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 setup and considerations ► ► Results

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 = ► 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)‏ Round(U*TOP)‏ i i

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 occur f desired *192 times per second

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 CBA A-B

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)

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 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

References Generate advanced PWM signals using 8-bit mCs Michael Copeland, Infineon AP16097: Different PWM Waveforms Generation for 3-Phase AC Induction Motor with XC164CS Infineon Infineon 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