Energy Saving with Variable Speed Drives

With the rising costs of energy and growing concern about climate change, any energy-saving solutions in industrial plant and commercial building installations are welcome. For the business manager minimising energy costs is an ongoing concern.

Electric motor and drive systems account for over 60% of UK industrial electricity demand. A large proportion of these motor installations drive fans, pumps conveyors and compressors.

In this article, we discuss the energy-saving potential of variable speed drives in motor control applications.

What is a variable speed drive?

Variable speed drive (VSD) devices change the speed of AC or DC motors. They increase the efficiency and deliver greater control over fixed-speed motors.

Variable frequency drives (VFD) are similar but the way they operate is subtly different to VSDs. A variable speed drive controls the motor by changing the input voltage. Variable frequency drives exert control by changing the input voltage and frequency.

You will find Variable speed drives in AC and DC motor applications. Whereas variable frequency drives only work with AC motors.

Matching a motor to the application

The speed of an AC motor depends on its number of poles and the input frequency. In DC motors the input voltage, magnet strength and wire turns on the armature determines the speed. Therefore, in both cases to change the speed, it is necessary to change input voltage or frequency.

With a fixed voltage and/or frequency it is important to match the fixed speed of the motor with the application. But the speed of the end application may vary for a variety of reasons including the speed, power, torque relationship (see below).

To solve this incompatibility issue, engineers often use gears, dampers or (in pumps) throttle control. These solutions are not ideal as they increase installation and maintenance costs, waste energy and reduce the efficiency of the system.

The motor power, speed, torque relationship

Torque is a rotational force or turning effect. If we take a fan as an example, at start-up the torque required to turn the fan is low. As fan speed increases so do air resistance and the amount of torque required to turn the fan.

If we now consider a conveyor, the load is relatively constant regardless of the speed, so the torque required remains constant. A fan is a quadratic load, a conveyor a constant load.

For a quadratic load, the relationship between power and speed is subject to the cube law. So, if you were to decrease speed by 10%, you decrease power by 27% (0.9×0.9×0.9). In contrast, with a constant load, if you decrease speed by 10% you decrease power by roughly 10%.

Other torque/power relationships to consider are constant power and constant power/constant torque. Examples of constant power include material rolling. Here the torque is inversely proportional to the speed. The paper industry is a good example of a constant power/constant torque application.

How variable speed drives save energy

Variable speed drives are not suitable for all applications. However, they can save significant amounts of energy and reduce costs in a wide range of industrial plant and commercial building operations. Some of these are covered below.

Motor speed adjustment – The cube law relationship between speed and power (see above) is one of the major drivers of variable speed drive installations. Fans and pumps are rarely required to operate at full speed. Hence, a VSD device can regulate motor speed to match the demands of the application and make significant power savings.

Motor sizing – In some applications, engineers match the motor size to the peak demand of the duty cycle. This peak may only occur at start-up or for short periods of time.

To deliver constant energy savings it is possible to combine a smaller motor with a variable speed drive (the Schneider ATV630 is one example). In this case, the motor is sized to cover normal application requirements while the VSD boosts output to cover short term peaks in the duty cycle.

Motor replacement – It is possible to replace older, less energy-efficient, motors with newer models combined with a variable speed drive installation.

Sensors and feedback loops – Variable speed drives have a level of intelligence and are programmable. When combined with sensors and feedback loops, they can control several motors. This can deliver significant energy savings across an installation.

Constant torque motors – Significant energy savings are possible on variable torque motors such as fans and pumps. That said, it is important not to disregard constant torque motors.

It can be financially advantageous to fit a VSD such as the Schneider ATV930 to air compressors where the average loading is 75 per cent of capacity or less. Energy savings are possible on conveyors using VSD controlled soft-starts. A Variable speed drive can also help ensure a conveyor runs at maximum efficiency.

There are many constant torque motor applications where a variable speed drive installation could pay for itself over the medium to long term.

The cost of a variable speed drive vs energy savings

A cost versus benefits analysis is always necessary before making any capital investment. Depending on the application, installation and maintenance requirements installing a variable speed drive may not be the best way forward.

Several online calculators will give an indication of the financial implications of installing a VSD. As a rough guide, they suggest a variable speed drive installed to replace a damped motor in a fan or pump application could pay for itself in saved energy costs in between six and nine months.

However, all calculators make many assumptions. It is best to try to understand the specific details of each potential application before deciding. Should you need to talk through the potential benefits of a VSD in your application, our engineering team are always available to help.

Although they deliver many benefits, variable speed drives are primarily energy-saving devices. They have a wide range of potential applications in industrial plant and commercial premises. However, to deliver the maximum energy savings it is important to select the right variable speed drive for the application.

The device should be correctly set up at the commissioning stage and correctly maintained throughout its in-service life. At Quantum-Controls we deliver a full VSD service including supply, installation and maintenance. Call today on 0330 9000 247 to discuss your requirements.

To learn more about Variable Speed Drives and the potential energy savings they could deliver in your application request our free 60-minute guide.

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