How can variable speed AC motor control reduce NVH and improve EV driving comfort?

The Real NVH Problem in EVs

 

Many drivers often hear a very sharp, high-pitched and irritating sound when their vehicles are starting to accelerate or undergoing regenerative braking. This noise, vibration and that roughness (commonly referred to as NVH) do not come from the road or wind noise, but are transmitted from the drive motor and its controller. The main drive motor is quite different from other auxiliary components. It has to operate within a wide range of rotational speeds and torque. If the control strategy is not properly implemented, the electromagnetic force will produce annoying noises, disrupting the originally quiet interior environment. This is where variable speed AC motor control becomes essential for real-world driving comfort.

Precisely regulate pressure and reduce noise

Many engineers have a misconception that the AC drive system can easily achieve smooth torque output. In fact, the traditional six-step commutation or the inadequate parameter adjustment of PWM control can easily cause torque fluctuations at specific harmonic frequencies. When stopping at the intersection and then starting again, the vibrations of the motor and the bracket will directly turn into harsh metallic sounds. Listening to these sounds for a long time can easily make people feel tired and agitated. This long-term, subtle vibration will significantly affect the driving experience of passengers and reduce the ride comfort of electric vehicles. To solve this problem, precise and cycle-by-cycle real-time regulation of voltage and frequency is necessary. Over time, these minor vibrations can make passengers feel uncomfortable, making electric vehicles seem cheap. The solution requires precise and cycle-by-cycle adjustments of voltage and frequency.

Smooth start without any jolts

A well-designed speed control algorithm will allow the current in the motor winding to gradually build up rather than immediately output the maximum torque.When the vehicle is traveling at a low speed on a congested section of road, the controller can smoothly increase the fundamental frequency from 0 Hz to 20 Hz within 1 second, while actively suppressing low-order harmonics. As a result, the driver will only experience a smooth and quiet driving sensation, without any annoying noise.The same technology can also eliminate the "stuttering" sensation that often occurs when driving in parking lots.

Parameter matching

To fully suppress torque ripple in a production vehicle, automakers rely on a purpose-tuned ac motor and controller as an integrated system. The controller’s field-oriented control (FOC) algorithm must know the exact flux linkage and inductance profile of the motor’s magnets. When these parameters are correctly matched, the controller cancels the 6th and 12th harmonic currents that cause the most audible noise. Without this combination, even with the use of top-notch hardware, it is impossible to achieve the ultimate silence in the cabin.

The genuine driving experience

A fleet test of a popular electric sedan confirmed this difference. In a light-load condition, the original controller produced a buzzing noise of 72 decibels (A) at a speed of 30 kilometers per hour. After applying the AC speed control technology that utilizes optimized PWM pulse width modulation and active harmonic suppression, the noise level of the same vehicle can be reduced to 58 decibels (A). The noise that the human ear can clearly perceive has been reduced by nearly three quarters. Many drivers have stated that even when driving at high speeds, the interior of the vehicle becomes as quiet as a library, and the driver's fatigue level has significantly decreased.

The Path to Premium Comfort

 

As electric vehicle buyers become increasingly discerning, the NVH (noise, vibration and harshness) of the power system has become a new battlefield for enhancing brand reputation. By adopting the AC variable-frequency motor control technology, whether it is combined with an integrated system or using a flexible independent controller, the driving comfort will no longer be just a verbal claim, but will actually happen to the users.