Inter-harmonics, specially first and second order inter-harmonics, in modulation with fundamental frequency, 2nd order harmonic, or also (with lower probability) higher order harmonics, can produce intensive flicker.

One of the main sources of inter-harmonics is VSD (Variable Speed Drive). Specially in places where lighting loads are powered by the UPS, due to the low short circuit  level of  DC bus,  inter-harmonics have a high potential to generate flicker.

Based on “William Stanley Jevons’  theory” , this problem also can be theorized as follows:

1 -Increasing electricity consumption prices leads to using more and more energy saving technologies such as VSDs.

2 – Using VSDs leads to increasing of inter-harmonics and also flicker.

3 – Flicker leads to Increased eye diseases.

4 – Increased eye diseases  leads to increased treatment costs.

So costs will be balanced again near the previous point.