Variable-speed drives combine better environment with lower energy use
In any building with sophisticated and extensive building services, there are ample opportunities to exploit variable-speed drives to improve the quality of the environment — and reduce energy costs.
Building-services engineers have fought shy of installing variable speed drives or inverters in the past, but GUY KENNETTsays that their apprehensions are based on older technologies. He also admits that few drives engineers have taken the trouble to understand the needs of environmental installations.Variable-speed drives have been fitted to machines in factories with great success for many years, but to date they just do not seem to have done the business in building services. In fact, many building-services engineers will tell you that whenever they have tried to use a drive it has been a struggle to get the promised performance. But today’s drives are far more intelligent and optimise motors and loads for easily overcoming many of the problems of poorly set up drives. Reliability
The argument is often advanced that adding an extra element such as a variable-speed drive compromises reliability. In fact the protection and monitoring capabilities of a drive are much superior to a direct-online or star/delta system. Drives control the speed of motors, and thus the pumps, fans, doors, lifts or escalators to which they are attached, to match the needs of the moment. Intuitively this seems like a good thing, and most people will immediately see that there is potential for considerable energy saving if equipment is not run at full speed all the time. But that is not really why drives should be used in building services. Energy saving is a secondary issue, even though it is the one that most people quote. The primary role of a building-services engineer is to make the internal environment comfortable, to provide the security, access, safety etc. Drives can help with all these, and saving energy in the process is a bonus. Many types of equipment benefit from being able to run at different speeds to match the needs of the moment. For instance, when a room is full of people, a ventilation fan will need to run fast, but it can slow down when the room is only partly occupied and slowing even further or stop overnight. However, the motor may need to quickly run up to maximum speed if it is venting smoke during a fire. Fortunately, a drive either can make these changes happen — either manually or under automatic control. But this is an over-simplification of the needs of a system for controlling a building or optimising the environment. Industrial engineers who have applied such thinking to building services have come unstuck because they have not thought through all the issues. Taking fans in an office again, you could have a basic timer to switch the fan off at night, churn it over at low speed from 8 to 9 a.m. to freshen the air prior to the arrival of staff and go to normal operating speed at 9 a.m. Working with such a set-up, presence sensors, temperature monitors, fire alarms etc. can take over and readjust the speed should a need arise. Most drives have sufficient on-board computer intelligence to easily respond effectively to such requirements. They can be made even more intelligent by using them with a control unit such as a building-management system (BMS), a PC or a programmable logic controller (PLC). Such controllers monitor various sensors to assess the environment, make decisions and then and adjust the speed of the drive appropriately. Even this is a simplification, as can be explained by considering the ventilation in a sports hall. If there is a large high-impact aerobics class or 5-a-side football tournament going on, humidity and carbon-dioxide levels will rise markedly. This could become unpleasant for the participants and thoroughly off-putting for the next users of the hall, particularly if it is a yoga or tai-chi class where serenity is the ideal. Fortunately, a modern drive can be used with environmental sensors and ramp up the ventilation for the athletic users. Because an optimum air quality has been maintained, the fans can wind down as soon as they leave to a nice quiet level, ideal to aspiring yogis. Similarly in the event of a fire, a BMS can instantly reverse some fans to suck out smoke, and speed up other to keep escape routes clear. The issue gets even more complex when you account for associated loads. In the sports hall example, the footballers’ air will need cooling, so a water pump comes into play. The electrical load from the pump must be modelled into the BMS’s control algorithm if the environment is to be maintained at its optimal level. Water supply
Drives are also regularly used with pumps. A tall building will often include a booster pump in the basement to keep a header tank of water at the top of the building filled. But water demand is characterised by sudden surges, say at bath time in a block of flats, so the header tank could empty and the pump may not be able to refill it promptly. One way to overcome this is to use a very large tank, but the weight can have implications on structural engineering and there could also be problems with stagnating water. Another way is to oversize the pump or have a cascade of several pumps, with progressively more kicking in as the surge continues, but this is expensive, maintenance-heavy and provides only crude control. Far better control can be achieved by fitting a variable-speed drive to the pump’s motor so that the flow can be continually adjusted to keep the tank nicely topped up automatically. Lifts and escalators
Drives respond immediately to adjust the speed of the motor they are controlling, a fact that can be exploited to create precise speed profiles. A passenger lift in a tall building has to travel fairly quickly to get to the top in a reasonable time. Its speed could easily reach 7 m/s. However, the lift must accelerate and decelerate fairly gently to give a comfortable ride. A drive can achieve this easily, even using s-curve rather than linear acceleration. The situation becomes more complex when you realise that sometimes the car will only be required to travel a floor or two. Here a much slower speed makes sense, which leads to different acceleration requirements. The number of people in the lift has a significant effect on the power required from the motor. During busy periods, many people will be in the lift and expect a high speed journey of many floors. At other times, just one person may be travelling a single floor. Again, the drive can take all of these variations in its stride. The world’s tallest buildings have the fastest lifts and extra complications have to be weighed in. Air is pumped from the lift shaft ahead of the car to reduce resistance, while air pressure in the car has to be constantly varied to prevent ear-popping and motion sickness. This is another case of multiple drives working in synchronism, controlled by a BMS or similar controller. Drives can also achieve significant energy savings in lifts. Instead of consuming power during descent, the loaded car causes the motor to generate power. The drive can use this energy to maintain power, rather than take it from the mains supply. Extra power may also be fed back into the supply itself, increasing the overall efficiency of the system. Interestingly escalators on London Underground use electricity in the morning when they are running upwards, but regenerate almost as much again in the evening. In summary, drives work on two levels. First, they make electrically driven equipment far more responsive, user-friendly, and intelligent. Secondly they achieve significant energy savings. Without drives intelligent buildings cannot exist. With them, even a humble flat or house can have features that were considered to be science fiction as recently as the last millennium. Guy Kennett is with Mitsubishi Electric UK Ltd, Automation Systems Division, Travellers Lane, Hatfield, Herts AL10 8XB. The energy savings of variable-speed drives
Pumps, fans and all electrically driven equipment consume power. Turning the speed down even slightly can result in substantial energy savings. Because they are moving volumes, pumps and fans comply to a cube law. If the speed is reduced by 20% to 80% of maximum, energy consumption is reduced by 0.8 x 0.8 x 0.8, or 51%. By halving the speed power consumption is cut to 12.5%; allowing for a few losses, power consumption will be cut to just 17%.
The energy and cost-saving potential of variable-speed drives is a value feature — but they achieve this at the same time as improving the quality of the environment in a building.
Pumps and fans often have to be sized to cope with occasional massive demand, so the energy savings achievable by trimming their speed constantly to match demand can be enormous. With increasing environmental awareness and accompanying new standards such as ISO 14000, drives must become more and more attractive. Additionally, analysts all agree that electricity costs will be rise steadily for the next decade or more after the post-privatisation price squeezes that supply companies endured to win market share.