Interharmonics are distortions in the current or voltage waveforms, like ordinary harmonics. However, they differ from normal harmonics in that the frequency of these waveforms are not integral to the fundamental. The waveforms are found between the normal harmonics of voltage and current.

For instance, while normal harmonics have frequencies such as 150Hz (3rd Harmononic) and 250Hz(5th Harmonic), interharmonics appearing in frequencies between these harmonics such as 160 Hz, 198 Hz, etc. However, they can appear as discrete frequencies in a spectrum. Interharmonics are believed to be caused due to transient changes in the value of current and voltage.

They often accompany normal harmonics. Fast Changes in the phase angles of currents and voltages can also cause interharmonics. Another cause of interharmonics can be asynchronous switching of semiconductor switches such as in systems which use pulse width modulation. Arcing loads, such as welding machines and arc furnaces, are also believed to cause interharmonics.

The Effects of interharmonics are saturation of current transformers, disturbance of telecommunication signals, etc. Interharmonics are known to cause low frequency mechanical oscillations.

Filters can be used to mitigate the effects of interharmonics. However, factors such as resonance, power loss, etc need to be kept in mind while making the choice of filters. Series filters are generally used against interharmonics.

Subharmonics is a term used to refer to Harmonics which have a frequency less than the fundamental frequency i.e. 50 Hz.