Abstract
Electromagnetic fields in the air gap of an electric
machine produce electromagnetic forces between the rotor
and stator. The total force exerted on the rotor due to
the eccentric rotor position is called the unbalanced
magnetic pull. This eccentricity force is directed
roughly over the shortest air gap. At low frequencies,
the vibration amplitudes of flexural modes may be large
enough to couple the electromagnetic system to the
mechanical one. This electromechanical interaction
changes the vibration behaviour of the system.
The main purpose of this dissertation is to reveal the
main rotordynamic consequences induced by the
electromechanical interaction in cage induction motors.
Another goal is to achieve this by deriving a simple and
representative electromechanical rotor model with
physical variables and parameters.
In this study, a new parametric model was derived for the
unbalanced magnetic pull induced by an arbitrary rotor
motion in transient operation. The parameters of this
model can be determined analytically from the basis of
the machine characteristics or estimated numerically as
in this study. To estimate the parameters, an efficient
numerical method was developed from the analysis of
impulse response. The numerical results showed that the
simple electromagnetic force model, together with the
estimated parameters, predicts the unbalanced magnetic
pull fairly accurately.
An electromechanical rotor model was derived by combining
the Jeffcott rotor model with the simple electromagnetic
force model, including two additional variables for the
harmonic currents of the rotor cage. Applying this model,
the rotordynamic effects of electromechanical interaction
were studied. Three induction motors were used in the
numerical examples. The results obtained show that the
electromechanical interaction may decrease the flexural
frequencies of the rotor, induce additional damping or
cause rotordynamic instability. These interaction effects
are most significant in motors operating at, or near, the
first flexural critical speed. Excluding the potential
rotordynamic instability, the numerical results indicate
that the electromechanical interaction effectively
reduces the unbalance response close to the first
flexural critical speed.
Original language  English 

Qualification  Doctor Degree 
Awarding Institution 

Supervisors/Advisors 

Award date  24 Aug 2004 
Place of Publication  Espoo 
Publisher  
Print ISBNs  9513864049 
Electronic ISBNs  9513864057 
Publication status  Published  2004 
MoE publication type  G5 Doctoral dissertation (article) 
Keywords
 unbalanced magnetic pull
 electromechanical interaction
 rotors
 electric motors
 vibration characteristics
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Holopainen, T. P. (2004). Electromechanical interaction in rotordynamics of cage induction motors: Dissertation. VTT Technical Research Centre of Finland. http://www.vtt.fi/inf/pdf/publications/2004/P543.pdf