Powerful Performance under Pressure – Fans for Inverter Drive Cooling

Beginning at the turn of the century the Kyoto Protocol has focussed countries and economic areas on reducing greenhouse gas emissions and minimising the harmful effects of global warming. More recently in Europe, the introduction of the Energy Performance of Buildings Directive has seen a swathe of regulations aimed at minimising power consumption and maximising efficiency in residential and non-residential buildings. At a local level, the UK building regulations require that energy efficiency is at the heart of building new and refurbishing existing buildings.

More recently, all rental buildings in the UK have become subject to the Minimum Energy Efficiency Standard (MEES), which specifies a minimum efficiency rating that the premises must meet. All rental buildings can be classified in terms of their efficiency rating with a range from A (highest) to G (Lowest). There are now restrictions in place on renting properties which have a building efficiency rating less than an ‘E’.

Within the current stock of commercial buildings in the UK, there are several motor-driven services that can be running at 100% capacity regardless of what the demand is at the time. For a quick win, some of those services can benefit from variable speed drive technology to reduce their operating point and hence lowering the building power consumption. Inverter drives are a technology that has been used to achieve power consumption reduction in new and existing building services.

Why choose an Inverter?

An inverter is most commonly used with industrial motors of internal rotor motor design (Type IMB), suitable for variable speed control by changing the frequency of the voltage at the motor terminals. Changing the frequency applied to the motor changes its synchronous speed which can be calculated from the simple formula:

Speed = Frequency (Hz) x 60

                                                                 No of pole pairs

Example 1)

For a 2-pole motor running on a 50Hz supply the synchronous speed is calculated by

                                                Speed = 50 x 60        = 3000            = 3000 rpm

                                                                 2 / 2                   1

Example 2)

For a 4-pole motor running on a 60Hz supply the synchronous speed is calculated by

                                                Speed = 60 x 60        = 3600            = 1800 rpm

                                                                 4 / 2                   2

In addition to changing the frequency, the inverter drive also varies the voltage: whilst reducing the frequency the voltage level is reduced as well. Changing the voltage level and frequency together optimises the motor efficiency which ensures that slip losses and the motor operating temperature are minimised.

How does an inverter work?

Manipulating the frequency using an inverter drive is a three-stage process.

The initial stage is to convert the incoming mains voltage from AC to DC using a full wave rectification circuit. There is then a second DC link stage that smooths the waveform and ensures that the inverter stays charged during its use. The final stage involves creating a square wave pulsed voltage output to simulate an AC waveform as shown below…

The final stage of the process has several issues associated with the pulsed square wave output. In modern frequency inverters, the voltage pulses are produced by insulated Gate Bi-polar transistors switching at high frequency. When using a high-frequency waveform on a supply line to control a motor, electromagnetic compatibility, harmonics, high voltage rise (dv/dt), uneven voltage distribution across the windings, high-frequency voltage reflections, high-frequency interference and bearing currents are all issues that must be managed.

Inverter Drive Efficiency

The process of converting AC to DC then simulating an AC waveform output using high-frequency switching is not without losses. Switching mains power via electronic components dissipates electrical energy converting it to heat. Some of the higher power inverter drives available have low energy losses leading to high-efficiency operation (up to 95%).

Incorporating an Inverter Drive into an Installation

One of the considerations of introducing an inverter drive into an existing installation is where to put it. Long runs of shielded cable are expensive so the inverter is commonly installed near to the motor it will be driving. In most applications space is at a premium, meaning inverters need to be compact whilst delivering the required power output.

The miniaturisation of the drive creates a high density of electronic components dissipating a significant amount of energy lost as heat into a small space. To maintain reliable performance, the temperature of the drive components must not exceed operational limits which means forced cooling is usually required.

Fans for Inverter Drives

High-density electronic enclosures require large amounts of air delivered at high pressure to overcome the internal resistance to flow. This can be achieved using a high-speed compact axial fan, a diagonal mixed flow fan or a backward curved motorised impeller.

Axial / Mixed Flow fans

High-speed DC powered compact fans capable of rotating at speeds up to 14,000 rpm can deliver high volume flows at pressures up to 500Pa. This level of performance can force air through the most densely packed electronics. In addition to high performance, variable speed drives are incorporated on board with rotational speed feedback can vary the performance based on demand and provide early warning if the fan is not functioning as expected.

High Performance 24V DC axial fans available via RS components include:

            400 series       - (840-5560)  

            600 series       - (878-1050)  

            8000 series     - (749-6966)  

            4000 series     - (055-7701)  

High performance 24V DC Diagonal flow fans available from RS Components include:

           DV4000 series - (826-1165)  

           DV5000 series - (826-1171)  

           DV6000 series - (826-1193)  

Backward Curved Centrifugal Fans

Centrifugal fans by their nature deliver air at higher pressures however they need a larger space envelope to incorporate them into the drive. This type of fan, when used with a low voltage DC motor, can provide high volume flow at pressures exceeding 500 Pa.

Variable speed drives incorporated on board with rotational speed feedback can vary the performance based on demand and provide early warning if the fan is not functioning as expected.

High-Performance 24V DC Centrifugal fans available from RS Components include:

            RG160 series  - (825-7901)  

            RER101 series - RS Extended Range

            RER175 series - (860-0223)

            RER190 series - (920-9206)  

In Summary…

To meet the demands of reducing energy consumption in the existing building stock, variable speed drives are being used to match the supply of building services to the demand. Frequency Inverters are one method of achieving this however due to size restraints, high power density and efficiency losses, forced cooling is required to maintain performance. High-speed compact axial, mixed flow and backward curved centrifugal fans can overcome high flow and high back pressures can be used to manage internal temperatures and ensure reliable and efficient operation.