Application Note, V1.0, Aug. 2008 AP08086 XC88x/XC878 Series CORDIC and MDU for Constant V/F Control of Induction Motor Microcontrollers Edition 2008-08-27 Published by Infineon Technologies AG 81726 München, Germany © Infineon Technologies AG 2008. All Rights Reserved. LEGAL DISCLAIMER THE INFORMATION GIVEN IN THIS APPLICATION NOTE IS GIVEN AS A HINT FOR THE IMPLEMENTATION OF THE INFINEON TECHNOLOGIES COMPONENT ONLY AND SHALL NOT BE REGARDED AS ANY DESCRIPTION OR WARRANTY OF A CERTAIN FUNCTIONALITY, CONDITION OR QUALITY OF THE INFINEON TECHNOLOGIES COMPONENT. THE RECIPIENT OF THIS APPLICATION NOTE MUST VERIFY ANY FUNCTION DESCRIBED HEREIN IN THE REAL APPLICATION. INFINEON TECHNOLOGIES HEREBY DISCLAIMS ANY AND ALL WARRANTIES AND LIABILITIES OF ANY KIND (INCLUDING WITHOUT LIMITATION WARRANTIES OF NON-INFRINGEMENT OF INTELLECTUAL PROPERTY RIGHTS OF ANY THIRD PARTY) WITH RESPECT TO ANY AND ALL INFORMATION GIVEN IN THIS APPLICATION NOTE. Information For further information on technology, delivery terms and conditions and prices please contact your nearest Infineon Technologies Office (www.infineon.com). Warnings Due to technical requirements components may contain dangerous substances. For information on the types in question please contact your nearest Infineon Technologies Office. Infineon Technologies Components may be used in life-support devices or systems only with the express written approval of Infineon Technologies, if a failure of such components can reasonably be expected to cause the failure of that life-support device or system, or to affect the safety or effectiveness of that device or system. Life support devices or systems are intended to be implanted in the human body, or to support and/or maintain and sustain and/or protect human life. If they fail, it is reasonable to assume that the health of the user or other persons may be endangered. AP08086 Constant V/F Control of Induction Motor AP08086 Revision History: 2008-08 Previous Version: none Page V1.0 Subjects (major changes since last revision) We Listen to Your Comments Any information within this document that you feel is wrong, unclear or missing at all? Your feedback will help us to continuously improve the quality of this document. Please send your proposal (including a reference to this document) to: [email protected] Application Note 3 V1.0, 2008-08 AP08086 Constant V/F Control of Induction Motor Table of Contents Page 1 1.1 1.2 1.2.1 Introduction ...................................................................................................................................5 Overview .........................................................................................................................................5 Motor Theory ...................................................................................................................................5 Principle of Operation......................................................................................................................5 2 2.1 2.2 2.3 2.3.1 2.3.2 2.3.3 Speed control of Induction motor ...............................................................................................6 Speed Control .................................................................................................................................6 Principle of Constant V/F Control....................................................................................................7 Principle of SVPWM Technique ......................................................................................................8 Three Phase Inverter ......................................................................................................................8 Six Step Mode .................................................................................................................................9 Space Vector Pulse Width Modulation..........................................................................................10 3 3.1 3.2 3.2.1 3.2.2 3.2.3 3.2.3.1 3.2.4 3.2.5 3.3 Software Implementation ...........................................................................................................13 Control System Overview..............................................................................................................13 CCU6 Timer T12 Period Match Interrupt ......................................................................................14 T12 Period Match ISR Disabled....................................................................................................14 Angle Calculation ..........................................................................................................................15 Space Vector Modulation ..............................................................................................................15 Calculation of PWM duty cycle for SVM .......................................................................................15 Update of CAPCOM6 Registers....................................................................................................17 Current Measurement ...................................................................................................................17 Timer T2 Overflow Interrupt ..........................................................................................................19 Appendix A Application Note Code for Switching Time Calculation...............................................................................21 4 V1.0, 2008-08 AP08086 Constant V/F Control of Induction Motor Introduction 1 Introduction 1.1 Overview This application note describes the implementation of a constant V/F control algorithm for control of an Induction motor using space vector modulation (SVPWM). Constant V/F control is a popular algorithm for open loop speed control of AC induction motors. This method is most suitable for applications without position control requirements or the need for high precision speed control. Examples of these applications include air conditioning, fans and blowers etc. In this application note, the principles of constant V/F control, SVPWM and the software implementation for the XC88x/XC878 microcontrollers are discussed. Also the advantages of the microcontroller peripherals: CAPCOM6E (Capture and Compare Unit for modulation and PWM generation) and the fast 10-bit ADC (Analog-to-Digital Converter), which are specifically designed for the motor control applications are discussed. This motor control software makes use of the advantages of the XC88x/XC878 peripherals, like the implementation of SVPWM using the CORDIC and MDU coprocessor units. The software for Induction motor control is written both in C and assembly, specifically the main algorithms (e.g. V/F control, SVPWM and current calculation subroutines) are written in assembly to reduce the execution time. 1.2 Motor Theory AC Induction motors (ACIM) are widely used in industrial and residential motor applications due to their simple construction and durability. These motors have no brushes to wear out or magnets that add to the cost. An induction motor has basically two parts, the Stator and the Rotor. The stator is made up of a number of stampings with slots to carry three phase windings. It is wound for a definite number of poles. The windings o are geometrically spaced 120 apart. Two types of rotors are used in induction motors – Squirrel cage rotor and Wound rotor. The most common type of rotor is the squirrel cage rotor. The rotor consists of a stack of steel laminations with evenly spaced conductor bars around the circumference. The conductor bars are mechanically and electrically connected with end rings. 1.2.1 Principle of Operation When a three phase AC voltage is applied to stator windings of an induction motor, a rotating magnetic field is produced. The rotating magnetic field travels at an angular speed equal to its stator frequency. It is assumed that the rotor is at standstill. The rotating magnetic field in the stator induces electromagnetic forces in the rotor windings. As the rotor windings are short circuited, current circulates in them, producing a reaction. As known from Lenz’s law, the reaction is to counter the source of the rotor current, i.e., the induced emf in the rotor creates a rotating magnetic field in the rotor. The induced emf will be countered if the difference in the speed of the rotating magnetic fields from the stator and the rotor becomes zero. When the differential speed between the rotor and magnetic field in the stator becomes zero, there is zero emf and hence zero rotor current resulting in zero torque production in the motor. This is called the synchronous speed of the machine. Now, depending on the shaft load, the rotor will settle down to a speed, always less than the speed of rotating magnetic field and torque will be produced,.The speed differential is known as the slip speed. Synchronous speed is given as ωs = 2πfs [rad/sec] where fs - supply frequency (1.1) Synchronous speed or speed of the stator magnetic field in rpm, given as Ns = Application Note 120 fs [RPM] where Pp – Number of poles Pp 5 (1.2) V1.0, 2008-08 AP08086 Constant V/F Control of Induction Motor Speed control of Induction motor 2 Speed control of Induction motor The induction motor always runs at less than its synchronous speed and by controlling the synchronous speed, the actual rotor speed can be controlled. The relationship between the synchronous speed, stator poles and the supply frequency is given in equation (1.2). 2.1 Speed Control For inverter driven induction motors, the speed can be controlled by changing the supply frequency. In order to maintain the constant air gap flux (and not allow it to saturate), the magnitude of the applied voltage needs to be varied in accordance with the frequency variation. The rms value of air gap induced emf in an induction motor is given by 1 kwΦm ωs T [v] 2 Eemf = (2.1) Eemf = 4.44kwΦmfsT [v] Where kw (2.2) - Stator winding factor Φm - Peak air gap flux fs - Supply frequency [Hz] (ωs =2π fs ) T - Number of turn per phase in the stator Neglecting the stator impedance Rs+jXls, the induced emf approximately equals the supply phase voltage. Hence, Vph ≈ Eemf (2.3) The flux is then written as Φm = Vph Kb fs (2.4) Where kb = 4.44kwT Kb is constant, so flux is approximately proportional to the ratio between the supply voltage and frequency. This is represented as Φm α Vph α Kvf fs (2.5) Where Kvf is the ratio between Vph and fs From equation (2.4), it is seen that, to maintain the flux constant, Kvf has to be maintained constant. Therefore, whenever stator frequency is changed, the stator input voltage has to be changed accordingly so as to keep Kvf constant. A number of control strategies have been formulated, depending on how the voltage to frequency ratio is maintained: a. Constant V/F control b. Constant slip-speed control c. Constant air gap flux control d. Vector control Application Note 6 V1.0, 2008-08 AP08086 Constant V/F Control of Induction Motor Speed control of Induction motor 2.2 Principle of Constant V/F Control If the effect of stator impendence, Rs+jXls, is not neglected, than relation between applied voltage and induced emf is given below Vph = Eemf + Is(Rs+jXls) Where Is (2.6) - Fundamental stator phase current, A Rs - Stator resistance per phase, Ω Xls - Stator leakage reactance per phase,Ω Induced emf is give as Eemf = j(LmIm)ωs = j λmωs Where Lm (2.7) - Magnetizing inductance per phase, H Im - Magnetizing current per phase, A ωs - Supply angular velocity, rad/sec λm - Mutual air gap flux linkages, V-s Substituting equation (2.6) in equation (2.5), Phase voltage is derived as follow Vph = j λmωs + Is(Rs+jXls) = IsRs + j (λmωs + IsXls) Vph = IsRs + jωs (λm + Is Lls) (2.8) Where Xls = Llsωs Lls - Stator leakage inductance per phase, H Hence the magnitude of phase voltage is given as 2 2 2 |Vph| = √ ((IsRs) + ωs (λm + Is Lls) ) (2.9) From equation (2.8), it is clear that V/F ratio needs to be adjusted based on, the supply frequency, the air gap flux magnitude, the stator impedance and the magnitude of the stator current. Such a complex implementation is not desirable for low performance applications, such as fans and pumps; therefore it is usual to have a preprogrammed voltage to frequency relationship as shown in Figure 1. Also an offset voltage should be added at low stator frequency to overcome the stator resistance drop. The relationship between the applied voltage and frequency is written as Vph = Voffset + Kvffs (2.10) Where Voffset = IsRs Application Note 7 V1.0, 2008-08 AP08086 Constant V/F Control of Induction Motor Speed control of Induction motor Voltage [V] Vrated Voffset Frequency [Hz] Figure 1 Voltage and Frequency profile for Constant V/F control 2.3 Principle of SVPWM Technique frated Pulse with modulation can be used to create a sinusoidal voltage by adjusting the duty cycle. The inductance of the motor will filter the PWM into a smooth signal. There are different ways to generate sinusoidal voltages with a three phase inverter. Space vector pulse with modulation is one of the popular methods to produce three phase sinusoidal voltages because it generates higher voltages with lower total harmonic distortion than sinusoidal PWM techniques. 2.3.1 Three Phase Inverter An inverter is an electronic circuit for converting direct current to alternating current. The structure of a typical three phase voltage source power inverter is shown in Figure 2. Va, Vb and Vc are the phase voltages applied to the windings of the motor. Q1 through Q6 are the six MOSFETs which are controlled by the input + + + PWM signals (A , A , B , B , C and C ), that shape the input voltages supplied to the motor terminals. Figure 2 Three Phase Voltage Source Inverter Application Note 8 V1.0, 2008-08 AP08086 Constant V/F Control of Induction Motor Speed control of Induction motor + - Note that whenever the MOSFET A is switched on, MOSFET A must be switched off and visa versa, to prevent damaging shoot-through current. This makes it easy to adopt a simple notation for describing the state of the inverter. For example, the state + when transistor A , B and C are on can be represented with the notation (100). The state where transistors + + A , B and C are on is denoted bye (011) 2.3.2 Six Step Mode In a three phase inverter, the three outputs can supply voltage at the motor terminals in any one of six active states “100”, “110”, “010”, “011”, “001” and “101”. In addition, two inactive states that produce no voltage at the motor terminals are “000” and “111”. Six resultant nonzero voltage vectors generated from six switch states respectively is illustrated in Figure 3. o They are of the same magnitude and each shifted by an angle of 60 in space. Two zero vectors (inactive states) are at the origin and supply zero voltage to a motor. These eight vectors are called the basic space vectors and are denoted by U0, U1, U2, U3, U4, U5, O000 and O111. (0,1,0) U2 (1,1,0) U1 Sector 1 Sector 2 (0,1,1) U3 Sector 0 Sector 3 Sector 5 (1,0,0) U0 Sector 4 U4 (0,0,1) Figure 3 U5 (1,0,1) Space Vector Pulse Width Modulation – Sector Consider the following sequence of states: (100, 110, 010, 011, 001, 101) Running the inverter through this switching sequence will produce the line-to-neutral voltages shown in Figure 4. This mode of operation is called “six-step mode”. Application Note 9 V1.0, 2008-08 AP08086 Constant V/F Control of Induction Motor Speed control of Induction motor Figure 4 Phase Voltage in Six Step Mode 2.3.3 Space Vector Pulse Width Modulation Space vector pulse width modulation is based on six step mode, but smoothes out the steps through averaging techniques. For example, if a voltage is required that is between two step voltages, the corresponding inverter states can be activated in such a way that the average of the step voltages produces the desired output voltage. A space vector is denoted by a magnitude and an angle. As time increases, the angle of the space vector 0 increases, causing the vector to rotate. This produces three line-to-line sinusoidal voltages that have 120 phase shifts. Application Note 10 V1.0, 2008-08 AP08086 Constant V/F Control of Induction Motor Speed control of Induction motor U1 (1,1,0) Us Sector 0 Tb U0 (1,0,0) Ta Figure 5 Approximate of Space Vector Us by Ta and Tb The goal of space vector pulse width modulation is to generate the appropriate PWM signals so that any vector can be produced. Consider a space vector (Us) located in the sector 0 defined by U0 and U1, as shown in the Figure 5. The desired space vector (Us) can be obtained by applying U0 for a percentage of time (Ta) and U1 for a percentage of time (Tb). In other words, U0 (100) state is active for time Ta, U1 (110) is active for time Tb and one of the null vectors Uz is active for T0 , where T0=Tp- Ta - Tb. The pulse period is called Tp. Space vector is represented as US TP = U0Ta + U1Tb+UzT0 (2.11) When the modulation index (the magnitude of Us) is less than 0.866, the sum of Ta and Tb will be less than Tp. The maximum modulation index for space vector pulse with modulation is 0.866. Figure 6 shows a symmetric or center aligned Space vector modulation implementation. For a given space vector (Us), switching times can be calculated using the following formulas: Ta = U[cos(α) Tb = sin(α) 3 ] 2 U sin(α) 3 (2.12) Where U = |Us| and α = angle (Us) Application Note 11 V1.0, 2008-08 AP08086 Constant V/F Control of Induction Motor Speed control of Induction motor Figure 6 SVPWM using symmetric PWM for Sector 0 Application Note 12 V1.0, 2008-08 AP08086 Constant V/F Control of Induction Motor Software Implementation 3 Software Implementation In this chapter, the implementation of a constant V/F control and SVPWM generation in the XC88x/XC878 microcontrollers are discussed in detail. 3.1 Control System Overview An implementation of a constant V/F control algorithm for inverter fed induction motors in open loop is shown in Figure 7. To implement this application in the XC88x/XC878 microcontroller, three on-chip peripheral modules are needed and they are CCU6E (CAPCOM6E), ADC (Analog-to-Digital Converter) and Timer T2. Vdc Speed Reference Speed Rampup ∫ α Space Vector Modulation PWM Unit M Inverter 3 phase Induction Motor |U| Voltage Amplitude Calculation Figure 7 Current Calculation Idclink Rshunt Implementation of Constant V/F control for Induction motor. To generate the PWM control signals for the inverter, the CCU6E module is used. For this purpose, timer T12 and the CC60SR, CC61SR, CC62SR compare registers are used. Timer T12 operation is configured for center aligned Mode. Dead-time control is enabled for the six PWM signals to avoid shoot-through current. The control algorithm and over current protection algorithm are executed in timer the T12 period match ISR. The timer T13 is configured in single shot mode. It starts synchronously with a T12 zero match. The timer T13 period match and CC63SR compare events are used to trigger ADC module for current measurement. The period register of timer T13 (CCU6_T13PR) and CCU6 channel 3 compare register (CCU6_CC63SR) are updated every second timer T12 period. And this value is always greater than 1.5 times of T12 period value. The timer T12 period match interrupt is blocked in the timer T12 period match ISR and released in timer T13 period match ISR. As a result of this setup, the timer T12 period match ISR is triggered in every second T12 period match. The ADC module is used for measurement of the motor current and speed reference value. Channel 3 and Channel 4 are used for the measurement of current at different time instants from a single current shunt. The measurement results of channel 3 and channel 4 are stored in result register 0 and 1 respectively. Also channel 0 is configured to measure the speed reference value and the result is stored in result register 2. In the Timer T2 overflow interrupt service routine, the voltage amplitude is calculated from the input speed reference value which is stored in result register 2. Also the Speed ramp up rate is controlled using Timer T2. Application Note 13 V1.0, 2008-08 AP08086 Constant V/F Control of Induction Motor Software Implementation 3.2 CCU6 Timer T12 Period Match Interrupt During this interrupt routine, all calculations necessary for constant V/F control is executed like the angle calculation, SVPWM generation, update of the CCU6E compare registers and over current check. T12 Period Match ISR Disable T12 PM Interrupt Vector Angle Calculation Sector and Sector Angle calcaulation SVPWM Update CCu6 Register Motor Current Calculation If current > set value Yes Stop Motor No Return Figure 8 Flow chart T12 Period Match ISR 3.2.1 T12 Period Match ISR Disabled Exit As the execution of all calculations takes longer than one T12 period cycle, T12 period match ISR has to be disabled during calculation. It is enabled after measuring the current value by timer T13 period match ISR. Application Note 14 V1.0, 2008-08 AP08086 Constant V/F Control of Induction Motor Software Implementation 3.2.2 Angle Calculation The vector angle is derived by integrating the motor speed over a time period. As the control strategy used here is open loop, the actual motor speed is neither measured nor calculated by any feedback mechanism. Thus the reference speed is used for the vector angle calculation. In the actual implementation, integral calculation of vector angle is achieved by simple summation of the reference speed over a time period. Now the scaling of all parameters involved in this angle calculation should be taken care of both at the input and output side as the final result of the angle calculation will be used by the SVPWM algorithm for which the 15 15] CORDIC coprocessor is used. The value range of input to the CORDIC processor is [2 -1, 2 . Hence the 15 15 output vector angle range [-π,π] is represented as [2 -1, 2 ] At the input side, the scaling for speed should be taken care of in such a way that the summation of this value gives the vector angle value. The speed scaling value is calculated as shown below: Nspeed = Where Pp ΔT 2 15 * 60 (3.1) ΔT * 2 16 * Pp - Number of Poles - Function call time (2*T12PM) The relation between actual value, normalization value and target value is given below T arg et_Value = Actual _ Value * 2 15 Normalization _ value 3.2.3 Space Vector Modulation 3.2.3.1 Calculation of PWM duty cycle for SVM (3.2) To generate six PWM signals (two signals for each phase), a reference voltage vector (Us) is required to determine the switching states and the corresponding duty cycle values. The voltage vector is represented in terms of magnitude (U) and sector angle (α). The voltage magnitude (U) can be obtained by multiplying the reference speed value with V/F constant. This calculation is done in every Timer T2 overflow ISR. Sector and sector angle (α) are calculated from vector angle, and this calculation is done using the MDU coprocessor. The Code for the sector angle calculation is shown in Figure 9 Application Note 15 V1.0, 2008-08 AP08086 Constant V/F Control of Induction Motor Software Implementation //Sector = (int)(((Angle)*6)>>16) & 0x0007); MOV MOV MOV MOV MOV MDU_MD0,Angle+1 MDU_MD1,Angle MDU_MD4,#006h MDU_MD5,#000h MDU_MDUCON,#10H; MOV JB A,MDU_MDUSTAT ACC.2,loop1 //Vector Angle Low Byte Value //Vector Angle High Byte Value loop1: // Check whether MDU is ready MOV Sector+1,MDU_MR2 // Sector value //AngleTab= (unsigned int)Angle-10922*Sector; MOV MDU_MD0,MDU_MR0 MOV MDU_MD1,MDU_MR1 MOV MDU_MD4,#006h MOV MDU_MD5,#000h MOV MDU_MDUCON,#11H; //Vector Angle*6 Low Byte Value //Vector Angle*6 Low Byte Value MOV JB // Check whether MDU is ready loop2: A,MDU_MDUSTAT ACC.2,loop2 MOV SectorAnlge,MDU_MR1 MOV SectorAnlge+1,MDU_MR0 Figure 9 // Sector Angle value Sector angle calculation using MDU Given the voltage amplitude (U) and sector angle (α), switching times (Ta and Tb) can be calculated using the following formulae. Ta = U[cos(α) Tb = sin(α) 3 ] = 2 U sin(60 α) 3 2 U sin(α) 3 (3.3) Where U = |Us| and α = angle (Us) In general, a look-up table could be used for calculating sine and cosine of the sector angle. In a look-up table approach, the table needs to be updated if the user changes PWM period (timer T12 period value) or modulation index. In this current implementation, it is possible to avoid the look-up table by using the CORDIC coprocessor for the sine and cosine calculations. For the calculation of the switching times, CORDIC coprocessor is used. CORDIC Configuration Circular Rotation Mode Xinital = U Yinital =0 Zinital = α Xfinal = K[X cos(Z) –Y sin(Z)] Yfinal = k[Ycos(Z) + X sin(Z)] Zfinal =0 MPS=0 Tb = Yfinal Interrupt Disabled Table 1 CORDIC Configuration for switching time calculation Application Note 16 V1.0, 2008-08 AP08086 Constant V/F Control of Induction Motor Software Implementation Compare values are calculated from Ta and Tb values using following formulae: VTa = VTb = VTc = VTd = Where T12P 3.2.4 1 2 1 2 1 2 1 2 (T12P Ta Tb) (T12P + Ta Tb) (T12P + Ta + Tb) (T12P Ta + Tb) (3.4) - PWM period (Timer T12 period value) Update of CAPCOM6 Registers The detailed compare values of CAPCOM6 compare registers at each sector is given in Table 2. Based on the current sector, the corresponding compare values are loaded into the respective shadow registers. Shadow transfer will happen during timer T13 period match ISR. Sector PWM Generation ADC Trigger CC60SR CC61SR CC62SR CC63SR T13PR 0 VTa VTb VTc (3T12P +Tb)/2 +D (5T12P +Ta)/2 +D 1 VTd VTa VTc (3T12P +Tb)/2 +D (5T12P +Ta)/2 +D 2 VTc VTa VTb (3T12P +Tb)/2 +D (5T12P +Ta)/2 +D 3 VTc VTd VTa (3T12P +Tb)/2 +D (5T12P +Ta)/2 +D 4 VTb VTc VTa (3T12P +Tb)/2 +D (5T12P +Ta)/2 +D 5 VTa VTc VTd (3T12P +Tb)/2 +D (5T12P +Ta)/2 +D Table 2 3.2.5 Compare Values for CAPCOM6 compare registers at each sector Current Measurement In order to trigger the current measurement very accurately, the amplified voltage of the current shunt is fed to two ADC channels, which are measured at different time intervals and stored in separate result registers. The ADC conversions are consecutively started every second period (2*T12) of the modulation. The first current measurement is triggered by the compare match of CC63 and sampled by the ADC channel 3. The second measurement by ADC channel 4 is triggered by the period match of timer T13. ADC channel 3 is used to measure the positive current and channel 4 measures the negative current. Figure 10 shows the usage of timers T12 & T13, the interaction of the timer interrupts, and the event triggers for the start of ADC conversions. Application Note 17 V1.0, 2008-08 AP08086 Constant V/F Control of Induction Motor Software Implementation Figure 10 Timing Diagram of Timer T12 and T13 The following table describes, the motor phase current measured at each sector during both trigger events. Sector Idc link Table 3 0 1 2 3 4 5 CC63 CM ISR Ia Ib Ib Ic Ic Ia T13 PM ISR -Ic -Ic -Ia -Ia -Ib -Ib Phase Current measured at each Sector Two phase currents are calculated for each sector from the ADC registers. Motor current is computed from these two phase current values using the MDU coprocessor unit. Over current protection is also implemented in this software. If the motor current value exceeds the set limit value, motor will be stopped. The maximum current range is defined as follows: Im ax = Where Vadcref Vadcref Rshunt * Gop (3.5) - ADC reference Voltage RShunt - Current shunt resistor value GOP - Amplifier gain In this implementation 10 bit ADC value is multiplied by 8. The current scaling is given below. NI = Application Note Im ax * 2 15 (3.6) 8 * 2 10 18 V1.0, 2008-08 AP08086 Constant V/F Control of Induction Motor Software Implementation 3.3 Timer T2 Overflow Interrupt In this motor control application, Timer T2 is used to generate interrupts for every 300 µS. During this interrupt routine, the speed reference value is read from ADC result register 2 and voltage amplitude value is calculated. Figure 11 Flow chart Timer T2 Overflow ISR Given speed reference value, the voltage amplitude value can be calculated using the following formulae U =V0 + KVF*fs Where V0 - Voltage offset value (Is*Rs) KVF - V/F Constant fs - Supply frequency fs Where Ns (3.7) = Ns * Pp 120 (3.8) - Speed reference value Application Note 19 V1.0, 2008-08 AP08086 Constant V/F Control of Induction Motor Software Implementation The voltage amplitude value depends on the maximum duty cycle of the PWM which is defined by the period value of timer T12. So V/F constant (KVF) value is scaled depending upon PWM period value (Timer T12). Normalization value of KVF is given below. NVF = Where NV 2 15 * 120 * NV Nspeed *Pp (3.9) - Voltage Normalization value Nspeed - Speed Normalization value Pp - Number of poles Voltage normalization value calculation is given below NV = Where Vdclink 2 15 * 3 * Vdclink 2 * fpwm * 3*2 fcpu (3.10) - DC link Voltage [v] fPWM - PWM frequency fCPU - CPU frequency Speed slew rate is controller by Timer T2. Slew rate calculation is given below Slew _ Rate = Where Nspeed Nspeed (T2 * TR * 2 15 ) [RPM/S] (3.11) - Speed Normalization Value T2 - Timer Overflow (µS) TR - Rampup Counter Application Note 20 V1.0, 2008-08 AP08086 Constant V/F Control of Induction Motor Appendix A Code for Switching Time Calculation Appendix A ORL CLR MOV MOV MOV MOV MOV MOV MOV MOV Code for Switching Time Calculation SYSCON0,#01 a CD_STATC,#00 CD_CON,#10 CD_CORDYL,#0 CD_CORDYH,#0 CD_CORDZH,AngleTab CD_CORDZL,AngleTab+1 CD_CORDXH,Amplitude CD_CORDXL,Amplitude+1 //switch to mapped SFRs //sector angle Value //Voltage Amplitude Value //CORDIC starts autmatically loop3: MOV JNB A,CD_STATC ACC.2,loop3 //wait for CORDIC with circular rotation MOV JB MOV MOV SJMP a,Sector+1 ACC.0,sector_odd1 Tb+1,CD_CORDYL Tb,CD_CORDYH sector_exit1 //switching time calculation Ta+1,CD_CORDYL Ta,CD_CORDYH //if sector 1,3,5; Ta=M*sin(sector_angle) sector_odd1: MOV MOV sector_exit1: SETB MOV SUBB MOV SUBB JNC CLR MOV SUBB MOV MOV SUBB MOV SJMP value_high: CLR MOV MOV value_exit: C a,AngleTab+01 A,#0AAH A,AngleTab A,#02AH value_high //if sector 0,2,4; Tb=M*sin(sector_angle) //Calculation of 60-sector angle Value C A,#0AAH A,AngleTab+01H AngleTab+01H,A A,#02AH A,AngleTab AngleTab,A value_exit A AngleTab,A AngleTab+01H,A Application Note 21 V1.0, 2008-08 AP08086 Constant V/F Control of Induction Motor Appendix A Code for Switching Time Calculation ORL CLR MOV MOV MOV MOV MOV MOV MOV MOV SYSCON0,#01 a CD_STATC,#00 CD_CON,#10 CD_CORDYL,#0 CD_CORDYH,#0 CD_CORDZH,AngleTab CD_CORDZL,AngleTab+1 CD_CORDXH,Amplitude CD_CORDXL,Amplitude+1 //switch to mapped SFRs MOV JNB A,CD_STATC ACC.2,loop4 //wait for CORDIC with circular rotation // 60-sector angle Value //Voltage Amplitude Value //CORDIC starts autmatically loop4: MOV JB MOV MOV SJMP sector_odd2: MOV MOV sector_exit2: ANL a,Sector+1 ACC.0,sector_odd2 Ta+1,CD_CORDYL Ta,CD_CORDYH sector_exit2 //switching time calculation //if sector 0,2,4; Ta=M*sin(60-sector_angle) Tb+1,CD_CORDYL Tb,CD_CORDYH //if sector 1,3,5; Tb=M*sin(60-sector_angle) SYSCON0,#0FEH //switch to standard SFRs Application Note 22 V1.0, 2008-08 http://www. inf ineon.com Published by Infineon Technologies AG

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