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1/15 author: m. di guardo, m. lo presti, v. marino may 2000 AN1255 application note new circuital solution to efficiently drive an ac monophase motor or ac load by st52x420 introduction this application note shows a new solution to efficiently drive a monophase motor or any ac load. this new modulation technique, together with fuzzy logic algorithms, allows to obtain high efficiency systems at low-cost for home appliances applications. the aim of the solution is to vary the voltage applied to the motor or ac load starting directly from the mains, without inverter topology. this is obtained by using only an ac/ac constant frequency conversion thus optimizing the filtering part needed to respect the emc/emi regulation. nowadays, a lot of consumer and industrial products are based on ac monophase or universal motors: washing machines, dishwashers, air conditioning systems, refrigerators, fans and ventilators. efficiency and system cost are the leading factors in the choice of the motor type together with electronic drivers and control technique. though the choice of three-phases induction or brushless motors with inverters, can guarantee an im- provement in overall performances, the higher system cost reduces their applicability in home appliances systems and, in general, to low/medium-end applications. most of todays control techniques for monophase and universal motors are based on phase angle partial- ization and a simple triac as a driver. the economic advantages offered by this solution are counterbal- anced by the high harmonic distortion introduced in motor signals leading to low efficiency. to achieve better results in terms of efficiency, more complex driver topologies can be chosen. therefore, this application note describes a new cost effective solution for monophase motors that allows to improve results with respect to triac drivers though keeping system cost lower than with inverter topologies. also, advantages offered by fuzzy control techniques implemented by st52x420 microcontroller are shown. standard topology for motor control as already introduced above, today among the most commonly used drivers for monophase motors, triac driver and phase angle partialization based techniques are more and more subject to criticism due to the high harmonic distortion and low efficiency provided. the new european regulations require an optimiza- tion in the efficiency of systems and heavy filtering stages, especially in the case of medium/high powers, for emi and emc constraints towards the mains. inverter driver systems, based on topologies like the ones shown in figure 1, allows to obtain appreciable results in this direction. figure 1. inverter based topology for motor control ac motor ac/dc ac mains conversion filter m + - dc/ac converter
AN1255 - application note 2/15 figure 2. phase partialisation method however, inverter systems need a double conversion, (ac/dc and dc/ac) and furthermore require com- plex techniques of modulation and control implemented on medium/high end microcontrollers. in the over- all the gain in performances is balanced by an electronic system cost that is not acceptable in low-end applications. another method to change the rms voltage applied in ac motor is the well known phase partialisation with triac. in this case the voltage is a function of the firing angle of the triac (figure 2). the driver topology proposed provides a more cost-effective solution, though keeping the harmonic con- tents of motor signal at a satisfactory level. single phase motors the solution proposed is well suitable for single phase motors with capacitor for their similarities with nor- mal squirrel-cage three-phase induction motors. the stator windings of a single-phase motor are two: one connected with the mains and the other, the aux- iliary one, is physically in quadrature with the main winding. this, can be generally wound with turns of small wire to produce a winding of very high resistance. the main winding has a relatively low resistance and high inductance, which results in a phase difference between the two windings when the voltage is applied to the terminals. a centrifugal switch or relay opens the auxiliary winding when the rotor reaches a certain speed; then the motor runs on the main winding only. in practice , where not much starting torque is required, instead of a high resistance winding, the starting winding is designed to operate in conjunction with a capacitor permanently in series with the auxiliary winding during the operation of the motor (figure 3). this arrangement gives a 90 -shift of the two magnetic fields generated respectively by l1 and l2 winding currents. this configuration generates a rotating field similar to the three-phase induction motors. figure 3. schematisation of an ac monophase motor t ~ z vsource i load controlled phase angle control vsource ac load l1 l2 c main winding auxiliary winding 220 vac mains
3/15 AN1255 - application note new circuital solution for monophase motor control this circuit allows to modify the rms values of the motor phase voltage, limiting the third harmonic mag- nitude. in addition, only an ac/ac conversion is performed and an appreciable reduction in the electronic complexity with respect to inverter bridges can be appreciated (figure 5). the method is based on a bi- directional chopper mains with a fixed frequency and variable duty-cycle. the effect generated will be a sinusoidal current in the load with a level given by the duty-cycle value (figure 4). figure 4. motor current level the monophase motor m1 is powered with a chopped sinusoidal voltage. transistor q1, (either an igbt or a power mos), that is driven by a pwm signal coming from st52x420 microcontroller, allows, together with bridge d1-d4, to chopper mains voltage during either positive and negative half periods. the duty cycle of the pwm signal allows to change the motor voltage level and then the motor current. current freewheeling for the magnetization of the motor is provided by the diodes d7 and d9 which are selected respectively by bipolar transistors q2 and q3, alternatively on in one of the two motor current half periods. the turning on of these transistors is given automatically by employing d6-d8 and d10-d11 v-forward diode network (figure 5). figure 5. new circuital solution for monophase motor (only for high cos f load) vline t t increasing duty cycle bi-directional pwm chopping duty-cycle results in load current pate nted r1 d7 q3 q2 d9 r2 220vac 1 j1 c2 d5 d6 c3 m1 d8 d11 c4 d10 d12 d1 d2 d3 d4 pwm q1 1 j2 current during q1 of f vbe(sat) vbe(sat ) line cur rent during p ositive grid voltage q3 is off
AN1255 - application note 4/15 thanks to this vforwards diode, the base current i bq2 and i bq3 is positive only in the positive half period of the current for ibq2 and in the negative half period for ibq3. capacitor c3 and c4 allow to hold v-forward during toff of the switch q1 hence ensure the base currents of q2 and q3. the base current of each transistor (q1, q3) however, can be negative during the half pe- riod of the mains, thanks to the forward of d5 and d12, this avoids possible turn-on of q1 and q3 when they must be off (figure 6). the figure below shows the voltage control of the one freewheeling transistor and the collector current during toff of the main switch. figure 6. q 2 v be control during positive half period of the mains figure 7. q3 vbe control during negative half period of the mains v be ,i c q 2 on q 3 off q 2 off q 3 on freewheeling current in d7 v be voltage 0 1 2 3 0 1 2 3 v be ,i c freewheeling current in d9 v be voltage q3 off q3 on q2 on q2 off
5/15 AN1255 - application note hardware implementation in the following schematic is shown the power section of the new solution driven by st52x420 microcon- troller. the switching frequency is fixed at 20 khz in pin 7 (timer1 out), duty-cycle value establishes the power level in the motor. the peripheral pwm_timer1 (as pwm_timer0 and pwm_timer2_) allows in fact to produce a square wave signal, at a certain frequency (from 78.4 khz to 1.2 hz), with a program- mable duty cycle from 0% to 100%. therefore, varying the duty cycle it is possible to vary the power sup- plied to the motor. an open loop control is implemented by using the potentiometer r7. as shown in figure 7, it is possible to fix a power level by variable resistor r7, connected to the analog input ain0. the speed reference is ob- tained by changing the value as r7. this value is used to load the timer register thus changing the duty- cycle. to avoid acoustic noise, the pwm is fixed at 20 khz and it is available in output in pin 7 (t1out). to drive the igbt, the transitor q4, q5 and q6 are implemented as level shifter. instead of this solution, a driver like l638x series can be used. figure 8. schematic of the board ab420_2 vcc vcc d9 stta106u q4 bc108 c14 0.47uf 630v r6 0 d2 byw100/50 d5 byw100/50 j1 1 + c10 470uf10v d1 byw100/50 d7 stta106u j4 con1 1 j2 1 t2 6+6v220v + c8 1000uf 25v j3 con1 1 q2 bul310fp q5 2n2222 q1 bul310fp q3 p3nb60 c19 0.1uf d3 byw100/50 ~ ~ + - d10 0.5a bridge05a q6 2n2907 c9 0.1uf r12 2.2k u2 st52t420dip 9 10 11 12 15 16 17 18 1 4 32 28 27 26 25 24 23 22 21 20 14 13 19 5 6 7 8 ain0/pb0 ain1/pb1 ain2/pb2 ain3/pb3 ain4/pb4 ain5/pb5 ain6/pb6 ain7/pa7/pb7 reset test oscin oscout vdd vss vpp pa0/tores pa1/t0outn pa2/t1outn pa3/t2outn pa4/t0strt pa5/t0clk gnda vdda pa6 pc0/int pc1/t0out pc2/t1out pc3/t2out r8 10k 250v20l rv1 u1 lm7805 1 2 3 vin gnd vout d4 byw100/50 u3 ts831 21 3 in out gnd c20 0.1uf ~ ~ + - d8 2a500v c1 1uf c18 100pf c2 1uf r9 10k d6 byw100/50 r5 15 c15 0.1uf j11 pb 1 2 3 4 5 6 7 8 y1 5mhz 1 2 3 1 gn d 3 j9 pc 1 2 3 j10 pa 1 2 3 4 5 6 7 8 f1 f2al250v + c17 1uf/35v-tantalio j7 con2 1 2 c16 3.3nf r10 5.6-1/4w r7 22k motor
AN1255 - application note 6/15 in case of closed loop control, it is possible to have all the i/os of the microcontroller available (figure 9), for example for a speed control loop or flux minimization of the ac motor for energy saving purposes. figure 9. pcb board figure 10. board ab420_2 st52x420 i/o i/o
7/15 AN1255 - application note software description the figure below shows the flowchart of the application and this can also be implemented by using fuzzystudio tm 4.0 as displayed in the following figures. figure 11. flowchart pwm1 interrupt routine set pwm_1 count disable pwm1 interrupt exit main variables and peripherals initialization wait for interrupts a/d interrupt routine number of samples is n sum_samples= sum_samples+chan0 duty=sum_samples / n enable pwm1 interrupt exit yes no pwm1 routine a/d routine
AN1255 - application note 8/15 peripherals configuration to fix the power supplied to the motor, it is necessary to vary the duty cycle of the signal timer1; in order to avoid noise effect, this level is determined reading a mean value of voltage determined by the variable resistor r7. in order to generate the pwm signal, the peripheral pwm/timer1 is used. this peripheral is configured to obtain a pwm signal at 19.2 khz frequency and to send an interrupt request at each rising edge of the timer1_out_signal, as you can see in the following figure: figure 12. timer1 configuration the voltage value determined by r7 is read by means of the a/d converter peripheral configured with a single channel (ain0) that is continually converted. the working frequency of the a/d converter is ob- tained dividing by two the clock master frequency (5mhz); the conversion is obtained each 32 m s. figure 13. port pin and a/d configuration
9/15 AN1255 - application note main program description the purpose of the main program is to initialize the variables used in this project, to enable the a/d and timer1 interrupt and to start the a/d and timer1 peripherals. the control action will be developed in the interrupt routines. figure 14. fuzzystudio tm 4.0 main program window interrupt routines description the purpose of the ad interrupt routine is to carry out an average arithmetic on n samples in order to filter noises on the analog channel. at every conversion, the value read on chan0 is added to a word type variable denominated `sum_samples', that at the end, after n conversions, will contain the sum of all the converted values. the mean value is obtained dividing by n then it is loaded in the duty variable. the load- ing of this variable in the counter pwm_1_count is performed in the pwm1 interrupt routine only when the average has been calculated. figure 15. a/d converter interrupt routine
AN1255 - application note 10/15 figure 16. pwm timer1 interrupt routine conclusion the advantages offered by the proposed driver solution can be summarized in the following points: it allows a modulation where the third harmonic is limited, therefore noise reduction is observed with respect to phase partialization angle at low speed (fig. 11, 13 and 14). the voltage applied to the motor can be controlled linearly by changing the duty cycle of the pwm signal on transistor q1. this allows to improve the control performances that can be obtained with respect to a simple phase angle control where the voltage applied to the motor is quadratic. if compared to the inverter driver solution, this driver does not need a separate power supply section. due to the absence of ac/dc conversion, the filtering stage is smaller therefore less expensive than in the case of an inverter driver. absence of torque pulsation with respect to phase partialization allows to reduce the vibrations of the mechanical support thus decreasing the acoustic noise. clearly a drawback of the proposed topology, with respect to the inverter-based one, is that no voltage/ frequency control can be done with obvious limitations on high speed control. the following table shows the power contents of spurious harmonics introduced by the two different mod- ulation techniques on a 0.2 kw monophase motor (phase angle partialization control by using a triac ver- sus pwm modulation allowed by the solution proposed in this application note). a constant load (with fixed speed) is applied to the motor. comparative table harmonics power content 1 st (watt) 3 rd (watt) 5 th (watt) triac solution 150 28 12 new proposed solution 100 0.1 0.0
11/15 AN1255 - application note it is possible to notice that the proposed solution allows to optimize the system efficiency reducing losses generated by harmonic distortion as it occurs with the triac based regulation (table). figures 11, 13 and 14 show the harmonic contents of motor current for the two techniques. figure 17. motor current harmonic contents with pwm technique and proposed solution figure 18. motor current and igbt voltage with pwm technique and proposed solution 50 hz/div. 50 hz 150 hz harmonic content of the motor current i load (a) 0 0.1 0.2 0.3 0.4 0.5 0.6 motor current igbt voltage 0 1 im (a) vm (v) 0 100 200 300 2ms/div. time
AN1255 - application note 12/15 figure 19. motor current harmonic content with phase angle partialization technique figure 20. motor current with phase angle partialization technique 50 hz/div. 50 hz 150 hz harmonic content of the motor current i load (a) 0 0.1 0.2 0.3 0.4 0.5 0.6 250 hz motor current 0 1 2 im (a) 2ms/div. time
13/15 AN1255 - application note figure 21. motor current and fft obtained with the new solution (25% of the total power)
AN1255 - application note 14/15 figure 22. motor current and fft obtained with the new solution (40% of the total power) references st52x420 datasheet, stmicroelectronics , march 2000 mohan n., undeland t. and robbins w. apower electronicso . john wiley & sons, 1989 lo presti m., pagni a., poluzzi r., rizzotto gg. aautomatic synthesis, analysis and implementation of a fuzzy controllero by - ieee second international conference on fuzzy systemso, san francisco (usa) - march 28 - april 1, 1993 lo presti m., d'angelo g. oa fuzzy control strategy for different classes of electrical engineo, computer design, 1994 international technical conference series, san diego, 13-15 september, 1994 leonhard w. acontrol of electrical driveso. narosa publishing house, new delhi diguardo m., vinci c. aasynchronous steureng mit fuzzy microcontrollerno. elektronik informationen, sept. 1999 for further information on the current application please refer to the file AN1255.fs4 designed with fuzzystudio tm 4.0 that can be downloaded from the st52 fuzzy microcontrollers web site at the following address: http://ww w.st.com/stonline/pr odpres
15/15 AN1255 - application note othe prese nt note which is for guidanc e only aims at providin g custome rs with informa tion regarding thei r products in order for them to save time. as a result, stmicroe lectronic s shall not be held liable for any direct, indirec t or consequen tial damages with respect to any claims arising from the conte nt of such a note and/or the use made by customer s of the informa tion contained herein in connecti on with thei r products.o information furnished is believed to be accurate and reliable. however, stmicroelectronics assumes no responsibility for the consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. no license is granted by implication or otherwise under any patent or patent rights of stmicroelectronics. specification mentioned in this publication are subject to change without notice. this publication supersedes and replaces all information previously supplied. stmicroelectronics products are not authorized for use as critical components in life support devices or systems without express written approval of stmicroelectronics. the st logo is a trademark of stmicroelectronics ? 2000 stmicroelectronics - all rights reserved fuzzystudio tm is a registered trademark of stmicroelectronics stmicroelectronics group of companies http://www .st.com australia - brazil - china - finland - france - germany - hong kong - india - italy - japan - malaysia - malta - morocco- singapore - spain - sweden - switzerland - united kingdom - u.s.a.

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