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Transcript
Activity: Read about the control of the BLDC motors and note the diagrammatic drawing of the synchronous machines with the main directions and coil markings; the purpose of the fits; the reasons for pulsating moment formation; the differences between BLDC and synchronous motors; and in connection with magnet arrangements, the surface-mounted bread loaf shaped, ring-shaped and buried arrangements, as well as types with external rotors. 1. The principle, regulation and features of permanent magnet synchronous motors controlled by a current vector in vehicle drives. 1.1 Essential properties of permanent magnet synchronous motors Essential properties of classical synchronous motors, as have learnt from the previous chapters, are as follows: -inflexible, maintaining speed; -not self-starting, therefore completely unsuitable for vehicle drives (but); -this electrical machine has the highest power density, with its cos φ=1. Making synchronous machines and synchronous motors depicted in previous chapters capable of vehicle driving has been an attractive challenge for a long time. This is due to the best indicators of the synchronous machines that are related to volume utilisation. The rigid assignment to the frequency was the most important hindrance, but this hindrance could be eliminated after the appearance of the inverters. The ability to construct it with permanent magnets can provide the same torque in a decreased machine size and also the pole wheel. In addition the DC excitation system can be omitted. The special change in the principle enables synchronous motors, known as electric motors that have the most inflexible nature, to operate dynamically with rapidly changing velocity and with a positive sign thus, having the ability to be built into servo drives or in vehicle drives. High speed operation of the latter one might lead to unwanted excess voltages due to the permanent magnets, which is an additional challenge for this construction. The self-controlled and computer aided synchronous motor controlled with a current vector is a special self-guiding system (see later), with properties free from the above-mentioned disadvantages: • appeared in electric locomotives (P>2000kW): 1974 • in robot and industrial servo drives (P<1 kW): 1986 • in cars (P>30kW) : 1997 1.1 Construction. Decreasing pulsating moment Material and arrangement of the magnets Vehicles with electric drives and servo drives run with synchronous motors excited by permanent magnets and the general arrangement of such motors is shown in figure 1. Stator coils a, b and c are shown with simplified graphical symbols, which occupy the stator bands, with a 1/6 width of phases and current directions. The symmetrical axis of the stator coil "a" is, as usual, the vertical centre-line of the stator. The synchronous motor in the figure has 2 poles and the rotor made with permanent magnets indicates a North-South direction as well as an axis d of the rotor and this is also the direction of Φ main flux. Figure 1. A simplified sketch of the synchronous motor. The motor torque is produced by the magnetic interaction between the induction field created by the permanent magnet rotor and the current flow in the stator coil. The circumferential value of the torque depends on the product of the actual values of the two aforementioned factors, that is, assuming a quasi-stationery state, the torque will have a constant value in the function of the angular displacement if these factors fit to each other in such a way that their product can be constant. On the basis of this, supply provided with normally sinusoidal voltage will require such an induction distribution and fit along the circumference. If this condition fails to occur pulsating torques are generated and the rate of such torques may show considerable differences. Appearance of such torques lead to disturbances in the position control of the servo drives, therefore the circumferential torque curve of the motors used there can only include alternating constituents below 1%. In vehicle drives the appearance of pulsation is disturbing, especially during start-up, and pulsation is not desired for higher speeds due to its effect of generating vibration in the masses, which may cause a surplus load and fatigue in the structure. The non-sinusoidal and square-wave supply can be realised at the lowest cost. It needs a matching and simple implementable magnet arrangement that results in an induction distribution with a similar shape and this can mitigate the torque pulsation. These motors are the so-called "brushless direct current" (BLDC) electrical machines, which are, from the aspect of their properties, designed mainly according to the synchronous motors. Their control and supply are simpler and cheaper therefore a lot of motors like them are used in motorized bicycles, scooters and models. On the other hand, pulsating torque may be caused by other reasons, namely, when a magnet arcs passes in front of the stator teeth, or more precisely, when such arcs osculate the edges of the teeth, this causes rapid changes in the induction distribution. The rate of this depends on the air-gap induction, the size of the air-gap, the shape of the teeth, and the magnet edge shape at the pole changes, taking into consideration one edge-pair of each. If the ratio of the slot number and the pole pairs is an integer, which is an advantageous construction from other aspects, pulses will appear simultaneously and, considering nominal torque, the lump sum may range between 1 and 20 %. Reasons for appearance of pulsating torque may be eliminated or may be made quasi undetectable by means of sinusoidal supply of synchronous motors, through the arrangement induction distribution aimed at this, and through the avoidance of osculation of said edges by using buried magnets. However, R&D and partly additional manufacturing activities and costs would be needed for this. In synchronous machines phase voltage and flow of the stator are sinusoidal in an ideal case. The main flux is generated by the rotor permanent magnets, producing a sinusoidal distribution for the air-gap induction in the air gap, in an ideal case, figure 2 b), or in the case of BLDC type motors, this will be a square-wave type, figure 2 a). Figure 2 Circumferential induction distribution in permanent magnet synchronous motors The following figure (figure 3) shows a potential realisation of the sinusoidal induction distribution: Figure 3 Curves of induction distribution in permanent magnet synchronous motors There are several construction modes for the arrangement and fitting of the magnets: - fitted in arc-shaped arrangement onto the rotor surface and "bread loaf" shaped, - buried arrangement, consisting of several lineal or arc-shaped sections (figure 4). Figure 4 Different kinds of permanent magnet arrangements in permanent magnet synchronous motors The following figure, Figure 5 represents a potential realisation of the constructions shown in the diagrammatic figures for poles consisting of two magnet parts. On the right, a magnet arrangement entitled "buried" is shown: Figure 5 Buried type magnet arrangement. Versions with external rotors and reversed arrangement use a surface-mounted arc-shaped arrangement, which are mainly used in motorized bicycles and scooters. Figure 6 shows 1/4 of the arc of a motor with an external rotor. Figure 6 A version with and external rotor that is used mainly in scooters. The arrows sign the direction of the winding by a specified layout. 1.3 About magnet materials used Magnets are mainly fabricated from rare earth metals over the last decades. The application of Nd-FB (neodymium-iron-boron) based magnets has gained ground over the last 10 years. Their coercivity (H rc) is 3 or 4 times of that of the rare earth metal based magnets and their remanent induction (Br) is higher by 20-30 %. Today approx. 30 different versions (Table 1) are available for this magnet. The appropriate type can be selected according to the Br value (from 1,05 to 1,35 Vs/m 2) and partly according to the planned operating temperature. For this new type of magnet material it is an unusually important aspect to choose and keep the appropriate operating temperature, since its remanent induction shows a highly negative temperature-dependence. The value of such dependence varies but it may exceed -1 %/ oC. A negative phenomenon of this nature in the case of an operation at a temperature above the planned value is the reduction of the Br induction and will entail the reduction of the flux value at the same rate, and due to this reason, the torque at the same motor current will also decrease by similar rate. Although a current increase applied to compensate this phenomenon resets the value of the torque, the efficiency will decrease due to the I 2R ohm increase in the coil loss. Torque resetting through increasing the current can result in only a few percentages of increase in the current intensity, taking also into consideration the surplus load of the inverter any further decrease in the induction above this value will entail torque loss, which can only be reinstated after cooling-down. The final reduction and loss of the remanent induction occurs when reaching the Curie-point, 260-300 oC, however, the original value can be reset after cooling down by magnetisation. For the appropriate engineering the particular Nd-FB types of the magnet materials are indicated by the manufacturers on the bases of the matching values of Br and the planned operating temperature. Column 1 shows the magnet type: Table 1 The properties of the magnet materials