Ordinary generators are designed by constant frequency constant pressure, it is impossible to fully meet the requirements of frequency control. The following is the impact of the inverter on the digital inverter generator:
1, the efficiency of the generator and the problem of temperature rise No matter the form of frequency converter, in operation have different degrees of harmonic voltage and current, the generator in non-sinusoidal voltage and current operation. Refused data, the current commonly used sine wave PWM inverter, for example, the low-order harmonic is essentially zero, the remaining higher than the carrier frequency about twice the higher harmonic components: 2u +1 (u For the modulation ratio). High harmonics cause stator copper loss, rotor copper (aluminum) loss, iron loss, and additional losses, most notably rotor copper (aluminum) consumption. Because the asynchronous generator rotates at a synchronous speed corresponding to the fundamental frequency, the rotor loss is generated when the higher harmonic voltage cuts the rotor bar with larger slip. In addition, the additional copper consumption due to the skin effect needs to be considered. These losses will make the generator extra heat, reduce efficiency, reduce output power, such as the ordinary three-phase asynchronous generator running in the inverter output of non-sinusoidal power supply conditions, the temperature increase is generally increased by 10% - 20%.
2, the problem of generator insulation strength At present, small and medium-sized inverter, many are using PWM control. His carrier frequency is about a few thousand to a dozen kilohertz, which makes the generator stator windings to withstand high voltage rise rate, which is equivalent to impose a large steep voltage on the generator, so that the turn of the generator turn Insulation to withstand the more harsh test. In addition, the rectangular chopper impulse voltage generated by the PWM inverter superimposed on the generator operating voltage, will pose a threat to the generator insulation to ground, insulation under high pressure repeated impact will accelerate aging.
3, Harmonic electromagnetic noise and vibration Ordinary induction generator inverter power supply, will make the vibration and noise caused by electromagnetic, mechanical, ventilation and other factors become more complex. Each time harmonic contained in the frequency power and the inherent space harmonic of the electromagnetic part of the generator interfere with each other to form various electromagnetic exciting forces. When the frequency of the electromagnetic force wave and the natural frequency of the generator body are the same or close to each other, a resonance phenomenon will occur, thereby increasing the noise. Due to the wide operating frequency range of the generator and the wide range of rotation speed, it is difficult to avoid the natural frequency of various components of the generator due to the frequency of various electromagnetic force waves.
4, the generator on the frequent start, the ability to adapt to the brake As a result of frequency inverter power supply, the generator can be low frequency and voltage without impact current start, and can use the inverter for a variety of systems The dynamic braking mode of rapid braking creates the conditions for frequent starting and braking. Therefore, the mechanical system and the electromagnetic system of the generator are under cyclic alternating force, which brings about fatigue and accelerated aging of the mechanical structure and the insulation structure.
5, low-speed cooling problems First of all, asynchronous generator impedance is not ideal, when the power supply frequency is low, the power of the higher harmonics caused by larger losses. Secondly, when the normal speed of the asynchronous generator is reduced, the cooling air volume decreases proportionally with the third power of the rotation speed, causing the low-speed cooling condition of the generator to deteriorate and the temperature rise to increase sharply, making it difficult to achieve constant torque output.