A Variable-Speed Sensorless Drive System for
Contents
1. October 1997 Interface Between Data Terminal Equipment and Data Circuit Terminating Equipment Employing Serial Binary Data Interchange A Variable Speed Sensorless Drive System for Switched Reluctance Motors 5 VB TEXAS SPRA600 INSTRUMENTS 1 2 2 Digital Motor Controller The board is designed and manufactured by Spectrum Digital Incorporated and utilizes the Texas Instruments TMS320F243 DSP This controller board has an ac to dc converter that generates a full wave rectified and filtered 162 volt dc from an ac supply input of 115 vac at 50 60 Hz With placement of on board jumpers the board can be configured in a voltage doubler mode to generate 320 volts dc if the intended motor application requires the higher voltage It also has a three phase power inverter powered from the 162 volt dc bus This power inverter can be configured with proper placement of jumpers to drive typical three phase ac induction motors three phase brushless dc motors or three phase SRMs In the SRM configuration the power driver uses the popular and standard two switches per phase topology as shown in Figure 2 Vbus Phase Winding Figure 2 SRM Power Driver Topology Current sensing resistors are included in each low side power driver leg with variable gain buffer amplifiers to output current feedback samples in each phase winding of the motor In this example application of the SRM drive system the gain of these buffer amplifiers has been set to giv2. SRM Magnetization Curves 0 cece cee eee eee eae 31 Figure 18 Analog Flux Measurement Circuit 2 2 20cse eaneeesaseyee cy censdae Coe gue p led geese 32 Figure 19 Flux Linkage Curves for Demo Platform SRM 00 c eee eee eee eee 33 List of Tables Table 1 Performance Parameters of the Demo SRM Drive System 0 0 cece ee eee 4 Table 2 Switched Reluctance Motor Characteristics 000 cece tee 5 Table 3 Description Of COMMANGS sstessrerspness egm iar aea EEE E E RRR E 9 Table 4 Sensorless SRM Performance Summary 0000 00 eee ee eee eee teens 10 Table 5 Execution Times of Foreground Activity 0 00 eects 13 Table 6 Serial Communications Module Commands 00 cee eee eee eee eee 26 A Variable Speed Sensorless Drive System for Switched Reluctance Motors 3 k TEXAS SPRA600 INSTRUMENTS 1 1 1 1 2 4 Hardware to Demonstrate Sensorless Control of the Switched Reluctance Motor Using the Texas Instruments TMS320F243 DSP This section describes the hardware used to demonstrate the sensorless control of a typical switched reluctance motor SRM drive using the Texas Instruments TMS320F243 digital signal processor DSP Research completed in March of 1998 and documented in the Application Report titled Developing an SRM Drive System Using the TMS320F 240 literature number SPRA420 led to a baseline software algorithm for conventional operation of the
3. the SRM drive system in this demonstration hardware Key features of the TMS320F243 EVM are e 544 words of on chip data memory e 28K words of onboard memory e on chip FLASH memory e on chip UART e MP7680 four channel digital to analog converter e 5 volt only operation For additional information see the Technical Reference on this TMS320F243 EVM published by Spectrum Digital Incorporated in 1998 To operate the demonstration as described in the operational procedures section the sensorless control software must be embedded in the TMS320F243 DSP For details on embedding the software in FLASH please refer to the TMS320F20x F24x DSP Embedded Flash Memory Technical Reference literature number SPRU282 Magtrol Dynamometer The dynamometer used to control load torque on the SRM in this demonstration hardware platform is manufactured by Magtrol Incorporated It is a load cell dynamometer model 705 6 that features a hysteresis brake for precise torque loading up to a maximum of 50 0 in lb of torque and has a maximum speed capability of 10 000 RPM Power rating for the model 705 6 dynamometer is 300 watts continuous and 1400 watts for less than five minutes To achieve the best accuracy capability of 0 25 attention must be given to the calibration of the unit for zero offset and scale factor as specified in the user s manual The motor shaft is attached to the dynamometer through a precision aligned flexible coupling that has been cu
4. B Ongr 8 where is a number close to 1 0 before it is passed on the velocity loop This additional filtering deliberately reduces the velocity loop bandwidth to prevent the velocity loop from acting on noisy estimates This filter block also improves operation at high speeds where the instantaneous velocity can vary significantly from estimate to estimate due to the small number of samples in a commutation period By providing the velocity loop with a velocity measurement averaged over many commutation periods speed oscillations are avoided A Variable Speed Sensorless Drive System for Switched Reluctance Motors Wy TEXAS INSTRUMENTS SPRA600 2 2 6 Stall Detector If the instantaneous shaft velocity drops below 60 RPM after the motor startup has completed a stall detector in the decision logic will engage After the logic detects a stalled condition it calls a subroutine which immediately cuts off the motor This routine sets all PWM generator duty cycles to zero switches off the lowside power transistors zeroes out all of the desired currents illuminates all LEDs on the EVM board and goes into an infinite loop A processor reset is required to restart the system This safety feature prevents the motor from overheating if the shaft suddenly becomes locked in place 2 2 7 Low Speed Operating Mode lt 400 RPM As the shaft speed is lowered velocity updates arrive with decreasing frequency and the bandwidth of the velocity loo
5. SRM drive using a shaft position sensor Follow on research completed in August of 1999 extended the performance range of a specific SRM without a shaft position sensor using a sensorless control algorithm Detailed information on the sensorless software control algorithm can be found in Section 2 Hardware used in this follow on research was the 3 phase 12 8 stator pole configured SRM manufactured by Emerson Electric Company and the digital motor controller board designed and manufactured by Spectrum Digital Incorporated www spectrumdigital com which utilizes the Texas Instruments TMS320F243 DSP The hardware and software algorithm as described are intended to be used primarily for potential customer demonstrations and to serve as examples of extended performance sensorless control of SRM drive systems Demonstration Goals The basic goals of this research work were to build upon the baseline software algorithm as described in the application report referenced above and to extend the performance range of the SRM drive using sensorless control Performance parameters listed in Table 1 of the SRM drive were set to cover a wide range of potential customer applications such as white goods washing machine compressor pumps and blower fan applications Table 1 Performance Parameters of the Demo SRM Drive System Speed range 150 to 4500 rpm Load torque no load to 48 oz in Speed regulation 10 over full speed torque range In addition to th
6. below the current command limit Otherwise the assigned duty cycle is the saturation limit This saturation limit is set to 50 for start up but is relaxed to 90 after the motor reaches its steady state operating speed A 100 duty cycle is never permitted because the drivers need time to refresh For negative errors the duty cycle is set to zero this soft chops the channel and the current decays until the error once again becomes positive Ramp Controller The ramp controller receives speed and direction commands from the serial comms module and applies the necessary sequence of commands to the commutation controller velocity loop and current loop to achieve the new target speed and direction It runs in the background as a software state machine After the motor has settled to a new target speed and direction the ramp controller enters its wait state In this state it continually checks with the serial command processor to see if a new command has arrived across the RS 232 link If a new target speed arrives the ramp controller shifts into its ramp state as shown in the state transition diagram of Figure 14 Depending upon whether the new target speed is above or below the current operating speed the ramp controller will either increment or decrement the current command speed by 1 RPM As the command speed is increased or decreased the conduction angle is adjusted for efficient torque production and the current command limit is also ad
7. flux linkage exceeds the switching or reference flux To be precise the motor is commutated when A gt agha 1 where is an estimate of flux in the active phase winding a is a scalar between 0 and 1 which is analogous to a conduction angle and 2 is the flux at the aligned rotor position This commutation condition is shown graphically in Figure 4 As indicated by condition 1 properly timed commutation depends on an accurate estimate of the flux in the active phase winding a knowledge of the magnetization curve at the aligned rotor position and a carefully selected firing angle A Variable Speed Sensorless Drive System for Switched Reluctance Motors 13 4 TEXAS SPRA600 INSTRUMENTS 2 2 1 Commutation Reference Pa Flux aa t Figure 4 Graph of Flux Estimate and Flux Threshold vs Time Flux Estimator As shown in the software block diagram of Figure 3 the quantity 4 is generated by the flux estimator block which calculates flux based on the pulse width modulation PWM duty cycle and the current in the active phase winding Its principle of operation is the same as a classical flux estimator which uses the update law n Ant Vn In Sw S 2 VEmMF to integrate the back EMF in the active phase winding In Equation 2 vp is the motor terminal voltage is the coil current and r is the winding resistance However unlike this classical flux estimator which requires terminal voltage and
8. flux measuring circuit used to obtain the aligned magnetization curve to a high degree of accuracy With this procedure users can adapt the sensorless algorithm for SRMs with different electrical properties Stator Flux Estimation Besides a knowledge of the magnetization curves at the aligned rotor position an accurate flux estimate must be developed by the motor control software to successfully commutate the motor To estimate the flux linkage the back EMF of the active phase winding is integrated using the update law Anti Ant Vn in tw 9 Veur In Equation 9 vp is the motor terminal voltage is the coil current and fy is winding resistance To reduce the cost of the motor drive system the terminal voltage is not measured explicitly Instead it is approximated using the formula Vn Vous In Virans n Vaiode n 10 where Vpys is the bus voltage dp the duty cycle Virans the voltage drop across the power transistor and Vgiode the diode voltage drop Notice that Equation 10 assumes that the bus voltage is a stiff source and that the v curves of the power devices are known Substituting Equation 10 into the original formula the new update law becomes Anat An F Vous dn Virans in Vdiode in in Tw 11 SSS Fh Ver For simplicity the drops across the power transistor diode and winding resistor are combined into a single term called the loss voltage permitting Equation 1
9. give reasonable values within the dynamic range of the circuitry for the SRM used in this demonstration platform Other SRMs may require different scaling to give correct results of flux measurement Demo SRM Flux Measurements Magnetization flux data was collected on the SRM used in the demonstration platform from low current levels to a maximum current of 4 amps Figure 19 shows this flux linkage data over this current range at the aligned rotor position Note that the curve is very linear to the maximum current level of 4 amps indicating no magnetic saturation at these current levels Also the slope of the curve as measured is the inductance of the SRM at the aligned position and indicates an inductance of about 52 mH Flux linkage data from measurements of all three phases of the SRM are shown and indicate excellent balance between the phase windings for this particular SRM A Variable Speed Sensorless Drive System for Switched Reluctance Motors 4i TEXAS INSTRUMENTS SPRA600 Magnetization Curves at Aligned Rotor Position for Phases A B C 0 25 Phase A 0 2 Phase B Phase C T 0 15 S x 0 1 5 u 0 05 oE 0 05 Current A Figure 19 Flux Linkage Curves for Demo Platform SRM 3 5 3 Look up Table Generation Method To generate a look up table from the raw magnetization data a polynomial curve fit is performed on the data For the motor used in the demostration the magnetization c
10. 1 to be written as n 1 4n Vpus An Voss n where Voss n Virans n Vaiode in intw 12 VEmF In Equation 12 the update is performed in the software with two additions a scalar multiplication and a single look up operation A Variable Speed Sensorless Drive System for Switched Reluctance Motors 27 k TEXAS SPRA600 INSTRUMENTS 3 2 28 Measuring the Voltage Loss Function To implement Equation 12 the loss voltage is measured as a function of current anda look up table is constructed This is accomplished by applying a fixed duty cycle PWM waveform to the motor winding After the transients die down the flux on the left and right hand side of Equation 12 will be equal and can be cancelled out leaving the steady state equation Vioss in Vous An 13 A simple program whose flow diagram is shown in Figure 15 can be written on the F243 automate data collection for the look up table construction The program begins by setting up the PWM generator and clearing the index counter At the start of the program loop the compare register is assigned the index value which generates a fixed PWM duty cycle corresponding to d N Nmax where Nmax is the counter value that corresponds to a 100 duty cycle For a F243 running at 20 0 MHz with a carrier frequency of 20 0 kHz Nm ax 1000 After applying the fixed duty cycle the program waits for several seconds until the current reaches
11. Driver Topology 3 ccecidi nie nti pune veered eres aaeeea 6 Figure 3 Software Block and Timing Diagrams 0c ee cee 12 Figure 4 Graph of Flux Estimate and Flux Threshold vs Time 000 eee e eee ee 14 Figure 5 SRM Power Driver Topology 00 c cece eee eee eee eens 15 Figure6 Flux Estimator Code 12000 coeduanswseene saneseseesuseseueseeaeseesaeeteseee aus 16 Figure 7 Flux Estimate and Threshold at 2500 RPM Under Full Load 000200 0 17 Figure 8 Illustration of Sensorless Commutation Algorithm With Lockout Window 18 Figure 9 Low Speed Operating Mode With a Second Flux Threshold for an Additional Velocity Update 00 ccc eens 19 Figure 10 Commutation and Velocity Update Flow Diagram 00 cece eee eee eee 21 Figure 11 Block Diagram of the Velocity Control Loop 0 0 cece eee eee eee 22 Figure 12 Instantaneous Velocity Estimate and Velocity Loop Output During Start up 23 Figure 13 Block Diagram of the Current Control Loop 00 cece eee eens 23 Figure 14 State Transition Diagram for the Ramp Controller 0 00 c cece eee eee eee 25 Figure 15 Flow Chart for Voltage Loss Measurement Program 0000 ce eee eee eee 28 Figure 16 Voltage Loss vs Current for the Emerson Electric SRM and the Spectrum Digital Motor Control Board ciccusenksexeeetsenea ested oiaeadewdaed te amp 30 Figure 17
12. ETTLE agit_cent 0 settle_cnt reset settle_cnt Real time delay settle_cnt 0 Figure 14 State Transition Diagram for the Ramp Controller If the controller receives an agitation command it resets its agitation counter before moving into the agitation brake state Once there a braking routine is called disables the velocity loop and commutation controller To passively brake the motor phase 2 of 2 is energized with a constant current command of 3 0 A The software delays four seconds while the motor brakes and the shaft aligns Next the phase is de energized the motor data structure various counters and flags are re initialized and a new target speed of 1000 RPM is established and the commutation direction is reversed To restart the motor phase 0 or 1 is energized depending on the direction of rotation the A D MUX is switched to the appropriate channel and the commutation and velocity loops are re enabled Once the motor has started in the opposite direction the agitation counter is decremented the settle counter is reset and the ramp controller enters the agitation settle state It remains in this state for about 20 seconds which allows the motor to settle to its 1000 RPM target speed After the settle counter expires the processor repeats until the agitation counter reaches zero at which point the controller transitions to the settle state and eventually to the wait state ready to accept a new serial com
13. NTS 2 2 8 20 Commutation and Velocity Update Algorithm Summary The flowchart in Figure 10 summarizes the entire commutation and velocity update algorithm After the algorithm commutates the motor it resets the flux estimator the lockout sample counter and the second velocity counter Every sample period the ISR calls the sensorless commutation routine which increments both velocity counters If the lockout counter is non zero it is decremented just before exiting the subroutine However if the lockout counter has expired a commutation can occur and further tests are performed If low speed mode is enabled lt 400 RPM and the first velocity update has not occurred the flux estimate is compared against the first threshold If the estimate has crossed the threshold a new instantaneous velocity is calculated using the value in the first update counter and a flag is set to indicate that the first update has occurred This flag is important because if it is not used then the velocity update will occur every time the ISR is invoked until the commutation cycle ends each time with a velocity estimate of 12 500 RPM Following the first threshold crossing the algorithm will check the flux estimate against the second flux threshold When the second threshold is crossed the velocity is updated using the second counter and the motor is commutated When the motor is commutated the current request for the active winding is zeroed and the so
14. Ureae Reeves 4 1 2 1 Switched Reluctance Motor Characteristics 000 c cece cece eee eee 5 1 2 2 Digital Motor Controller s 5ic0ce0sccaceebes esos Leeiee edhe eee Hee we Ewe Bes 6 1 2 3 TMS320F243 Evaluation MoOduIGs 251602 os085h40208 Babee die de ei ERe Ene PEs 7 1 2 4 Magtrol Dynamometer isoc s ceed opnar eneren deavede Meee ree meee pia e eens 7 1 2 5 Dynamometer COnmtrollSr vcwisecungyedewneeesugesannaud eo anaes eoapecuaened 8 1 3 Operational Procedures siris ncnept cent ka rne EENE Ean EE EE E Lee eaew edie nes 8 1 4 Sensorless SRM Peronmance s va tdweeees ci deh 50 seue ds eee ee ia beede ia haee eats 10 2 Control Software for a Sensorless SRM Drive System 0 cee eee eee eee 11 2 1 Overview of the Sensorless SRM Control Software 0 0 c cece eee 11 2 2 Sensorless Commutation and Velocity Update Algorithm 0 0c eee eee 13 22 1 FIUX Estimator sien tends Renee a ee Oh eat eed hee eda E e Ge ke 14 22 2 Flux Estimator Implementation oc oiceockectberene tes vane tees Marendeun rece re 15 22 9 Flux Reterence Generale acs icecweawedewesasduswdeedxe as see ene eon ERE S 16 228 LOCKOULWINOOW 4 22600 it erence se vedio E daake EERO E EENES 16 2 2 9 Velocity Estimation 34 5 0 kin cous bkvdeed cose e u kee ds 8 bhned hae deed daw lee eee Fe 18 22 0 Stall DOGG pcxecis ortine rebated beret eo eer Siee rer Sew ho ENER EEEE xe 19 2 2 7 Low Speed Operating Mode lt 400 RPM 0 0 0 cee eee e
15. XAS INSTRUMENTS SPRA600 Table 3 Description of Commands Commandt Description gt t Turn on drive system gt SXXXX Set new target speed in RPM 0150 lt xxxx lt 4500 gt b Brake motor and reverse direction gt a Agitate the motor gt C Cut off the drive system t Each command must be followed by a carriage return These commands can set new target speeds reverse the motor direction agitate or cut off the motor If the speed command gt s followed by a four digit number between 0150 and 4500 is issued the motor will ramp up or down at 100 RPM sec to the new requested target speed provided it is between 0150 and 4500 RPM If a speed is requested above the top speed of 4500 RPM the new target speed will be set to 4500 RPM Likewise if the requested target speed is below 150 RPM the new target will be set to 150 RPM After the ramp is completed the software will wait for a settling period of several seconds and then wait to receive the next speed command If the next command is a brake command gt b the software will apply passive braking which is accomplished by injecting a constant current into phase 2 After the motor slows down and aligns with phase 2 it will start up in the opposite direction spool up to the initial target speed of 1000 RPM and then continue ramping up or down to its former speed The agitate command is quite similar to the brake command When it is issued the motor simulates the agitatio
16. a steady state value When this happens the current voltage pair is recorded in an array Next the index is advanced and if the new index value is less than ngjna the process is repeated Figure 15 Flow Chart for Voltage Loss Measurement Program A Variable Speed Sensorless Drive System for Switched Reluctance Motors Ww TEXA 3 3 3 4 S INSTRUMENTS SPRA600 To avoid blowing fuses that are located on the digital motor controller board or damaging components Nfinal must be carefully chosen To calculate an approximate value for Nfinal calculate the loss voltage at the maximum current using the formula Vioss imax Viiode imax Virans imax imax Tw 14 From this calculate the duty cycle needed to generate this loss voltage Viggg max Minal Vv N max 15 bus Using an imax 4 0 A a diode drop of 0 70 V and a voltage drop of 1 1 V across the IGBT a winding resistance of 2 5 ohms a typical bus voltage of 170 0 V and an Nmax 1000 Nf inal 70 Generating a Voltage Loss Look up Table After the voltage current data is recorded in the array it is exported via the XDS510 to the PC the data is fitted with a polynomial curve with voltage as a function of current To simplify calculation of the table index the function is evaluated at the current points AEE 5 Y n 0 255 16 Using these points an index into the 256 point look up table can be rapidly ca
17. and which must be issued to initiate the motor start up sequence Commands which are issued at other times such as when the ramp controller is ramping the motor to a new target speed are ignored When the command processor is called it checks to see if a character is waiting in the SCI receive buffer If a character has arrived that character is processed otherwise the routine promptly exits and returns control to the ramp controller The action which is taken depends on the newly received character and the current state of the command processor In its default lead in state the command processor waits for a lead in character Once a lead in character is received the processor transitions to the command state In this state it waits for a valid command character If the turn on command t is received the command processor waits for a carriage return and then sets the turn on flag This begins the motor startup sequence If the speed command character s is received the processor waits for four digits converts these four characters to a number and then waits for a carriage return At this point the processor checks to see if the target speed is within bounds If not the processor limits the upper speed to 4500 RPM and the lower speed to 150 RPM Next the processor notifies the ramp controller that a new target speed has arrived by setting a flag and then returns to the lead in state If the agitation command a is re
18. c has been omitted from the integration A Variable Speed Sensorless Drive System for Switched Reluctance Motors 15 4 TEXAS SPRA600 INSTRUMENTS 2 2 3 2 2 4 void update_flux_estimate anSRM_struct anSRM int phase long templ temp2 long dflux phase anSRM gt Active update flux linkage estimate Kf 1 templ VBUS anSRM gt dutyRatio phase long_table_read VoltTable anSRM gt iFB phase gt gt 2 amp temp2 dflux templ temp2 anSRM gt fluxEstimate phase anSRM gt fluxEstimate phase dflux if anSRM gt fluxEstimate phase lt 0 anSRM gt fluxEstimate phase 0 Figure 6 Flux Estimator Code Flux Reference Generator To check the commutation condition the decision logic must compare the flux estimate against a reference flux 4 which is the product of a conduction angle a and the flux at the aligned rotor position 4 The conduction angle a is established by the ramp controller which varies a as a function of operating speed to maximize the motor s efficiency The quantity 24 is returned by the subroutine get_alignedFlux This routine looks up what the flux would be at the aligned rotor position given the current i and the active phase winding As in Section 2 2 2 the long_table_read subroutine must be invoked to retrieve the desired table element from FLASH memory Lockout Window The decision logic which has bee
19. ceived the agitation counter is reset the ramp controller is placed in the agitation brake state starting the agitation motion sequence and the processor returns to the lead in state If the reverse command b is received the processor goes through passive breaking and alignment starts the motor in the opposite direction places the ramp controller in the ramp state to bring the motor up to its previous speed This is the same sequence as a single agitation If the cut off command c is received the processor disables all interrupts cuts off the motor in the same manner as the stall detector and returns to the lead in state A Variable Speed Sensorless Drive System for Switched Reluctance Motors Ww TEXA 3 3 1 S INSTRUMENTS SPRA600 Calibration for a Sensorless SRM Drive System The sensorless algorithm presented in Section 2 depends on an accurate estimation of flux in the active phase winding To enhance estimation accuracy this algorithm includes the voltage drops across the power devices which must be obtained as a function of current The first part of Section 3 describes a technique for measuring the combined voltage drop across the power devices and the winding resistance as a function of current using the existing demonstration hardware Besides an accurate flux estimate the algorithm requires a knowledge of the magnetization curve at the aligned rotor position The second part of Section 3 documents an analog
20. coil current measurements this modified estimator only relies on current measurements Instead of measuring the terminal voltage it is approximated using the formula Vn Vous dn Virans n Voiode n 3 which takes into account the voltage drops across the active devices in the power inverter whose topology is shown in Figure 5 In Equation 3 Vbus is the bus voltage dp the duty cycle Vtrans the voltage drop across the power transistor and Vgioge the diode voltage drop Notice that Equation 3 assumes that the bus voltage is a stiff source and that the v i curves of the power devices are known Substituting Equation 3 into the original formula the new update law becomes Ant n t Vous An Virans in Vaiode in In Tw 4 Veur For simplicity the drops across the power transistor diode and winding resistor are combined into a single term called the loss voltage permitting Equation 4 to be written as Ansa Ant Vous In Yioss in where Voss in Virans n Vaiode n intw 5 Ver A Variable Speed Sensorless Drive System for Switched Reluctance Motors ki TEXAS INSTRUMENTS SPRA600 With the loss voltage tabulated as a function of current the update is performed in the software with two additions a scalar multiplication and a single look up operation For more information on how to construct the voltage loss table please refer to Section 3 Vbus Phase Wind
21. d Sensorless Drive System for Switched Reluctance Motors Ww TEXA 2 2 S INSTRUMENTS SPRA600 The execution time of each foreground activity shown in the software block diagram is listed in Table 5 Table 5 Execution Times of Foreground Activity Activity Execution Time usec Flux Estimator 8 3 Sensorless Commutation 15 5 to 34 8 Current Control Loop 10 6 Velocity Control Loop 11 3 Note that execution time of the sensorless commutation algorithm varies If the motor needs to switch phases and perform a velocity update the run time will be 34 8 usec otherwise the run time will be 15 5 usec Overall execution time of the ISR varies from 34 4 usec to 53 2 usec depending on which activities must execute On average the interrupt service routine utilizes 55 of the processing time leaving the remaining 45 for the background loop which includes a serial command processor and ramp speed controller Sensorless Commutation and Velocity Update Algorithm At the core of the sensorless algorithm is the commutation controller which contains a flux estimator flux reference generator and decision logic Ideally the decision logic should commutate the motor when the rotor and stator poles are nearly in alignment For this particular algorithm 1 the flux in the active phase winding is compared with a reference flux which is a scaled version of the flux at the aligned pole position The decision logic commutates the motor when the
22. duced to less than 4K by replacing the look up tables used by the commutation algorithm with polynomial interpolating functions As shown in Figure 3 the software is composed of five key modules an algorithm for sensorless commutation an outer loop for velocity control an inner loop for current control a serial command processor and a ramp controller The first three of these modules execute in the foreground They are called from a timer interrupt service routine ISR which is fired every 66 7 usec 15 0 kHz by a free running onboard timer As shown in the timing diagram of Figure 3 the commutation controller and current control loop execute every interrupt cycle at 15 0 kHz while the velocity loop only executes every sixth interrupt cycle at 2 5 kHz XDS510PP is a trademark of Texas Instruments Incorporated A Variable Speed Sensorless Drive System for Switched Reluctance Motors 11 SPRA600 Foreground Activities Flux Estimator Decision Logic and Velocity Estimate Align Flux Calculation Serial Command Processor Background Activities 15 0 kHz Foreground Sensorless Commutation active Velocity Control 2 50 kHz Current Control WB TEXAS INSTRUMENTS PWM PWM PWM3 Background Legend Current Loop EE Commutation Control ES Velocity Loop Figure 3 Software Block and Timing Diagrams GE Ramp Control GE Serial Command Processor 12 A Variable Spee
23. ductor switch Q1 controls the current through the SRM phase winding and is driven by a pulse generator at the gate drive input with a pulse width that is adjusted to allow the current to reach a steady state level For the SRM used in this demonstration platform with an electrical time constant of 20 msec the pulse width of the gate drive should be set to approximately 100 msec A Variable Speed Sensorless Drive System for Switched Reluctance Motors 31 3 5 2 32 k TEXAS SPRA600 INSTRUMENTS VBUS 2N4858 winding ris AN 10k set amp gain at 4 00 5 62k Output Scale 1 volt 404 set amp gain at 5 62 with motor winding R 2 5 ohms gate drive 0 15 V IRF740 Q1 36k set amp gain at 4 14 Figure 18 Analog Flux Measurement Circuit The voltage level at Vpyg should be adjusted for the maximum current desired for the flux measurement Due to limitations in the measurement circuitry the Vpys should never be set greater than about 40 volts dc Differential instrumentation op amps U1 and U2 measure the voltage across the phase winding and the current flow through the winding with a current sensing resistor of 1 ohm The summation amplifier U3A subtracts the in drop from the voltage across the winding to v irw form and the result is then integrated in the reset integrator U3B to form the flux measurement 1 Gains shown in the schematic have been set to
24. e a current sensing scale factor of 1 0 amp volt On board low voltage power supplies of 5 volts dc and 15 volts dc are also included so that this board can operate independently from 115 volts ac at 50 60 Hz Other features of this controller board are current sensing resistors on the bus to enable power factor correction capability This feature is not used in this example application Some minor modifications to the board have been made for this example application and include removal of the R5 0 03 ohm bus current sensing resistor Current sensing resistors R2 R3 and R4 have also been changed from 0 04 ohm to 0 2 ohm to adjust the current feedback scale factor and reduce electrical noise sensitivity A serial communications interface RS 232 port is also provided on the board and is used in this example application to introduce input set speed commands so that the motor speed can be changed on the fly while the motor is running 6 A Variable Speed Sensorless Drive System for Switched Reluctance Motors 4i TEXAS INSTRUMENTS SPRA600 1 2 3 1 2 4 TMS320F243 Evaluation Module This evaluation module EVM based upon the Texas Instruments TMS320F 243 digital signal processor is designed and manufactured by Spectrum Digital Incorporated It is an excellent platform to develop and run software on the F24x family of processors and was used extensively in the development and testing of the software algorithm for the sensorless control of
25. ee 19 2 2 8 Commutation and Velocity Update Algorithm Summary 00200 ee eee 20 2 3 Velocity LOOP eerste imeen n R E th coy ne teu deve led cere creed ends abesuaex 22 23 1 Motor Start up Under Load 2 3 cceies cree tee sreee iis vane esate lend ear ekews 22 2 4 Current Control LOOP esiri piera e ew ipnemee ET EO B E EOE canes pes eqere eh eqepeemeeaas 23 2 5 Ramp Controler cccecns tusencoaeanen a E A E E E E EEEN 24 26 Seral COMMS ereire areia EAD E a a a aE erha ET 26 3 Calibration for a Sensorless SRM Drive System sssssnnnnnnsnnnunnnnnnrnnnsnno 27 31 Stator Fl x SUMAN 6 2 6 6 inde ik risiste akidar ia E DE EE Wie e a a aa EER 27 3 2 Measuring the Voltage Loss Function sssssaeeeaaae eee eee ees 28 3 3 Generating a Voltage Loss Look up Table 0 ccc cece cent eee ees 29 3 4 Measuring the Voltage Loss Data eee 29 3 5 Flux Measurement Method 000 c cette ent eee ae 30 3 5 1 Flux Measurement Hardware 000 c cece cc cence eee e eens 31 3 5 2 Demo SRM Flux Measurements 0 0000ce see reese eee eee needa es 32 3 5 3 Look up Table Generation Method 00 cece cee ranr 33 FRETOFENICOS a oorner E E E E EE EN 34 2 A Variable Speed Sensorless Drive System for Switched Reluctance Motors Ww TEXA INSTRUMENTS SPRA600 List of Figures Figure 1 Interconnection Diagram for SRM Demo Hardware 05 c cece cece eens 5 Figure 2 SRM Power
26. ese basic requirements other goals were to successfully start the SRM from standstill under a full load torque of 48 oz in Hardware Description Hardware used for this demonstration is described in the following sections A diagram of the interconnections between the various hardware elements that make up this SRM demonstration platform is shown in Figure 1 A Variable Speed Sensorless Drive System for Switched Reluctance Motors Ww TEXA sS INSTRUMENTS SPRA600 115 vac n 60 Hz 3 115 vac X v Magtrol Model 5240 Dynamometer Computer Dual Power Controller Monitor Supply RS 232 24 vdc 45 vde SRM Ribbon Spectrum Digital Inc Cable Digital Motor Controller N 9 115 vac 60 Hz Flex Magtrol Coupler Model HD 705 6 x Dynamometer 50 in lb Spectrum Digital Inc TMS320F243 EVM Figure 1 Interconnection Diagram for SRM Demo Hardware 1 2 1 Switched Reluctance Motor Characteristics The characteristics of the switched reluctance motor are shown in Table 2 Table 2 Switched Reluctance Motor Characteristics Number of phases 3 Number of stator poles 12 Number of rotor poles 8 Phase resistance 2 5 ohms Aligned inductance 52 mH Unaligned inductance 9 5 mH Phase current max 4 amps Manufactured by Emerson Electric Company for the Maytag Neptune Auto Washer Maytag and Neptune are trademarks of Maytag Corporation Tt TIA EIA Standard 232 F
27. ftware advances to the next active phase based on the direction of rotation Next the velocity loop s output is assigned to the current request for the next active winding While this assignment will be made the next time the velocity loop is executed it may take up to six sample periods for this to happen While a variable delay of one to six sample periods is acceptable at low operating speeds this causes problems at high speeds where there is a much smaller number of samples in a commutation cycle Finally the A D MUX is switched to the next channel and the lowside insulated gate bipolar transistor IGBT is turned on for the next phase winding and the lowside IGBT in the previous phase is turned off A Variable Speed Sensorless Drive System for Switched Reluctance Motors 4i TEXAS INSTRUMENTS SPRA600 Wait 1 Sample First update counter Second update counter Lockout counter 0 Lockout counter First update flag 0 Reset flux estimator Reset lockout counter Reset first update flag Update velocity using second update counter Reset second counter Commutate motor Update velocity using first update counter Reset first update counter Set first update flag Figure 10 Commutation and Velocity Update Flow Diagram A Variable Speed Sensorless Drive System for Switched Reluctance Motors 21 k TEXAS SPRA600 INSTRUMENTS 2 3 2 3 1 22 Velocity Loop The velocity loop w
28. hich executes at a frequency of 2 50 kHz employs a discretized proportional integral PI control law to control motor shaft speed The proportional term in the control law damps out speed oscillations and the integral term drives the DC speed errors to zero As shown in the block diagram of Figure 11 for safety reasons a windup limit of 6 25 A in the positive direction and the same in the negative direction is imposed on the integrator In addition an upper and a lower current command limit are placed on the output of the velocity loop The upper limit is imposed for safety reasons and varies with operating speed the lower positive limit ensures that there is sufficient amount of current to make commutation decisions Figure 11 Block Diagram of the Velocity Control Loop Motor Start up Under Load When the motor is started under load the software counters that gauge the time between commutations can roll over corrupting the speed estimates and causing the velocity loop to command an improper amount of current When this happens the motor may experience start up hesitation may start up in the wrong direction or may even stall altogether To solve this problem the integrator in the velocity loop is given a large initial value which exceeds the current command limit This causes the initial velocity estimates to be neglected and a large amount of torque to be applied to the motor In effect the velocity controller runs open loop When the
29. ing Figure 5 SRM Power Driver Topology 2 2 2 Flux Estimator Implementation The subroutine update_flux_estimate updates the flux estimate based on the duty cycle applied to the PWM generator and the current measured in the active phase winding As shown in Figure 6 the code implements Equation 5 in a straightforward manner with the exception of the look up operation To perform the look up operation an index into the 256 point table is generated by shifting a 10 bit unsigned integer two places to the right The address of the desired table element is formed by adding this index to the table s base address and the table is accessed by invoking the assembly language subroutine 1ong_table_read This subroutine reads the desired table element from FLASH program memory using a table read operation TLBR and returns the value in a desired data memory address After completing the table read both terms in Equation 5 are added to the previous flux estimate to form the new one and the update is stored in the SRM data structure This update is a scaled version of actual flux linkage in the winding To convert it to physical units of V sec multiply by the following scale factor x anSRM gt fluxEstimate 6 1000 x 15000 In Equation 6 the factor of 1000 reflects the fact that the actual volts have been scaled by the maximum duty cycle count of 1000 and the factor of 15 000 reflects the fact that the time step of 66 7 use
30. iods 200 usec at 15 0 KHz at the beginning of the commutation cycle current noise is unable to prematurely trip the commutation logic While improving the noise rejection of the commutation algorithm this lockout interval does impose an upper limit on the motor s commutation rate which in turn limits the motor s maximum speed to 12 500 RPM A Variable Speed Sensorless Drive System for Switched Reluctance Motors 17 2 2 5 k TEXAS SPRA600 INSTRUMENTS Commutation t Earliest moment commutation can occur Lockout Figure 8 Illustration of Sensorless Commutation Algorithm With Lockout Window Velocity Estimator To estimate shaft velocity a software timer counts the number of sample periods between successive commutations When a commutation occurs the instantaneous velocity is evaluated in RPM using the formula A 37 500 INST 7 K conv 42 Koor fs 42 N 7 where Nis the sample count Kggny converts rads sec to RPM and fs is the sampling frequency of 15 0 KHz Since Equation 7 involves a reciprocal calculation this formula is implemented as an assembly language function This function performs the reciprocal operation using 16 back to back conditional subtract instructions SUBC As a result the entire operation only requires 2 0 usec After the instantaneous velocity is calculated the estimate is processed by a first order infinite impulse response IIR filter of the form FLT Brut 1
31. justed If the command speed falls below 400 RPM the low speed operating mode is activated which doubles the velocity update rate A real time delay is also inserted to ensure specific ramp up and ramp down rates of 100 RPM sec or 50 RPM sec respectively The ramp controller remains in this state until the command speed has reached the target speed When this happens the controller resets the settle counter and transitions into the settle state The purpose of this state is to allow any speed transients to die down before accepting any new target speeds from the serial command processor The controller remains in the settle state until the counter expires which happens after approximately two seconds When the counter expires the controller returns to the wait state and awaits a new target speed A Variable Speed Sensorless Drive System for Switched Reluctance Motors 4i TEXAS INSTRUMENTS SPRA600 Agitation Command Reset agit_cnt WAIT AGIT BRAKE Process serial Brake rotor commands agit_cnt Speed agit_cnt gt 0 Command reset settle_cnt settle_cnt 0 RAMP If command speed lt Lee AGIT SETTLE command_speed settle _cnt If command speed gt target Real time delay command_speed Real time delay Adjust conduction angle Adjust maximum current If command speed lt 400 RPM turn on low speed mode settle_cnt 0 command_speed target settle_cnt 0 reset settle_cnt S
32. k TEXAS Application Report INSTRUMENTS SPRAG600 October 1999 A Variable Speed Sensorless Drive System for Switched Reluctance Motors Stephen J Fedigan Ph D and Charles P Cole DSPS Research and Development Center ABSTRACT With the advent of high speed digital signal processors DSPs specialized for motion control applications ithas become possible to control motors without mechanical speed or position sensors This is achieved by algorithms that estimate the desired quantities in real time based on the electrical signals in the motor windings Benefits include cost savings and improved reliability due to reduced component count This application report presents a low cost sensorless drive system for a switched reluctance motor SRM based on the Texas Instruments TMS320F243 DSP Detailed descriptions of the hardware configuration the sensorless commutation algorithm its implementation details and procedures for measuring motor parameters should enable readers to rapidly duplicate and customize a sensorless SRM drive system to meet their specific application k TEXAS SPRA600 INSTRUMENTS Contents 1 Hardware to Demonstrate Sensorless Control of the Switched Reluctance Motor Using the Texas Instruments TMS320F243 DSP 0 cece cece eee eee eee 4 11 Demonstration Goals ci cevia ckvednesheeeed shee ded 6 eee e ceded e heen die han alee ses 4 1 2 Hardware Description ccc siisvexeved rex eiesare nink eager ereeeeeeartores
33. lculated by right shifting the 10 bit current measurement over two places Measuring the Voltage Loss Data Figure 16 shows a loss table constructed exactly in the manner described in the preceding section This figure contains two curves which compare the total Viggg against purely ohmic losses At low currents the voltage drops across the diode and the transistor dominate whereas at higher currents this drop becomes almost a constant 1 8 volts and the ohmic losses dominate As the figure shows accounting for the active components significantly improves the accuracy of the flux estimator particularly at low currents where the voltage losses are due primarily to the high dynamic impedance of the diode and the transistor By including the power devices motor commutation is improved considerably under no load and light loading conditions resulting in more accurate speed regulation XDS510 is a trademark of Texas Instruments Incorporated A Variable Speed Sensorless Drive System for Switched Reluctance Motors 29 k TEXAS SPRA600 INSTRUMENTS 0 0 5 1 1 5 2 2 5 3 3 5 4 Current A Figure 16 Voltage Loss vs Current for the Emerson Electric SRM and the Spectrum Digital 3 5 30 Motor Control Board Flux Measurement Method In most sensorless commutation methods it is very desirable to have a full set of SRM magnetization curves as in Figure 17 showing typical flux linkage V sec versus phase current amps o
34. lete All products are sold subject to the terms and conditions of sale supplied at the time of order acknowledgement including those pertaining to warranty patent infringement and limitation of liability Tl warrants performance of its semiconductor products to the specifications applicable at the time of sale in accordance with Tl s standard warranty Testing and other quality control techniques are utilized to the extent TI deems necessary to support this warranty Specific testing of all parameters of each device is not necessarily performed except those mandated by government requirements CERTAIN APPLICATIONS USING SEMICONDUCTOR PRODUCTS MAY INVOLVE POTENTIAL RISKS OF DEATH PERSONAL INJURY OR SEVERE PROPERTY OR ENVIRONMENTAL DAMAGE CRITICAL APPLICATIONS TI SEMICONDUCTOR PRODUCTS ARE NOT DESIGNED AUTHORIZED OR WARRANTED TO BE SUITABLE FOR USE IN LIFE SUPPORT DEVICES OR SYSTEMS OR OTHER CRITICAL APPLICATIONS INCLUSION OF TI PRODUCTS IN SUCH APPLICATIONS IS UNDERSTOOD TO BE FULLY AT THE CUSTOMER S RISK In order to minimize risks associated with the customer s applications adequate design and operating safeguards must be provided by the customer to minimize inherent or procedural hazards Tl assumes no liability for applications assistance or customer product design TI does not warrant or represent that any license either express or implied is granted under any patent right copyright mask work right or other intellect
35. mand A Variable Speed Sensorless Drive System for Switched Reluctance Motors 25 k TEXAS SPRA600 INSTRUMENTS 2 6 26 Serial Comms The serial communications module receives and processes character command strings which are sent by users across the RS 232 link from the host computer The serial communications interface SCI on the F24X is configured to run at 19 200 baud with 7 data bits 1 stop bit and odd parity The communications module has five commands in its command set which are summarized in Table 6 Table 6 Serial Communications Module Commands Commandt Description gt t Turn on drive system gt SXXXX Set new target speed in RPM 0150 lt xxxx lt 4500 gt b Brake motor and reverse direction gt a Agitate the motor gt C Cut off the drive system t Each command must be followed by a carriage return These commands are used to turn on the drive system set a new target speed reverse motor direction agitate the motor and cut off the drive Note that all commands start with a gt lead in character followed by a command character and must end with a carriage return which is not shown in the command column The speed command s character is followed by four digits and a Carriage return to indicate a new target speed The command processor is implemented as a software state machine and is invoked only when the ramp controller is in the wait state The exception to this is the turn on comm
36. n action of a washing machine i e it repeatedly brakes aligns reverses direction and spools up to 1000 RPM After repeating this sequence ten times the software exits the agitation mode and waits for a new command To turn off the motor the user can either turn off the bus power or can issue the cut off command gt c which de energizes all of the motor phases and spinlocks the processor Once this command is issued the user can restart the system by following the procedure outlined above For further details concerning the ramp controller and serial communications please refer to Section 2 Note Commands issued while the motor is ramping and settling at a new target speed will be ignored A Variable Speed Sensorless Drive System for Switched Reluctance Motors 9 4 TEXAS SPRA600 INSTRUMENTS 1 4 Sensorless SRM Performance 10 Table 4 summarizes the performance of the SRM drive system Table 4 Sensorless SRM Performance Summary Parameter Value Units Speed range 150 4500 rpm Load torque 48 oz in Speed regulation lt 8 low speed lt 8 mid range lt 1 hi speed lt 2 Start up load torque 48 0 oz in For operating speeds between 1000 and 3500 RPM regulation is tighter than 1 within the design load of 48 0 oz in For higher speeds regulation is more challenging and grows to 3 at the top speed of 4500 RPM This happens because the speed measurement resolution decreases as the
37. n described to this point will time the commutations properly if the motor has been well characterized and if a low noise current measurement is available However under certain operating conditions noise levels in the current signal will rise and this may trip the decision logic at the wrong moment While the integral action of the flux estimator naturally filters out noise the flux threshold is more sensitive since it depends only on the instantaneous current At low current levels present at the beginning of the commutation cycle the noise may be sufficient to suddenly drop the switching threshold and accidentally trip the commutation logic This problem is particularly apparent at high load torques and manifests itself in the form of motor speed oscillations which are shown in Figure 7 A Variable Speed Sensorless Drive System for Switched Reluctance Motors Wy TEXA S INSTRUMENTS Flux V s 0 12 f Estimated Flux and Flux Threshold for Commutation Estimated Flux Flux Threshold 0 08 0 06 F 0 04 hy 0 02 0 5 1 5 Time sec 2 2 5 x 1073 SPRA600 Figure 7 Flux Estimate and Threshold at 2500 RPM Under Full Load The flux threshold comes perilously close to tripping the commutation logic at the beginning of the commutation cycle By enforcing a lockout window depicted in Figure 8 which prohibits commutation for three sample per
38. ocity Estimate and Velocity Loop Output During Start up In Figure 12 it is evident that the velocity is overestimated immediately following start up This causes a momentary drop in the integrator output from saturation but the integrator winds up again the velocity controller saturates for approximately the first 0 50 seconds Shortly after reaching the initial target velocity the integrator unwinds enough to bring the velocity controller out of saturation and the system resumes closed loop operation The velocity loop overshoots about 30 under no load conditions but under the design load of 48 0 oz in the overshoot is less than 10 Current Control Loop The current control loop illustrated in Figure 13 which executes at the sampling frequency of 15 0 kHz employs a proportional control law to realize the current requests which arrive from the velocity loop cmd V k i a n Figure 13 Block Diagram of the Current Control Loop A Variable Speed Sensorless Drive System for Switched Reluctance Motors 23 k TEXAS SPRA600 INSTRUMENTS 2 5 24 Every sample period the current controller reads the latest current sample and calculates the current error which is the difference between the measured and requested current For positive errors the controller calculates a duty cycle to the active PWM channel in proportion to the error This calculated duty cycle becomes the applied one provided that the calculated value is
39. p suffers If the motor is suddenly loaded at a low operating speed lt 400 RPM the integrator in the velocity loop may not respond before the shaft velocity dips low enough to trigger the stall detector To help improve loop bandwidth under these conditions at speeds below 400 RPM the commutation algorithm enters a special low speed operating mode In this mode the velocity update rate is doubled by estimating the velocity twice every commutation cycle As shown in Figure 9 this is done by adding a second flux threshold which triggers only a velocity update Figure 9 Low Speed Operating Mode With a Second Flux Threshold for an Additional Velocity Update As before this threshold is a scaled version of the aligned rotor flux Due to the sawtooth shape of the flux waveform by setting amp m a 2 the new velocity update will occur approximately midway through the commutation To implement the midway velocity update a second counter is used to keep track of the number of samples between successive crossings These overlapping measurement intervals permit the update rate to be doubled and using this approach it has been possible to extend the lower speed range from 300 RPM to 150 RPM As in the commutation a lockout interval is enforced to prevent current noise from accidentally triggering a velocity update at an unintended moment A Variable Speed Sensorless Drive System for Switched Reluctance Motors 19 k TEXAS SPRA600 INSTRUME
40. result in hardware damage injury or even death Commands can be issued through a terminal program such as a Hyperterm which is included in the Windows operating system To properly configure Hyperterm the connection speed must be set at 19 200 baud with 7 data bits odd parity and 1 stop bit and all flow control must be disabled With the connection established the demonstration rig can be turned on using the following procedure 1 Power up the PC load controller and dynamometer brake 2 Run the terminal program and the Magtrol M TEST software 3 Turn on the dual power supply 4 Plug in the bus supply line on the Spectrum Digital board Once power has been applied to all of the components the terminal program can be used to issue the turn on command which is gt t followed by a carriage return When the EVM receives the turn on command the F243 injects an alignment current into phase 2 After the shaft has settled at the aligned position the F243 begins commutating the motor causing it to spool up in the counterclockwise direction to its initial target speed of 1000 RPM At this point the F243 is ready to receive new commands which are summarized in Table 3 Windows is a registered trademark of Microsoft Corporation Tt ANSI IEEE Standard 488 1 1987 IEEE Standard Digital Interface for Programmable Instrumentation A Variable Speed Sensorless Drive System for Switched Reluctance Motors Wy TE
41. rther details concerning the hardware and performance specifications for this drive system refer to Section 1 The control software features a flux based sensorless algorithm for 3 phase SRMs operating on 170 V of bus voltage and up to 4 0 A of phase current The algorithm is capable of two quadrant speed control between 150 and 4500 RPM with better than 8 speed regulation lt 1 between 1000 and 3500 RPM and reliable start up under a design load of 48 0 oz in An RS 232 serial link permits users to issue commands from a host PC to turn on cut off and change motor speed and the direction of rotation on the fly This software can be launched via the EVM s JTAG port using a XDS510PP emulator board inline with a SPI 110 optoisolator or the software can be directly embedded in the EVM s FLASH memory For further details on FLASH programming refer to the TMS320F20x F24x DSP Embedded FLASH Memory Technical Reference literature number SPRU282 Overview of the Sensorless SRM Control Software The software for the sensorless drive system is written primarily in C with the exception of a few assembly language subroutines for high speed computations The code fits into the TMS320F243 s 8K of FLASH program memory and the DSP s internal RAM blocks and requires no external memory It consumes about 6K of program memory and 300 words of data memory To execute the code on a TMS320C242 DSP controller the program storage requirements can be re
42. speed squared due to sample rate effects At the low end speed regulation is affected by the reduced frequency of speed updates This update rate is tied to the shaft speed and these updates occur less and less frequently as the speed is decreased To achieve the 10 performance specification below 400 RPM the controller enters a special low speed operating mode which doubles the update rate and in turn preserves the bandwidth of the velocity loop This keeps the regulation error below 8 at 150 RPM For further details concerning the low speed operating mode please refer to Section 2 With the help of an open loop start up procedure the motor can also be started reliably under a full load of 48 0 oz in This is valuable in pump and compressor applications where the load is constant throughout the entire operating speed range A Variable Speed Sensorless Drive System for Switched Reluctance Motors Ww TEXA 2 2 1 S INSTRUMENTS SPRA600 Control Software for a Sensorless SRM Drive System This section describes the control software for a sensorless SRM drive system using the TMS320F243 DSP The drive system is intended for demonstration purposes and is composed of a TMS320F243 evaluation module EVM a digital motor controller board available from Spectrum Digital Incorporated a 3 phase 12 8 stator pole configured 0 40 HP SRM manufactured by Emerson Electric Company and a Magtrol 50 0 lb ft dynamometer and load controller For fu
43. stom modified for this demonstration platform Alignment of the motor shaft through the flexible coupling is critical to prevent vibration and mechanical induced noise at higher operation speeds above 2500 RPM This type of flexible couplers can be obtained through Magtrol Incorporated A Variable Speed Sensorless Drive System for Switched Reluctance Motors 7 k TEXAS SPRA600 INSTRUMENTS 1 2 5 1 3 Dynamometer Controller Control of the dynamometer is provided by a Magtrol model 5240 controller that is a speed controlled power supply designed to interface with any type of IBM compatible computer using an IEEE 488T general purpose interface bus GPIB instrument controller The model 5240 controller can be used to control any Magtrol Load Cell Dynamometer In addition it can be set to return torque speed data to the computer when used with appropriate computer software In this demonstration hardware testing of the SRM performance was accomplished with an automatic motor testing software M TEST version 2 03 provided by Magtrol Operational Procedures To operate the demonstration a serial communications link must be established between the F243 and the host PC by using a standard RS 232 through pin serial cable This cable is attached to the DB9 connector on the Spectrum Digital board and the COM port on the PC WARNING Do not use the DB9 connector on the EVM board This connector is not isolated and ignoring this warning could
44. ual property right of TI covering or relating to any combination machine or process in which such semiconductor products or services might be or are used Tl s publication of information regarding any third party s products or services does not constitute Tl s approval warranty or endorsement thereof Copyright 1999 Texas Instruments Incorporated
45. urve is very linear up to the maximum operating current of 4 0 A and a first order curve accurately fits the data However other motors may have magnetization curves which are nonlinear and a higher order polynomial may be needed to capture the knee of the curve After the fitting the function is evaluated at the currents in imax 325 Y n 0 255 17 These points are chosen to allow an index into the 256 point look up table to be calculated by right shifting the 10 bit current measurement over two places After generating the look up table itis multiplied by the scaling factor Nnmax s maximum duty cycle count times the sampling frequency to make it compatible with the scaling of the flux estimator The reasons for this particular flux scaling are discussed in Section 2 A Variable Speed Sensorless Drive System for Switched Reluctance Motors 33 vy TEXAS SPRA600 INSTRUMENTS References 1 Lyons J S MacMinn and M Preston Flux Current Methods for SRM Rotor Position Estimation IEEE IAS Meeting Conf Record 1991 34 A Variable Speed Sensorless Drive System for Switched Reluctance Motors IMPORTANT NOTICE Texas Instruments and its subsidiaries Tl reserve the right to make changes to their products or to discontinue any product or service without notice and advise customers to obtain the latest version of relevant information to verify before placing orders that information being relied on is current and comp
46. velocity estimates start to exceed the initial target velocity of 1000 RPM the integrator begins unwinding at a rate that depends on the difference between the estimated and desired velocity As the integrator output decreases eventually the velocity loop output falls below the current command limit and the velocity controller resumes closed loop operation and regulates the speed to 1000 RPM This initial integrator value should be sufficient to start the motor up reliably under full load but should be no larger than necessary A oversized value will cause too much velocity overshoot perhaps more than the application can tolerate whereas an undersized value may not start the motor up reliably In the code an initial integrator value has been chosen which is sufficient to start the motor up reliably under a load of 48 0 oz in The start up procedure which has been described is shown in Figure 12 which graphs the estimated velocity and the command current versus time for a no load condition A Variable Speed Sensorless Drive System for Switched Reluctance Motors Ww TEXA S INSTRUMENTS 2 4 SPRA600 S 1500 p an 7 1000 F g N _ ult B 500 HHH se i av ye w We S ii 0 L 0 5 1 1 5 Time sec z 5o 3 i vd i hel re V 5 O W N O g 3 Wy rockon D E 2 o i 0 5 1 1 5 Time sec Figure 12 Instantaneous Vel
47. ver the full angular range from unaligned to fully aligned rotor position These flux linkage measurements require a precision angle indexer to hold the SRM shaft while making the measurement of flux linkage versus current flowing through the motor phase winding This SRM magnetic model is then used along with an estimate of the flux linked by a phase to determine rotor position The commutation algorithm used in the SRM demonstration platform only requires the flux linkage characteristic at the aligned rotor angle Since injecting a DC current into the phase winding causes the rotor poles to align with the stator no special alignment hardware is required After the rotor shaft settles into alignment with the stator flux data is then taken at this aligned rotor position using the flux measurement hardware as described in Section 3 5 1 A Variable Speed Sensorless Drive System for Switched Reluctance Motors Wy TEXAS INSTRUMENTS SPRA600 3 5 1 Flux Linkage V s Current A Figure 17 SRM Magnetization Curves Flux Measurement Hardware Figure 18 shows the schematic of the analog flux measurement hardware used to collect magnetic data for the SRM used in the demonstration platform In the SRM flux linkage in each phase of the motor can be calculated from the basic equation V iR dt relating flux linkage to voltage across the phase winding and current flow through the winding Referring to the schematic the semicon
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