Traditionally, throttling is used to regulate flows of air or fluid. While throttling reduces the flow, the motor is still running at full speed and works even harder as it has to work against a restriction. By reducing the speed of the motor, the variable speed drive ensures no more energy than necessary is used to achieve the required flow.
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) (Tµn2).
Furthermore, to produce 64% of the torque only requires 51% of the power (0.64x0.8) (Pµn3), 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.
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 building services engineer will find impossible to justify.
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.
ABB’s Energy Saving Calculator helps you work out exactly how much you can save by replacing a fixed speed damped or throttled centrifugal load with a variable speed drive controlled solution.
You can specify up to 5 different applications, including multiples of each type. You may be surprised by exactly how much you can save.
Replacing a single 400kW motor damped to 70% power with a variable speed drive solution can save over £25,000 per annum, the cost of the drive being paid back in less than half a year. A saving of over £200,000 after 10 years.
Electricity bills are increased by 0.43p per kWh from 1st April 2001 as part of the Government's Climate Change Levy (CCL) - an increase of around 12%. By switching to drive controlled solutions, you can negate the impact of this levy.