How does a VFD (Varaible Frequency Drive) works?

Question

How is it applied to motors? What does is look like?

Answer 1

See: http://en.wikipedia.org/wiki/Variable_frequency_drive

Answer 2

VFD's are primarily used to vary the speed of a three phase AC Motor. The speed of an three phase AC motor is given by the formula N = (120 x f) / p, where N is the speed, f is the frequency and p the number of poles of the motor. Since the number of poles in a motor is constant, the only parameter that can be varied is the frequency.

A variable speed drive (VFD) has a converter and an inverter. The converter converts the input AC into DC. The inverter then converts the DC again to AC of required voltage and frequency.

This conversion is done due to two primary reasons. One, it is not possible to convert to the required voltage and variable frequency directly from the input supply. Secondly, using a programmable inverter, it is possible to vary both the output voltage and frequency (V/f) as per the characteristics of the motor.

Two types of VFD's are used. One gives a linear Voltage / Frequency Graph. The other, called sensorless vector drives adjusts to the dynamic characteristics of the motor and outputs frequency and voltage as per torque requirement at various stages from start to full speed.

Answer 3

We know voltage control the speed. Ampere control the torque from our old school.

Once we control the frequency, we can actually control both the torque and speed. Reducing the frequency cycle will control the speed. Increasing hte peak to peak will increase the torque.

It is more advance and more advantageous VVD and VAD. Cheaper as well.

The looks is very much depending on what kind of motor you want to control. Generally for a 5 to 10 hp motor, the VFD is just like a box measured at 6in x 8 in x 12 in.

Answer 4

Try this website. It gives the basic theory behind VFDs and using HVACs.