CHAPTER 1 INTRODUCTION

Transcription

1 1 CHAPTER 1 INTRODUCTION 1.1 ELECTRICAL MOTOR This thesis address the performance analysis of brushless dc (BLDC) motor having new winding method in the stator for reliability requirement of electromechanical actuator application. There is a critical need to reduce the weight and size of the electrical motor used for the space mechanism while providing better reliability. By incorporating functional redundancy, the brushless dc motor can be used for this high critical engine gimbal control actuator mechanism. The study has been motivated by the increasing potential and interest in the use of brushless dc motor in high performance actuator application. The bldc motor has been considered as the viable choice for the applications related to aerospace, automobile, machine tools, medical, industrial automation, electric propulsion etc. The primary objectives of the thesis is to a) Introduce a new winding method in the stator of BLDC motor such that the motor has quadruplex functional redundancy. b) Develop the prototype motor and analyse the performance output of each quadrant to study the four quadrant redundancy. c) Simulate and study the performance of the BLDC motor with a new high permeability magnetic material for the stator core instead of conventional silicon steel material.

2 2 1.2 BACKGROUND The traditional dc commutator motor with electronically adjustable voltage has always been prominent in motion control. It is easy to control, stable, and requires relatively few semiconductor devices. Developments in electronics have helped to keep it competitive in spite of efforts to displace it with ac drives. The objections to the commutator motor arise due to the operational problems associated with the commutator and brushes. The permanent magnet dc motor emerges replacing the field winding and pole structure with the permanent magnet. It permits considerable reduction in the motor diameter due to efficient use of radial space by the permanent magnet assembly. The commutation is effective as the permeability of the magnet being near to air reduces the armature reaction effect. A brushless dc motor known as electronically commutated motor consists of permanent magnet rotor assembly, armature coil stator assembly and sensor assembly. The bldc motor has inverse configuration to the conventional dc motor with inner field assembly and outer armature coil assembly. The phase coils are electronically switched to supply current flow into the phase windings. The fixed armature winding eliminates the problems of connecting current to the moving armature. The torque is exerted in the rotor by the interaction of field produced by the permanent magnets and the electromagnetic field generated by the armature coils. Electronic commutation circuit in the bldc motor replaces the mechanical brushes and commuator segments to provide the voltage to the phase windings to keep the rotor rotating. The controller performs similar timed power distribution by using a solid state circuit rather than the brush and commutator segment.

3 3 The brushless dc motor is more efficient in converting electrical energy into mechanical power than brushed dc motors. Due to the absence of physical contact between the brushes and commutator the friction loss is significantly reduced, frequent maintenance is eliminated, higher current can flow into the phase windings, motor can be used in critical environment and the motor can operate in high speeds without being limited by a physical contact. An advantage of the brushless configuration in which the rotor is inside the stator is that more cross-sectional area is available for the armature winding. At the same time the conduction of heat through the frame is improved. The bldc motor provides large torque to weight ratio making it suitable to high performance applications. The brushless dc motor configuration is shown in Figure 1.1 alongside the permanent magnet dc commutator motor. Figure 1.1 (a) Brushless dc motor (b) PM dc commutator motor The stator structure is similar to that of a polyphase ac induction motor. However, the windings are distributed in a different manner. Most bldc motors have three stator windings connected in star fashion. One or more coils are placed in the slots and they are interconnected to form a phase. The

4 4 coil windings are distributed over the stator periphery to form an even numbers of poles. The function of the magnet is the same in both the brushless motor and the dc commutator motor. In both cases the airgap flux is ideally fixed by the magnet and little affected by armature reaction. Figure 1.2 shows the inner rotor outer stator brushless dc motor having three star connected winding. (a) (b) Figure 1.2 (a) Brushless dc motor (b) 3-phase star connected winding The rotor consists of permanent magnets and can vary from two to many pole pairs with alternate North (N) and South (S) poles. Based on the required magnetic field density in the rotor, the proper magnetic material is chosen to make the rotor assembly. Ferrite magnets are traditionally used to make permanent magnets. As the technology advances, rare earth alloy magnets are gaining popularity. The ferrite magnets are less expensive but they have the disadvantage of low flux density for a given volume. In contrast, the alloy material has high magnetic density per volume and enables the rotor to compress further for the same torque. Also these alloy magnets improve the size to weight ratio and give higher torque for the same size motor using

5 5 ferrite magnets. Neodymium Iron Boron (NdFeB) and Samarium Cobalt (Sm2Co17) are the examples of rare earth alloy magnets. Unlike a brushed dc motor, the commutation of a bldc motor is controlled electronically. To rotate the bldc motor, the stator windings should be energized in a sequence. Rotor position is sensed using Hall effect sensors fixed into the stator assembly. Whenever the rotor magnetic poles pass near the Hall sensors, they give a high or low signal indicating the N or S pole is passing near the sensors. Based on the combination of these three Hall sensor signals, the exact sequence of commutation can be determined. The major disadvantage of brushless dc motor is production of undesired cogging torque and ripple torque. The cogging torque is generated due to the interaction of permanent magnet flux and stator reluctance variation in the airgap. The rotor tends to align to the stator teeth even without winding excitation. This cogging torque superimposed on the desired output torque causes vibration and acoustic noise in the motor during running. The ripple in the torque output is due to the six step commutation used in the switching technique to drive the motor. 1.3 MOTIVATION Many engineers have the impression that the technology of the electrical motor is saturated. But there is more analysis and technology development activity now a days in the motor design than in the past. The reason behind the design, analysis and development of advanced electrical motor is due to availability of new high permeable magnetic materials, computer based analysis tool, advancement of power electronics devices and requirement of special motors for high critical application such as aerospace industry.

6 Application Requirement Rotary actuators finds many applications in space industry namely: (i) for thrust vector control (TVC) of launch vehicle by gimballing the thrusting engine; (ii) for maintaining the engine thrust of the launch vehicle by regulating the mixture ratio of fuel and oxidizer; (iii) for positioning the antenna of the spacecraft; (iv) for the motion of the robots etc. These actuators are generally designed for the performance requirements by taking into account of the weight, size and power constraints. The actuators can be electromechanical or electrohydraulic mainly based on the actuation performance requirements. For the class of actuators mentioned above, hydraulic actuators are larger in size and weight. Additionally avoiding leaks and ensuring reliability have been persistent challenges in application of hydraulic technology. Actuator manufacturers currently hold the opinion that purely electromechanical actuators will eventually replace electrohydraulic actuators in the near future. For this actuator application, permanent magnet brushless dc motors are perfectly suited due to their efficiency, reliability, long life, less size, high power density and torque density. The benefits of electromechanical actuators includes simple configuration and less number of components, reduced system weight and cost, easy fault detection and isolation scheme, no life restricting elements. The above reasons motivate the development of brushless dc motor for electromechanical actuator replacing conventional hydraulic actuators New Magnetic Materials Availability of new materials such as soft magnetic composite materials, Cobalt Iron alloy, Nickel Iron alloy and magnetic stainless steel increases the usage of electrical motor for high critical electromechanical application. Figure 1.3 shows the evolution of permanent magnet materials in the past decades.

7 7 Figure 1.3 Evolution of Permanent magnet materials The sustained success of permanent magnet materials from Ceramic, Alnico to high energy product Samarium Cobalt and Neodymium Iron Boron increases performance output of the permanent magnet motors. The size of the motor is also reduced due to improved magnet characteristics which plays important role in aerospace industry Power Semiconductor Devices Earlier the Gate Turn On thyristors has been widely adopted in the power circuit to control the low power rating motors. With recent advancement and capability of power semiconductor switches such as MOSFET and IGBT to switch higher current at higher frequency enables the precise control of the advanced electrical motor. This leads to the use of electrical motor for electromechanical energy conversion in variety of applications such as aerospace and automobile industry.

8 Computer Aided Design Motor design has been computerized since the early days of computers. More recently electromagnetic field analysis has emerged from the academic world into the design office as a tool for optimizing designs, particularly with respect to the efficient utilization of materials and optimization of geometry. The most popular methods for analysis are based on the finite element method. The recent advancement in analysis tool enables the designer to evaluate the analytical design calculations. Most recently coupled field analysis tools are emerged where the designer can analyse electromagnetic, thermal and structural behaviour of the motor in the common workbench. 1.4 BENEFITS Some of the problems of the brushed dc motor are eliminated in the brushless design. In this motor, the mechanical commutator and brush assembly is replaced by an external electronic switch synchronized to the rotor position. Brushless motors are typically 85 90% efficient whereas dc motors with brushes are typically 75 80% efficient. The life of a brushless dc motor is significantly longer compared to a dc motor. The switching of coil currents in the commutator segments produces electrical and RF noise. Brushless motors have no chance of sparking, unlike brushed motors, making them better suited to aerospace environment. Also the major requirement of the electromechanical actuator system in space mechanism is the reliablity, less weight, size and compactness. The proposed method of introducing quadruplex winding redundancy technique in single stator assembly of the brushless dc motor reduces volume of the motor while providing better reliabiltity. The proposed

9 9 method of four motors in a single unit reduces the size and weight compared to the dual hybrid stator assembly and rotor assembly configurations. 1.5 OUTLINE OF THESIS The thesis is organized as follows Chapter 1 presents the literature of permanent magnet machines, limitation of conventional dc motor and theory of permanent magnet brushless dc motor. It also presents the motivation and benefits of the present work based on the advantages and safety critical application requirement. Chapter 2 involves the introduction to the electromechanical actuator system requirement for aerospace mechanism. The present issues and problem statement are illustrated. The scope of the work and original contribution in this thesis are pointed out. The literature survey related to permanent magnet brushless dc motor design, analysis and its application has been dealt. Chapter 3 focuses on the design of permanent magnet brushless dc motor with new winding technique for quadruplex redundancy requirement. The requirement specification, design approach, quadruplex winding design and permanent magnet rotor design conforming to the quadruple redundancy are presented. Chapter 4 contains the design of 24 slots quadruplex winding redundancy armature stator and 8 poles permanent magnet rotor configuration. The prototyping issues of the designed 24 slots motor are illustrated. To overcome the limitations the 48 slots and 60 slots armature stators and 16 poles permanent magnet rotor configurations are designed. The quadruplex redundancy winding scheme for the both the motor types are

10 10 given. The Hall sensor assembly configuration for the quadruplex redundancy stator is also provided. Chapter 5 contains the finite element modeling and analysis of the analytically designed motor. The preliminary designed motor is modeled in the two dimensional finite element based electromagnetic software (MagNet 7.0). The profile of the airgap flux density, core flux density and torque output of the motor are plotted. Chapter 6 deals with the fabrication of prototype motors. The materials for the stator assembly and rotor assembly are listed. Winding technique, back-emf and torque measuring setup are illustrated. The passive tests such as resistance, inductance and insulation resistance values are tabulated for the prototype motors. The no-load speed, back-emf and stall torque measurements are tabulated. The actuator level and system level response characteristics of the motors are plotted for slow sine and step input. Based on the experimental results the torque constant improvement in 60 slots stator is studied. Chapter 7 introduces the method to improve the performance output of the fractional slot (60 slots motor) quadruplex redundancy permanent magnet brushless dc motor. The performance output of the motor is compared for conventional M19 29 gage silicon steel stator magnetic core material and Cobalt Iron alloy stator magnetic core. The simulation results are shown in this chapter. Chapter 8 deals with the conclusion and future research work.