Does Motor Noise Relate to Power Frequency? Insights on Design and VFD Challenges

Is Motor Noise Related to Power Frequency?

Generally speaking, motor noise has little direct correlation with the power supply frequency. When designing a motor, engineers focus heavily on ensuring stable performance under the specified frequency. They rarely adopt unconventional electromagnetic or mechanical designs. Instead, they tend to follow proven and validated electromagnetic structures and mechanical frameworks. Even innovative designs are based on deep experience and repeated verification processes, aiming to minimize any potential electromagnetic or mechanical noise that could be coupled with the power frequency.

For example, considerations such as shaft deflection calculations, housing rigidity checks, skewed slots in cast aluminum rotors, skewed stator slots in permanent magnet motors, and offset arrangements of rotor magnets — all these design elements stem from established practices or are inherited from proven industry solutions. Of course, this principle relies on the premise that a motor is designed by seasoned experts employing either standard or masterful design strategies. No serious manufacturer would take major risks in this area lightly.

However, exceptions do exist — otherwise, motor design would be too simple! Like a brilliant doctor reversing a critical condition with a single treatment, outstanding motor designers and experts often show extraordinary insight and problem-solving ability when faced with exceptional challenges.

To achieve ultra-low noise under specific operating conditions, top-tier designers and industry veterans have developed sinusoidal or low-harmonic windings. Even then, it's not a simple matter of copying; careful selection of electromagnetic and structural strategies is essential. Otherwise, only a few manufacturers would be able to win such specialized orders.

As variable frequency drives (VFDs) become more widely used in motor applications, new issues have emerged that were previously rare. A typical example is the piercing, whistling noise that sometimes occurs at certain frequencies during operation — and avoiding it can be quite tricky. In these cases, noise phenomena related to power frequency, which were once rare, now appear. If a designer lacks enough practical experience, solving such issues can be extremely difficult.

The Development Trend of Variable Frequency Motors

With the rapid advancement of power electronics and new semiconductor technologies, AC motor speed control has become more refined and widespread. Modern VFDs offer excellent output waveforms and highly competitive performance-to-cost ratios, leading to their broad adoption in industries using AC motors.

Applications include large motors for steel rolling, small and medium-sized motors for roller tables, traction motors for railways and urban transportation, elevator motors, crane motors for container handling, as well as pumps, fans, compressors, and household appliances. All have seen great success using AC motors with frequency control.

In induction motors, when the slip is relatively small, speed is roughly proportional to supply frequency. Thus, adjusting the frequency directly changes the motor speed. In variable frequency operation, maintaining a constant main magnetic flux is critical. If the flux is too high, it causes magnetic saturation, increasing excitation current and lowering power factor. If the flux is too low, motor torque will drop.

In recent years, the global market for variable frequency drive systems has been growing rapidly at an annual rate of 13% to 16%, with a clear trend toward replacing traditional DC drive systems.

However, conventional asynchronous motors, originally designed for constant-frequency, constant-voltage operation, face major limitations when applied to VFD systems. In response, special-purpose variable frequency motors have been developed internationally, tailored to specific applications and requirements. These include low-noise, low-vibration motors, motors optimized for enhanced low-speed torque, high-speed motors, motors equipped with tachogenerators, and motors for vector control applications.