aim aim of ofthis this project project was was to tocontrol control the the CM CM speed speed with with a reliable a reliable and and robust robust method method to toachieve achieve anan efficient efficient diesel-generator diesel-generator operat operation.ion. This This technology technology alleviates alleviates system system complexity complexity by by eliminating eliminating powerpower converters converters from from generators’ generators’ rotors rotors and and stator statorss [20]. [20 A]. much A much smaller-scale smaller-scale electronic electronic controller controller ensuresensures CM CM control control and and operation. operation. This This results results in inbetter better fuel fuel consumption consumption profile, profile, a robust a robust speed speed controllercontroller method, method, and and a vast a vast range range of ofvariable variable speeds speeds without without limitation limitation by by converter converter capacity capacity [22]. [22 ].

4. Sensor-Less4. Sensor-Less Space Space Vector Vector Modulation Modulation (SVM (SVM)) Speed Speed Controller Controller Based Based on on Constant Constant V/F V /F FrequencyFrequency drives drives offer off ermany many advantages advantages such such as aslow low harmonic harmonic distortion, distortion, bidirectional bidirectional power power flow,flow, and and fast fast dynamic dynamic response response [25]. [25 ].In In this this wo work,rk, we we connected connected a aprogramme programmedd microcontroller microcontroller with with a a powerpower drivedrive usingusing the the direct direct power power control control and an spaced space vector vector modulation modulation (DPC-SVM) (DPC-SVM) method. method. Figure 3 Figureshows 3 shows the configuration the configuration of the voltage of the voltage source convertersource converter (VSC) based (VSC) on based a constant on a constant switching switching frequency frequencyoperation. operation. To eliminate Tohigh-switching eliminate high-switching frequency, we introducedfrequency, awe passive introduced resistor /capacitora passive (RC) resistor/capacitorfilter [26,27]. However, (RC) filter in [26,27]. this structure, However, three-phase in this structure, power is three-phase used as variable power feedback is used as for variable the PWM feedbackcontroller for tothe mitigate PWM controller low order to harmonics mitigate low in case order of unbalancedharmonics in loads. case of unbalanced loads.

Figure 3. Direct power control of the induction motor (IM) based on sensor-less space vector modulation Figure 3. Direct power control of the induction motor (IM) based on sensor-less space vector (SVM) applied to the VSDG. modulation (SVM) applied to the VSDG. In this conﬁguration, L is an inductor to repel the current oscillations, the opto-isolators are the In this configuration, L is an inductor to repel the current oscillations, the opto-isolators are the ﬁlters to regulate the input signals, and the drives are transistors to precisely trigger the IGBTs. The filters to regulate the input signals, and the drives are transistors to precisely trigger the IGBTs. The DPC method makes it possible to send appropriate signals by estimating the instantaneous position of DPC method makes it possible to send appropriate signals by estimating the instantaneous position the rotor. AC motors need the precise AC voltage signals to have the oscillations that are proportional of the rotor. AC motors need the precise AC voltage signals to have the oscillations that are to the frequency. For this reason, variable frequency drives (VFDs) are always arranged in a way to proportional to the frequency. For this reason, variable frequency drives (VFDs) are always arranged in a way to regulate output Voltage/Frequency to a constant ratio and value. VFDs create variable voltage and frequency to change AC motor speed. Accordingly, the speed of the AC motor varies with the change in frequency, and the higher the frequency applied to the motor, the faster the motor rotates [28]. The control principle of the VSC uses the SVM-PWM method by separating the voltage and current components. One of the reasons for connecting the feedback control loop in the SVM

Energies 2020, 13, 2224 6 of 15 regulate output Voltage/Frequency to a constant ratio and value. VFDs create variable voltage and frequency to change AC motor speed. Accordingly, the speed of the AC motor varies with the change in frequency, and the higher the frequency applied to the motor, the faster the motor rotates [28]. The control principle of the VSC uses the SVM-PWM method by separating the voltage and current components. One of the reasons for connecting the feedback control loop in the SVM method is to adjust the instantaneous active power (P) toward its reference value. The second reason is to follow and evaluate the reactive power (Q) to directly regulate the DC-Link. The unit power factor can also be acquired by monitoring the reactive power. In this method, both P and Q are calculated in α and β reference frames using Clark’s transformation as follows: " # " #" # P E E i = α β α Q E Eα i β − β where Eα, Eβ, iα, and iβ represent components on α and β axes for the output voltage and current, respectively. DPC-SVM estimates the reference stator voltage to minimize the torque and current ripples. Based on SVM modulates, PWM creates appropriate signals to generate reliable sinusoidal waveforms using constant switching frequency. v s (ideal motor speed) is a produced voltage signal coming from a microcontroller card. The power converter receives a treated voltage signal vs, which is 0–10 V and creates the requested frequency at the output point to control the motor speed. In conclusion, for this part, the VFD using the DPC method receives treated vs as input from the microcontroller and produces constant torque and variable speed using a constant ratio V/F strategy.

5. AC Motor Speed Control with Microcontrollers Microcontrollers are programmable, reusable, and cost-effective devices that create sine pulse width modulation signals for switching purposes [29]. In this research, we used a Freescale FRDM–KL25Z microcontroller card programmed with the MCUXpresso software. After compiling the required code, the FreeMASTER software was used as a run-time developer application to monitor the real-time experiment. The aim of this section was to propose a simple control algorithm to achieve ideal speeds for an induction motor coupled with the stator of a VSDG. There were several objectives associated with this approach: 1. High reliability achieved by a self-correction loop. 2. Ease of integration with VFDs. 3. Fast processing speed and high accuracy. 4. High efficiency due to accurate switching command. Figure4 shows a complete diagram of a simple and robust application for controlling motor speed based on the load fluctuation. Several parameters must be set on the digital signal processor (DSP) controllers to create a sine wave simulator. The default mode of this controller is only able to process 12-bit DC signals and send the requested response to its output ports. In this application, we needed a controlled 0–10 V AC voltage at the output port of the DSP card to trigger the VFD system. To start encoding the analog-to-digital conversion board (ADC), we needed to initialize some parameters including the signal processing time and the number of samples per cycle. The FreeMASTER software could recognize the internal and external ports of the DSP for its control algorithm. However, the input and output port voltages were 0–3 and 0–5 V, respectively. This board is capable of receiving DC current and voltage signals from its input and sending the appropriate voltage signal to the AC power converter. Therefore, additional devices were required to adapt the microcontroller card to the power converter and the measuring devices (sensors). Figure5 illustrates the complete bit space definition and its domain based on DSP card limits. The input values were converted into DC signals and inserted in the control algorithm based on specific current and voltage limits. Near to 250 sample signals were embedded each 1 ms to create high-quality sinusoidal waveform and reduce graph discretization. Our objective was to regulate the AC motor Energies 2020, 13, x FOR PEER REVIEW 7 of 15

Energies 2020, 13, 2224 7 of 15 speed based on the load oscillation. Therefore, both feedbacks illustrated in Figure4 came from the current and voltage measurement devices. They were able to track the load variation and send an Energiesappropriate 2020, 13 signal, x FOR toPEER the REVIEW DSP input ports. 7 of 15

Figure 4. Closed-loop control diagram of variable speed motor application.

Figure 5 illustrates the complete bit space definition and its domain based on DSP card limits. The input values were converted into DC signals and inserted in the control algorithm based on specific current and voltage limits. Near to 250 sample signals were embedded each 1 ms to create high-quality sinusoidal waveform and reduce graph discretization. Our objective was to regulate the AC motor speed based on the load oscillation. Therefore, both feedbacks illustrated in Figure 4 came from the current and voltage measurement devices. They were able to track the load variation and send an appropriateFigure signal 4. Closed-loopClosed-loop to the DSP control control input diagram diagram ports. of variable speed motor application.

FigureFigure 5.5.Domain Domain andand frequencyfrequency deﬁnitiondefinition onon aa 1212 bitbit analog-to-digital analog-to-digital conversion conversion board board (ADC). (ADC).

6.6. DesignDesign ofof thethe SpeedSpeed ControlControl AlgorithmAlgorithm andand NumericalNumerical SimulationSimulation TheThe objectiveobjective ofof thisthis sectionsection waswas toto developdevelop thethe controlcontrol algorithmalgorithm forfor thethe ACAC motormotor usedused asas aa compensatorcompensator forfor thethe rotatingrotating statorstator VSDG.VSDG. TheThe ACAC motormotor drivesdrives thethe statorstator ofof aa VSDG,VSDG, suchsuch asas thethe dieseldiesel engine,engine, andand thethe rotorrotor reducesreduces their their speed speed to to avoid avoid producing producing unnecessary unnecessary energyenergy duringduring lowlow electric demand. electric demand. UnlikeUnlike numerousnumerous studiesstudies focusingfocusing onon thethe generator’sgenerator’s electricelectric outputoutput treatmenttreatment withwith aa powerpower converter,converter,Figure wewe 5.varied Domainvaried the theand speed speedfrequency ofof thethe definition DG DG using using on a a a12 new new bit analog-to-digital technologytechnology basedbased conversion onon aa rotatingrotating board (ADC). stator.stator. Thus,Thus, thethe AC AC motor motor shaft shaft was was coupled coupled to to the the rotating rotating stator stator and and compensated compensated for for the the required required speed speed of of the the 6. Design of the Speed Control Algorithm and Numerical Simulation electricelectric generator generator to to avoid avoid harmonic harmonic distortion distortion when when the the diesel diesel engine engine slowed slowed down. down. As As a aresult, result, the the controlcontrolThe algorithm algorithmobjective wasofwas this ableable section toto regulateregulate was to the thedevelop speedspeed th ofofe the thecontrol compensator compensator algorithm motormotor for the byby AC trackingtracking motor the theused electricelectric as a compensatorloadload variation.variation. for Di Differenttheﬀerent rotating loopsloops stator andand VSDG. conditionsconditions The inACin thisthis motor algorithmalgorithm drives ensuretheensure stator thatthat of motor motora VSDG, performanceperformance such as the isis dieselalwaysalways engine, within within and the the intended theintended rotor area. reducesarea. TheThe their microcontrollermicrocontrolle speed to avoidr receives receives producing voltage voltage unnecessary and and current current energy signals signals during to to calculate calculate low the instantaneous active power every one millisecond following the control algorithm structure. The electric demand. algorithmUnlike structure numerous adds studies every focusing voltage andon the current genera valuetor’sbased electric on output the time treatment domain towith produce a power an converter, we varied the speed of the DG using a new technology based on a rotating stator. Thus, the AC motor shaft was coupled to the rotating stator and compensated for the required speed of the electric generator to avoid harmonic distortion when the diesel engine slowed down. As a result, the control algorithm was able to regulate the speed of the compensator motor by tracking the electric load variation. Different loops and conditions in this algorithm ensure that motor performance is always within the intended area. The microcontroller receives voltage and current signals to calculate

Energies 2020, 13, x FOR PEER REVIEW 8 of 15 theEnergies instantaneous2020, 13, 2224 active power every one millisecond following the control algorithm structure.8 The of 15 algorithm structure adds every voltage and current value based on the time domain to produce an appropriate voltage signal. The microcontroller sends voltage signal to the VFD to control the CM appropriate voltage signal. The microcontroller sends voltage signal to the VFD to control the CM speed. Figure 6 shows the structure of the control algorithm based on load power calculation. The speed. Figure6 shows the structure of the control algorithm based on load power calculation. The three most important steps to define the control algorithm are as follows: three most important steps to define the control algorithm are as follows: 1. Workspace1. Workspace Generation. Generation. The The first first step step to toprogram program every every microcontroller microcontroller is is to to provide provide an an ideal ideal environmentenvironment with with accurate accurate classifications classifications and specifications.and specifications. The workspace The workspace may consist may consist of several of sourceseveral code source files that code create files a largethat create environment. a large Forenvironment. this application, For this voltage application, and current voltage sensors and andcurrent a host computer sensors and were a thehost external computer peripheral were the devices external to beperipheral recognized devices by the to software be recognized workspace by andthe DSP software ports. Workspaceworkspace and unfolds DSP inputports. signalsWorkspac everye unfolds time the input control signals algorithm every time requires the control them. Moreover,algorithm appropriate requires DSP them. ports Moreover, found on appropriate the electric card’sDSP ports data sheetfound should on the be electric well-recognized card’s data by the softwaresheet should parameters. be well-recognized by the software parameters. 2. Speed2. Speed Control Control Implem Implementation.entation. Two Twomain main inputs inputs were wereembedded embedded in the in controller the controller coming coming from fromvoltage voltage and and current current measurement measurement devices. devices. Thes Thesee components components were were transformed transformed into into voltage voltage signals,signals, using using limiters, limiters, before before being processedbeing processe by thed algorithm.by the algorithm. Both voltage Both and voltage current and parameters current measuredparameters from measured the demand from side the are demand used inside the are equation used in belowthe equation to determine below to the determine load power. the load The principlepower. of The the controlprinciple algorithm of the control is based algorithm on the instantaneousis based on the power instantaneous formula. Accordingly,power formula. the controlAccordingly, program runsthe bycontrol sending program commands runs by from send theing host commands computer from to the the microcontroller host computer card. to Thethe controlmicrocontroller algorithm processes card. The the control input signalsalgorithm and proce producessses the appropriate input signals signals and for produces the power appropriate converter basedsignals on instantaneous for the power electric converter grid based consumption. on instanta Oneneous advantage electric of grid such cons a controllerumption. isOne the advantage fast motor responseof such and a controller high electric is the torque fast motor production. response and high electric torque production. P ∑((Value [[0]]×Value [])[1]) InstantaneousInstantaneous Power Power= = × Number o f samples RealReal electric electric load load varies varies asymptotically, asymptotically, and and it it consists consists of of the the active active and and reactive reactive parts. parts. Apparent Apparent powerpower represents represents the the total total electric electric power power value value considering considering the the angle angle between between voltage voltage and and current. current. AsAs this this project project aimed to synchronizesynchronize thethe DEDE production production with with demanded demanded mechanical mechanical torque torque applied applied on onthe the DE DE shaft, shaft, for thefor mathematicalthe mathematical calculations, calculat onlyions, theonly active the poweractive waspower considered; was considered; the apparent the apparentpower was power not calculated.was not calculated. In addition, In theaddition, instantaneous the instantaneous calculation calculation of active power of active reduced power the reducedcomplexity the ofcomplexity the control of algorithmthe control and algorithm resulted an ind a resulted fast microcontroller in a fast microcontroller response. response.

Figure 6. Autonomous AC motor speed control algorithm based on variable input surveillance. Figure 6. Autonomous AC motor speed control algorithm based on variable input surveillance.

Energies 2020, 13, x FOR PEER REVIEW 9 of 15 Energies 2020, 13, 2224 9 of 15 The proposed algorithm plan (Figure 6) was implemented in the microcontroller. Small ADC cards were integrated with motor peripherals to reduce application complexity and increase reliability. The The proposed algorithm plan (Figure6) was implemented in the microcontroller. Small ADC motor terminals were connected with the microcontroller using the VFD. The algorithm showed a rapid cards were integrated with motor peripherals to reduce application complexity and increase reliability. time response by using high-sampling frequency and different control loops. Moreover, these control The motor terminals were connected with the microcontroller using the VFD. The algorithm showed a schemes reduced switching noise by giving a precise signal to the VFD. rapid time response by using high-sampling frequency and different control loops. Moreover, these control3. Rules schemes and reducedConditions. switching The final noise step by before giving sending a precise the signal voltage to the signal VFD. to the VFD input is to 3.compare Rules and the Conditions. load power The with final the step predetermi before sendingned thresholds. the voltage signalThree todifferent the VFD load input power is to compareintervals the load define power the with ideal the predeterminedspeed condition thresholds. into the control Three di algorithmfferent load as power follows. intervals The definegoal of the idealhaving speed such condition conditions into thewas control to adapt algorithm the main as follows.application The (VSDG) goal of havingwith the such load conditions variation. was toTherefore, adapt the mainduring application a low electric (VSDG) load, with the the di loadesel variation.engine slows Therefore, down during to avoid a low producing electric load, theunnecessary diesel engine mechanical slows down torque. to avoid On the producing other hand, unnecessary the AC mechanical motor with torque. the robust On the control other hand,algorithm the AC motor starts with compensating the robust control for the algorithmrequired speed starts for compensating the stator of for the the electrical required generator. speed for the stator of the electrical generator. 600 W < Load Power (Per Phase) ≤ 1000 W------Motor Speed = 1200 RPM 600 W < Load Power (Per Phase) 1000 W——————-Motor Speed = 1200 RPM 400 W < Load Power (Per≤ Phase) ≤ 600 W------Motor Speed = 1500 RPM 400 W < Load Power (Per Phase) 600 W——————–Motor Speed = 1500 RPM 200 W < Load Power (Per≤ Phase) ≤ 400 W------Motor Speed = 1800 RPM 200 W < Load Power (Per Phase) 400 W——————–Motor Speed = 1800 RPM Figure 7 and Table 1 reproduce ≤the response data of the control algorithm using different pure Figure7 and Table1 reproduce the response data of the control algorithm using di fferent pure resistive load values. These responses were from the motor speed and VFD output. I axis is a state- resistive load values. These responses were from the motor speed and VFD output. I is a state-space space model created in the workspace, and the time axis is an indicator to demonstrateaxis the sampling model created in the workspace, and the time axis is an indicator to demonstrate the sampling time time on which motor reaction was evaluated. More precisely, Table 1 shows the specific speed and on which motor reaction was evaluated. More precisely, Table1 shows the specific speed and active active power of the electric load separately for every graph. power of the electric load separately for every graph.

(a)

(b)

FigureFigure 7. 7.Received Received signals signals from from motor motor phase phase terminal terminal voltages voltages and and currents. currents.

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Energies 2020, 13, x FOR PEER REVIEW 10 of 15

Table 1. Supplementary details of Figure7. Table 1. Supplementary details of Figure 7. RPM Sample Counter Number Samples Active Power Frequency Figure7 RPM Sample Counter Number Samples Active Power Frequency Figure 7 12001200 126 unit 126 unit 245 DEC 245 DEC 653.736 653.736 W W 40.8163 40.8163 Hz Hz a a 15001500 120 unit 120 unit 197 DEC 197 DEC 485.672 485.672 W W 50.7614 50.7614 Hz Hz b b 18001800 118 unit 118 unit 169 DEC 169 DEC 270.041 270.041 W W 59.1716 59.1716 Hz Hz c c

FigureFigure 8 focuses8 focuses on onthe thetransient transient effect e ff ofect the of theload load variation variation on different on different parameters parameters of the of theAC AC motor.motor. In this In thisscenario, scenario, the theelectric electric load load increased increased one-step one-step from from 300 300 to to500 500 W W per per each each phase. phase. Figure Figure 8a 8a representsrepresents the the required required time time for for the the control control application application to toregulate regulate the the motor motor speed speed based based on the on the loadload variation. variation. Figure Figure 8b indicates8b indicates the theVFD VFD output output voltage voltage during during thethe load load fluctuation. fluctuation. The The results results showedshowed an appropriate an appropriate sine PWM sine PWMwaveform waveform for the forinduction the induction motor. The motor. fast electromagnetic The fast electromagnetic torque stabilizationtorque stabilization process appears process in appears Figure in 8c. Figure The8 c.VFD The using VFD usingDPC DPCand andpower power correction correction loops loops programmedprogrammed on the on themicrocontroller microcontroller achieved achieved this. this. Figure Figure 8d illustrates8d illustrates the thetotal total harmonic harmonic distortion distortion (THD)(THD) of the of theVFD VFD output output while while the themotor motor was was running running at 1500 at 1500 rpm. rpm.

Figure 8. Transient eﬀect of load variation on: (a) motor speed; (b) variable frequency drive (VFD) Figurephase-to-phase 8. Transient outputeffect of voltage; load variation (c) motor on: electric (a) motor torque; speed; and ((db)) currentvariable harmonic frequency distortion drive (VFD) rate. phase-to-phase output voltage; (c) motor electric torque; and (d) current harmonic distortion rate. 7. Implementation and Experiments 7. ImplementationThe control and algorithm Experiments operated on the FDRM-KL25Z signal processor board. The load power trackingThe control control algorithm strategy operated and DPC-PWM on the FDRM-KL2 were used5Z in thissignal application processor to board. achieve The a fastload and power reliable trackingresponse control of motor strategy speed. andThe DPC-PWM experimental were setupused appearsin this application in Figure9. to Table achieve2 gives a fast more and details reliable of the responsemotor of speed motor control speed. application. The experimental setup appears in Figure 9. Table 2 gives more details of the motor speed control application.

Energies 2020, 13, 2224 11 of 15 Energies 2020, 13, x FOR PEER REVIEW 11 of 15

Figure 9. Hardware implementation. Figure 9. Hardware implementation. Table 2. Parameters of variable speed induction motor. Table 2. Parameters of variable speed induction motor. Application Specification Quantity ApplicationSquirrel-Cage Specification Motor Quantity2 KW Squirrel-Cagepole Motor 2 4KW Statorpole Winding Star or4 Delta Torque 10.8 N m · StatorEffi ciencyWinding Star 80% or Delta MicrocontrollerTorque Card FRDM-KL25Z 10.8 N·m EfficiencyFrequency 48MHz 80% Sensor MMA8451Q Microcontroller Card FRDM-KL25Z Connectivity MCU IMCU I/O PowerFrequency Converter ABB ACS355-048MHz 3E SensorPN MMA8451Q 4 KW (5HP) ConnectivityU1 MCU3–400 IMCU V/ 480 V I/O I 14 A /6.4 A Power Converter1 ABB ACS355-0 3E f1 48–63 Hz VariablePN Resistance 4 KW2 KW (5HP) ResistanceU1 2403–400/120/60 V//60 480/30 ΩV Accuracy 5% I1 14 A /6.4 A Nominal Voltage 120 V—AC/DC Current/Voltagef1 Isolator 48–630.2 KW Hz MaximumVariable Continuous Resistance Current 12/ 5KW A Resistance 240/120/60/60/30 Ω As a variable speed diesel generatorAccuracy uses an AC motor to rotate5% the generator’s stator, we used a synchronous generator as a mechanicalNominal load Voltage for the induction 120 motor V—AC/DC instead of the rotating stator. The induction motor and the synchronousCurrent/Voltage generator Isolator were coupled.0.2 Consequently, KW the mechanical load applied on the AC motorMaximum shaft varied Continuous by changing Current the electrical 1/5 load A connected to the synchronous generator. The induction motor was equipped with rotor damper winding to limit induced voltage and increaseAs a variable motor speed torque. diesel The generator controlled uses system an AC supplies motor to 208 rotate V at 50the/60 generator’s Hz and operates stator, we at threeused dia synchronousfferent rpm steps. generator In this as experiment, a mechanical electrical load for loads the induction were manually motor changedinstead of to the see rotating the reaction stator. of diThefferent induction components motor and simultaneouslythe synchronous monitor generato ther motorwere coupled. behavior Consequently, and the control the algorithm. mechanical load applied on the AC motor shaft varied by changing the electrical load connected to the 8.synchronous Results and generator. Discussion The induction motor was equipped with rotor damper winding to limit induced voltage and increase motor torque. The controlled system supplies 208 V at 50/60 Hz and The results below indicated the response of a 2 kW induction motor during the electric load step operates at three different rpm steps. In this experiment, electrical loads were manually changed to variation. The three-phase Fluke multimeter analyzer measured the power quality of the VFD output see the reaction of different components and simultaneously monitor the motor behavior and the and motor frequency. In Figure 10, we present the results for two different load power levels. For the control algorithm. first step, the 482 W electric load per phase was applied to the synchronous generator. Based on the conditions defined on the controller, the AC motor speed increased to 1497 rpm. This behaviour proved 8. Results and Discussion

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The results below indicated the response of a 2 kW induction motor during the electric load step variation. The three-phase Fluke multimeter analyzer measured the power quality of the VFD output Energiesand motor2020, 13 ,frequency. 2224 In Figure 10, we present the results for two different load power levels. For12 ofthe 15 first step, the 482 W electric load per phase was applied to the synchronous generator. Based on the conditions defined on the controller, the AC motor speed increased to 1497 rpm. This behaviour that motor speed variation followed the control algorithm. While the experiment was running, the proved that motor speed variation followed the control algorithm. While the experiment was electric load was manually reduced to 295 W per phase. Consequently, the control program increased running, the electric load was manually reduced to 295 W per phase. Consequently, the control motor speed to 1798 rpm to meet the predetermined operating conditions. Therefore, during low load program increased motor speed to 1798 rpm to meet the predetermined operating conditions. values, the VSDG speed control was able to compensate for the necessary rpm while the diesel engine Therefore, during low load values, the VSDG speed control was able to compensate for the necessary slowed down. Figure 10 (a1, b1) represent the motor frequency response, as dictated by the control rpm while the diesel engine slowed down. Figure 10 (a1, b1) represent the motor frequency response, algorithm conditions. The green and blue graphs shown in Figure 10 (a2, b2) are voltage and current as dictated by the control algorithm conditions. The green and blue graphs shown in Figure 10 (a2, signalsb2) are coming voltage from and signalcurrent limiters signals apparatuses. coming from Thesignal pink limiters graph apparatuses. indicates a reference The pink signalgraph producedindicates bya the reference microcontroller signal produced itself. The by yellowthe microcontroller graph shows itself. the load The currentyellow usedgraph by shows the microprocessor the load current to calculateused by the the instantaneous microprocessor power. to calculate the instantaneous power.

(a1) (b1)

(a2) (b2)

FigureFigure 10. 10.Results Results for for the the 2 kW2 kW AC AC motor motor speed speed control control application application using using the the FDRM-KL25Z FDRM-KL25Z DSP DSP card; (a1card;,b1) motor(a1, b1 frequency) motor frequency response response to sudden to sudden increase increase/decrease/decrease of electric of electric load; ( a2load;,b2 )(a2 voltage, b2) voltage (green) and(green) current and (blue) current response (blue) toresponse sudden to increase sudden/decrease increase/decrease of the load of (reference the load (reference signal of microcontrollersignal of inmicrocontroller pink and load current in pink inand yellow). load current in yellow)

TableTable3 indicates 3 indicates the the CM CM and and VFD VFD behavior behavior during during electric electric load load switching switching processprocess betweenbetween the twotwo conditions conditions deﬁned defined in in the the control control algorithm algorithm implemented implemented inin aa microcontroller.microcontroller. The three-phase powerpower and and its its corresponding corresponding frequencyfrequency for different diﬀerent loads loads appear appear below. below. Precise Precise speed speed adjustment adjustment for forthe the CM CM was was achieved achieved using using load consumption tr trackingacking strategy. Power Power factors, factors, currents, currents, and and harmonicsharmonics were were received received from from the the VFD, VFD, and and load load powerpower waswas measuredmeasured separately from from the the load load panel. panel.

TableTable 3. 3.Measured Measured values values of of the the induction induction motor motor andand VFDVFD outputoutput voltage.voltage.

PhasePhase A A Phase BPhase B Phase C Phase C PowerPower (KW) (KW) 0.482 0.482 0.259 0.259 0.527 0.527 0.276 0.276 0.433 0.433 0.295 0.295 CurrentCurrent (A) (A) 77 5 5 9 9 66 77 4 4 Frequency (Hz) 49.24 59.96 49.24 59.96 49.24 59.96 Frequency (Hz) 49.24 59.96 49.24 59.96 49.24 59.96 Harmonic (%) 7.3 7.7 7.3 7.7 7.3 7.7 V (RMS) 119.37 121.34 119.3 121.7 119.4 122.2 Power Factor 0.69 0.61 0.63 0.38 0.41 0.37 Energies 2020, 13, 2224 13 of 15

9. Experimental Validation This section presents a comparison between the numerical and experimental simulations of the 2 kW induction motor speed control application. The numerical simulation used MATLAB/Simulink environment following the algorithm described in Section6. In Table4, we present the target speed values of the AC motor for each load and a comparison between the achieved values of the rpm using numerical and experimental approaches. The comparative analysis of load vs rpm showed reliable performance of the control algorithm. During the operation of the test bench, the motor speed increased and decreased several times according to predetermined load variation. Each time, a steady state condition was achieved within 1–3 s, which demonstrated a good synchronicity between the VFD and controlled program in producing electromagnetic torque. This response time showed that the AC motor speed adjusted rapidly in the case of a sudden load change. Finally, the fast and robust response of the AC motor to the load variation resulted in minimum electric disturbances at the VSDG output and motor shaft oscillations.

Table 4. Validation results.

Speed (rpm) 1000 1200 1500 1800 Load Power Per Phase (W) 1000 600 400 200 Numerical (rpm) 971 1182 1482 1792 Experiment (rpm) 955 1167 1497 1798 Error % 1.64 1.26 1.01 0.33

10. Conclusions One of the solutions to increase electricity production efficiency using gensets is to use VSDGs. A recently developed technology uses a rotating stator as a solution to allow the diesel engine to slow down and operate at better efficiency with reduced loads. An AC compensator motor drives the generator’s stator and adjusts its speed according to the VSDG electric load such that the stator speed increases when the VSDG electric load decreases. In this paper, we developed a control algorithm for the AC compensator motor that follows the load variation. The numerical and experimental results demonstrated an efficient and robust control algorithm. The main advantages for introducing this control approach with the rotating stator VSDG technology are the increased efficiency operation and reduced fuel consumption and GHG emissions of the diesel engine. The fast time response of the AC motor ensures current quality without the use of complex power electronics.

Author Contributions: Writing the paper, M.M.; experimental design and setup, B.T.; refining the manuscript, M.R. and A.I. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Conflicts of Interest: The authors declare no conflict of interest.

Abbreviations The following abbreviations are used in this manuscript:

VSDG Variable Speed Diesel Generator DG Diesel Generator DE Diesel Engine AC Alternative Current GHG Greenhouse Gases CVT Continuous Variable Transmission PID Proportional Integral and Derivative ED Electric Drive CM Compensatory Motor SG Synchronous Generator Energies 2020, 13, 2224 14 of 15

DPC Direct Power Control VSI Voltage Source Inverter SVM Space Vector Modulation DQ Direct Quadrature VSC Voltage Source Converter RC Resistor/Capacitor PWM Pulse Width Modulation IGBT Insulated-Gate Bipolar Transistor VFD Variable Frequency Drive ADC Analog to Digital Converter THD Total Harmonic Distortion RPM Revolution Per Minute

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