Key points
Variable speed drives offer enormous advantages when applied to compressors in a variety of applications. But only if they are sized properly and specified appropriately, warns Mark Whitmore, General Manager at BOGE.
They are called many things – variable speed drives (VSDs), inverters, adjustable speed drives, variable frequency drives, the list goes on. But, however you couch your terms, VSDs are often touted as the panacea that will inevitably boost motor efficiency and cut energy efficiency in a range of applications.
This is a myth; indeed, it is entirely possible for VSDs actually to reduce system performance by introducing harmonics into the system that can cause additional heating of the motor and which, in turn, will introduce power losses. According to some estimates, a VSD run at fixed speed suffers inherent losses of around 3 to 5%.
However, if they are specified well, nine times out of ten VSDs can make a huge positive difference to system performance, saving energy and cutting other operational costs by controlling the speed of motors.
Comparison between fixed speed compressor and VSD
There is a useful analogy here that helps explain what it’s like when a fixed speed compressor is used where a VSD version would be more appropriate. Imagine you are driving an automatic car with the accelerator pedal perpetually to the floor and controlling the speed using just the brake.
As well as wasting an awful lot of fuel, you would also cause a great deal of wear on the brake pads and place enormous strain on the engine.
Before VSDs were invented that is effectively how motors in compressors, fans and pumps were run. While the efficiency of your car’s engine is not intrinsically improved by the presence of the accelerator, having one – and using it appropriately – enables you to boost your miles per gallon, reduce wear and tear, and experience a better ride than you would by relying on the brake alone.
Why? Because an accelerator pedal enables the driver to control the speed of the car and match it to his or her precise needs at any given moment, or depending on road conditions.
So VSDs align power input and energy demand to the load variances of the specific application. One application in which their use is growing is in the control of compressors.
VSDs allow air compressors to deliver precisely what is needed for the application at the right pressure while, at the same time, compensating for line losses and coping with the variability of demand. Indeed, a VSD-controlled air compressor uses an AC drive to control the speed of the unit, typically saving energy compared to a fixed-speed equivalent.
The ‘load-unload’ control scheme
Craig van Wyk, founder of South African environmental consultancy, VWG Consulting, has pointed out that the most common control philosophy employed with screw compressors is the ‘load-unload’ control scheme.
He explains: “In this scheme, the compressor motor runs continuously, but may be loaded (producing air) or unloaded, depending on the discharge pressure. While an unloaded screw compressor does not produce compressed air, the problem is that it still consumes a significant amount of power – in my experience, often 30% of the power required when on load.“Running a screw compressor unloaded for extended periods of time is therefore inefficient, since energy is being consumed with no benefit in terms of compressed air production.”
A VSD compressor modulates the speed of the screws, and hence the flow of air, but this only occurs when the demand for air fluctuates to levels below the capacity of the compressor.
Mr van Wyk again: “The machine therefore spends most of the time producing air (it can also go off-load or even be switched off at very low air demand levels), but using less energy than would have been used with a load-unload control scheme. The flow of air and the power required are proportional to the speed of the compressor motor.”
The Carbon Trust makes the point that, when more than one compressor is used on a single system, giving one compressor a variable speed drive allows the other fixed speed compressor(s) to run at optimum efficiency on base load with the VSD compressor varying its output so the compressor installation accurately matches actual demand.
It says you can estimate the load on a compressor in two ways: first, by:
- timing the on-load and off-load periods – you can usually hear the change between the two states. It is not possible to determine the load with modulating control by this method so these compressors may need switching to on/off load to determine the average load.
- asking a compressed air equipment supplier or independent firm to install monitoring equipment on the system. This will measure energy consumption and variations in airflow and pressure.
Compressors can be big energy users – 10 to 12% of all power generated in the UK is dedicated to the production of compressed air. Anything that can be done to reduce this energy burden is to be welcomed and – provided they are specified appropriately – VSDs can go a long way to reducing this.
Single-speed compressors make sense when:
- Running at 100% capacity for fixed periods and then turned off when no longer required Variable speed drive compressors make sense when:
- There are variable capacity demandsAdvantages of well-specified VSDs include:
- Less mechanical stress on the motor and therefore reduced maintenance and repair costs, and better equipment uptime
- Greater control so the operator can modify the response to specific application demands and enjoy better process control, and therefore higher productivity and quality
- An ability to start/stop under full system pressure without the need to unload, saving time
- Big energy savings (between 5 and 35%) at less than full capacity where there are varying loads
- Reduced system leakage because of the lower system pressure
- Providing ‘soft starting’, thus eliminating high inrush current
VSDs in action
Two real-world examples of variable speed drive (VSD) compressor installations highlight the benefits of this technology when it is applied well.
First, take the example of a plastics and extrusion blow moulding company in Merseyside, which introduced VSD technology and achieved savings as a result. The existing compressed air infrastructure comprised three systems using 12 compressors with a total capacity of 975hp, or 727kW.
The system was inefficient and therefore offered the company an opportunity to save energy, reduce the carbon footprint and prepare for future expansion by replacing these existing systems.
The new system uses a fixed speed compressor to deliver the company’s base-load air requirements, combined with a VSD compressor that provides additional capacity to match air output precisely with demand.
The two units are controlled by a master compressor control system, which senses demand patterns to deliver the required air using the most efficient combination of compressors.
Following the implementation of this new system, the potential energy savings were calculated to be more than £74,000 a year, while air capacity increased by 35%. Reducing the number of compressors from 12 to two also resulted in a £7,000 reduction in annual maintenance costs.
In a second example, a UK-based flooring company was able to cut its energy bills following the installation of an 11kW variable speed compressor.
The unit replaced an old 18.5kW machine, one of two at the site. Just by employing a smaller variable speed compressor, the company is saving around £2,500 annually in energy costs alone.
The frequency-controlled compressor automatically adjusts to the air demand while controlling the pressure perfectly. The air end operates at the necessary speed to generate as much compressed air as is required, which means expensive idling as well as load/no load cycles are eliminated.
