Danfoss - Principles / Advantages of Inverter Drives

Principles Advantages of Inverter Drives - Part L Compliance

The VLT® HVAC Drive, built on Danfoss’ modular plug-and-play platform and dedicated to HVAC applications, keeps operation simple. Danfoss has unequalled experience in advanced drive technologies for Building Services applications. This experience has been used to optimise its features and make it the perfect, cost effective match for fans, pumps and compressors in buildings fitted with sophisticated HVAC systems - VLT® HVAC Drive a must for intelligent buildings.

Frequency Converter Control

It is universally acknowledged that frequency converter control of centrifugal fan, pump and compressor motors brings significant efficiency benefits along with operational benefits in terms of simplicity of control and overall controllability. Compatibility with a host of fieldbus communication systems means VSDs today are cheaper to install, are easily integrated with complex control systems, can be controlled and monitored remotely and can even monitor energy savings while in operation.

Building Regulations Part ‘L’

The Building Regulations Part L imposes severe targets on energy performance of new build and existing buildings undergoing refurbishment. In the case of air-conditioned buildings, a Target Emission Rating (TER) is mandated. Designers have the choice of technologies enabling them to achieve these improvements, but key among them are variable speed drives (VSDs) for the fans, pumps and compressors on air conditioning systems. The regulations are demanding to the point that only variable air volume (VAV) systems employing VSDs will satisfy the mandatory standards. VSDs offer the single most effective means of reducing energy consumption in an air conditioned building, with savings of up to 40% of the power previously used to run air conditioning systems. There is a substantial benefit in power factor correction and the improved overall controllability, while adding to comfort levels, also improves overall efficiency.

Fan Systems

Consider three of today’s principal methods of controlling air flow:-

• Dampers
• Guide Vanes
• Variable Speed Control

Assuming a typical operating point of 80% max air flow, a damper uses typically 93% motor power, a guide vane around 70% and a VSD only around 50% - just over half of the energy absorption of a damper system. At 50% airflow, VSD absorbed power drops to as little a 15%, with the guide vane still taking 50% and the damper approx 70%. The life cost of mechanical systems, which offer poorer controllability and require greater maintenance, is therefore very much greater than that of electronic control.

Applications with variable speed fan control include: VAV, constant volume (air quality control), plus / minus lab or clean-room systems, dust extraction, cinema or theatre (including operating theatre) supply and extract, cooling towers, boiler and kiln FD & ID fans and many more.

Pump Systems

Pumps also benefit greatly from variable speed control. All of the benefits applicable to VSD fan operation apply to centrifugal pump operation. As well as boosting efficiency, there are operation benefits in terms of smoother control, reduction in system stress and reduced pump wear and tear.

Control valves are notoriously inefficient. Flow reduction of 30% over a valve results in little more than 5% energy reduction - raising the cost of moving each litre of fluid by 30%-40%. However, flow reduction by speed control means energy demand falls with volume and can be as much as halved. This means that not only has the overall cost fallen, but the cost of moving each litre of water has fallen, increasing system efficiency.

Compressor Systems

In the same way that variable speed control ensures the most efficient operation of fans and pumps, so it also offers major advantages in controlling compressor systems. The VLT® HVAC Drive has been designed to offer flexible, intelligent control of scroll, screw and centrifugal compressors making it easier to optimise cooling capacity and pressure levels for water chillers and other typical compressor applications in HVAC.

The Temperature Set-point feature enables calculation of the actual room temperature from the pressure in the cooling medium and refines compressor operation accordingly. The flexibility of the drive also facilitates the economy of using a single compressor as opposed to a cascade of several small units. Compressor life is enhanced by the programmable Maximum Start/Stop feature while the Quick Start-Up feature means the drive can control a bypass valve reducing load and speeding start-up, also extending compressor life.

Smart Logic Controller

The integral 4 PID controllers enable the drive to control air handling units, valves and dampers. Building Management DDCs are thereby released and valuable data points are saved. When integrated into the BMS network, all the HVAC Drive I/O points are available as remote I/Os to extend the BMS’s capacity. Room temperature sensors can be directly connected to the drive. The drive also handles logical rules and inputs from sensors, real-time functionality and time related actions. This enables the drive to control a wide range of functions, including :-

• Weekend & working-day operations
• Cascaded P-PI for temperature control
• Multi-zone pressure control
• Flow balancing between fresh & outlet air

Fire Override Mode

The drive will ignore control signals or alarms, continuing to run until eventual self destruction. This is critical in ventilation systems where in the event of a fire, the vent fans are required to continue in operation regardless of the destruction of the drives or motors. Stairwell pressures can be maintained at a higher level than other parts of the building, ensuring that fire escapes remain free of smoke.

Automatic Motor Adaptation (AMA)

AMA tunes the drive to the motor ensuring that under all speed and load conditions, motor magnetization and thus flux is optimized. The drive is designed to tune the widest possible range of induction motors. It calculates a cos Ø with a more valid value than normally found on the motor label. This value is also adopted for optimal motor magnetizing and energy optimisation.

Tax Efficient

Recognising the highly beneficial role VSDs play in energy conservation and reducing greenhouse emissions, VLT® HVAC Drive is listed as eligible for Enhanced Capital Allowances, allowing the whole of the capital cost of the drives to be set against corporation tax in year 1, reducing overall capital costs and reducing the payback period of the capital investment.

Contact Danfoss for your free copy of the new VLT Energy Box software and your free copy of the HVAC Planet CD guide on the application of AC drives on air conditioning solutions.