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Magnetic pump 1. Magnetic pump (1) Overview: Magnetic drive pumps, also simply referred to as magnetic pumps, are pumps in which a magnetic coupling is integrated with the pump itself. It is completely different from mechanical couplings, as it uses the principle of magnetic induction to transmit torque. Figure 23-5 is a structural diagram of the magnetic pump. It consists of the main pump head (impeller, pump body), external magnetic steel (active rotor), internal magnetic steel (passive rotor), and isolation sleeve. The prime mover drives the outer magnetic steel to rotate, and due to the effect of the magnetic field, the inner magnetic steel rotates in sync. It should be an integrated unit of the internal magnet and the impeller, with the impeller rotating together with the internal magnet to pump the liquid. There is a isolation sleeve between the inner and outer magnetic magnets; therefore, as shown in Figure 23-5, the liquid being pumped is contained within this isolation sleeve, eliminating the shaft seal problems associated with conventional pumps. Thus, a magnetic pump is a leak-free pump. Since it completely prevents liquid leakage, its areas of application are expanding. In addition to use in the aerospace industry, it is employed in sectors such as chemicals, metallurgy, light industry, and food processing for pumping highly corrosive, toxic, flammable, and valuable liquids. 1 — Pump body; 2 — Impeller; 3 — Pump cover; 4 — Sliding bearing; 5 — Pump shaft; 6 — Inner magnetic rotor; 7 — Outer magnetic rotor; 8 — Bracket; 9–11 — Intermediate coupling; 12 — Shield; 13 — Base; 14 — Electric motor. (2) Basic structure of magnetic pump. There are two structural forms for magnetic drive: cylindrical magnetic coupling and disc-shaped magnetic coupling, as shown in Figure 23-6. Cylindrical magnetic couplings are widely used because they have a high utilization rate of magnetic materials; under the same magnetic circuit parameters, they can generate a greater transmitting force per unit volume of magnetism. In disc-type magnetic couplings, a large axial force is generated between the two magnets; therefore, measures must be taken to balance this axial force and the isolation sleeve must be made thicker. Otherwise, the isolation sleeve may deform or get damaged. For these reasons, despite their simple design and ease of manufacture, disc-type couplings are rarely used. The following mainly introduces the structural form of the cylindrical coupling, as shown in the figure. The inner magnetic cylinder is coaxial with the pump rotor, and can also be made of ordinary magnetized steel. 8 to 28 permanent magnet blocks are installed on the outer cylindrical surface of the inner magnetic cylinder in the direction parallel to the axis; separators made of austenitic stainless steel are placed between these magnet blocks. In addition to supporting and protecting the magnet blocks, these separators also serve to prevent magnetic flux from leaking laterally between them. A covering layer must be applied over the inner magnet. If stainless steel is used for the coating, arc welding is employed; if plastic is used, the magnet is enclosed through molding or welding methods. The coating layer must be of high quality, free from pores; after it is wrapped, it is then turned. 2) Outer magnetic cylinder: The outer magnetic cylinder is coaxial with the motor shaft, and can also be made of ordinary magnetized steel. On the inner cylindrical surface of the outer magnetic cylinder, 8 to 28 permanent magnet blocks are also installed in the direction parallel to the axis. Isolation blocks made of austenitic stainless steel are installed between the magnetic blocks. External magnetic steel is generally not coated with stainless steel or plastic; a corrosion-resistant coating may be applied to prevent corrosion. 3) Isolation sleeve: It is placed between the inner and outer magnetic steel pieces, fixed to the pump body, and serves to tightly seal the inner magnetic steel piece from the medium being pumped. Isolation sleeves usually come in three different forms. Metal molds: Made from materials that cannot be magnetized and have high electrical resistance, such as precipitation-treated austenitic stainless steel, titanium alloys, etc. Non-metallic type: Made from fluoroplastics, carbon fiber plastics, etc. Composite type: plastic outer wall with a metal cladding. The isolation sleeve is a key component of magnetic drive sealing; it has a flange at one end, is relatively thick, and the flange end features a sealing groove in which an O-ring is placed. The isolation sleeve is placed between the inner and outer magnetic steel pieces and bears internal pressure; its thickness is generally 1–2 mm for metal materials and also 1–2 mm for plastic materials. Magnetic steel: Magnetic steel is the key component in magnetic drive systems, and the three most commonly used materials for this purpose are ferrite, rare earth magnets, and iron-boron magnets. The main parameters for evaluating the performance of magnetic materials are: ① Maximum magnetic energy product (BH). The magnetic energy product is the product of the magnetic induction intensity and coercivity; a high magnetic energy product allows for greater power transmission at the same size. ②Magnetic induction coercivity (Hc): A higher coercivity results in a lower demagnetization rate. ③Remaining induction strength (Br): A higher magnetic induction strength results in greater power transfer. The performance parameters of some magnetic materials are shown in the table. (3) Characteristics of magnetic pump: 1) Since the central shaft does not extend out of the housing, there is no issue with shaft seal leakage, allowing for complete sealing. 2) The power transmission has overload protection. 3) Due to the losses associated with magnetic drive, the efficiency of magnetic pumps is about 5% lower than that of conventional pumps. It is not suitable for use in high-power pumps. 4) Sliding bearings are lubricated by a pumped medium, resulting in lower reliability compared to ordinary pumps, and the temperature of the pumped medium is limited. 5) High requirements are placed on the material and manufacturing precision of the isolation sleeve. 6) Axial force balance is crucial for magnetic pump systems, and self-balancing devices should be used.