1 Introduction
People have used DC generators, electromagnetic AC generators, claw-pole generators, reluctance generators, and inductor generators in small wind power generation devices. With the technological advancements in permanent magnet materials, the magnetic energy product of permanent magnet materials has greatly increased, and permanent magnet generators are now the primary type used. This type of generator surpasses the previous types in both electrical performance and safety and reliability. Because its applications differ from general generators, its technical requirements are unique, and its performance must be well-matched to wind turbines. Therefore, this paper analyzes and discusses several issues related to this type of generator.
2 Technical Requirements
Figure 1 shows the schematic diagram of a small wind power generation device. Wind drives a wind turbine to rotate, converting wind energy into mechanical energy. The turbine then drives a generator to rotate, converting the mechanical energy into electrical energy, which is then rectified and output. The design of this type of generator first requires selecting the generator type and rectifier circuit; determining the calculated rectified power, rated power, voltage, and speed. Its main technical requirements 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 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 operate under overload for 2 minutes at the rated voltage; (8) Under no-load conditions, the generator should be able to withstand twice the rated speed for 2 minutes without damage or harmful deformation to the rotor structure; (9) The generator should be able to withstand rain, snow, sand and lightning.
In addition, it should also meet the general technical requirements for motor insulation, withstand voltage, mechanical strength, etc.
Technical requirements (5), (6), (7), and (8) are special requirements for wind turbine generators, which will be analyzed separately below.
3 Selection of Electromagnetic Load
Modern motor manufacturing practices and long-term operational experience have roughly established the ranges for the line load As and magnetic load Bδ in motor design. When the product of As and Bδ is the same, the ratio between As and Bδ determines different parameters, power performance indicators, and quality of the generator. When Bδ is large and As is small, the generator is iron-rich; when As is large and Bδ is small, the generator is copper-rich.
The magnitude of the motor's electrical load is measured by the current density j (A/mm²) of the motor windings and the line load As (A/cm). The larger the electrical load, the greater the copper loss. For small-power wind turbines, low voltage and high current are generally used. Especially for generators below 1kW, most use 24, 36, or 48V (rectified DC), and these motors have relatively large rated currents. For small-power generators from 1 to 10kW, excessively high rated output voltages should also be avoided. This is because these generators primarily use batteries for energy storage; higher voltages require more batteries, increasing the overall cost, which is generally unacceptable to customers. In short, small-power wind turbines have relatively high line loads and are copper-rich generators, resulting in high copper losses, accounting for approximately 70% of the total motor losses – this is an objective reality. Furthermore, the generator's output power increases with wind speed, as shown in Figure 2. Increased generator power leads to increased heat generation, but with increased wind speed, heat dissipation conditions improve significantly. Therefore, for this type of generator, the standard As selection for general motors should not be strictly adhered to; a higher As value can be selected, which is both necessary and permissible. For example, the As value for a typical small-power motor is 60 to 80 A/cm; while for such generators, the As value can be 100 to 150 A/cm; and for aircraft generators using high-efficiency oil-injection cooling, the As value can reach around 300 A/cm. Therefore, the selection of As value should comprehensively consider motor losses, efficiency, heat dissipation, and application scenarios to obtain a reasonable value.
The selection of the magnetic load Bδ can be based entirely on the general principles of motor theory, and will not be elaborated here.
4 stators
4.1 Stator Gear
Given the high electrical load and large copper losses of this type of generator, when designing the generator, while ensuring sufficient mechanical strength and permissible magnetic flux density, the tooth width and yoke thickness should be minimized as much as possible to increase the slot area, enlarge the stator winding conductor area, reduce copper losses, and improve generator efficiency. This is not something every manufacturer considers. Often, because the stator winding conductors are relatively thin, the design requirements can be met during the initial operation of the generator. However, after 2 to 3 hours of operation, the temperature rises sharply, and the output power drops rapidly, causing the rated output power to fall short of the requirements.
4.2 Stator winding
The technical requirements (5) for small-power wind turbines introduce the concept of generator starting torque. This is because small wind power generation devices generally operate at speeds of tens to hundreds of revolutions per minute. To reduce the number of components, lower costs, and improve reliability, the wind turbine is directly coupled to the generator shaft. This requires minimizing the starting torque generated by the generator cogging effect so that the wind turbine can start quickly and generate electricity as soon as possible when the wind speed is low (2 to 3 m/s). To this end, the national standard GB10760.1-89 puts forward requirements, as shown in the table below.
Power (W) 50 100 200 300 500 1000 Maximum starting resistance torque (Nm) 0.20 0.30 0.35 0.50 1.20 1.50
From a theoretical perspective, using stator skewed slots, rotor skewed poles, and stator fractional slot windings can all reduce the resistive torque caused by cogging effect and meet technical requirements. However, practice has shown that fractional slot windings are the most effective way to reduce resistive torque.
Using stator skewed slots is relatively easy to implement in terms of technology, but the effect is not obvious, and if the skewed slot distance is too large, the electrical performance of the generator will be affected. Using rotor skewed poles, which involves twisting the rotor magnets and poles to a reasonable size, is more difficult in terms of technology, and the effect is not obvious. Therefore, fractional slot windings are mostly used.
Fractional slot winding:
Number of slots per pole per phase: q = Zs/2mp = ac/d
Number of slots per pole Q = Zs/2p = AC/D
In the formula: Zs is the number of stator slots; m is the number of unwound phases; p is the number of generator pole pairs; A and a are integers; c/d and C/D are irreducible fractions.
Theory and practice have proven that the larger the value of D, the smaller the starting torque of the generator [5]. Furthermore, as the value of q increases, the negative sequence impedance decreases and the leakage reactance decreases, which is desirable. However, excessively increasing the value of q reduces the generator's ability to suppress higher harmonics, which should be avoided. Therefore, as long as the torque requirements specified in the national standard are met, a larger q value is not always better.
We calculated and tested the torque of several generators to determine the gear-pole fit, as shown in Figure 3.
5 rotors
The rotor speed of small wind power generation devices ranges from tens to hundreds of revolutions per minute, and the generator rotor is directly coupled to the rotor. The rotor speed determines that the generator is a multi-pole low-speed generator; the rotor generally uses ferrite and neodymium iron boron magnets with a tangential structure; the rotor structure must be robust and able to withstand the impact of rapid changes in wind speed without damage or deformation. This is as clearly stated in technical requirements (7) and (8). The rotor issue will be discussed in a separate article.
6 features
6.1 DC Output Voltage
The generator outputs AC voltage, which is then rectified to charge the battery. National standards stipulate that the rectified voltage should be 2V higher than a standard 12V battery, meaning the generator output voltage can be 14V, 28V, 42V, 56V, etc. However, practice has shown that this regulation is feasible for areas with abundant wind resources, but less so for areas with moderate but usable wind resources. For example, in aquaculture areas in Jiangsu's inland lakes, a 42V (DC) generator connected to two series-connected batteries (24V) worked well without serious problems. Therefore, when designing a generator, the wind conditions in the area where the wind turbine will be used should be considered; generally, the voltage should be at least 4V higher to fully utilize valuable wind resources.
6.2 Output Characteristics
The relationship between output power P and rotational speed n is not generally required for generators, but it is important for this type of generator. 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 the output characteristics should be as soft as possible above the rated wind speed. Therefore, when designing the generator, the magnetic circuit should be saturated as much as possible to prevent the generator output power from rising sharply due to the frequent overspeeding of the wind turbine, which could cause excessive impact on the charger and inverter, and overheating of the generator, thus damaging it.
6.3 Matching of wind turbine characteristics and generator output characteristics
(1) After the wind turbine starts, the generator is required to generate electricity as soon as possible, that is, to capture wind energy in the low wind speed range. This is as required by technical requirement (6), that the starting torque of the generator should be as small as possible so that the wind turbine can start running as soon as possible.
(2) It is hoped that the generator P=f(n) will be a quadratic parabola before the rated point, so as to obtain the best wind energy by matching the generator with the wind turbine.
