The Pump & Fan Curve

Energy Saving with ABB

Saving Energy with Variable Speed Drives

A variable speed drive regulates the speed of the motor, and in turn the speed of the pump or fan, by controlling the energy that goes into the motor, rather than restricting the flow of a process running constantly at full speed. Running a motor at full speed while throttling the output is like driving a car with one foot on the accelerator and the other on the brake; a part of the produced output immediately goes to waste. A variable speed drive can save over 60% of the energy. This is possible as it controls the energy at source, only using as much as is necessary to run the motor with the required speed and torque – much in the same way as the accelerator in the car controls the engine revs. As acceptance of the technology increases, variable speed drives are gaining market share across a range of industries. In particular, variable speed drives can dramatically reduce energy consumption in fan and pump systems.

 

The Pump Curve

 

A pump or fan running at half speed consumes only one-eighth of the energy of one running at full speed. Or, put differently: the power required to run a pump or a fan is proportional to the cube of the speed. This means that if 100% flow requires full power, 75% requires (0.75)3= 42% of full power, and 50% flow requires (0.5)3= 12.5% of the power.

Illustration 1: Using a variable speed drive is the most efficient way of controlling pumps

As a small reduction of the speed can make a big difference on the energy consumption, and as many fan and pump systems run at less than full capacity a lot of the time, a variable speed drive can make huge savings compared to a motor driving an application under mechanical control. A variable speed drive can also make it possible to stop a motor completely when it is not required as re-starting with a variable speed drive causes far less stress than starting direct on line – soft start is an inherent feature of the drive.

Regulating the motor speed has the added benefit of easily accommodating capacity rises without extra investment, as speed increases of 5-20% is no problem with an AC variable speed drive as long as there is enough spare capacity in the system.

The Fan Curve

Low running Costs

With 50% airflow, the commonly used guide vane system results in the fan motor using over three times the energy (kWh) of a motor controlled by ABB low voltage variable speed drives.

Energy Saving Fan Curve

Illustration 2: Fan applications are also extremely well suited for variable speed drives

By matching the performance of the motor to the needs of the process, variable speed drives can give major savings, compared to the wasteful practice of running the motor at full speed against a restriction to modulate output.