Electromagnetic Interference (EMI, also called radio-frequency interference or RFI when in radio frequency) is a disturbance that affects an electrical circuit due to either electromagnetic induction or electromagnetic radiation emitted from an external source.
All motors create magnetic fields because of the currents passing through the windings - or just because they have strong magnets (permanent magnet designs). Stronger magnets will create stronger fields - which will extend to greater distance from the magnet surface.
The switching of a magnetic field will in turn create a current swing in neighboring conductors. In a DC machine, the switching action is in the rotor circuit, with the mechanical interaction between brush and commutator occurs. Both the speed of the switching (or slope, or ramp rate, which is really high for commutation) and the frequency can create interference.
I have never read about in texts but I would very much assume it is for two reasons:
- The current reverses direction in the armature continuously but it is a very sudden current reversal with very fast voltage rate of change (dv/dt). These quick changing currents (almost square waves) will produce electromagnetic fields into the Radio Frequency spectrum, linked to their Fourier spectrums.
- No matter how good the commutation is, of the brushes against the commutator - there will always be sparks (however small) or arcing between these components. Sparks have a very wide-bandwidth radio frequency emission - also naturally producing radio interference.
Most DC motors are used for variable speed/positioning applications. When AC motors are used for these applications the input to those motors must have a switching power supply, such as a VFD. That causes EMI also. The AC motor is the load. Without the load there would be no EMI.
An AC induction motor with broken rotor bars can create EMI. Wound Rotor induction motors (slip ring induction motors) can cause EMI because of the brush/slip ring action. The old repulsion/induction AC motors can create EMI. Not to mention the universal AC/DC motor used in everything from toys to electric tools can definitely cause EMI.