Electromagnetic Noise in Motors:Definition, Characteristics And Causes

Electromagnetic noise in motors is a common issue during motor operation. It originates from electromagnetic force waves generated by the alternating magnetic field in the air gap between the stator and rotor, as well as the magnetostrictive effect of the iron core, which induces vibrations of the stator iron core and frame and further radiates noise. Distinct from mechanical noise and aerodynamic noise, electromagnetic noise has complex causes and impairs the operational performance of motors. An in-depth understanding of its definition, characteristics and root causes serves as a fundamental prerequisite for motor performance optimization and noise reduction. Specifically, it refers to airborne noise radiated outward when periodic electromagnetic force waves produced by the alternating magnetic field in the stator-rotor air gap trigger forced vibration of the stator core and frame, together with vibration noise induced by the magnetostrictive effect of the iron core.

Electromagnetic noise in motors can be clearly differentiated from mechanical noise and aerodynamic noise via the following typical characteristics:

After the motor is powered off, the rotor keeps rotating due to inertia, yet the noise disappears instantly and completely. This is the simplest method to distinguish electromagnetic noise from other types of noise. In practical tests, the motor speed before power cut shall be higher than the speed where suspected electromagnetic noise occurs. Since the rotating speed drops rapidly after power disconnection (especially for permanent magnet motors), a relatively high shutdown speed is required to ensure the operating speed range after power-off covers the speed band where electromagnetic noise appears.

Low-frequency electromagnetic noise usually presents as a dull resonant humming sound mainly caused by low-order magnetic field harmonics; high-frequency electromagnetic noise manifests as sharp whistling or squeaking noise, which primarily stems from PWM modulation harmonics, slot harmonics and high-order magnetic harmonics.

Resonance will occur when the excitation frequency of electromagnetic force waves coincides with or approximates the natural vibration frequency of the stator core and frame, leading to an exponential rise in noise level and resulting in harsh abnormal noise.

The formation mechanism of electromagnetic noise follows the chain: air-gap magnetic field distortion → alternating electromagnetic force waves → forced structural vibration → noise radiation, with air-gap magnetic field distortion as the core cause.

1. Radial Electromagnetic Force Fluctuations Induced by Air-gap Magnetic Field Distortion

The air gap between the stator and rotor is the core medium for magnetic field transmission inside a motor. Under ideal conditions, the air-gap magnetic field follows a standard sine waveform with uniformly distributed and symmetric electromagnetic forces free of periodic fluctuations, thus generating no vibration or noise. In practical engineering applications, however, various design and manufacturing deviations such as pole-slot matching, stator-rotor eccentricity, asymmetric magnetic circuits and unbalanced phase currents will distort the magnetic field and generate numerous harmonic magnetic fields. When superimposed on the fundamental magnetic field, these harmonic fields produce radial electromagnetic force waves of different orders and frequencies. Acting as periodic alternating loads on stator teeth and the iron core, radial forces continuously stretch and squeeze the stator structure to induce high-frequency stator vibration. The vibration energy is transmitted to the motor frame, agitates ambient air, and eventually forms electromagnetic noise. This principal mechanism accounts for more than 80% of total electromagnetic noise energy.

2. Slot Harmonic Excitation Caused by Slotting Effect

Slots machined on the stator and rotor break the uniformity of magnetic circuits in the air gap and trigger slot harmonic magnetic fields. Stator and rotor slot openings lead to periodic fluctuations of air-gap magnetic flux density, generating slot harmonic electromagnetic forces with fixed orders and stable amplitudes. Such harmonic forces feature fixed frequencies and strong excitation capacity, which constitute the leading cause of high-frequency electromagnetic whistling in small and medium-sized permanent magnet motors and induction motors. Particularly when the stator and rotor slot numbers are unreasonably matched, the amplitude of slot harmonic forces increases sharply, which easily resonates with the stator structure and aggravates noise defects. Therefore, stator-rotor slot matching is a critical parameter in motor design. It must be confirmed in the initial design phase, as modifications in the later stage will be extremely difficult.

3. Magnetostrictive Effect and Interference from Control Harmonics

On one hand, silicon steel sheets of the motor iron core undergo periodic tiny expansion and contraction deformation under alternating magnetic fields, known as the magnetostrictive effect. Such high-frequency micro-deformations drive overall vibration of the iron core and produce low-frequency electromagnetic humming, which is especially prominent in high-power motors with large-sized iron cores. On the other hand, variable-frequency adjustable-speed motors are powered by frequency converters adopting PWM modulation. Square-wave pulse modulation introduces abundant high-frequency voltage and current harmonics, which further distort the air-gap magnetic field, generate high-frequency electromagnetic force waves and cause the unique high-frequency whistling noise of variable-frequency motors. In addition, air-gap eccentricity, asymmetric winding distribution, winding turn errors, inconsistent magnetic permeability of silicon steel sheets and other defects will worsen magnetic field distortion and indirectly intensify electromagnetic noise.

The essence of electromagnetic noise in motors reduction lies in harmonic optimization, among which slot harmonics are the top priority. Other types of harmonics are relatively easier to optimize and exert minor influences on electromagnetic noise. Key factors affecting slot harmonics include pole-slot matching, slot opening width and slot opening depth, which shall be comprehensively evaluated together with other motor performance indicators during design.