3 Phase Wave Generation

1
3 Phase Wave Generation

• 3 Phase waveform generation using Timer/Counter1
  and 3 on atmega128

2
Topics

• Sinusoidal Wave Generation Theory
• Atmega128 setup and considerations
• Results

3
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

4
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
5
Topics

• Sinusoidal Wave Generation Theory
• Atmega128 setup and considerations
• Results

6
uC Sinusoidal PWM

• Line to neutral voltages approximated using PWM
  switching
• Average output voltage controlled by duty cycle
• uC duty cycle OCRnAC/Top
• Top and pre-scaler are set so switching frequency
  is at 20kHz

7
Lookup Table Generation

• Using Excel duty cycle of was approximated
• One cycle broken up into 192 steps
• Sin(2PIi/192), i 0..191
• OCRnAC Round(Sin(2PIi/192)TOP)
• TOP value is 400 for my 20kHz switching frequency
• Looking up values frees up uC for other tasks

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sine Look-Up Table
Round(UTOP)?
sin(2PIi/192)?
9
Sin lookup table array
const uint16_t sin_lookup1923
//OCRnA, OCRnB, OCRnC 0, 0, 346, 13, 0, 353
, 26, 0, 359, 39, 0, 364, 52, 0, 370, 65, 0
, 374, 78, 0, 379,
10
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

11
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
• OCRnAC value generated from lookup table
• OCRnA sin_lookupcounter
• OCRnB sin_lookupcounter1
• OCRnC sin_lookupcounter2
• Interrupts occur fdesired192 times per second

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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 OCR1AC of one Timer/Counter1 and the
  counters three output pins a three phase waveform
  can be generated
• Bonus I dont have to buy anything to
  implement the design

13
Viewing Wave forms
RC filters and Resistive loads were used to view
the waveforms
PortB.5 PortB.6 PortB.7
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Topics

• Sinusoidal Wave Generation Theory
• Atmega128 setup and considerations
• Results

15
Results
Waveform growth
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Results
C
B
A
A-B
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Shoot Thru

• If S and S- on at same time the circuit would
  short

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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 OCRA1 (set later)

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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
20
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/c
  hannel.html?channelff80808112ab681d0112ab6b2dfc07
  56
• 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