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BRUSHLESS DC MOTORS FOR WINCHESTER DISC DRIVES
Spindle motors and in particular Winchester disc drives are becoming very dominant among the many applications of brushless d.c. motors. The flexibility and simplicity of design makes these motors ideal in an application where management of space is of great importance. The switching circuits necessary for these motors add controllability to the performance and make a high degree of speed-control a possibility. Economically, the brushless motor is acceptable even with the additional cost of the switching circuit. This has been made possible by the lower cost of electronic integrated circuits in the past few years.
THE IMPACT OF HIGH SPEED IRON LOSSES ON BLDC
MOTOR OPERATION AT SPEEDS ABOVE 10,000 RPM
The brushless dc (bldc) motor is now being used in a number of high performance applications where motor speeds of 10,000 to 30,000 rpm are achieved. The impact of iron losses cannot be ignored and must be included in any design procedure for these high speed applications. This paper will compare the various design tradeoffs of the internal motor losses in small bldc motors, specifically a 2.0" O.D. and a 3.0" O.D. bldc motor, when operating at high speeds. These losses include eddy current, hysteresis and copper losses. The results will be presented and a number of computer simulation solutions will be shown. These will be used in optimizing different performance parameters. Some of these results will be matched to various applications in the US market. The impact of the motor's thermal performance will limit the actual dissipated power that the brushless dc motor can achieve. A series of thermal resistance targets will be established which will be used as practical limits to the motor's ability (or inability) to dissipate heat.
Magnetics Design of Brush and Brushless DC Motors
Through Lumped Reluctance and FEM Simulation Programs
When designing a permanent magnet dc motor, the magnetics design engineer usually faces a myriad of design conditions that are of an interactive nature. He must determine the flux values from the main field (magnets) as well as from the winding. Knowledge of the resultant magnetic field and the armature reaction is also required. He must also keep a continuous check on the iron losses in the different sections of the magnetic circuit. These losses must be established in concert with the copper winding configuration and fill. The outcome of these optimization activities is a magnetic design that integrates both electrical and mechanical performance. This design would define the geometry of all the parts of the magnetic circuit and also the materials involved.
A number of simple lumped reluctance simulation programs that can run on a personal computer have been developed. These programs require a very short computer time and eliminate many of the typical variations on configuration. They can achieve sufficient accuracy for the majority of motor design application. They can be used in place of the slower but more accurate finite element mesh (FEM) simulation programs.
This paper will simulate a set of brush and brushless dc motor designs to demonstrate the development of a "high tech" (rare earth magnet) motor. Some basic formulae used in these programs will also be presented. The intent is to demonstrate that PM motors can be designed with simpler program techniques, as long as the user understands that the resulting boundary conditions will impact simulation accuracy.
Brushless DC motor operation at
high speeds as affected by the core losses
The brushless dc (bldc) motor is now being used in a number of high performance applications where motor speeds of 10,000 to 30,000 rpm are achieved. The impact of iron losses cannot be ignored and must be included in any design procedure for these high speed applications. This paper will compare the various design tradeoffs of the internal motor losses in small bldc motors, specifically a 2.0" O.D. and a 3.0" O.D. bldc motor, when operating at high speeds. These losses include eddy current, hysteresis and copper losses. The results will be presented and a number of computer simulation solutions will be shown. These will be used in optimizing different performance parameters. Some of these results will be matched to various applications in the US market. The impact of the motor's thermal performance will limit the actual dissipated power that the brushless dc motor can achieve. A series of thermal resistance targets will be established which will be used as practical limits to the motor's ability (or inability) to dissipate heat.
Computer Program Aids
Magnetics Design of Brush and Brushless DC Motors
A computer program that can run on a personal computer employs simple lumped reluctance simulation to determine the magnetics design. The program has sufficient accuracy for the majority of motor design applications and can be used in place of the slower but more accurate finite element mesh (FEM) simulation programs. To demonstrate use of this program, we will simulate a set of brush and brushless DC motor designs for a rare earth magnet motor. Our intent is to show that a simpler program can be used as long as the user understands that the resulting boundary conditions will impact simulation accuracy.
Brushless DC motors for disk drives
The use of brushless dc motors in rigid disk drives has become an important segment of the motor market. The flexibility and simplicity of design makes these motors ideal in an application where management of space is of great importance. The switching circuits of these motors add controllability to the performance and make a high degree of speed-control a possibility. The unique operating conditions in disk drives are reflected in the specifications describing these motors. In addition to the electrical and mechanical requirements, the materials used are strictly controlled and specified.
Magnetizing Fixture Provides 3-D Flux Flow
Magnetizing fixtures produce the fields required to coerce permanent magnets permanently. These fixtures come in different shapes and sizes determined by the associated permanent magnet. A recent fixture requirement called for magnetization of a high-energy-product magnet ring with eight asymmetric poles and alternating 30° and 60° spans. This fixture required an axial flux path as well as the normal X-Y path. The three dimensional flux flow would enable the fixture to produce a higher magnetizing force than previously possible.
Voice-coil actuators
Voice-coil actuators are electromagnetic devices which produce accurately controllable forces over a limited stroke with a single coil or phase. They are also often called linear actuators, a name also used for other types of motors. A related form is the swing-arm actuator, which is used to rotate a load through a limited angle (usually 30 degrees or less). These devices are capable of extremely high accelerations (more than 20 times gravitational acceleration, or "g"), and great positioning accuracy when suitably controlled (one-millionth of an inch or better). A properly designed device may have a settling time (the time required for structural vibration to settle down to below the measurement threshold after a high acceleration move) of two milliseconds or less. The major use of this type of actuator is in computer peripheral disk drives. They are also used in shaker tables, lens focusing, medical equipment, laser cutting tools and elsewhere.
VOICE-COIL ACTUATORS: INSIGHT INTO THE DESIGN
Voice-coil actuators are a special form of electric motor, capable of moving an inertial load at extremely high accelerations (more than 20 times the rate of acceleration of gravity "g" at the Earth's surface) and relocating it to an accuracy of millionths of an inch over a limited range of travel. Motion may be in a straight line (linear actuators) or in an arc (rotary, or swing-arm actuators). After completing a motion (called a "seek" in the computer peripheral memory industry) the moving parts must stop vibrating very quickly. This period, called the settling time, may be a few milliseconds or less.
Optimal Design of Magnetizing Fixtures
Permanent magnets are used in large numbers in brushless DC motors, actuators, sensors, instruments, and other electromagnetic devices. Magnetic materials are rapidly becoming more powerful and the devices which use them are constantly being redesigned into smaller sizes, with equal or better performance. The reduced dimensions of the magnets, along with increased number of poles, in materials of very high coercivity, make the magnetizing process much more difficult. Given a particular magnetizer (electric pulse generator) it may be impossible to properly charge a given magnet with a specified pole pattern, even though the equipment is capable of supplying the required energy. In this article, methods will be considered which lead to a strategy for defining an "optimal" magnetization process for a given permanent magnet when charged by a particular pulse generator, depending on the competing needs of cycle rate, sharpness of magnetic transitions, etc. and show when no solution is possible. A different magnetizer, or modifications to the existing magnetizer, might then be needed
A Review of Steel Materials in Motion Devices
Electrical steel is the name given to the steels used in motion devices like motors and actuators. Major developments over the past century have given us a wide selection of new steel alloys while constantly improving on the old ones. The general trend is driven by variety, economy and efficient devices. Following is a discussion of some of the most common parameters and features associated with electrical steel which are crucial in developing the ability to choose the appropriate material for the particular device.
ENGINE MANAGEMENT FOR HYBRID ELECTRIC VEHICLES
A hybrid electric vehicle comprising an internal combustion engine and a battery bank as sources of power is being investigated. In this study, the conversion of mechanical work from the engine to electrical power using an alternator and phase converter is investigated. The unique characteristics of the three components above are analyzed to identify the corresponding limitations. More importantly, the favorable range of operation of these units operating together is identified.
An experimental van using the hybrid concept is constructed and its starting and control modes are described. An analysis of engine, alternator and converter is performed to formulate a control strategy. Special attention is given to the converter and its algorithm due to its important interface role between the alternator and battery bank.
THE USE OF POWDER METALS IN DC MOTORS
Motors, like other electromagnetic devices, are benefiting from a rapid development in powder metallurgy. Several grades of ferrous powders are now available in very fine particle sizes. Together with the development in die making and pressing techniques, it has become possible to produce parts whose density is very close to that of wrought iron. The process of making parts form powder metals is essentially compaction followed by sintering. There are many advantages in using powder metals, primarily in providing an economic solution to sourcing parts with complicates shapes. However, it must be understood that parts intended for the magnetic circuit can only operate at lower levels of flux density and permeability than solid steel.
FIXTURES FOR STRAIGHT-THROUGH MAGNETIZING
A large number of magnets are magnetized in a uniform field along a straight-through axis. While fixtures used for radially oriented and other specially-shaped fields must be designed for the specific magnet shape (and often, for the material properties as well), fixtures used for straight-through magnetization are able to handle a variety of tasks. Straight-through magnetizing fixtures are of two basic types, the solenoid (which may or may not have steel backing), and the C-frame. Of these, the C-frame is heavier, more expensive and takes up more space. Why then, would anyone prefer this type of fixture? The purpose of this paper is to explore the advantages and disadvantages of each type, and to present considerations for a choice between them, for particular applications. Complete design criteria for these designs is not, however, within the scope of this paper.
Radially-Oriented Magnet Rings
The rotors of permanent magnet dc electric motors have been constructed by a number of means in the past. Perhaps the most popular has been to bond a set of magnet arc segments onto a steel backing. Now, however, a few magnet manufacturers have begun to produce complete rings of radially-oriented magnet materials. Some are isotropic, others anisotropic. These rings have definite advantages (and a few disadvantages) compared with arc segments and other construction methods. The effect of the magnet shape on the performance of the motors is discussed. Magnetic and physical properties of presently available ring magnets are also briefly discussed
Flux Density/Coercivity (B/H) of Round Rod magnets in Air
Under some conditions, a magnet which has been magnetized in a fixture or coil, which is then placed in a magnetic circuit, is found to produce a weaker field than was calculated, based on its fully magnetized B/H curve. Another form of the same problem occurs when a magnet which has been magnetized in place, in a magnetic circuit, is found to produce its intended field initially but would become weaker if removed from the circuit and then returned to it. Magnets under these conditions are said to have undergone "shape demagnetization". The problem is much more severe with older Alnico materials which have a high flux-density B but relatively little coercivity H, and also a B/H curve which is more of a continuous arc than a straight line with a fairly sharp knee. Designers who have worked only with the newer magnet materials may not be aware that the danger of partial demagnetization by shape exists in these materials too.
Motor Design Advancements Using NdFeB Magnets
Electric motors with permanent magnets have seen a significant rise in market share and are being used in an increasingly wide range of applications. The availability of these magnets has played a major role in this growth. The advent of neodymium iron magnets and the improvements in their manufacturing in the last decade have dramatically impacted these motors. The parameters which are directly affected by these magnets are reviewed to further the understanding of their role in the industry. The record of annual sales of this type of magnet speaks for itself. The reasons for this success lie partially in the lower cost of the material, but mainly in its being coupled with the higher energy-products being achieved. This paper will only discuss how the magnet characteristics benefit the motor performance.