normally in VFD we taking ac convert into dc then chopped the dc at desired frequency and voltage then again convert vaiable dc into ac.my question is that why we require to convert ac to dc and control it why we not control ac power and vary the speed.
The speed of an AC motor is mostly determined by frequency, not voltage. However the voltage must also be controlled to maintain a sufficient magnetic field without saturating the iron. The ratio of voltage to frequency should be maintained nearly constant at rated voltage / rated frequency.
Asynchronuos motor speed depends on frequency not on voltage. AC has fixed frequency so we need first to convert it to DC and then generate AC of desired frequency to get appropriate motor speed. Voltage controls output power.
There was a device called TRIAC that was used to control AC directly to vary the speed from AC motor. It was widely used to control motor speed before the invention of VFD after power FET is available. VFD is a complex electronic device that requires to run on DC. That was why all VFD must first convert income AC 60hz power source into DC to let the VFD circuit running.
An AC voltage is a series of pulses, not a continuous flow of current like DC. With 60Hz we have a positive and a negative pulse 60 times a second. If we want say 37 Hz output, we have to store the 60Hz pulses in capacitors and then use transistors to output pulses from that voltage store at the different rate. We do not have control of the 60 Hz power frequency because that is set at Hover dam etc for the entire nation and the power grid depends on it remaining synchronized with every other power plant.
You can't convert AC power at 60 Hz directly to other frequencies.
