3rd February 2022

How to Design a Quiet Motor

With the onset of electric vehicles, noise and vibrational analysis of the electric motor has become a very important part of the design of electric powertrains. The electric traction motor is now the only propulsion component in an electric vehicle and its noise is no longer masked by an engine. Hence it is crucial to design a quiet motor as any unwanted motor whine would cause unpleasant noise and vibration to the driver. Reducing unwanted motor noise is by no means limited to electric traction motors. In many other motor applications such as industrial drives, pumps, wind turbines, reducing motor noise and vibration are important design aspects.

How Noisy Is My Motor Design?

Noise and vibration are caused by a combination of excitations (electromagnetic forces generated by the motor) and the structural dynamics of the motor (the natural frequencies of the structure). An unlucky combination leads to high pitched ‘whining’, which is typically caused by a high excitation force aligning with a dominant natural frequency of the motor structure.

We can demonstrate how to assess the noise levels in an electric motor using an example. A typical automotive traction motor is shown below. This is an interior Permanent Magnet (IPM) motor topology with 8 poles and 48 slots.

Figure 1: Example of an Automotive Traction Motor

An easy way of assessing at what frequencies we are likely to get peaks in the acoustic response is by means of a Campbell Diagram. This diagram shows the dominant excitation frequencies (orders) as diagonal lines with a linear relationship between frequency and rotating speed. The natural frequencies of the stator structure of the electric motor are shown as vertical lines.

Figure 2: Campbell Diagram

Peaks in the noise typically occur where lines intersect, meaning at that frequency the electromagnetic force excites a structural mode of the stator. For our example the most important structural modes are F0, a breathing mode excited by the average force around the stator changing over time, and F8, an octahedral mode excited by the fundamental forcing frequency due to the motor having 8 poles.

A spectrogram shows the acoustic response for each frequency order. Where the frequency order crosses a natural frequency the amplitude of the acoustic response—or sound power—is maximum, as shown in the spectrogram below.

Figure 3: Spectrogram

By taking a cross section of each dominant order—a so-called order cut—we can plot the acoustic sound power as a function of the operating speed of the motor. This allows us to identify at which speeds/frequencies the largest response occurs.

Figure 4: Graph of Acoustic Sound Power vs Operating Speed of Motor

 

How, and When, To Improve The Design To Reduce Noise

The design of electric motors is a complex process involving multiple physical domains. A motor designer needs to assess how noisy their design is early in the design process when changes can easily be made to avoid any noise issues. For this, a motor designer needs access to a fast Noise Vibration Harness (NVH) analysis tool during the concept design of their electric motors. Often a motor designer would look to reduce the electromagnetic excitations, but the example below shows that it is very important to consider the acoustic response of the motor to these excitations if we want to reduce noise.

For our example motor, the highest peaks in the acoustic response occur at 8,400rpm and 9,600rpm. From the Spectrogram we can clearly see that at these speeds a peak occurs as the harmonic orders (12th and 14th) cross the natural frequencies of the stator structure (F0 and F8). A 2D FFT analysis of the radial force excitations shows that the biggest noise response for our example motor comes from three small excitations highlighted in the plot below. The reason these small harmonics generate such a large response is because the stator structure is more flexible for these particular shape and frequency orders.

 

Figure 5: FFT Analysis of the Radial Force Excitations

This highlights the importance of an approach that combines forcing and response, which enables engineers to rapidly evaluate noise and vibration from the early stages of the design process. If we were focussed only on reducing the electromagnetic excitations we wouldn’t worry about these small excitations, yet these are key to the noise generated by this motor. In this case to reduce the noise we can focus on reducing the stator response, rather than reducing the already small excitations further.

The stator response is defined by the modal behaviour of the stator structure, which is determined by its dimensions and materials. One way of reducing the stator response is by thickening the stator back iron, which increases its stiffness and natural frequencies. This will reduce the noise amplitude at resonance peaks and increase the motor speed at which resonance occurs. Ideally, we will increase this speed until the resonance occurs above the motor’s maximum operating speed.

Figure 6: Impact of increasing stator back iron thickness

For our example, the increase in the stator back iron thickness of the electric motor means the frequencies of structural modes F0 and F8 shift rightwards. Comparing the resulting response shows a reduction in the resonance peak caused by the 12th harmonic by 4dB. This is due to increasing the stator stiffness for the structural mode F0. The resonance peak due to the 14th harmonic has disappeared completely as the 14th harmonic no longer crosses structural mode F8.

Figure 6: Order Tracking Graph with Increased Back Iron Thickness

The NVH analysis solutions shown above are the latest addition to Ansys Motor-CAD (Motor-CAD v15 / 2022 R1 release). Ansys Motor-CAD is the dedicated software for design of electric motors. It provides fast and accurate design and analysis tools including all major physics, i.e. electromagnetic, thermal, mechanical and now also NVH. The NVH analysis uses a combination of FEA and analytical methods so that simulation times are kept short. This enables a motor designer to bring NVH analysis into their multi-physics concept design and make design changes to avoid noise issues early on.

Presented by Dr. Melanie Michon, Head of Engineering at MDL

  • Find out more about the new Motor-CAD release here.

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