Prestressed Modal Analysis of a Rotor Assembly, E-Motor

Intro

This is a short case on prestress effects for a modal analysis of a rotor. For simplification rotor assembly includes only rotor core and shaft. This example serves the purpose to demonstrate briefly some aspects of pre-stress effects. Generally speaking prestress effects are important phenomena that can alter the behavior of a system dramatically. There is quite a deep theory behind pre-stress effects. However, the following simplified case is presented here that provides some “screenshots of possible results” depending on the level of detail involved in the virtual model.

Various simulation subcases are examined such as:

  • glue connection of rotor core and shaft
  • press-fit connection
  • press-fit connection and various rotational speeds

The solution is a modal analysis solution. On further future examples various other solutions are going to be examines such as frequency response, shock and random cases.

Below is a picture which describes the following cases from left to right

  1. Modal analysis with only glue connection for PF.
  2. Modal analysis with interference fit
  3. Modal analysis with interference fit and 1k rpm rotational velocity (1000 rpm)
  4. Modal analysis with interference fit and 10k rpm rotational velocity (10000 rpm)
  5. Modal analysis with interference fit and 15k rpm rotational velocity (15000 rpm)
  6. Modal analysis with interference fit and 22k rpm rotational velocity (22000 rpm)

Results

Results are shortly described on the following picture. The axial eigenmode is displayed for the 6 subcases

 

Axial eigenmode is displayed for the 6 subcases cases from left to right 1. Modal analysis with only glue connection for PF. 2. Modal analysis with interference fit. 3.Modal analysis with interference fit and 1k rpm rotational velocity (1000 rpm) 4. Modal analysis with interference fit and 10k rpm rotational velocity (10000 rpm). 5. Modal analysis with interference fit and 15k rpm rotational velocity (15000 rpm). 6. Modal analysis with interference fit and 22k rpm rotational velocity (22000 rpm)

 

The 1st case has no pre-stress effects and the last case has large modal displacements since loosening effect is taking place due to the overspeed. The case in between have PF+RPM.

 

The results on the axial eigenfrequency for the cases are the following

 

Stiffness is following a descending order and so does f. See also picture below. Interference fit reduces the stiffness of the connection. In addition, the rotational velocity causes spin softening effects.

 

Table below summarizes the analysis and short comments are provided for each case.

 

Motor Abnormalities

Various motor abnormalities are linked with a wrong selection of interference fit and a non-proper analysis of the various phenomena that can occur during duty operation of an e-motor, such as spin softening effects, pre-stress effects introduced with interference fits, etc.

Some e-motor abnormalities are summarized below:

  • High Noise
  • High Unbalance
  • High Vibrations
  • Low Efficiency