Hello, Motor Fans:
In this blog I discuss how to use the Mechanical physics module in Ansys Motor-CAD to perform E-NVH analysis and determine which modes of motor operation contribute to the vibration, deformation and noise of the motor. The engineer can know at which frequencies to not operate the motor to avoid resonance with the natural frequencies of the motor.
MODEL
Open the IPM Mech Traction tutorial project via the the path in the Motor-CAD installation folder,
PHYSICS
The Mechanical physics module is selected. This module will provide the E-NVH analysis and post processing capability.
E-NVH Calculations
Here is a brief review of sources of NVH and some of the physics concepts used in NVH analysis.
Sources of NVH
Electromagnetic Forces: Variations in the electromagnetic field can cause vibrations and noise. This includes forces from magnetic attraction and repulsion within the motor.
Mech Imbalances: Any imbalance in the rotating parts, such as the rotor, can lead to vibrations.
Structural Resonance: The natural frequencies of the motor's components can resonate with operational vibrations, amplifying noise.
Bearing Issues: Defects or misalignment in bearings can cause significant noise and vibrations.
Aerodynamic Forces: The flow of air around and through the motor can create noise, particularly at higher speeds.
Gear Mesh: In motors that use gears, the interaction between gear teeth can generate noise and vibrations.
Physics Concepts
Time Order, Electrical: Describes an electrical signal with respect to time.
Time Order, Vibration: Describes how vibrations and noise levels from a location vary in time.
Space Order, Vibration: Describes the spatial distribution of vibrations over the motor.
Vibration Frequency: Describes the rate of vibrations at points in the motor which act as sources of noise.
Natural Frequency: This is the frequency that the motor naturally tends to vibrate. There may be multiple natural frequencies that are multiples of a fundamental natural frequency that produces the first mode.
Mode (Shape): This is the shape that the motor deforms into due to the vibrations. Figures may exaggerate the depiction of the shape for Illustrations purposes. Modes manifest as a result of the applied forces with natural frequencies which resonate with motor. There are as many modes as there are natural frequencies and for each mode there is a different structural stiffness value. Modal analysis reveals which modes are dominant.
Dynamics
Repulsive and attractive radial forces are applied to the stator as the magnet poles pass by the stator teeth as the rotor rotates. These repulsive and attractive radial forces on the stator oscillate with the speed of rotor rotation and the faster the rotor rotates, the faster the forces oscillations applied on the stator. The stator has natural frequencies and structural stiffnesses associated with it that depend on the stator structure and mass. Vibrations will develop on the stator if the radial force oscillations on the stator matches the natural frequency of the stator and the stator will deform into a unique shape corresponding to the particular natural frequency the rotor is running at. The relationship of the stator deformation and rotor speed is analyzed in Modal Analysis.
The mode number, or shape order, corresponds to the integer that vibration distribution with spatial periodicity can be divided into evenly. For example, Mode 0 has spatial period 0, Mode 2 has spatial period 2, Mode 4 has spatial period 4, etc. The mode number is also the order of the natural frequency.
E-NVH and Forces Calculations
Go to Input Data >> Settings >> Calculation, and adjust the Force Calculation and Torque Calculation settings.
We can define a Torque Speed Curve using operating points we want the motor to operate at. After setting the operating points, click "Generate E-NVH and Forces".
Motor-CAD will use the E-Magnetic module to calculate EM forces. Then the Mechanical module will use this solution data to calculate the torque, and the required Peak Line Current and Phase Advance.
E-NVH and Forces Results
The modal stiffness increases with Mode number because as the Mode number increases (increasing rotor speed) the shape order increases accordingly and deformation is more difficult. Also, as the Mode number increases, the vibrations penetrate deeper into the stator and more stator mass is involved in the vibration. The rotational frequency is not linearly proportional to the Mode number in this motor due to the harmonics. Under certain simplified conditions such as a string attached at one end and free to oscillate at the other end, the Mode number will be linearly proportional to the oscillation frequency.
We can view how the Torque varies with Electrical angle. Also, we can see how Torque Harmonics vary with Electrical angle.
The Space Orders and Time Orders dominating the Force Density on the stator can be visualized, and adjustments to the motor design can be made to reduce the strongest forces.
The structural response in terms of velocity in the direction of the vibration can be analyzed here for each operating point.
The vibrations which depend on rotor speed are sources of noise which has a sound frequency. The rotor speed and sound frequency correspond to radiated sound power which can be analyzed using the plot below.
There are too many results to show in the E-NVH module for one blog. However, here are views of the E-NVH module tabs that are available for post processing results.
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