Basic Principle

The synchronous speed of an induction motor is primarily a function of the number of poles and the frequency:

A slip between actual and synchronous speed is generated by the applied load torque. This slip is usually small, typically 1-3% of synchronous speed. A way to achieve a variable speed motor is to vary the frequency. Such a device is called a frequency converter. The performance of the pump, i.e. the flow, is proportional to the impeller speed. This implies that the flow from the pump can be controlled by varying the frequency.

In theory the basic idea is simple, the process of transforming the supply frequency to a variable frequency is basically made in 2 steps.
1. Rectify the sinusoidal voltage to a DC-voltage.
2. Artificially recreate an AC-voltage with desired frequency.
This is done by chopping the DC-voltage into small pulses which approximates an ideal sine wave.

A VFD consists basically of 3 blocks: The rectifier, the DC-link and the inverter.

Fig 1. Schematic functions of a VFD
The most common type of VFD, and the one recommended by FLYGT, is the PWM - Pulse Width Modulation type. The PWM type VFD normally uses a constant voltage which is pulsed with integrated bi-polar power transistors (IGBT). The sine wave is generated by varying the width of the pulses. The frequency which the transistors are turned on and off with is called switching frequency. The higher the switching frequency is the better the reproduction of the ideal sine wave becomes.

Fig 2. Schematic output signal from a VFD of PWM type.

The affinity laws state that the flow is proportional to impeller speed for a specific point on the pump curve. Head and NPSH are proportional to the square of the speed, while the power is a cubical function of the speed. The hydraulic (pump-) efficiency remains constant when the speed is reduced.

Affinity laws:

The speed of the impeller is, as mentioned before, directly proportional to the frequency. This implies that the impeller speed in the affinity law equations can be substituted with frequency.

Fig 3. Frequency regulated pump curve.