Drive for better building services

Guy Kennett of Mitsubishi Electric challenges why building services engineers have fought shy of installing variable speed drives or inverters in the past.

Building services engineers have fought shy of installing variable speed drives or inverters in the past, but Guy Kennett of Mitsubishi Electric says that their apprehensions are based on older technologies.

He also holds his hands up and admits that few drives engineers have taken the trouble to understand the needs of environmental installations.

Drives have been fitted to production machines in factories with great success for many years, but to date they just don't 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's 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.

The argument is often advance that by adding an extra element, ie a drive reliability will be compromised; in fact the protection and monitoring capabilities of a drive are much superior to a DOL of star/delta system.

In essence drives control the speed of motors, and thus the machines, 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 things are not run at full speed all the time.

Energy Saving Pumps, fans, and indeed all electrically driven equipment consume power, but 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 you trim their speed to say 80 per cent maximum, the energy consumption is reduced by 0.8 x 0.8 x 0.8, or 51 per cent.

By the same token by running at half speed power consumption is cut to 12.5 percent, allowing for a few losses, the power consumption will be cut to just 17% of it's normal power.

Given that pumps and fans often have to be sized to cope with occasional massive demand, the energy savings achievable by trimming their speed constantly to match demand can be enormous.

In these days of increasing environmental awareness and accompanying new standards such as ISO 14000 drives can only become more and more attractive.

Additionally analysts all agree that electricity costs will be rising steadily for the next decade or more after the post-privatisation price squeezes that the supply companies endured to win market share.

But that is not really why drives should be used in building services.

Energy saving is a secondary issue, even though it's 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 of these, and the fact that they save energy in the process is a bonus.

Many types of equipment benefit from the ability to run 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 partially occupied, slowing even further or even stopping completely overnight.

However the motor may need to quickly run up to maximum speed if it is venting smoke during a fire situation.

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 building control or environment optimising system.

Industrial engineers who have applied this sort of 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 that switches the fan off at night, churns it over at low speed from 8-9am to freshen the air prior to the arrival of staff and goes to normal operating speed at 9am.

Working in conjunction with this, 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 in conjunction with a control unit such as a building management system (BMS), a PC or a programmable logic controller (PLC).

Such controllers will be monitoring various sensors to assess the environment, make decisions and then adjust the drive appropriately.

And even this is a simplification.

Consider the ventilation in a sports hall: if there is a large high-impact aerobics class or five-a-side football tournament going on, humidity and carbon dioxide levels are going to 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 in conjunction with environmental sensors and ramp up the ventilation for the athletic users.

Because an optimum air quality has been maintained, as soon as they leave the fans can wind down 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 complicated 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, for example, the job of which is to keep a header tank or 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 of this can have ramifications on structural engineering and it can also lead to 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.

Consider the passenger lift in a tall building.

To get right to the top floor in a reasonable time the car needs to travel quickly; in the tallest buildings that speed can easily be 7m/sec.

But the lift car must accelerate and decelerate fairly gently in order to give a comfortable ride.

With a drive you can achieve this easily, even using s-curve rather than linear acceleration.

The situation becomes more complex when you realise that some times 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.

Also 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 there may be just one person travelling a single floor.

Again the drive can take all of this in its stride.

The world's tallest buildings have the fastest lifts and extra complications have to be weighed in.

In order to achieve the speed, air is pumped from the lift shaft ahead of the car to reduce resistance, while air pressure within 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, because instead of consuming power during down travel 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 the 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.

Firstly they make electrically driven equipment far more responsive, user-friendly, and intelligent; secondly they achieve significant energy saving.

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 last millennium.

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