Modern Building Management Systems (BMS) provide a wide range of control functions: fire control, security, power monitoring and air conditioning control. The latter of these is perhaps the best known function of a BMS, and one that can be enhanced and improved by using electronic variable speed drives as an integral part of the control system.
Variable speed drives, or VSDs, provide infinite control over the speed of motors driving pumps and fans. This ensures that the BMS achieves maximum controllability over the building’s environment, matching the temperature and humidity to the demands of the prevailing weather and the number of occupants. Compared to cruder systems, such as dampers or valves which choke off the flow of air or liquid produced by a fan or pump, a VSD can drive the fan at the exact speed needed to maintain the building’s internal conditions, making the most of the capabilities of the BMS’ software and sensors.
Energy Saving
The other major advantage of VSDs is energy saving – by controlling the speed of the motor so it runs at only the speed needed, VSDs help the BMS control energy usage. As an example, a 55kW motor in continuous duty costs £24,000 per year to run at a cost of 5 pence/kilowatt hour. If 50% airflow is required all the time, there are various ways to achieve this. The variable speed drive would use 12.5% of the rated motor power to produce this, compared to the least efficient method, the outlet damper, which would use 80%.
The difference between the most and least efficient method in the example above is £16,000 worth of electricity – £16,000 that need not be spent every year if variable speed drives are used.
An example of the benefits possible is an air conditioning project for Coutts & Co, the international private banking arm of Natwest Group. Coutts is now saving some £70,000 per annum with the help of drive technology from ABB, a cut of 90 per cent on its previous energy bill.
One 4kW, one 22kW and two 75kW drives are driving the pumps for chilled, hot and condenser water, as well as medium pressure water for the boiler room. The pumps are now running at about 40% of maximum speed, whereas before, they would be running at 100% continuously.
Sensors in the pipework, connected to the drives, help regulate the speed of the pumps to ensure optimum usage of the water’s energy content. The system is co-ordinated with the Building Management System, enabling easy control of the indoor climate. The lower pump speed also reduces mechanical wear, saving maintenance costs.
Building the system
Drives are also relatively straightforward to connect to the BMS. In many applications, they require only three hard-wired I/O connections; a start/stop, an analogue 0-10V input for speed control, and a fault relay output. The other main connection option is to use a field bus system, which has the advantage of requiring only two wires rather than individual hard-wired connections for each drive. Fieldbus is sufficiently fast for building control, yet there is reluctance among BMS specifiers to move from the tried and trusted dedicated I/O method.
Fieldbus can also support an unlimited number of drives – systems incorporating hundreds of VSDs are not uncommon. This gives BMS designers more options, allowing them to use more VSD features.
When choosing a VSD for a BMS, there are various criteria and issues to take into account. If it is intended for use on a Fieldbus system, it should have interfaces to all the major protocols, such as LonWorks, Profibus, Trend IQ and Modbus. Trend IQ is one of the most popular BMS protocols on the UK market, but few drives use it and so an interface is needed, such as the Compass Point developed by North Communications. Used by ABB drives, up to 20 drives can be connected to a single Compass Point unit.
It is also important for the drive to have low harmonics as standard – drives, fitted as standard with internal harmonic chokes do produce less harmonic current and therefore reduce the level of harmonic pollution seen by the supply network. This is particularly important now that the new G5/4 standard has been introduced.
The VSD should also have a good quality EMC filter that meets the requirements of first environment restricted distribution.
Perhaps most important is that the drive should be a dedicated HVAC unit, with software suitable for building applications. HVAC drives are quite different from process control drives and should not be confused. The drive’s software suite should be one that can be programmed to meet the particular demands of HVAC applications – for example, the drive should ideally have an dedicated exhaust fan function (sometimes referred to as a fireman’s override) to disperse smoke in the event of a fire and one that responds to a loss of input signal from the BMS by switching to top speed or to a speed set by the user.
Another thing to look for is the IP enclosure rating of the drive – an IP54 rating will allow the drive to be sited next to the motor in potentially dusty or wet conditions. Cabling costs can thus be kept to a minimum, equally important whether the drive is being designed into a new building or retrofitted into an older one.
Summary
Variable speed drives bring major advantages, helping the BMS to achieve one of its primary goals of managing the building environment. Combined with the high energy savings they can bring, drives are becoming an integral part of a modern BMS system.
Drives used in building control need to be dedicated HVAC drives, equipped with the appropriate software