Electricity currently accounts for 40% of the world’s energy usage, and some 30% of this is used by electric motors. However, AC drives, which control the speed of electrical motors, can lead to massive energy savings, not to mention a longer lifetime for the machines. AC drives can be used for almost any electrical motor and are gaining ground worldwide. Today, they are most commonly used in elevators, fans, pumps and air conditioning equipment, but there are also interesting developments in the renewable energy sector.
What is an AC drive?
As I am not from an engineering background I will not spend a lot of time on the technical details. In a nutshell, an AC drive is a low voltage frequency converter or an “energy regulator” that ensures that an electrical motor consumes the optimal amount of power at all times, rather than wasting power when it is not required. It consists of three main components: a rectifier unit (connects a drive to the electrical network), a DC circuit (stores electrical energy from the rectifier) and an inverter unit (takes electrical energy from the DC unit and supplies it to the motor).
Benefits of an AC drive
So what are the benefits with an AC drive? Applying an AC drive to an electric motor can lead to: cost savings in energy use, a longer life span for the motor and better control of the industrial process.
Massive energy savings potential
The energy savings can be substantial. For instance, if a particular motor needs to spin at a rate of 500 revolutions in a minute in order to carry out a given task, but actually spins at 750 revolutions a minute (due to unregulated energy supply) it is wasting a great deal of energy.
According to research published by Vacon, a Finnish AC drive manufacturer, it has been estimated that using AC drives could lead to annual savings of 50 billion kilowatt hours (kWh) in Europe alone. Roughly speaking, the energy saving potential from full penetration of AC drives in Europe means that we could do without 10 GW of power generating capacity. This is equal to 11 nuclear power units and 32 fossil power units. The same study concludes that up to 76 billion kWh could be saved annually in the US and 134 kWh in China.
To see the potential benefits, consider the following facts: about 40% of the world energy consumption is in electrical energy and 65% of the electricity is produced by burning fossil fuels, such as oil, coal and gas creating huge amounts of CO2 emissions. Some 30% of the electrical energy is used in electric motors. Using AC drives the saving potential is 20-50% in electric motors.
In Europe and North America, about 40% of all energy is consumed by buildings. The biggest areas of consumption are heating, ventilation and air conditioning systems. Quite astoundingly, up to 90% of the lifetime costs of fans and pumps in these systems come from energy. Using an AC drive can significantly reduce the energy consumption.
Improved efficiency creates other benefits as well
Apart from energy savings, the AC drive has other benefits. When the AC drive is coupled to the electric motor it regulates the electricity supply that is fed into the motor and therefore only allows the required amount of electricity to be fed into the motor to make it work efficiently. In other words, it ensures that the motor does not work too hard and as a result, it prolongs the life of the machine and delays the need for service and maintenance work. AC drives can also reduce the noise level.
AC drives can be used in a wide range of applications and are gaining ground
AC drives are also known as adjustable speed drives (ASD), adjustable frequency drives (AFD), variable frequency drives (VFD) or variable speed drives (VSD).
Two years ago less than 10% of electric motors were equipped with AC drives but the figure is increasing as the drives are gaining ground. The drives can be used with almost any electrical motors but the most commonly used areas are pumps, fans, elevators, hoists, air conditioning equipment as well as compressors.
An example from Australia – Murray Goulburn
Depending on the price of energy, the payback time can be very short. An example here in Australia is the large food producer Murray Goulburn. As part of its efforts to cut its carbon footprint it chose to implement a new system to supply all the compressed air required at its Leongatha plant, which is used for both plant and packaging processes, to include new compressor units, air supply pipes and dryers. The company replaced four existing units with just one 300-kW high-speed compressor run by an AC drive and leaving two units for standby air. As a result, the compressed air system’s power consumption was reduced from 9.1kW per cubic metre of air per minute to 6.12kW, i.e., 33% less power to generate higher air output. The project, which was initiated in November 2007 is expected to pay back, pre-tax, in 22 months. There are other case studies where the payback time has been as short as six months.
Significant potential in renewable energy
AC drives are also used in a number of renewable energy areas, such as hydro power and wind power. The wind power industry is subject to technological changes and AC drives can play a big part in the future. One thing that has increased in popularity is the combination of permanent magnet generators and full-power converter systems. Optimised with AC drives that function as power generation plants as well as motor drives these can lead to less mechanical stress and higher efficiency.
Another trend in the wind energy sector is the technological shift from large and heavy transmission gearboxes to smaller ones. Also, there are technological changes in between the gearbox and the grid, such as larger and more powerful inverters. These new inverters are capable of dealing with much higher capacity; from about 600kW up to about 1.8MW of power. This new technology reduces noise, weight and the total cost of wind turbines.
Time for companies to invest in energy efficiency
The current recession has meant that new clean-tech investments have often been put on hold. However, companies that have strong balance sheets and the time to invest in new technology will likely be significantly ahead of competitors when the economic wheels start spinning again. AC drives offer significant energy efficiencies at relatively short pay-back time and can lead to long-term competitive advantages.