Electrical – Phase Cut to Control Speed of AC Induction Motor with Arduino

^{simulate this circuit – Schematic created using CircuitLab}

So I'm trying to speed controlling a single phase AC Induction Motor(For Fan). And I am trying to use phase cutting to control it. So I use arduino to detect zero cross of AC Wave, and then cut the wave with triac, also I am adding a snubber circuit. But I can't reach 90% of the speed, and when I try to set the speed to 50% the motor goes humming.

What is the cause that I can't reach 90% of the speed and why the motor goes humming when 50%? Also, is there another available option to control single phase AC induction motor using arduino? I already try to search about VSD/VFD but after some google searching it won't work with a single phase(only 3-phase motor).

For the diagram, all ground in schematic is arduino ground, and voltage source is from the same source.

int AC_LOAD = 3;    // Output to Opto Triac pin
int dimming = 128;  // Dimming level (0-128)  0 = ON, 128 = OFF

void setup()
{
  pinMode(AC_LOAD, OUTPUT);// Set AC Load pin as output
  attachInterrupt(0, zero_crosss_int, RISING);  // Choose the zero cross interrupt # from the table above
}

//the interrupt function must take no parameters and return nothing
void zero_crosss_int()  //function to be fired at the zero crossing to dim the light
{
  // Firing angle calculation : 1 full 50Hz wave =1/50=20ms 
  // Every zerocrossing thus: (50Hz)-> 10ms (1/2 Cycle) 
  // For 60Hz => 8.33ms (10.000/120)
  // 10ms=10000us
  // (10000us - 10us) / 128 = 75 (Approx) For 60Hz =>65

  int dimtime = (75*dimming);    // For 60Hz =>65    
  delayMicroseconds(dimtime);    // Wait till firing the TRIAC
  digitalWrite(AC_LOAD, HIGH);   // Fire the TRIAC
  delayMicroseconds(10);         // triac On propogation delay (for 60Hz use 8.33)
  digitalWrite(AC_LOAD, LOW);    // No longer trigger the TRIAC (the next zero crossing will swith it off) TRIAC
}

void loop()  {
  i = 90; 
    dimming=i;
    delay(10);       
}

I got this code from http://www.alfadex.com/2014/02/dimming-230v-ac-with-arduino-2/ and I just modify the loop so I can get a constant speed. I give i = 90, so I can get approximately 70% of the speed( 90 / 128 ).
Thanks!