WORKING PRINCIPLE OF VFD

It is important to be familiar with the working principle of VFDs as they are extensively used in AC motor-driven applications. VFD has greater functionality and operation capabilities than conventional motor drives, which will be explained in detail in this article.

Let’s take a 3-phase load as an example to see how VFDs work. Firstly, when AC power is supplied from the mains, it directly passes through the first VFD stage – Rectifier stage. The current passes through six diodes that convert the AC-supplied current into DC. In short, each of the three phases is connected to one pair of diodes, which only allow the peak of each phase to pass through it.

Therefore, the output of the 3 phases passing through diodes, when measured by an oscilloscope.

the output current looks like a rough DC or just a current wave that doesn’t include the negative side of the current. Therefore, this current needs to be converted into a healthy DC and the best way to achieve that is by connecting a capacitor that will smooth the output current.

This capacitor is the VFD’s second stage, called the DC filter. Because of the charging and discharging effect of the capacitor, the current passing through the DC filter will have a wave.

After the DC is considered smooth, it is ready to pass through the third stage of the VFD – the IGBTs inverter to convert it into AC again. When the current reaches this stage, the converted AC’s frequency could be varied accordingly. The connected number of IGBTs here will be turned on and off very fast to produce an AC-like waveform that will run the connected motor as if it was connected directly to an AC supply.

The IGBTs here act like switches that when rapidly switched on and off, create the Pulse-Width Modulation (PWM) that is a key to allowing a VFD to control the speed of the motor.