What makes the inductive load like a motor heat up when used on a modified sine wave inverter. Is it because of the back emf caused by sudden drop of voltage by the square wave?
I think the idea is as follows: any repetitive wave form may be decomposed in a sum of sine waves with frequencies multiples of the base one (Fourier series). The base frequency part produces work, but other frequencies are two fast and the slip is too much so their energy is lost whithout producing work, or better said the ratio of losses to work produced is too high.
Modified sinewave inverters do not necessarily produce a pure sine wave: -
Image from here.
Because of the harmonics of the "modified sinewave", eddy current losses are increased (due to the higher frequency harmonics) and, this causes heat increases over and above the heat produced when driven by a pure sinewave: -
Image taken from this article.
Maybe this is what you refer to?
Is it because of the back emf caused by sudden drop of voltage by the square wave?
Motors heat up, regardless of being on a "modified sine wave" inverter or standard AC main for a variety of reasons. Those include:
- Resistive losses in the windings.
- Eddy current losses in the core materials.
- Friction losses in the bearings of the moving parts.
Back EMF does not cause losses.
Back EMF will be a sinewave.
The difference between the drive waveform and the back EMF is the voltage driving current in the motor. Normally, the drive would also be a sinewave, so the difference would be relatively small and proportional to the torque demand from the load.
With modified sine, even at no torque load (above friction, windage) the difference is relatively large, (and far from sinusoidal) causing relatively large currents to produce heating.