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3 Phase Wave Generation 3 Phase waveform generation using Timer/Counter1 and 3 on atmega128 Topics ► Sinusoidal Wave Generation Theory ► Atmega128 setup and considerations ► Results 3 Phase line to line voltages ► uC used to control switch states ► +E or 0 is shown to the load on each phase Depends on if upper or lower switch is on 3 Phase line to line voltages ► 2/3 on switching scheme used ► Allowing switches to be off for 1/3 of the time reduces switching power losses ► Line to neutral voltages are not sinusoidal, but line to line are!!! *U, V, W are the desired line to neutral voltages Topics ► Sinusoidal Wave Generation Theory ► Atmega128 ► Results setup and considerations uC Sinusoidal PWM ► Line to neutral voltages approximated using PWM switching ► Average output voltage controlled by duty cycle ► uC duty cycle = OCRnA:C/Top ► Top and pre-scaler are set so switching frequency is at 20kHz Lookup Table Generation ► Using Excel duty cycle of was approximated ► One cycle broken up into 192 steps ► Sin(2*PI*i/192), i = 0..191 ► OCRnA:C = Round(Sin(2*PI*i/192)*TOP) ► TOP value is 400 for my 20kHz switching frequency ► Looking up values frees up uC for other tasks sine Look-Up Table sin(2*PI*i/192) i i 0-63 64-127 128-191 Round(U*TOP) Sin lookup table array const uint16_t sin_lookup[192*3] = { //OCRnA, OCRnB, OCRnC 0, 0, 346, 13, 0, 353, 26, 0, 359, 39, 0, 364, 52, 0, 370, 65, 0, 374, 78, 0, 379, … … } Timer/Counter 1/3 Setup ► Phase & Frequency correct PWM ► Dual Slope Operation ► Used to control switches Timer/Counter1 Upper Switches Timer/Counter3 Lower Switches ► Timer/Counter 3 switches inverse of Timer/Counter 1 ► Output bit is set or cleared on compare match Timer/Counter 2 ► Updates Timer/Counter1/3 OCRnA,OCRnB,OCRnC to control duty cycle ► Counter variable incremented by 3 every time Timer/Counter 2 interrupts ► OCRnA:C value generated from lookup table OCRnA = sin_lookup[counter] OCRnB = sin_lookup[counter+1] OCRnC = sin_lookup[counter+2] ► Interrupts second occur fdesired*192 times per My epiphany + ≈ + •One output port of uC looks like one switching stage •Switches between 0 and Vcc •Upper and Lower switches never on at same time (no shoot-thru) •Using OCR1A:C of one Timer/Counter1 and the counters three output pins a three phase waveform can be generated •**Bonus** I don’t have to buy anything to implement the design Viewing Wave forms RC filters and Resistive loads were used to view the waveforms PortB.5 PortB.6 PortB.7 Topics ► ► Sinusoidal Wave Generation Theory Atmega128 setup and considerations ► Results Results Waveform growth Results A B A-B C Shoot Thru If S+ and S- on at same time the circuit would short Shoot Thru To avoid shoot-thru add a switching delay Amount to increment or decrement by depends on switching frequency OCR1A = OCRA-1 (cleared sooner) OCR3A = OCRA+1 (set later) Other Considerations Make sure TCCR1 and TCCR3 are synchronous Using SFIOR Stop pre-scalers (stop the clocks) Set TCCR1 and TCCR3 to zero Restart prescalers Setting TSM, asserts a reset signal to PSR0 and PSR321. Upon clearing TSM PSR0 and PSR321 are set to zero and timers/counters begin counting synchronously References Generate advanced PWM signals using 8-bit mCs Michael Copeland, Infineon http://www.edn.com/article/CA52686.html AP16097: Different PWM Waveforms Generation for 3-Phase AC Induction Motor with XC164CS Infineon http://www.infineon.com/cms/en/product/channel.html?channel=ff80808112ab681d0112ab6b 2dfc0756 AVR447: Sinusoidal driving of three-phase permanent magnet motor using ATmega48/88/168 AVR494: AC Induction Motor Control Using the constant V/f Principle and a Natural PWM Algorithm AVR http://www.atmel.com/products/AVR/mc/?family_id=607