By Greg Shreeve, Energy Consultant, Future Climate.
Variable speed drives (VSDs) have one of the fastest paybacks of all the energy saving measures available to industry. But many firms are not aware they have multiple instances where VSDs are suitable and quite how much energy they could be saving.
What are VSDs?
You are likely to have AC motors operating in several areas of your building and not just on a production line. AC motors run at fixed speeds, regulated by the incoming alternating current. They operate most efficiently where a constant motor output speed is required. However, AC motors are more often than not used for tasks where motor demand is variable and therefore operating less efficiently.
A variable speed drive is an electronic controller that will adjust the frequency and voltage of the supply to an AC motor to adjust its speed and torque output. By controlling the supply voltage, output varies with only marginal losses in overall efficiency.
What types of processes are they suitable for?
Around half of all motor applications have some kind of varying demand. These include any processes that might involve moving air and liquids (e.g., centrifugal pumps and fans), machinery with winders or precisions tools such as drills or saws). The description below sets out which of these types of variable demand can benefit from savings with the installation of a VSD by classifying motor loads into three categories:
1. Variable torque and power
Typical of fans and pumps, the power and torque output of the motor is proportional to the cube of the motor’s speed. This means that reducing the motor’s speed can significantly reduce the electricity consumption. In instances where fans are used, for example in ventilation, demand is often controlled inefficiently using dampers. Whereas a VSD could lead to considerable savings.
2. Variable power constant torque
These loads include conveyors, crushers, agitators, surface winders and air compressors. As the speed of the motor increases so does power, but torque remains constant. With these loads, as speed is directly proportional to power, a fifty percent reduction in speed results in a fifty percent reduction in power consumption. VSDs are certainly suitable for these applications as they could lead to significant savings.
3. Variable torque constant power
These loads are typical of machine tools and centre winders. In these cases, the motor’s power remains constant regardless of its speed. A VSD is unwarranted as an energy saving measure as the power input to the motor will not vary.
Costs and paybacks
VSDs are relatively inexpensive to install. A typical 7.5kW drive might cost around £650, with smaller drives 1kW drives starting at around £300. For anything over 10kW prices roughly start at £750 plus £55 per additional kW +VAT.
When applied to loads with variable torque and power, the power reduction is proportional to the cube of the speed reduction. That means a pump or a fan running at 80% of its normal speed will reduce its power consumption by 50%. Considering that a 2.2KW motor can cost over £500 a year to run, with a VSD reducing the average speed by 20%, payback could easily be made within 2 years.
How are VSDs installed?
VSDs are simple to install, but must be commissioned and installed by a suitably trained and competent professional. The installer should assess your motors usage to determine its intended operation profile and programme them to suit the motor’s required load. Generally, the VSD will operate in conjunction with sensors to assess demand and to automatically regulate the motor’s speed and torque.
Other benefits of VSDs
Features like soft starting can reduce stress on the motor and drive train, prolonging equipment life. This also limits start-up electrical currents reducing motor heating and demand on mains supply. VSDs can rapidly adjust the power and torque required by a motor, giving better control in high-speed applications. VSDs can also reduce noise and vibration in heating and ventilation ductwork by eliminating the need for dampers.
Smarter Choices can help your company assess the potential for energy savings measures like VSDs and access finance with no upfront investment. Contact us on firstname.lastname@example.org or 0207 721 5031 to see how Smarter Choices can help you save money and reduce your carbon footprint.