Abstract: In recent years, brushless DC motors have been widely used in many fields, and slotless brushless DC motors are increasingly coming into view. There's even a saying that carbon brush DC motors are the first generation, brushless DC motors are the second generation, and slotless brushless DC motors are the third generation. So, what is a slotless brushless DC motor? Compared to "carbon brush" and "brushless," what are the differences or advantages of "slotless brushless" DC motors? This article, based on my experience of nearly ten years of research and development on slotless brushless rare-earth permanent magnet DC motors, aims to offer some preliminary thoughts on slotless brushless motors, specifically in relation to brushless DC motors. Keywords: slotless brushless motor, overload, electromagnetic vibration 1. Changes in the manufacturing process of motors after slot removal 1) Since there is no slot for embedding the armature coil, the armature winding must be made into an integral cylindrical structure—a cylindrical coil—and then placed on the smooth annular surface of the armature core. 2) Because there are no slots to withstand electromagnetic torque, the cylindrical coil requires good overall integrity, high strength, and a firm connection with the armature core, which necessitates the use of special technical means. 3) Since the position detection Hall element cannot rely on slots to ensure its correct installation position, its logical position relative to the winding must be determined using specialized instruments during manufacturing. 4) In terms of the overall structure of the motor, its complexity is reduced, which is conducive to specialized production. 2 Changes in Product Performance After Slotless Motors 1) High efficiency: One reason is that the inner circle structure of the core laminations in slotted motors (and the outer circle structure in external rotor motors) is a combination of teeth and slots. The slots are used to embed the coils, and the teeth form the magnetic circuit to conduct magnetic flux. Because the teeth are very narrow and have high magnetic density, they generate significant heat, resulting in high tooth losses. Slotless motors do not have this type of loss. Another reason is that copper wire, like air, is non-magnetic, and the magnetic resistance is very high when magnetic lines of force pass through it. Slotless motors inevitably increase the magnetic resistance of the entire magnetic circuit. Therefore, compared to slotted motors, slotless motors have a weaker magnetic circuit, resulting in relatively lower eddy current losses. A third reason is that eliminating the teeth and slots in the core laminations expands the winding arrangement space. This allows for either increasing the cross-section of the copper wire to increase the motor's current rating, increasing the number of turns in the windings to increase the motor's voltage rating, or a combination of both. Ultimately, this achieves a higher power-to-volume ratio than slotted motors while maintaining relatively small losses. With the same amount of loss, the higher power naturally results in higher efficiency. Therefore, the increased efficiency of slotless motors is self-evident. 2) Lightweight and compact: See the explanation for reason three in 1). 3) Strong overload capacity: Brushless DC motors have a strong overload capacity. Torque is proportional to current, and its overload capacity is assessed through the allowable overload current. However, there is still a significant difference between slotted and slotless structures. Due to the small magnetic gap in slotted structures, the gap is equal to the mechanical gap when the magnetic pole is unsheathed, and equal to the mechanical gap plus the sheath thickness when the magnetic pole is sheathed. The single-sided magnetic gap varies from 0.04 to 0.80 mm depending on the product size. Such a small magnetic gap has limited resistance to armature reaction after the winding is energized. Therefore, the magnitude of the winding overload current must be limited to prevent the permanent magnet poles from demagnetizing due to the reverse magnetization of the overload current, which would lead to the loss of motor function. The slotless structure is different. The single-sided magnetic gap varies from 3 to 12 mm depending on the size of the product. The magnetic reluctance is quite large, which effectively suppresses the demagnetizing effect of overload current. The main reason for limiting overload current is that the winding is damaged due to heat. Therefore, it is common for slotless motors to be overloaded by more than 10 times in an instant. Its overload capacity is unmatched by any motor, including slotted brushless DC motors. 4) Low vibration and noise, smooth operation: One reason is the absence of slots, meaning no teeth, thus eliminating the "cogging effect" of conventional slotted motors—the high-frequency vibration caused by the permanent magnet poles sometimes aligning with teeth and sometimes with slots during rotation. Another reason is the large magnetic gap; the difference in axiality between the stator and rotor formed during manufacturing is significantly reduced relative to the magnetic gap, effectively suppressing the significant unilateral magnetic pull in slotted motors, allowing for smooth operation. A third reason is the small winding inductance, an order of magnitude lower than in slotted motors, resulting in a small electrical time constant, faster response under control, and reduced electromagnetic vibration. These factors are the inherent reasons for the low vibration, low noise, and smooth operation of slotless motors. 3. Reliability Analysis 1) As mentioned above, slotless motors have outstanding overload capacity, which can withstand unexpected accidents during use. Under normal circumstances, the motor remains intact; in other words, slotless motors are durable. 2) As mentioned above, the winding is made into an integral cylindrical structure to improve its overall mechanical performance. However, from the perspective of electrical insulation, it is a filled and integrally encapsulated insulation structure, which makes the inter-turn insulation performance more reliable and the insulation performance to ground improved. In particular, the insulation resistance value to ground can withstand double the test. 3) As mentioned above, the cylindrical coil is placed on the smooth armature core ring (cylinder) surface. Except for some contact surfaces, most of the outer surface of the winding is exposed in the gaps inside the motor. That is to say, the heat of the winding can be released into the motor (of course, it will eventually achieve thermal equilibrium through the contact between the casing and the air), thereby reducing its own temperature gradient and avoiding the phenomenon of local overheating of the winding. In slotted motors, the overheating problem of the winding located in the central slot of the core must be considered. Therefore, in terms of its working conditions, the armature winding, which is called the heart of the motor, is safer and more reliable than that of slotted motors in slotless motors. 4) The issue of torque transmission is a primary concern for industry professionals. Specifically, is it reliable to transmit electromagnetic torque solely through the connection between the coil and the core after slotting the iron core? This is not a concern. Firstly, slotless motors have been in use for at least twenty years, and the technology is mature and reliable; no such failures have occurred. Secondly, destructive testing has confirmed their high reliability. We heated the armature and clamped the iron core and cylindrical coil separately for torsional failure. When the torque increased to more than 60 times the rated torque, the test fixture began to deform and fail, while the connection between the coil and the iron core remained intact. In summary, the reliability of slotless motors is generally superior to that of slotted motors. 4 Design and Development Strategies and Product Outlook 1) Problem Statement and Analysis: Some argue that the superior quality of slotless motors is achieved by increasing the magnetic gap in slotless design, at the cost of increased magnet usage. This conclusion is only accurate when the motor diameter is greater than 80mm and the speed is less than 3000r/min. In small-diameter or high-speed motors, the amount of magnets used is not large. When the motor is so small that slotting is not possible or the amount of magnets cannot be reduced (for example, when the calculated thickness is thin and the strength is poor, it must be artificially thickened), slotless motors actually have an advantage. As the motor diameter gradually increases from 80mm and the speed gradually decreases from 3000r/min, the amount of magnets used will increase significantly. The design and development countermeasures mentioned below mainly refer to motors within this range. 2) Design and development countermeasures: This does not involve common problems of magnetization technology, but only principle improvements, with the aim of reducing the amount of magnets used by reducing the magnetic air gap. Therefore, when conditions permit, the following can be done: A. Use a long iron core and fewer coil turns to make the motor slender; B. Use multi-pole small-pitch coils to compress the length of the motor ends, thereby compressing the overall motor length; C. Use multi-wire parallel winding to increase the slot (virtual slot) fill factor of the windings; D. Choose the pole arc coefficient appropriately, because when the electrical angle of the magnetic poles reaches 120° and is increased further, the added effect is not economical compared to the increased volume; E. Choose magnets with a high selectivity/price ratio, such as N38SH, which is cheaper than N33SH, even though the former is significantly more expensive than the latter. 3) Product Outlook: Slotless brushless permanent magnet DC motors were originally developed for military needs. With the continuous development of electronic technology and permanent magnet materials, they are now emerging in civilian applications. Judging from the more than 100 products developed and applied by Jiangxi Xitai Motor Co., Ltd., these products have significant performance advantages, positive user feedback, high demand, and strong momentum in multi-field applications. Technically, these products have a power range of 5W-22kW, a current range of 1A-120A, a voltage range of 12V-480V, and a frame size range of φ34-φ360, comparable to the existing range of traditional brushless DC motors. Their applications span various industries. Therefore, they are frequently chosen as the preferred option when comparing with traditional brushless DC motors, and their market share is increasing. Especially today, with global resource scarcity, the energy-saving and consumption-reducing benefits of high efficiency and small size of slotless brushless permanent magnet DC motors will become increasingly important. I believe that slotless brushless motors are an inevitable trend and the direction of development.