This document is an abstract for a thesis submitted by Nagarajan S to Anna University in partial fulfillment of a Doctor of Philosophy degree. The thesis deals with detecting broken bars in three-phase squirrel cage induction motors using finite element modeling. It models an induction motor using finite element analysis software to simulate broken bar faults. The simulations show increased flux, flux density, and current but decreased stored magnetic energy with more broken bars. The thesis also simulates faults in voltage source inverters and current source inverters supplying induction motors and develops a fault-tolerant inverter using a leg swap module.

1. INVESTIGATIONS ON FAULT DETECTION OF
INVERTER FED THREE PHASE SQUIRREL
CAGE INDUCTION MOTOR
ABSTRACT
of the Thesis
Submitted by
NAGARAJAN S
in partial fulfilment for the requirement of award of the degree
of
DOCTOR OF PHILOSOPHY
FACULTY OF ELECTRICAL ENGINEERING
ANNA UNIVERSITY
CHENNAI 600 025
NOVEMBER 2013

2. 1
ABSTRACT
The induction Motor has been regarded as a workhorse in industrial
applications for many years. In the last few decades, the induction motor has
evolved from being a constant speed motor to a variable speed and variable
torque machine. When the application requires large power and torque
specifications, the usage of the induction motor comes into demand. This
results in the requirement of an efficient machine, which is stable during fault
conditions. The various faults that occur in induction motors are, rotor broken
bar, stator inter-turn fault, bearing fault and eccentricity fault. Out of these,
the rotor broken bar fault is very specific in squirrel cage induction machines.
The interior faults of the induction motor account for more than 70% of the
induction motor failures. Conventional methods make use of the test
machines; however, the machine has to be damaged to complete the
experiments of interior faults. Interior faults include the stator and rotor faults
of induction motors. Rotor faults are related to broken bars. Rotor failures are
caused by a combination of various stresses that act on the rotor. These
stresses are identified as electromagnetic, thermal, dynamic, environmental,
and mechanical. These lead to low frequency torque harmonics, which
increase noise and vibration. Hence, the detection of a broken bar fault is
essential for the protection of the induction motor against failures and
permanent damages.

3. 2
The Finite Element Method (FEM) is a general technique, for
numerical solution of the integral or differential equations governing the
behavior of systems. The finite element software accurately calculates
magnetic fields and related motor design parameters, for motors of
complicated geometry with saturation and/or permanent magnets, with
significant armature reaction and with or without eddy currents. The finite
element method saves the cost of the machines and the possibility of
improving the design during the planning stage. The finite element method is
distinguished from finite difference equations in determining the shapes that
the discrete steps can take. The magnet is the most advanced package
currently available for modeling electromagnetic devices.
This work deals with the detection of broken bars in a three phase
squirrel cage induction motor, using the finite element model of the induction
machine. FEM is more precise than the winding function approach method, as
it is based on the actual geometry of the machine. The machine model can be
easily modified to study the effects of faults on the machine’s performance.
The CAD package called “Magnet 6.26.6” is used for the Static 2D and
Transient 2D analysis. The various machine parameters like flux density, flux
function, magnetic energy and current are calculated, using this CAD
package, and their values are compared under healthy and faulty conditions.A
three phase squirrel cage induction motor is modeled on the basis of the finite
element method. The simulation results are obtained for the broken bar fault.
It is found that the faults due to broken bars saturate the magnetic force

4. 3
distribution on the rotor tooth, adjacent to the bars that are broken. In the
static analysis, the stored magnetic energy is decreased when the number of
broken bars is increased. The flux function and flux density are increased,
when the number of broken bars is increased. In the transient analysis, it is
found that the flow of current in the stator phases, and the flux linkage
produced in the motor are increased, whereas, the stored magnetic energy is
decreased, when the number of bars broken is increased. The simulated
results are compared with the theoretical results for parameters like the flux
function and flux density.
Simulation studies are also carried out for different fault conditions
in the Voltage Source Inverter (VSI), Current Source Inverter (CSI) and fault
tolerant inverter systems. The Total Harmonic Distortion (THD) of the VSI
fed drive is evaluated under healthy conditions and various faults in the
inverter circuit. From the simulation results, it is seen that there is an increase
in the harmonic distortion due to faults. The current harmonics get introduced
upon the introduction of faults in the inverter module. The THDs of the
normal VSI fed drive and fault tolerant VSI fed drive are evaluated, under the
open leg fault condition. From the simulation results it is seen, that there is a
decrease in the harmonic distortion by using a fault tolerant VSI fed drive
with Stator Phase Connection (SPC). However, the fault tolerant inverter with
a leg swap module replicates the performance of a healthy VSI. The hardware
is implemented in the VSI fed drive, and comparisons are made between the
healthy and faulty conditions. In a healthy condition, the motor runs at a

5. 4
speed of 1430 rpm. In the open leg faulty condition, the voltage waveforms
are distorted, and the motor fails to run.
Using FEM the fault can be easily detected and analyzed for the
three phase squirrel cage induction motor, without damaging the parts of the
machine. On the converter side, the THD analysis helps to find out the
various faults that occur in the VSI & CSI. A fault tolerant inverter, with a leg
swap module, was also developed for the VSI fed induction motor drive, to
overcome the effect of the fault in the inverter.