Primary function of a VFD in aquatic applications is to provide energy savings. By controlling speed of a pump rather than controlling flow through use of throttling valves, energy savings can be substantial. By way of example, a speed reduction of 20% can yield energy savings of 50%.

The term “variable frequency drive” is somewhat misleading, as is another generic name for it – “inverter.”

A variable frequency drive actually controls two parameters: frequency and voltage. It does this due to the electrical characteristics of the device being controlled: a three-phase induction motor.

When a three-phase AC voltage source is connected to the stator windings of an induction motor, a rotating magnetic field is established inside the stator body. The speed of this rotating magnetic field is described by the equation:

120 * F / P = S

Where F is frequency in Hz (cycles per second), P is the number of pole pairs (always a multiple of two) in the stator windings, and S is the speed of the field rotation in revolutions per minute. The constant 120 gives us the RPM units. If you wanted speed in radians per second, it would be a different constant.

However, the stator of an induction motor is an inductor, so there’s another equation that holds sway here. The impedance of an inductor is related to the applied frequency by the equation:

Again, F is frequency, but L is the inductance (fixed by the mechanical construction of wrapping a certain number of turns of a certain gauge of wire around the iron core of the stator) and 2π is a constant that yields X in Ohms.

Note that as frequency approaches zero, inductive impedance also approaches zero, so the inductive impedance at, say 30Hz is half of what it would be at 60Hz, and one quarter what it would be at 120Hz.

So, if you reduce the frequency applied to the motor stator and you DON’T reduce the applied voltage, the total impedance (wire resistance plus inductive impedance plus the negligible capacitive impedance) will reduce significantly and the stator will draw higher and higher current (I), eventually burning out due to I²R heating.

So a VFD has to control both voltage and frequency. How does it do that? Magic!

No, not really.

The overwhelming majority of VFDs today operate on the same design principles. They consist of three main subsystems:

- The Converter
- The Filter
- The Inverter

The converter takes in the incoming AC fixed-frequency power from the utility and converts it to DC. It doesn’t (from an engineering standpoint) matter if the DC power comes from a bridge rectifier, a solar panel, a DC generator, whatever, so long as there is DC power. The DC power is then filtered to make it smooth, and stored so that there’s a surplus of power available. The filter in the majority of VFDs is made up of capacitors, but it may also be a large inductor, or a combination of capacitors and inductors. Capacitors store power as charge. Inductors store power in the magnetic field around them established by the flow of current in the inductor itself.

Finally, the inverter takes the DC power and “chops it up” in a simulation of an AC sinewave, one that the inductor of the motor stator will accept.