#define NPWM  360
#define PPWM 800      // 800 = 20kHz PWM

uint16_t p1count,p2count,p3count;
int pm;
float fpm;
uint16_t l[NPWM];
int _v;

void setup()
 {
 int i;
 float f;
 // _v is used to reduce the amplitude of the SPWM.
 // I have code that takes a current sensor output on DC supply
 // and reduces amplitude to maintain a maximum current setpoint.
 // the AC servo motors have an inverse relationship of
 // current to RPM

 _v = 250;    // for now, maximum current or amplitude.

 for(i=0; i < NPWM; i++)
   {
   f = 2.0 * 3.14159 * (float)i/(float)NPWM;
   f = 0.8*(float)PPWM * (1.0 + 1.0*sin(f)) * 0.5;
   l[i] = (int)(f);  
   }
 t135();
 do_phase(0);
 Serial.begin(115200);
 pm=0;
 fpm = 0.0;
 }

void t135()
 {
// set up the 3 timers
 pinMode(46,OUTPUT);
 pinMode(5,OUTPUT);
 pinMode(11,OUTPUT);
 TCCR1A = TCCR3A = TCCR5A = _BV(COM5A1) |  _BV(COM5B1) | _BV(WGM51);
 TCCR1B = TCCR3B = TCCR5B = _BV(WGM53) | _BV(WGM52) | _BV(CS50);
 OCR1A = OCR3A = OCR5A = 1;
 ICR1 = ICR3 = ICR5 = PPWM;
 TIMSK1 |= (1 << TOIE5);
 TIMSK3 |= (1 << TOIE5);
 TIMSK5 |= (1 << TOIE5);
 }

ISR(TIMER3_OVF_vect)        
 {
// each timer needs the new PWM width every 1/20,000 second
 long c;
 c = ((long)l[p2count] * (long) _v) >> 8;
 OCR3A = c;
 }
 
ISR(TIMER1_OVF_vect)        
 {
 long c;
 c = ((long)l[p1count] * (long) _v) >> 8;
 OCR1A = c;
 }

ISR(TIMER5_OVF_vect)        
 {
 long c;
 c = ((long)l[p3count] * (long) _v) >> 8;
 OCR5A = c;
 }

void do_phase(int pc)
{
 // have counters pointing at 3 positions in the 360 deg sinwave
 // clock them forward or back at the speed obtained from the pot
 // on A0
 p1count = pc;
 p2count = pc + 120;
 if (p2count >= 360)    p2count = p2count - 360;
 p3count = pc + 240;
 if (p3count >= 360)    p3count = p3count - 360;  
}

void loop()
{
int p,i,cmax;
float v;

// read pot on A0 for phase increment or velocity
p = analogRead(0);
v =  (float)(p - 512)/100.0;

// update phase with phase increment from pot on A0
fpm += v;
if (fpm >= 360.0)
 fpm -= 360.0;
if (fpm < 0.0)
 fpm += 360.0;  
pm = (int)fpm;
do_phase(pm);

}