Snakeoil and disbelief
May 1st 2008

No matter how you focus on energy issues associated with the operation of building and manufacturing environments, the inclusion of inverter drives makes real sense says Jeff Whiting of Mitsubishi Electric

In this current market, where energy costs are so volatile, it is vital to take whatever opportunities we can to reduce power consumption and therefore costs.

Even in the unlikely event that costs are not an issue, taking the situation from a green viewpoint, it is clear that we need take carbon reduction seriously.

Manufacturers of variable speed drives or inverters claim that significant energy savings can be realised in most fan and pump applications, simply by fitting their equipment to the motors. In fact the high value of savings often presented by manufacturers seems almost to create a credibility gap. As a 'dyed in the wool' drives person, I have seen legitimate energy savings claims treated with suspicion rather like the 'snake oil' solutions of the travelling Wild West shows.

Worse still, even when real savings are identified, many building operators seem content to accept inevitable energy cost rises and even justify this as being 'budgeted for', therefore they feel don't need to address the energy savings.

It is a somewhat surprising fact that inverters currently are only used in about 10% of the building systems where they could be. Let's consider a typical building application, such as an air handling unit which is part of the climate conditioning system to understand the cost savings.

Whole of life costs Based on a typical 110kW motor, the inclusion of an inverter drive and controls is likely to add about £9000 to the cost of the project and would typically have a life expectancy in excess of 10 years. A 110kW motor operating 24 hours a day will have an annual energy cost of £70,000 at today's prices. Clearly, with this level of annual costs, saving energy should be part of the long term strategy as even a small saving will make considerable impact on the lifetime costs of the system.

Perhaps surprisingly, a good many motors in building control systems do run 24 hours a day. Others tend to run during the building's occupancy, plus an hour or two either side. In a building running two eight-hour shifts, the motors may run for 20 hours out of 24.

Given that a pump, fan or other motor installation is likely to have a working life of at least 10 years (this is not uncommon in building controls), the financial savings on the initial out lay can be truly massive. And even in short-lived installations an inverter can be very cost effective, the payback period for installing an inverter is often measured in months; indeed it rarely goes beyond a year.

Motors are fixed speed devices which run at a speed determined by the supply frequency (50Hz in the UK ), but fitting an inverter allows the speed to be adjusted, and the slower the motor goes the less energy it consumes. For most fan and pump applications the consumed power falls in proportion to the 'cube of the speed reduction' which in real world language means that if you can run the fan or pump at 90% of its rated speed then theoretically the power consumed would fall by 27%.

The inverter itself would add a few losses into the system, so a realistic saving would be around 24%. A great savings, which based on the 110kW motor would equate to £16,800 per year in cost reduction at today's prices. Payback for fitting the inverter of about 7 – 8 months with an ongoing reduction in operating costs of at least £16,800 for the life of the equipment.

When systems are designed, they are generally sized for peak operational loading, a condition that may only occur for a few days each year, the rest of the time having a lower power requirement. This can be clearly seen where an air handling system is designed for the hottest day of the year. General engineering practice would define the temperature range about 5 – 10% higher than the expected norm, just in case a weather anomaly occurred.

Further to this, systems are generally sized 'to be on the safe side'. This is partly because motors only come in standard sizes and installing one that is too small results in poor performance and high maintenance. Government figures have identified that today most installed motor driven fan and pumps are 25% oversized even for peak operation.

The oversized fan or pump does have 'spare capacity' and traditionally operates with the excess air flow (or other output) being restricted or vented off through mechanical restrictors such as baffles or valves. A very small degree of power reduction is experienced in this type of system, but it pales into insignificance when considering the savings which can be achieved using inverters to adjust the motor speed and therefore the required airflow.

The inverter can be integrated in building monitoring and control systems to run at the optimum working conditions whatever the weather, adding to the building comfort while making significant energy savings.

Modern inverters such as Mitsubishi's FR Series inverters have further tricks up their sleeves as they are able to tune themselves completely to the motor and dynamically reduce the power fed to the motor for the current load conditions. This automatic motor optimisation can typically save a further 3 – 10% of the power without any loss of motor performance. It gives exceptional results where systems have been oversized at it reduces heat and magnetic losses in the motor itself.

There are many other benefits for using inverters in a building. For instance, they can reduce maintenance of fans and pumps due to their soft starting of motors which reduces shock loading on motor and pump bearings. The inverters are also full of operational data which can be integrated into the building control and management systems.

Some people may always be concerned about 'snakeoil' where such large savings are claimed. As for me I just hate paying energy companies and the taxman money I don't need to! Best of all, I know I am helping create a sustainable environment for us all.

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