Variable speed drives have been proven to save large amounts of energy and money in applications where demand varies. The electric motor is by far the biggest user of electricity in industry and buildings, being behind virtually everything that moves.
The energy efficiency of many of these motor installations can be dramatically improved with a variable speed drive, which ensures that the motor only runs at the speed and times needed. Drives are inexpensive and easy to install, but despite this most motors are left running at a single speed, often without being needed for long periods.
The potential savings are significant - a pump or fan running at 80% speed only uses 51% of the energy compared to one running at full speed. The problem is made worse by the fact that many motors are oversized, using more energy than the application actually needs. This is because motors are only available with a certain number of fixed speeds. Users tend to fit the next biggest size relative to the requirement and then throttle the output, for instance by reducing water flow with a valve.
By reducing the speed of the motor, a drive can ensure that, for instance, a pump uses no more energy than necessary to achieve the required flow. In addition to the energy savings, this will enable accurate control and less mechanical wear, reducing maintenance and extending the life expectancy of the system. Payback is often less than six months on energy alone. Variable speed drives capable of all these savings can cost as little as EUR130 (AED617) to buy.
Because motors are so numerous, each may seem insignificant in isolation so they are often overlooked when considering energy efficiency measures. However, it is precisely because they are so numerous that they are so important.
To minimise running costs, users should look at the whole life cycle, starting when new equipment is bought and new plant is designed. How much less would the equipment cost over the lifecycle if a variable speed drive were fitted from the outset? Oversizing means paying for output that is not needed. It also means the motor is inefficient because it is not fully loaded.
Taking a proactive approach to conserving energy is a very successful way of saving large sums of money for companies of all sizes. Many pump and fan applications run at constant speed, although variable speed operation often would give far better performance at a much lower cost. For instance, a 100kW fan throttled by 50% in continuous operation can give payback in six months if equipped with a variable speed drive.
Drive operation
Drives operate by switching the fixed mains supply voltage to a variable voltage and frequency in response to an electrical control signal. When coupled to a fan or pump motor, the change in frequency will result in a corresponding change in motor speed. The motor speed can be decided in relation to outside parameters such as water flow or air temperature.
A centrifugal pump or fan running at half speed consumes only one-eighth of the power 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 (0.8 x 0.8). Further, in order to produce 64% of the torque, only 51% of the power (0.64 x 0.8) is needed as the power requirement is reduced in the same way.
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 low voltage ac drives. The difference between these figures represents wasted energy - waste that any engineer or financial manager will find impossible to justify.
Diagram one, illustrates the extent of energy savings that can be achieved in a ventilation system. 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.
Many heating, cooling and ventilation distribution systems operate at a constant flow rate, even though peak demand may only be required for a few hours. The conventional response to meeting the changing demand for heating and cooling within a building is to restrict flow to individual rooms, while maintaining peak flow in the central hvac system. However, through the use of this approach, considerable energy is used and equipment lifetime is shortened.
A much better approach is to use a variable speed drive on hvac pumps and fans to vary air or water flow to more precisely meet changing load demands. The motors can be controlled to maintain a constant pressure within air ducts or water pipes. A pressure sensor in the pipe or duct measures the system pressure and as this changes sends a signal to the building automation system, which in turn sends a speed demand signal to the drive. Thus, as valves and dampers close, the pressure rises in the ducts or pipes, this in turn reduces the speed of the fan or motor.
Gaining energy savings
In order for a company to reduce energy costs it needs to evaluate how it actually uses energy. An energy appraisal is a systematic examination of key pump and fan applications that includes the monitoring of energy consumed both before and after the change to variable speed drives.
It defines where energy can be saved and quantifies how much energy can be saved with the installation of variable speed drives. These figures are then translated into a possible monthly saving, the amount that will be saved, in energy costs alone, if the equipment is installed.
It is not unusual for users to dismiss the promise of 50% energy saving on a 20% speed reduction as the exaggerated claims of a manufacturer. However the savings can be verified and the best way to start is with an energy survey. This will allow you to see the potential savings in black and white, enabling you to make the decisions that will bring your company improved profitability.
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