How to Analyze Noise, Vibration and Harshness in Electric Powertrains (e-NVH) using Simulation

What are the Sources of Noise and Vibration in Electric Drives?

The general shift towards electrification in the electric vehicle (EV) market and beyond has created a need for higher fidelity simulation of electric powertrains. One aspect of this trend that has been getting attention is the desire for detailed analysis of an electric motor’s noise, vibration, and harshness (NVH) characteristics early in the design stage. 

The sources of the characteristic high-pitch whine of electric motors are the interaction between different airgap field harmonics inside the machine, as well as the switching voltage inputs from the inverter. These elements generate force waves in the airgap, which can excite the structure of the motor and cause vibrations, particularly at specific resonant frequencies. Imperfect torque and stator load profiles cause further vibration of attached machinery components and the gearbox housing (e-axle). Unlike internal combustion engines, where the engine sound is often a prominent feature that we want to accentuate, with electric drive units, any sound that is produced is usually undesirable, so the goal is to minimize it. 

A Complete & Fully Integrated Workflow

To properly analyze how this noise and vibration is created and to mitigate it, a system-level simulation of the motor, the inverter and the mechanical components is necessary. To capture the NVH characteristics of an electric drive unit, a new workflow, that spans GT-FEMAG’s electromagnetic finite element analysis simulations and GT-SUITE’s electrical and mechanical transient simulations, was developed (noted in Figure 1 below).

Figure 1: Complete and fully integrated eNVH workflow

Electrical Section 

The first step in this process is to use GT-FEMAG, a finite element electromagnetic modeling tool built for motor design to design a motor that meets speed and torque requirements for the traction motor. After the motor design has been finalized, GT-FEMAG can export a very high-fidelity model of the motor, used to populate the datasets of a new lookup-table–based permanent magnet synchronous motor (PMSM) template in GT-SUITE, that can capture the torque ripple and the spatial harmonics inside the machine. This motor template is coupled with a detailed 3-phase inverter and controlled with a closed loop feedback control (Figure 2). 

Figure 2: Closed loop inverter + motor model

This simulation outputs the 3-phase currents in the motor windings, at multiple different speeds (see Figure 3 below). These currents will be used in the next step to calculate the motor forces, in the mechanical part of the workflow. 

Figure 3: Detailed 3-phase current output for multiple operating point

Mechanical Section 

Moving on to the mechanical section of the workflow, using the ABC currents that were calculated previously, FEMAG evaluates the magnetic pressure as a function of space and time, which we can then use to predict the forces that are generated in the motor, as a function of rotor position and stator tooth, for each operating speed and torque combination (Figure 4). These forces will be the boundary conditions for the mechanical analysis in the next step. 

Figure 4: GT-FEMAG workflow for the load distribution

Next, these excitation loads are used in GT-SUITE as an input for a forced frequency analysis to get the structural steady–state response of the overall gearbox housing. By performing a Fourier transformation of the loads from the previous step, we can obtain the amplitudes of the applied loads at each frequency, resulting from the various speed and order combinations of the simulated drivetrain. With this information, it is possible to directly identify areas that end up in excessive surface vibration and react accordingly by modifying the system. Additionally, the surface vibration response can be used to perform an acoustic analysis using a rapid sound assessment method that will provide the sound pressure level at any location around that structure (see Figure 5 below).

Figure 5: Surface Normal Velocity at a given frequency, Campbell diagram at a given node, and sound pressure of the powertrain in 3D space

An All-in-One Package for e-NVH Analysis  

This workflow can offer a very straightforward and convenient way to analyze the NVH performance of any electric powertrain. As a tightly linked system, contained fully within GT’s library of tools, it enables users to run many iterations easily and quickly and to optimize their designs based on many parameters, like the geometric characteristics of the motor, the switching frequency, or the modulation strategy of the inverter etc., and see how these changes affect the NVH performance. The high degree of integration between the electromagnetic, the electrical and the mechanical domains of this workflow provides a seamless user experience, without having to resort to multiple different simulation tools, as is typically the case. 

Learn More About our e-Powertrain NVH Solutions 

The full workflow is presented in more detail by GT’s experts in this 30-minute SAE webinar. If you’d like to learn more or are interested in trying GT-FEMAG and GT-SUITE for e-powertrain and NVH simulation, contact us!