CHAPTER 1
INTRODUCTION
1.1 Motivation
Currently AC motors are favorably used in many industrial applications.
Squirrel cage induction motors are particularly popular because of their simple
structure, low cost production and less maintenance. However, the limitation of the
induction motors is the working speed which is lower than the speed of rotating
magnetic field and the changing slip depends on load torque. That is, an increasing in
load torque results in the decreasing in working speed. Hence, the induction motors
are not suitable for applications which require an accurate control of speed and
position such as servo systems. On the other hand, speed of synchronous motors can
be accurately controlled by varying the frequency of the rotating magnetic field which
is called synchronous speed. However, the synchronous motors suffer from high
production and maintenance costs.
Permanent magnet synchronous motors (PMSMs) have been widely used in
many industrial applications. Due to their compactness and high torque density [1],
the PMSMs are particularly used in high-performance drive systems such as the
submarine propulsion. The permanent magnet synchronous motor eliminates the use
of slip rings for field excitation, resulting in low maintenance and low losses in the
rotor. The PMSMs have the high efficiency and are appropriate for high performance
drive systems such as CNC machines, robotic and automatic production systems in
the industry.
Generally, the design and construction a PMSM must consider both of the stator
and rotor structures in order to obtain a high performance motor. However this thesis
focuses only on the design of the permanent magnet rotor and uses the stator structure
from an existing induction motor without changing the windings. That is, the squirrel
cage rotor is replaced by a newly designed permanent magnet rotor.
In the design of the permanent magnet rotor, it is important to determine the
optimum value of “magnet span” because it effects the harmonics of the back-emf and
the cogging torque. The design is based on the finite element analysis using the
software called Finite Element Magnetic Method or FEMM. The rotor is constructed
using the optimum magnet span obtained from the computation by FEMM. The
experimental results are compared with the results from the calculation.
This thesis is carried out as a part of the technical cooperation between Institut
National Polytechnique de Lorraine (INPL), FRANCE and King Mongkut Institute of
Technology North Bangkok, THAILAND. One of the objectives of this cooperation is
to develop technical knowhow on electric motor designs for the industry in our
country.
1.2 Thesis objectives
1.2.1 To apply the finite element analysis in the PMSM design.
1.2.2 To construct the permanent magnet rotor.
1.2.3 To compare the experimental results with the calculation results.
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1.3 Thesis boundaries
The scope of the thesis includes the design and construction of a permanent
magnet rotor based on the stator frame of a three-phase 1-Hp induction motor. Each
pole of the rotor is fixed with several magnet blocks. The optimum magnet span is
chosen to minimize the THD of back EMF of the motor. A finite element method is
used in the design calculation. Torque and cogging torque are considered in the
design.
1.4 Thesis procedure
1.4.1 Study research works regarding the design of PMSMs.
1.4.2 Study the fundamental of 3 phase induction motor and synchronous motor.
1.4.3 Study the fundamental of electromagnetic field.
1.4.4 Study the application of finite element method in motor design.
1.4.5 Test and find the parameters of the induction motor whose the stator frame
is used in the design.
1.4.6 Study the program for designing by finite element magnetic method:
FEMM.
1.4.7 Compute and design the permanent magnet for the rotor.
1.4.8 Construct the rotor.
1.4.9 Assemble the rotor to the stator and experiment.
1.4.10 Conclude the results.
1.5 Expected benefits
1.5.1 Obtain the method to design the permanent magnet synchronous motor.
1.5.2 Encourage the corresponding objective research.
1.5.3 Obtain the way to develop the high performance motor in the industry.
1.5.4 Support the potentiality for technology development in our country.