Introduction

Motors demand power from a variable speed drive according to the speed required and the type of load connected to the motor. Some loads, such as conveyors, are constant torque loads. This means the torque demand of the load remains broadly the same across the speed range, while the power demand changes approximately in proportion to speed. As a simple estimate, a 20% reduction in speed can give a 20% reduction in power consumption.

The largest energy-saving benefits associated with VSDs are seen on centrifugal loads such as fans and pumps. These are variable torque loads, where torque is proportional to the square of speed and power is proportional to the cube of speed. Because of this, even a modest speed reduction can produce a large reduction in power consumption. For example, a 20% reduction in speed can reduce the power demand by almost 50%.

Explanation

When a motor is connected directly to the mains supply, it receives a fixed voltage and fixed frequency. In the UK and Europe, this is normally based on a nominal supply frequency of 50Hz. Fan and pump manufacturers select motors to meet the required duty at this fixed speed, and the motor nameplate or equipment data usually states the power and flow at 50Hz.

In practice, site demand often does not match the full flow or pressure available at 50Hz. In these cases, a VSD can be used to reduce motor speed to better match the process requirement. This is where significant energy savings can often be achieved.

As mentioned above, the relationship between speed and torque on a centrifugal load is quadratic, while the relationship between speed and power is cubic. These relationships are often shown as curves, but the table below provides a practical guide to how speed reduction affects torque demand and power consumption.

Speed vs Torque and Power

Speed	75%	80%	85%	90%	95%	100%
Torque (required by load)	56%	64%	72%	81%	90%	100%
Power consumption	42%	51%	61%	73%	86%	100%

On centrifugal loads such as fans and pumps, torque demand reduces with the square of speed and power demand reduces with the cube of speed. This is one reason why many AC inverters include a variable torque or quadratic V/F setting for fan and pump duties. On general-purpose AC inverters, the default setting may be a linear V/F curve, which is more suitable for constant torque loads. A fan or pump will often still run with a linear V/F curve, but selecting the correct fan or pump mode can improve optimisation for the application.

Key Points

• The main energy-saving benefit on fan and pump applications comes from reducing speed. Some AC inverters offer additional energy optimisation features, but speed reduction usually provides the largest saving.
• Switching loads off completely when they are not required provides the greatest energy saving of all, as power consumption is reduced to zero. Fans and pumps are sometimes left running unnecessarily because they are seen as background services rather than part of the main process.
• The most basic way to control a fan or pump is to run it at a fixed reduced speed. This can provide a worthwhile and immediate energy saving.
• A better level of control can be achieved by using a pressure or flow transducer in the process. A 4–20mA signal can be connected directly to most AC inverters and used as a feedback reference.
• Many AC inverters include an internal PID controller, allowing the drive to automatically maintain the required pressure or flow by adjusting motor speed.
• It is often practical to begin with a simple fixed speed reduction and add closed-loop control later. In many cases, the same AC inverter can remain in place as the control strategy becomes more advanced.

Conclusion

Installing an AC inverter on a centrifugal load such as a fan or pump can deliver significant energy-saving benefits. As a simple starting point, the system can be set up with a fixed reduced speed. More sophisticated control can then be added later if required.

Additional Information

Fan Laws:
1)   Air velocity follows fan speed in a near-linear relationship (the First Fan Law).
2)   Pressure follows the square of air velocity, meaning doubling the speed will increase pressure by four times (the Second Fan Law).
3)   Power required therefore follows the cube of speed, meaning doubling the speed will increase power demand by eight times (the Third Fan Law).

The same laws apply when reducing fan speed, which is why significant energy savings can often be achieved.

Written by: Simone Inett. Last reviewed or revised: May 2026