For designers, installers and operators of heating, ventilation and air-conditioning (HVAC) systems, energy efficiency is rightly the number one selection criteria. This applies particularly to fan motors. However, while EC fans - the popular solution that integrates a fan, motor and speed control – are regarded as something of a de-facto guarantee for efficiency, that is not always the case.
When evaluating the efficiency of your fan, motor and speed control combination it is vital to consider the ‘wire to air’ efficiency. Therefore, the total effect of both electrical and mechanical performance must be taken into account.
Some EC fans have the motor mounted inside the fan wheel which disrupts the air flow. The result is decreased aerodynamic efficiency, and that impacts the overall efficiency of the fan unit. In contrast, a fan ‘package’ that comprises a high efficiency motor operating in combination with a variable frequency drive (VFD) that matches the motor speed with the required load could offer superior overall efficiency.
Part-load efficiency is crucial
In most cases, EC fans are fitted with brushless DC motors, known also as permanent magnet (PM) motors. When operating at full speed they can offer efficiency better than the IE4 standard. That sounds good until you consider that their motor efficiency can drop rapidly at part loads. In fact, they might not even deliver IE3 efficiency at speeds below 60 percent. This part-load efficiency is crucial since the majority of HVAC installations operate at less than 80 percent speed for 99 percent of the time.
A more cost-effective, and energy-efficient, alternative can be to reduce the dimensions of the mechanical components such as air handling units (AHUs), ducting and auxiliary equipment to match the most common operating conditions. This approach must make provision to over-speed the fan motors to handle the 1 percent peak requirement, accepting that for this brief period there will be slightly lower efficiency and increased audible noise.
EC fans cannot be over-speeded. Yet with VFDs it could, for example, be possible to over-speed a motor by up to 15 percent of its nominal speed.
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| ABB EC Titanium™ motors are a highly efficient integrated motor drive solution for HVAC applications |
Power factor makes an important difference
Harmonic currents are an additional energy efficiency consideration as they lower the total true power factor (PF) of the network by increasing the total line current. This extra current is not active current, but reactive current that results in higher system losses. Harmonics can result in added infrastructure costs due to the need to over-size equipment such as transformers, cables and fuses. Electricity bills might be increased while some utilities also impose penalties on their commercial customers with a low power factor.
When evaluating the efficiency of your fan, motor and speed control combination it is vital to consider the ‘wire to air’ efficiency
A traditional EC motor might have a power factor of around 0.78, drawing some 25 to 40 percent more current from the network than its actual load. That is why many EC fan systems require the installation of costly, centralized harmonic mitigation equipment.
Instead, a fan package approach enables the use of a state-of-the-art VFD with active front end (AFE) technology that eliminates harmonics at source. The benefit is a power factor of 1, so that the motor draws only the current it actually needs.
What happens when motors need replacing?
There is also a sustainability issue regarding the ease of servicing and replacement of the fan motors. Should an EC fan fail it is not serviceable and the complete unit will need replacing, with implications for both costs and the lead time to get a suitable replacement.
With a separate drive, motor and fan set up based on standard equipment, if the VFD fails it can be replaced the same day, with no need to go inside the AHU. If the motor fails it can also be replaced the same day.
