Design features of small AC permanent magnet wind turbine
DC generators, electromagnetic alternators, claw pole generators, reluctance generators, and inductive generators have been used for small wind power plants. With the development of the technology of permanent magnet materials, the magnetic energy product of permanent magnet materials has been greatly improved, and permanent magnet generators are mainly used at present. This type of motor is superior to the former types of generators in terms of electrical performance and safety. Since the application place of this type of generator is different from that of a general generator, its technical requirements have its particularity, and it must have a good match with the wind turbine in terms of performance. Therefore, some analysis on several problems in this type of generator is carried out. And explore.
2 technical requirements
Figure 1 is a schematic diagram of a small wind power generation device. The wind drives the wind wheel to rotate, converting the wind energy into mechanical energy. The wind wheel drives the generator to rotate, converts the mechanical energy into electrical energy, and rectifies and outputs. The design of this type of generator must first select the type of generator and the rectification line; determine the calculated rectification power, rated power, voltage, speed and so on. The main technical requirements for it are:
(1) rated output power PN (W); (2) rated output voltage (DC) UN (V); (3) rated speed NN (r / min); (4) generator efficiency η (); (5) Starting resistance torque TN (Nm); (6) at 65 rated speed, the no-load voltage of the generator should not be lower than the rated voltage; (7) at 150 rated speed, the generator should be able to overload operation under rated voltage 2min; (8) The generator should be able to withstand 2 times the rated speed under no-load conditions, lasting 2 minutes, the rotor structure should not be damaged and harmful deformation; (9) The generator should be able to prevent rain, snow, sand and lightning .
In addition, it should also meet the technical requirements of general motor insulation, pressure resistance, mechanical strength and so on.
Technical requirements (5), (6), (7), (8) are special requirements for wind turbines, which will be analyzed separately below.
3 electromagnetic load selection
The practice of modern motor manufacturing and the long-term operation of the motor generally give the range of the line load As and the magnetic load Bδ of the designed motor. When the product of As and Bδ is the same, then the ratio between As and Bδ determines the different parameters of the generator, the force energy index and the mass. When Bδ is large and As is low, the generator is iron-rich, and when As is large and Bδ is small, the generator is copper-rich.
The electrical load of the motor is measured by the current density j (A/mm2) of the motor winding and the line load As (A/cm). The larger the electrical load, the larger the copper loss. For low-power wind turbines, it is generally low-voltage and high-current. In particular, generators below 1 kW are mostly used at 24, 36 V or 48 V (rectified DC), and the rated current of such motors is large. For low power generators from 1 to 10 kW, high rated output voltages cannot be taken. Because this type of generator mainly uses battery energy storage, the voltage is high, more batteries must be used, which increases the cost of the whole machine, which is difficult for customers to accept. In short, the low-power wind turbine line load is relatively high, and it is a copper-rich generator. The copper loss of the motor is large, accounting for about 70% of the total loss of the motor. This is an objective situation. In addition, the output power of the generator increases as the wind speed increases, as shown in Figure 2. The power of the generator is increased and the heat is increased. However, as the wind speed increases, the heat dissipation condition is greatly improved. Therefore, for this type of generator, the standard of selecting the As for the general motor should not be adhered to, and the higher As value can be selected, which is both required and allowed. For example, the general small power motor As is 60 to 80 A/cm; and the As of the type of generator can be taken as 100 to 150 A/cm; and the aerogen generator As using high-efficiency fuel injection cooling can reach about 300 A/cm. Therefore, the choice of As should take into account the motor loss, efficiency, heat dissipation and application, and obtain a reasonable value.
The choice of magnetic load Bδ can be completely in accordance with the general principles of motor theory, and will not be described here.
4 stator
4.1 stator slots
Based on the high electrical load of this type of generator, the copper loss is large. When designing the generator, the tooth width and the yoke thickness should be minimized to ensure the groove area by ensuring sufficient mechanical strength and magnetic flux density. Increase the wire area of the stator winding, reduce copper consumption, and improve the efficiency of the generator. This is not something that every manufacturer has considered. Often due to the thinner stator winding wires, the design requirements can be met during initial operation of the generator. After 2 to 3 hours of operation, the temperature rises sharply and the output power drops rapidly, so that the rated output power does not meet the requirements.
4.2 stator winding
The technical requirements of low-power wind turbines (5) introduce the concept of generator starting resistance torque, because small wind power generation equipment generally rotates at tens to hundreds of revolutions, in order to reduce links, reduce costs and improve reliability. The wind wheel of the device is directly coupled to the generator shaft. This requires minimizing the resistance torque generated by the generator cogging effect, so that when the wind speed is low (2 to 3 m/s), the wind turbine can be started quickly and generate electricity as soon as possible. To this end, the national standard GB10760.1-89 put forward the requirements, see the table below.
Power (W) 501002003005001000 maximum starting resistance torque (Nm) 0.200.300.350.501.201.50
From the theory of the motor, the stator chute, the rotor slant pole and the stator fractional slot winding can reduce the resistance torque caused by the cogging effect and meet the technical requirements. However, it has been proved that the fractional slot winding is the most effective way to reduce the resistance torque.
The use of the stator chute is relatively easy to implement in the process, but the effect is not obvious, and if the chute distance is too large, the electrical performance of the generator will be affected; using the rotor oblique pole, the rotor magnet and the magnetic pole are twisted to a reasonable size. The process is difficult and the effect is not obvious; therefore, fractional slot windings are mostly used.
Fractional slot winding:
Number of slots per phase per pole q=Zs/2mp=ac/d
Number of slots per pole Q=Zs/2p=AC/D
Where: Zs is the number of stator slots; m is not the number of winding phases; p is the number of generator pole pairs; A, a is an integer; c / d, C / D is an irreducible fraction.
Theory and practice prove that the larger D is, the smaller the starting resistance torque of the generator is [5]. In addition, as the value of q increases, the negative sequence impedance decreases and the leakage reactance decreases, which is what we hope. At the same time, however, excessively increasing the value of q, the ability of the generator to suppress higher harmonics is reduced, and this should be avoided. Therefore, as long as the resistance torque requirement specified by the national standard is met, the larger the q value, the better.
We calculated and actually tested the torque of several generators, from which we can determine the coordination of the teeth, see Figure 3.
5 rotor
The wind turbine speed of a small wind power generation device is tens to hundreds of revolutions per minute, and the generator rotor is directly coupled to the wind wheel. The rotor speed determines that the generator is a multi-pole low-speed generator; the rotor is generally made of ferrite and neodymium-iron-boron magnet, tangential structure; the rotor structure must be firm and can withstand the impact of sudden changes in wind speed without damage or damage. And deformation. This is clearly stated in the technical requirements (7) and (8). The rotor problem will be discussed in a special article.
6 characteristics
6.1 DC output voltage
The generator outputs an alternating current voltage to rectify and charge the battery. According to the national standard, the rectified voltage should be 2V higher than the standard 12V battery, that is, the generator output voltage is 14V, 28V, 42V, 56V... However, it has been proved that this regulation is feasible for areas with very abundant wind resources, but for wind resources, the areas that can be used are low. Some people used to produce 42V (DC) in the inner lake culture area of Jiangsu Neihu. The generator was connected to two series of batteries (24V) and it worked well without serious problems. Therefore, when designing a generator, it should be known that the wind source in the area where the wind turbine is used should generally be higher than 4V in order to make full use of valuable wind resources.
6.2 Output characteristics
The relationship between output power P and speed n is not required for general generators and is important for such generators. Figure 2 shows the measured characteristics of the DYF-600 generator. Due to specific requirements, wind turbines require generators to generate electricity at low wind speeds, while output characteristics are as soft as possible above rated wind speeds. Therefore, when designing the generator, the magnetic circuit should be saturated as much as possible, so as not to cause frequent overspeed of the wind turbine, and the output power of the generator rises sharply, causing excessive impact on the charger and the inverter and overheating of the generator, thereby damaging. .
6.3 Matching characteristics of wind turbines with generator output characteristics
(1) After the wind turbine is started, the generator is required to generate electricity as soon as possible, that is, wind energy can be captured in a low wind speed range. This is as required by the technical requirements (6), the starting torque of the generator is as small as possible, so that the wind turbine can be cut into operation as soon as possible.
(2) It is hoped that the generator P=f(n) has a quadratic parabolic relationship before the rated point to obtain the best wind energy by matching the generator with the wind turbine.
