fig. 1

fig. 2

As can be seen, the differences are considerable. At 60% of its speed, a motor controlled by auto-transformer or Triac already absorbs 50% of the power and has a very steep absorption/speed ratio curve which means a high level of absorption even at low speeds. The current absorption of the motor controlled by inverter is directly proportional to the speed. Studying the two graphs, it can be seen that at 60% of its speed, the motor controlled by inverter consumes approximately 60% less than the motor controlled by transformers/Triac. Taking an average of the operating speeds (operating time for a given speed percentage) per cycle and/or per year, we find that the average fan speed is approx. 60% and therefore with an inverter control we have a saving in energy consumption of approximately 50%. Another important advantage is the longer mechanical and electrical life of the fan. If we examine the two graphs again, we can see that at low speed percentages, current absorption in the inverter system is clearly lower than with the Triac/transformer system; this means that the motor overheats less at the very moment when the fan blades are rotating more slowly and cooling it less efficiently. Noise level is another important factor and is much lower than with a traditional Triac system. This is due mainly to the drive waveform which, while varying in voltage and frequency, remains perfectly sinusoidal without causing any distortion and consequent saturation of the motor iron. Moreover, as regards to the electric system, the current absorption of the motors controlled by inverter is cosf = 1, while the cosf ranges from 0.5 to 0.6 with the other controls. This results into a lower overload of the lines and a lower voltage drop.