There are two types of generator excitation methods. One is called external excitation, which is to establish a magnetic field by supplying excitation current from the power grid. This method cannot be used in rural areas where there is no other power supply. The other is called self-excitation, which relies on its own residual magnetism and a set of capacitors connected to the stator windings for self-excitation. This method is widely used in rural areas. Selection of self-excited asynchronous motor and the conditions required for generators (1) In order to meet the power demand of both power and lighting loads, an asynchronous motor with a "Y" connection should usually be selected to facilitate the lead-out of the neutral line. (2) In order to reduce the cost, an asynchronous motor with a capacity of less than 15kW and a voltage of 380/220V should be selected. (3) The speed of the motor should be slightly lower than that of the prime mover. The speed of the prime mover should generally be about 5% to 10% higher than that of the motor at the same time. (4) There must be a certain amount of residual magnetism on the rotor of the motor. (5) An excitation capacitor of appropriate value must be connected in parallel. The correct selection of the no-load excitation and load parallel capacity is very important. If the capacity is too large, the no-load voltage will be too high, which may damage the equipment; if it is too small, the no-load voltage will be too low. The no-load excitation capacitor should be selected so that the voltage generated by the generator does not exceed the rated voltage specified on the nameplate. According to this condition, the excitation capacitor Co required under no-load (connected in "△") can be calculated by the following formula (1) where IO is the no-load excitation current A under UN, UN is the rated line voltage of the motor, and CO is the no-load excitation capacitor μ. For the total three-phase capacity connected in "Y", it can be calculated by the following formula C0=106＊3I0/314UNx (2) where UNX is the rated phase voltage of the motor. When the motor is loaded, the generator voltage will decrease with the increase of the load. Therefore, the capacity must be compensated when running under load. When the load power factor cosφ=1 (i.e., a purely resistive load) and is fully loaded, due to the reactive power loss caused by the active power in the power grid, a capacitor must be added to compensate for the reactive power in the load. The compensation capacitor value can be estimated by the following formula: C1=1.25CO (3) Where C1 is the compensation capacitor value when under load. When the load power factor cosφ is about 0.8, due to the inductive load, in order to compensate for the reactive power loss caused by the reactive power in the power grid, a corresponding compensation capacitor value must be added. The compensation capacitor value C2 can be calculated by the following formula: C2=0.6SN/314UN2 (4) Where C2 is the compensation capacitor value for reactive power. μ is the rated capacity of the asynchronous motor. The total load compensation capacitor C when under rated load is: (5) The total excitation capacitance of the three phases is C_total. (6) For example, there is a J02-71-8 motor with a rated power Pe=17kW, rated speed 720r/min, rated voltage 380V, rated current 35.8A, Y-connection, power factor 0.8, no-load current 5.5A (nameplate value). It needs to be converted into an asynchronous generator. Calculate the no-load capacitance and load capacitance. Substitute the above data into formulas (1), (3), (4), and (5) to obtain a no-load capacitance of 78μF and a total load compensation capacitance of 322.4μF. Precautions during operation (1) In order to ensure the quality of the power supply voltage, the compensation capacitance should be increased or decreased according to the load. (2) The load should be brought on after the no-load voltage of the generator has risen, otherwise it will be difficult to establish the generator voltage. (3) The load should not exceed 25% of the generator capacity, and the capacity of a single motor should not exceed 10% of the total generator capacity, otherwise the starting current will be too large, the voltage will drop sharply, and the motor will be difficult to start.