Many pumps are oversized or poorly matched to current process needs – that problem is solved with variable speed drives that adjust the speed of the motor to match the exact production needs and so save energy.
An average electric motor consumes its purchase price in electricity in just 1 month (30 days) so investing in the highest efficiency variants gives an ongoing saving from month one. New VSD’s also bring big savings compared with older VSD technologies, due to improvements in switching topologies, and quite often 15% savings can be realised.
The motor control is also far smoother and so mechanical parts, belts and seals are not damaged during running, which reduces maintenance costs.
Motor speed savings
Motor speed control offers industry the single largest opportunity for saving energy and money.
Most motors run at a fixed speed. By adjusting the speed to more accurately match the requirements of the loads, which generally vary over time, you can enhance the efficiency of motor drive equipment.
The benefits of speed variation can include increased productivity and product quality, less wear and mechanical stress, power factor correction (cos phi; close to 1), noise reduction along with energy savings of 50% or even more for some types of applications.
Pump and fan energy savings
One of the most effective ways to save energy is to target your pump and fan applications mainly because there are so many of them and the potential energy savings are so great.
Fan and pump applications are known as “variable torque” and in a variable torque load the power changes with the cube of the speed, for just a 7% speed reduction (that equates to 3.5Hz on a 50Hz system) the energy saving would be 20%, for a 20% speed reduction (10Hz), the energy saving rises to 50%.
The graph below shows the energy use of a fan system with three different control methods. The energy use of the drive closely follows the demands of the system, because this is the only control method that regulates the output in accordance with the system needs.
Both the inlet guide vane and the outlet damper control the output by placing a restriction in the way of the flow, with the motor still running at full speed. By contrast, the drive varies the speed of the motor and only uses the energy needed to achieve the required output. The same principles apply to pump systems.
Variable speed drive technology has improved significantly over many years. New switching technologies and switching devices mean new VSD’s can be 10 – 15% more efficient than older models, so targeting existing variable speed technologies also has great benefit.