Drive to cut energy

30 September 2012

Fitting drives to motors is one of the lowest hanging fruits when it comes to saving energy in your building. But even the simple drive has got a lot more to offer these days as Craig Fuller reports

Like most job roles, that of a facilities manager has changed significantly over the past decade, partly driven by the need to keep up with the dramatic developments in technology and how they affect the practice of building management.

Many of these technological developments have been in variable-speed drives (VSDs). Drives have been around now for some 40 years, long enough for them to be seen as a standard part of the building’s heating, ventilation and air conditioning (HVAC) system. As such, facilities managers have come to rely on the drive to ensure that the flow of air or liquid around the building is regulated so as not to waste energy.

Yet, today’s drives are not what they were even two years ago. It is well worth spending a little time and reassessing exactly what it is that today’s VSDs can offer your building.

Today’s drive is some 60 percent smaller than its predecessors. A 37 kW drive can have a footprint smaller than an A4 page, only 203mm x 262mm. Lower power drives are even smaller. That opens up an entire new area for installing drives. They can be discrete and tucked out of the way. Within that footprint comes 70 percent fewer components than 40 years ago – hence the massive increase in reliability. Thus, the mean time between failures (MTBF) of drives has increased fivefold. Today the MTBF is better than 1failure in 50 years.

The printed circuit boards of modern drives are also coated to improve resistance to environmental factors and withstand contamination.

In fact, today’s drives have a design lifetime of six years for fans and nine years for capacitors, based on permanent operation at full load in 50ºC ambient temperature. Modern drives should be able to pass their fully rated output current at full speed and load, avoiding the need for an oversized drive unit.

Greater reliability means manufacturers are prepared to give longer warranties, with 24 months not being uncommon.

Improved communication
Despite having fewer components than their predecessors, modern drives offer a lot of functionality. The serial communications protocols built into many drives today include BACnet, Modbus RTU and others, while a fieldbus adapter allows connection of LonWorks, Profibus-DP, CANopen, DeviceNet, Modbus/TCP ControlNet and Ethernet. This means the drive can connect to any building management system (BMS).

Fieldbus offers greater flexibility than point-to-point hardwiring. It can give as many as 70 points of information and frees up the drive’s I/Os, which can then be used to control the building’s plant.

The greatest flexibility is offered by open network standards, which can be used by any manufacturer and do not tie you in to working with one particular supplier in the future. BACnet, the open protocol serial communications standard, gives complete access to drive parameter information when connected to a BACnet control network. BACnet is one of the most accessible of all the open systems on the market, giving more scope for future development. All BACnet devices communicate in the same language, without the expense of additional hardware and gateways.

Such communications open up new levels of intelligence within a drive. For example, one particular modern drive has 13 I/O points. This provides extensive control options, either hardwired or via the serial communication network.

A real-time clock can be used, together with the drive’s timer functions, to tell the software to trigger a particular event, allowing the drive to be a stand-alone control unit without the need for an input from the BMS. The clock can also be used to date any fault which might occur, or to send alarm signals when critical equipment needs to be inspected.

Control at your fingertips
To see the most obvious sign of drives’ new-found intelligence, just take a look at the control panel or keypad. Today’s drives often have numerous HVAC specific macros such as supply fan, return fan, cooling tower, as well as other macros that the user can use for his own programs. All these save time in setting up the drive.

The panel can help users start up the drive, maintain it and diagnose faults. An on-board fault history assistant can show the actual time when the fault occurred, speeding up drive repair. The assistant can detect, for example, when there is a dirty filter in the air-handling unit and prompts users to replace the filters. It can even keep track of bearing and drive belt replacement intervals, as well as lubrication intervals for the driven equipment.

For stand-alone applications where no BMS is installed, drives with a built-in maintenance assistant can be used. This can flag up when maintenance is needed, either at specific times, after a set number of running hours or when equipment is no longer performing as it should.

Another useful function of modern drives is a run-time accumulator, which counts the number of hours a drive passes current to the motor. Used with the timer function, this can also be used to flag up maintenance intervals.

Efficiently meeting building regulations
One of the most basic functions of a variable-speed drive is to save energy. Some recent drives feature a kWh counter showing the consumed energy. This can help with electricity billing in accordance with Part L2 Building Regulations. It also allows verification of energy savings before making investments in capital equipment. VSDs offer the single most effective means of reducing energy consumption.

Further energy savings are achieved by using software that continuously monitors the performance of the motor, whilst adjusting the voltage to get the best performance. This can lead to a five percent decrease in input power, helping to further reduce energy costs.

Today’s drives should also feature an emergency override operation mode, ensuring optimum performance of the smoke extract system for the longest possible duration. Modern HVAC drives systems should use sensor technology to monitor carbon dioxide levels and initiate ventilation when the level rises above acceptable levels.

Combating interference
One aspect is electrical interference. Drives in buildings need to do their jobs without interfering with the jobs of other systems. One of the biggest problems is the harmonic frequencies that all drives produce. You need to ensure your supplier has the necessary expertise in harmonics and is able to advise. Also, ensure that your chosen drive has a DC choke that suppresses harmonics effectively across the speed range and not just at full speed.

Another interference issue is a drive’s electro-magnetic compatibility (EMC). This a measure of how well a drive can work with other electrical and electronic devices without causing problems. EMC filters suitable for 400 V network connection built into the drive as standard will save panel space, avoiding additional wiring, earthing and assembly costs.

As you can see there is a lot going on inside that grey drives box and a lot of new functions have more than likely been added to drives since you last upgraded. A good drives supplier will be able to offer all the advice you need to ensure you have the best VSD for your building. But it’s not just about the right product. Manufacturers should be able to offer advice on matched drive and motor packages so as to ensure the correct dimensioning of both. When purchased as a unit, the entire motor and drive installation qualifies for enhanced capital allowances (ECAs) for tax purposes.

So, why not take another look at your building’s drive applications and see how modern variable-speed drives can help you save more energy, cut costs and improve your maintenance?

Craig Fuller works for ABB Limited