View AN1112_2046753.PDF datasheet online --- IC-ON-LINE

Datasheet File OCR Text:

january 1999 AN1112 application note three-phase motor control by using st52x301 authors: m. di guardo, g. grasso, m. lo presti introduction induction motors with squirrel-cage are widely used in industrial environments because of their low cost and rotors rugged construction. the induction motor is a simple and robust machine, but its control migth be a complex task. when ma- naged directly from the line voltage, the motor operates at nearly a constant speed. to obtain speed and torque variations, it is necessary to modify both the voltage and the frequency, by using an electronic converter must be used to perform this operation. the best way to run the motor is by using a pwm sine-wave modulation, but in many applications this can result very expensive for the complex implementability. the six-step modulation (square wave) is a low-cost solution that allows to run the motor at various speeds. the aim of this application is to describe how st52x301 can easily work to obtain frequency and voltage variations in the inverter driver and how to perform a closed loop control. speed control by varying stator frequency and voltage the rotor speed can be controlled by varying the frequency of the stator voltage f. this is possible by varying vs in a linear proportion to f. varying the stator fr equency and voltage is the preferred technique in most variable-speed induction mo- tor drive applications. this technique is known as v/f=constant. as displayed in figure 1, for a small value of slip frequency (fsl= fs-fr) and for fixed flux values, a linear relationship between torque t and fsl (slip) takes place at any frequency f. fig . 1 torque speed characteristics f = constant 1/ 22
in order to eliminate one control variable, it is possible to fix the v/f ratio as f ollows: v f = const = k this is equal to fix the magnetic flux in the motor. v f minimum freq. rated freq (stator freq.) vmax vmin fig . 2 - voltage vs frequency relation v/f=k control method or v/f fuzzy variation can be successfully applied for a large class of applications. a way to choose this value could be, to determinate the max torque applied (load) and the correspon- ding speed; that means to evaluate the maximum requested magnetic flux. in order to change the stator voltage frequency it is necessary to use a variable frequency converter. the variable frequency converter, which acts as an interface between the utility power system and the induction motor, must satisfy the following basic requirements: ability to adjust the frequency according to the desired output speed ability to adjust the output voltage so as to maintain a const ant air gap flux in the constant torque region. ability to supply a rated current on a continuous basis at any frequency. rectifier inverter motor ac line filter dc variable frequency conv erter output (variable voltage & frequency) fig . 3 - variable frequency converter f = constant ? AN1112 - application note 2/ 22
the variable frequency converter can be implemented by using several techniques: pulse width modulated sinusoidal voltage source inverter (pwm vsi) square wave voltage source inverter (square wave vsi or six step modulation) square wave inverter (six step modulation) with square wave inverter operations, each inverter switch is on for 180 and a total of three switches are on at any instant of time. the resulting voltage is shown in figure 4. motor phase voltage t fig . 4 - one phase voltage in order to implement the six-step modulation, the inverter must be driven by using the following si- gnals: vdc link u u m r s t + - v v w w t t t u v w three phase inverter fig . 5 - bridge control signals since the inverter is operating in a square wave mode, the magnitude of the motor voltage is controlled by vdc link, i.e. the dc bridge supply voltage. ? three-phase motor control by using st52x301 3/ 22
changing the frequency of the three signals it is possible to change the frequency of the stator voltage. these three square wave signals can be easily obtained by using 3 digital i/0s and the timer of st52x301. in particular, these signals are obtained by writing sequentially on the 3 selected bits of the digital i/o port as shown in figure 6: t t t u v w 101 001 011 010 110 100 (binary) 5 1 3 2 6 4 (decimal) fig. 6 - digit sequency on st52x301 parallel port the timer counter is used to change the speed in writing sequence, in this way the frequency of the 3 square waves is modified. instead, the magnitude of the stator voltage is modified by using the st52x301 pwm. in order to change the value of the stator voltage, the 3 square waves are anded with a high frequency pwm whose duty cycle is managed by the triac counter register of the st52x301 pin (24). the high fre- quency pwm is then the switching frequency of the inverter transistors (fig. 7). t u t pwm u pwm u * t u * fig. 7 - bridge pwm control signal ? AN1112 - application note 4/ 22
u* u* m r s t + - v* v* w* w* st52t301 pwm digital i/o timer w v u u* v* w* a/d speed ref. fig . 8 - system schematic block by using this method the line to line r. m. s. voltage is about: v(rms) = 0.78*vd*d where d is the pwm duty cycle in the following figure is shown the schematic block of the st52x301 board that allows to reproduce a six step modulation. the main parts of this system are the microcontroller, the circuit utilized to provide the bridge control signals ( u*, v*, w* and u* , v* , w* ) and the three-phase inverter driver. ? three-phase motor control by using st52x301 5/ 22
fig.9 dead time circuit in order to avoid cross conduction problems a delay, "dead time", must be added between u*, v*, w* and u* , v* , w* . in figure 9 is shown the circuit used to create the "dead time". the rc value and the negative schmitt trigger threshold voltage, v n , determine the delay t dt as follows: t dt =-rc*ln(v n /5) ? AN1112 - application note 6/ 22
c4 0.1uf 1 2 3 4 5 6 7 8 9 10 11 12 j1 busi /o u v w 12 3 u4a 74h c 08 45 6 u4b 74hc 08 9 1 0 8 u4c 74h c 08 c3 0.1uf ain0 ain[0..3] vcc p0 9 p1 10 p2 11 p3 12 p4 13 p5 14 p6 15 p7 16 ready 17 p8 18 test 19 main2 20 main 1 21 pw m ou t 24 mode 25 reset 26 int 27 timerout 28 tres 29 tctrl 30 osc ou t 31 osc in 32 tclk 33 rxd 37 txd 36 ain0 43 ain1 42 ain2 41 ain3 40 bg 44 avdd 2 avss 3 vpp 6 u1 st 52t 301 1 2 3 4 5 6 7 8 j2 con8 8sip100 ain0 ain1 ain2 ain3 agn d 5 6 u2c 40106 r14 10k 1 2 u2a 40106 3 4 u2b 40106 r5 3.3k r10 3.3k d2 1n 4148 d3 1n 4148 r11 1m c10 1kf c11 1kf 9 8 u2d 40106 11 10 u2e 40106 r6 3.3k r7 3.3k d4 1n 4148 d5 1n 4148 r12 1m c12 1kf c13 1kf r2 10k rc05 r3 2.2k rc05 q1 bc 308 to92 vcc c9 100uf 16v cel04 13 12 u2f 40106 speed reference c1 22pf c2 22pf y1 5m h z ain1 ain2 ain3 agn d avdd vcc vcc l1 10uh cel04 1 2 3 4 5 6 7 j3 con7 7sip100 vcc avdd jp1 masse c5 0.1uf r1 27 c8 10uf 10v agn d t c6 0.1uf c7 1uf 10v r4 2.2k rc05 rs d1 4.3v do41 3-phase motor 5 6 u3c 40106 speed 1 2 u3a 40106 3 4 u3b 40106 r8 3.3k r9 3.3k d6 1n 4148 d7 1n 4148 r13 1m c14 1kf c15 1kf inverter u v v' w' u' w fig.10 six step modulation schematic for hw implement. ? three-phase motor control by using st52x301 7/ 22
six step modulation s/w implementation by st52x301 the software implementation is shared in a main program and an interrupt subroutine designed with fuzzystudio ? 3.0. peripherals configuration the first step of the program development is the peripherals configuration. the following figures show the peripheral configurations: the parallel port is configured with 3 output lines and 5 input lines; pin0 = u; pin1=v; pin2 = w the analog to digital converts: two channels, chan0 = speed feedback, chan1 = reference. fig.11 peripherals configuration ? AN1112 - application note 8/ 22
fig . 12 - peripherals configuration the peripheral named triac pwm driver, generates a pwm signal with a fixed period. this period, imposed by the prescaler value, 25, is 332 m s (3 khz). the duty cycle of this signal is utilized to modulate the voltage amplitude. ? three-phase motor control by using st52x301 9/ 22
fig . 13 - peripherals configuration the timer configuration allows to change the motor speed range. six step signal period changes, according to the timer counter register value, from 400 m s x 1 x 6 (416 hz) up to 400 m s x 255 x 6 (17hz), if the prescaler is 7999. ? AN1112 - application note 10/ 22
main program the following figure shows the main program window. the first two blocks (" timer_int_setting " and " ti- mer_int_priority ") allow the interrupts mask configuration. in this case, only the timer is enabled to sup- ply an interrupt, each time the timer reaches the counter register value. this interrupt is utilized to syn- chronize the phase switching. the block " variables_initialization " assigns a default value to the global variables while the " digi- tal_port_bit_set " block sets the parallel port u-v-w pins. the following blocks, " voltage_level_setting " and " start_pwm ", are used respectively to set and start the pwm s ignal on st52x301 triacout pin. the " start_ad " block enables analog to digital converter to work in continuous mode. by means of the following two blocks, the converted values of measured speed and speed reference are read and stored into two global variables. the " error_definition " block performs the error calculation as folllows: error=reference - speed after a time delay the resulting error value is sent to the " fuzzy_controller " block. this fuzzy block im- plements a fuzzy algorithm which produces the frequency increment or decrement d f to be added to the actual stator frequency, in order to obtain the desidered speed. details on this algorithm will be given later on. fig . 14 - programs main view the block named " integrator " updates the current speed and checks for possible under/overflow in the algebric sum. the " v_f_control " fuzzy block is used as fuzzy model of the voltage-frequency relations- hip, whose diagram is shown in fig.2. this fuzzy block outputs a voltage value for each frequency input value. " voltage_2_pwm " and " freq_2_timer " blocks updates triac counter register and timer counter register, in order to change the pwm mean value and rotor speed. ? three-phase motor control by using st52x301 11/ 22
the figure below shows the content of the " six_step_modulation " block. it is an arithmetic block and is used to sequentially repeat the 6 logic states of st52x301 (p0 .p1. p2) pins. the same decimal digits are reported in fig. 6. the variable named "ciclo" is incremented at each turn of the software loop with a timing imposed by the timer interrupts with " wait0 " block. the sixstep variable is sent to the port by the " square_wave_to_ioline " block inside timer interrupt routine. fig. 15 - arithmetic block fuzzy controller in figure 16 is reported the content of the " fuzzy_controller " block described in the main program. the global variable error" initializes the local fuzzy input. the global variable range (0 : 255) is mapped into the (-10:+10) universe of discourse. five membership functions cover the fuzzy input range in order to share the universe of discourse in 5 fuzzy subsets. each subset is named in order to give semantic meaning to the variable values. the first rule produces a strong increment of the stator frequency. in fact, error is "negative_big" means that "speed >> reference", then the controller must decelerate the motor shaft rotation. the way to slow down the speed is to decrease the stator frequency. in term of wave period, this leads to an increment of the timer counter register value (wave period increment). the second rule performs the same action but with a lower action strength. in fact error is "negative" in- volves a membership function near to the zero error. this rule is activated when the shaft speed is clo- se to the reference speed, so control action is more soft. the third rule gives no correction to the stator frequency because it is activated when s peed is equal to the reference. the same reasoning could be done for the other rules. the output value of the " fuzzy_controller " block is obtained with the contribution of all activated rules. ? AN1112 - application note 12/ 22
v/f fuzzy definition this application implements a simple linear function between frequency and voltage. by using fuzzy ru- les, it is possible to implement more complex non linear functions between the two variables. furthermo- re, by using a fuzzy routine for the v/f definition, it is possible to avoid the use of a software division, that normally, in standard 8-bit microcontrollers, is time expensive and needs complex software routines. the global variable freq initializes the fuzzy local variable frequency. five mbfs have been chosen to cover the universe of the discourse (0 : 255). by using more mbfs, a very accurate model can be obtai- ned if higher precision is required. in order to obtain a linear variation of the ratio v/f, the rules have been defined by using experience and knowledge. analogous results, or better, can be obtained by using fuzzy modeller software tools such as adaptive fuzzy modeller (afm) provided by stmicroelectronics. fig . 16 - fuzzy controller block ? three-phase motor control by using st52x301 13/ 22
results and conclusions the ac 3-phase motor control described in this application note represents a good compromise be- tween system costs and motor performances. the implemented six-step controller is good enough for a lot of consumer applications where the motor power and cost do not justify very complex control systems (field orientated control and so on). in this case, the graphical programming environment reduces the development time also for not expert programmers. fig . 17 - v/f model implementation ? AN1112 - application note 14/ 22
from figure18 it is possible to observe the system response. to evaluate acceleration characteristics and control goodness, some trials were made during the software development. fig.19 shows free acceleration characteristics starting from a quiet state of the shaft. although the implemented system is very simple the control performances do not degrade in comparison with other controls methodologies. 0 8 0 500 1000 1500 2000 2500 time (sec) motor speed ref. signal fig. 18 speed reference following time (sec.) motor speed ref. signal -200 0 100 200 300 -100 0 8 fig.19 step response ? three-phase motor control by using st52x301 15/ 22
references [1] designers guide to power products - application manual, stmicroelectronics [2] mohan, undeland, robbins power electronics: converters, applications and design john wiley & sons [3] paul c. krause "analysis of electric mac hines", mcgraw-hill [4] fuzzystudio tm 3.0 - user manual, stmicroelectronics ? AN1112 - application note 16/ 22
appendix: st52x301 assembler code ; source file: p:\applic\ sixstep\acmotor.wcl ; compile time: tue oct 06 11:09:14 1998 ; device type: st52x301 ; compiler version: 01.00 (02.06.98) data 0 0 42 213 0 data 0 1 0 42 42 data 0 2 44 171 42 data 0 3 43 127 44 data 0 4 42 84 43 data 0 5 42 213 0 data 0 6 0 42 42 data 0 7 44 171 42 data 0 8 43 127 44 data 0 9 42 85 43 stop irq 3 timer_interrupt stop @wclstart@@: ldcf 0 255 ldcf 1 4 ldcf 2 6 ldcf 3 0 ldcf 4 243 ldcf5 1 ldcf 6 40 ldcf 7 12 ldcf 8 25 ldcf 9 0 ldcf 10 4 ldcf 11 0 ldcf 12 64 ldcf 13 0 ldcf 14 0 ldcf 15 228 start: timer_int_setting: ldcf 14 8 timer_int_priority: ldcf 15 210 ? three-phase motor control by using st52x301 17/ 22
variables_initialization: ldrc 15 0 ldrc 13 0 ldrc 14 1 ldrc 9 0 digital_port_bit_set: ldpr 2 9 voltage_level_setting: ldpr 1 15 start_pwm: ldcf 11 2 ldcf 10 7 start_timer: ldcf 6 41 ldcf 6 43 start_ad: ldcf 2 7 ad0_2_reference: ldri 12 0 ad1_2_speed: ldri 10 1 error_definition: mdgi ldrr 11 12 subo 11 10 megi delay: arth0: ldrc 7 0 condition0: mdgi ldrc 0 255 sub 0 7 megi jpnz @@00001 @00002: jp arth1 jp @@00003 @00001: jp return0 @00003: arth1: mdgi ? AN1112 - application note 18/ 22
ldrc 0 1 add 7 0 megi return0: @00000: fuzzy_controller: ldrr 0 11 stop ldp 0 1 ldp 0 1 fzand con 130 ldp 0 4 ldp 0 4 fzand con 128 ldp 0 3 ldp 0 3 fzand con 127 ldp 0 2 ldp 0 2 fzand con 126 ldp 0 0 ldp 0 0 fzand con 124 out 0 stop ldri 8 9 integrator: mdgi ldrc 0 128 add 13 8 add 13 0 megi mdgi ldrc 0 250 sub 0 13 megi jpz @@00005 jpns @@00004 ? three-phase motor control by using st52x301 19/ 22
@00005: ldrc 13 250 @00004: @00006: v_f_control: ldrr 0 13 stop ldp 0 6 ldp 0 6 fzand con 50 ldp 0 9 ldp 0 9 fzand con 64 ldp 0 8 ldp 0 8 fzand con 128 ldp 0 7 ldp 0 7 fzand con 192 ldp 0 5 ldp 0 5 fzand con 250 out 0 stop ldri 15 9 fuzzy_out0_2_voltage: ldri 15 9 voltage_2_pwm: ldpr 1 15 freq_2_timer: ldpr 0 13 six_step_modulation: mdgi ldrc 0 1 sub 0 14 megi jpnz @@00007 @00008: ldrc 9 5 ? AN1112 - application note 20/ 22
@00007: @00009: mdgi ldrc 0 2 sub 0 14 megi jpnz @@00010 @00011: ldrc 9 1 @00010: @00012: mdgi ldrc 0 3 sub 0 14 megi jpnz @@00013 @00014: ldrc 9 3 @00013: @00015: mdgi ldrc 0 4 sub 0 14 megi jpnz @@00016 @00017: ldrc 9 2 @00016: @00018: mdgi ldrc 0 5 sub 0 14 megi jpnz @@00019 @00020: ldrc 9 6 @00019: @00021: mdgi ? three-phase motor control by using st52x301 21/ 22
ldrc 0 6 sub 0 14 megi jpnz @@00022 @00023: ldrc 9 4 ldrc 14 1 jp @@00024 @00022: mdgi ldrc 0 1 add 14 0 megi @00024: wait0: waiti jp ad0_2_reference timer_interrupt: square_wave_to_ioline: ldpr 2 9 iret0: reti stop information furnished is believed to be accurate and reliable. however, stmicroelectronics assumes no responsibility for the co nsequences of use of such information nor for any infringement of patents or other rights of third part ies which may result from its use. no license is gran- ted by implication or otherwise under any patent or patent rights of stmicroelectronics. s pecification mentioned in this publication are subject to change without notice. this publication supersedes and replaces all information previously supplied. s tmicroelectronics products are not authorized for use as critical com ponents in l ife support dev ices or systems without express written approval of stmicroelectronics. the st logo is a registered trademark of s tmicroelectronics ? 1998 stmicroelectronics C printed in it aly C all rights reserved stmicroelectronics group of companies http://www.st.com australia - brazil - canada - china - france - germany - it aly - ja pan - korea - malaysia - malta - mexico - morocco - the netherlands - singapore - spain - sweden - switzerland - taiwan - thailand - united kingdom - u.s.a. ? AN1112 - application note 22/ 22

▲Up To Search▲

Price & Availability of AN1112

	To Download AN1112 Datasheet File
If you can't view the Datasheet, Please click here to try to view without PDF Reader .