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Variable Frequency Drive Load Types

The potential for variable frequency drive (VFD) energy saving from slowing down the load depend on the characteristics of the load being driven. There are three main types of load: variable torque, constant torque and constant power.

Variable torque load
Variable torque loads are typical of centrifugal fans and pumps and have the largest energy saving potential controlled by variable frequency drives. They are governed by the Affinity Laws which describe the relationship between the speed and other variables.
Variable torque load

The change in flow varies in proportion to the change in speed:
Q1/Q2 = (N1/N2)
The change in head (pressure) varies in proportion to the change in speed squared:
H1/H2 = (N1/N2)2
The change in power varies in proportion to the change in speed cubed:
P1/P2 = (N1/N2)3
Where Q = volumetric flow, H = head (pressure), P = power, N = speed (rpm)

The power – speed relationship is also referred to as the 'Cube Law'. When controlling the flow by reducing the speed of the fan or pump a relatively small speed change will result in a large reduction in power absorbed.

Constant torque load
Typical constant torque applications controlled by variable frequency drives include conveyors, agitators, crushers, surface winders and positive displacement pumps and air compressors.
Constant torque load

On constant torque loads the torque does not vary with speed and the power absorbed is directly proportional to the speed, this means that the power consumed will be in direct proportion to the useful work done, for example, a 50% speed reduction will result in 50% less power being consumed.

Although the variable frequency drive energy savings from speed reduction are not as large as that with variable torque loads, they are still worth investigating as halving the speed can halve the energy consumed.

Constant power load
On constant power loads the power absorbed is constant whilst the torque is inversely proportional to the speed. There are rarely any energy savings opportunities from a reduction in speed. Examples of constant power applications include center winders and machine tools.
Constant power load

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