V/f conversion control method

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 In the asynchronous motor speed regulation principle, focuses on frequency conversion air conditioner here is how to realize the V/f of the frequency conversion control, which is widely used in inverter PWM (pulse-width modulation) technology. Asynchronous motor with inverter drive box. The ac to dc rectifier, smooth ac electric motor loop after the pulsating dc smooth, by the inverter it transform into frequency adjustable alternating current (ac). As shown in figure 3 (a), a sine wave divided into N (N = 12), and then put the sine curve and the horizontal axis of each uniform surrounded area, are high with an equal in the area of rectangular pulse to replace, the halfway point of the rectangular pulse with sine wave every parts. With sine wave of equivalent continuous rectangular wave pulse sequence midpoint overlap (pictured above), in this way, by N continuous and wide range of rectangular pulse waveform with the positive half cycle of sine wave equivalent. Also, sine wave of negative half cycle can be equivalent with the same method. Figure 3 (b) a series of pulse waveform is expected by the PWM inverter (Pule Width Modulation) waveform. Because of the pulse amplitude is equal, so the inverter can be made of a constant dc power supply. 


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That electric motor price is to say, this kind of hands has been a make inverter in the inverter adopts uncontrolled diode rectifier. Inverter output pulse amplitude is the output voltage of the rectifier. Such as inverter all switching devices are working under ideal state, to drive the corresponding signal switching device should also be similar to figure 3 (b) the shape of a series of pulse waveform. As a result of the PWM output voltage waveform and current waveform is sine wave, with many higher harmonic components, which makes the energy input to the motor cannot be fully use, increase the wear and tear. In order to make the output waveform similar to sine wave, sinusoidal pulse width modulation (SPWM) is proposed. So-called SPWM modulation, simply speaking, is in the pulse width modulation, the duty ratio of pulse sequence shall be carried out in accordance with the law of sine wave of change, that is, when the sine wave amplitude is maximum, the pulse width is the largest also, when the sine wave amplitude is the minimum value, pulse width and minimum (as shown in figure 4). In this way, electricalmotor1.blogspot.com the output pulse sequence can be made to the motor load greatly decreases, and the higher harmonic current component to improve the efficiency of the motor. SPWM waveform is sum up the features of "continuous range wide, two narrow middle wide".