## Reducing unbalanced magnetic pull in induction machines

##### Abstract

Induction machines are the most widely used type of electrical machines
because of their robustness, simplicity, and relatively low cost. However, the small
airgap in the induction machine makes them more susceptible to Unbalanced Magnetic
Pull (UMP). This is because the magnitude of the UMP is a function of the degree of
eccentricity, which is the ratio between the length of misalignment and the mean airgap
length. The bearing-related failure accounts for approximately 41% of the total failures
of induction machines; the percentages of bearing-related failure would be higher for
applications in a harsher environment. In this thesis, the UMP caused by rotor
eccentricity is investigated, because a small degree of rotor eccentricity is unavoidable
due to the manufacturing tolerance and 80% of the mechanical faults could cause rotor
eccentricity in electrical machines.
When the rotor is not at the centre of the stator, the eccentric rotor causes an
uneven airgap around the rotor, in which the magnetic permeance with the higher
harmonics content will be created. The magnetomotive force (MMF) produces
additional pole-pair ±1 magnetic flux around the airgap. The interaction between each
magnetic flux with its pole pair ±1 magnetic flux produces UMP. As only the
magnetic flux that crosses the airgap causes UMP, the magnetic flux is categorised
into magnetising flux and airgap leakage flux, because both types of flux possess
different characteristics at a different rotor slip. As the airgap leakage flux is difficult
to calculate analytically, an empirical method is proposed to estimate the UMP caused
by the airgap leakage flux. Then, the UMP caused by the magnetising flux can also be
estimated by using the empirical method. The parameters for the empirical method can
be found by using either the FEA or the experimental results.
The damping effect of the magnetising flux in a parallel connected rotor bar is
discussed and a damping coefficient is introduced to explain this scenario. The
damping coefficient can also be used to calculate the UMP in a steady state analysis.
UMP comparisons between the cage rotor and wound rotor induction machines are
made. The wound rotor has a much higher UMP because the pole-specific wound rotor
could not damp the additional pole pair ±1 magnetic flux. Therefore, a damper
winding at the stator slot is also proposed in order to damp the UMP by producing a
counteracting flux. In addition, analytical equations have also been derived for
different scenarios, such as static eccentricity, dynamic eccentricity, axial-varying
eccentricity, and skew rotor bars. Finite Element Analysis (FEA) and experimental
work are used to demonstrate the derived analytical equation. Furthermore, the power
losses caused by the rotor eccentricity are investigated. Iron losses, copper losses, and
frictional loss are discussed and compared with both the analytical equation and the
FEA results.
In order to reduce the UMP in the induction machines, the two proposed
methods are the slip control method and damper windings topology. The slip control
method utilises the non-linearity characteristic of the UMP at different rotor slip. To
find the optimum operating slip with the lowest UMP, the UMP/Torque ratio is
introduced. The characteristics of the UMP/Torque ratio varies with the type and
design of the induction machines. However, this method is only applicable when the
machine is lightly loaded, because the magnetising flux is limited by the capped
terminal voltage and the core saturation of the machine. For the damper winding
topology, a circulating current flowing in the damper winding could produce a
counteracting flux to damp the UMP. The proposed damper windings configuration is
only suitable for the induction machine with an even pole pair number. Finally,
comparisons between both UMP reduction methods are made.