int PWMPin = 10;
int motorSpeed = 0

void setup()
{

}

void loop()
{

  for (motorSpeed = 0 ; motorSpeed <= 255; motorSpeed += 10)
  {
    analogWrite(PWMPin, motorSpeed);
    delay(30);
  }

  for (motorSpeed = 255 ; motorSpeed >= 0; motorSpeed -= 10)
  {
    analogWrite(PWMPin, motorSpeed);
    delay(30);
  }
}