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Danfoss - Principles / Advantages of Inverter Drives

Benefits / Principles / Advantages of Inverter Drives

The VLT® HVAC Drive

Dedicated for Building Services applications; its optimised features make it the perfect cost effective match for fans, pumps and compressors in buildings fitted with sophisticated HVAC systems – VLT® HVAC Drive is a must for intelligent buildings.

Motor Independency for Building Services

Frequency converter control of fans, pumps and compressor motors brings significant energy and operational benefits in terms of simple and flexible overall control. Legislation has reinforced the need for higher efficient systems, resulting in a more diverse offering of high efficient induction, permanent magnet and synchronous reluctance motors. Danfoss Drives are at the forefront by providing a solution where one drive fits all motor types. This gives motor independency to OEM, installers and end users, helping to future proof the building services system. Maximum possible energy savings can therefore be achieved using the best motor technology for the application whilst reducing total cost of ownership. Freedom of Choice – One VLT® Drive for all motors.

Automatic Motor Adaptation (AMA)

The AMA function ensures motor magnetisation and flux is optimised for induction, permanent magnet and synchronous reluctance motors. It calculates a cos Ø with a more accurate value than normally found on the motor label. This value is also adopted for energy optimisation, requiring only the motor’s nameplate details and taking up to 2.7 seconds to tune into the connected motor. The result is a true matched motor and VSD system as infinitely variable cabling and connection values are also taken into account.

Smart Logic Controller

With built in intelligence, the user can maximise the intelligence within the drive to save money on installation. The integral 4 PID controllers enable the drive to control air handling units, valves and dampers. Building Management DDCs are thereby released, reducing system costs. When integrated into the BMS network, all the HVAC Drive I/O points are available remotely to extend the BMS’s capacity. 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, enabling 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
  • AHU controller

Fan Systems – Fire Mode

The savings gained when using variable speed control is well documented, what is lesser known is the ability for drives to be fully integrated in building smoke control systems. FIRE MODE can be activated via a dedicated input from a fire alarm system, maximising availability by operating in extreme conditions such as phase loss; attempting to restart any tripped or stalled fan even working at reduced capability over and above the conditions (DOL fans may have ceased operation). The drive will continue to operate ignoring all non-critical alarms until the motor is destroyed by fire or heat. Whilst in FIRE MODE, stairwell pressurisation can still be controlled, enabling the easy exit of occupants whilst maximising the time to safely exit the building. When the fire alarm status is reset, the drives revert to normal operation.

Fans can be dual rated, performing their routine task within the comfort of a building and in the event of a fire can be utilised to control smoke, giving a clear exit route for all occupants. The benefit of dual use is that fans are operational just prior to the alarm – if it is turning before an alarm then it is highly likely to turn during the alarm situation. The more complex the building, the greater the need for the ventilation system to maximise the efficiency of the building whilst maintaining comfort and safety for its occupants. Variable speed drives offer flexible demand matched ventilation.

Pump Systems

Pumps also benefit greatly from variable speed control. All benefits applicable to VSD fan operation apply to centrifugal pump operation. As well as energy savings and increased efficiency, there are operational 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%.

Refrigeration Systems

The performance of a refrigeration system is expressed using the Energy Efficiency Ratio (EER) or the Co-efficient of Performance (COP). This is the ratio of the generated cooling or heating capacity to the power actually consumed and is usually based on full-load operation. However, it is not enough to rate refrigeration units at just one load level, since most refrigeration systems operate under partial load conditions. This means that significant energy savings can be obtained using speed control.

Refrigeration System without Speed Control

In a refrigeration system without speed control, the refrigerant compressor always runs at full speed, regardless of the cooling capacity actually required. The cooling output is regulated by the evaporator, which is filled by the expansion device.

Since the expansion valve constantly tries to fill the evaporator optimally, this adjustment causes the pressure to change and therefore creates oscillation in the system. As a result of this oscillation, the evaporator never properly fills and operates ineffectively and therefore the cooling capacity of the refrigerant is not optimal.

Refrigeration System with Speed Control

Continuous variable speed control by VLT® Refrigeration Drive FC103 makes intelligent capacity control possible. By creating stability whilst balancing the capacity to the actual load, system wide COP/EER improves providing significant energy savings. Intelligent compressor and condenser fan control is a “must” in any optimised refrigeration system.

The following positive effects can be achieved in a refrigeration system with variable-speed compressor operation:


  • Stable suction pressure
  • Increased capacity using smaller compressor
  • Built-in soft starter function
  • Reduced mechanical load
  • Fewer starts & stops extends lifetime
  • Eliminate mechanical capacity control

Condenser Fan Control

  • Load dependent capacity control
  • Stable condensing pressure
  • Reduced charge of refrigerant
  • Less dirt build up on condenser
  • Stand-alone optimised control with VLT® Refrigeration Drive FC103

Pumps in A/C or Indirect Cooling Systems & Fans in Air-Conditioning Systems

  • Coolant pump capacity according to demand
  • Stable coolant flow & pressure
  • Optimised operation of air handling units
  • High efficiency
  • Airflow according to demand
  • Stand-alone optimised control with VLT® Refrigeration Drive FC103
  • Operate from direct signal (0/4-20mA or 0-10v DC)

Depending on the application, speed control can result in energy savings ranging from 10% to as much as 70%.

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