Saving energy with variable speed drives

A variable speed drive can reduce energy consumption by as much as 60%. For a 90 kW motor in continuous duty, this can mean over £9,000 per year. This is because the variable speed drive reduces the amount of energy drawn by the motor.

Even a small reduction in speed can give significant savings. For instance, a centrifugal pump or fan running at 80% speed consumes only half of the energy compared to one running at full speed.

Calculate your application – go to the energy saving calculator

How to cut energy consumption by nearly 50%

A centrifugal pump or fan running at half speed consumes only one-eighth of the energy compared to one running at full speed. This is because the torque needed to run a pump or fan is the square of the volume. For instance, reducing the pump speed to 80% only requires 64% of the torque (0.8x0.8). Furthermore, to produce 64% of the torque only requires 51% of the power (0.64x0.8), as the power requirement is reduced in the same way.

The explanation for this lies in the pressure difference across the impeller. When less pressure is produced, less acceleration of air or fluid across the impeller is required. It is the simultaneous reduction of acceleration and pressure that multiplies the savings.

Even further improved energy efficiency is offered through a technique called motor flux optimisation. This is particularly beneficial in pump and fan applications and under light loading conditions. The overall efficiency of an ABB low voltage AC drive and its motor is typically 1% to 10% higher than conventional AC drives. At 25% load, efficiency is up to 10% better.

High efficiency
Comparing energy consumption of various mechanical and electrical airflow control methods of a typical centrifugal fan reveals that: At 80% airflow, the energy consumption is: 97% of maximum with damper control; and 76% of maximum using guide vanes; but only 51% using ABB low voltage AC drives. The difference between these figures represents wasted energy - waste that any engineer or financial manager will find impossible to justify.



Low motor losses
ABB’s low voltage AC drives automatically adjust the voltage to frequency ratio to achieve optimum current for pump and fan loads. This minimises motor losses and reduces motor noise.



Low running costs
The extent of energy savings is shown by the comparison on the right. 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. These extra running costs, compared to inverter control, are a powerful argument in favour of selecting ABB low voltage variable speed drives.



To learn more about ABB's range of variable speed drives, click here

To find a UK distributor, click here

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    This is how much energy a variable speed drive saves at different speeds compared to traditional control methods
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