Friday, 21 April 2017

Andy Preston, UK Drives Product Manager, ABB Limited, writes: Changes in building regulations are seeing the introduction of new technologies that may not be right for HVAC applications. Take electronically commutated motors (ECMs). These are brushless DC motors that function using a built-in inverter and a magnetic rotor, and as a result are claimed to achieve greater efficiency in air-flow systems than other kinds of AC motors.

ECMs are highly efficient and provide a consistent airflow over a range of downstream static pressures. From start-up they provide a gradual ramp-up to set-point flow rate and generate less noise than many other motor technologies. They are highly modular, helping with installation.

While ECMs achieve specific fan powers at smaller powers, they can generate large amounts of harmonic distortion. The motors cannot ride through power dips nor can the controller catch a spinning load, something that is critical in the HVAC industry, especially data centres.

It is not possible to overspeed ECMs to achieve best efficiency characteristics of the system. For those installations seeking to benefit from BACnet or other embedded communications, ECMs cannot be connected to fieldbus networks. Also the controller cannot be programmed to perform PID control or timed functions.

While the ECMs may be suitable for some applications, it is essential to consider the right motor technology.  ECMs are limited up to 7.5 kW. Above that customers and OEM’s are reverting to permanent magnet (PM) motors to achieve the efficiency levels. But there are other technologies.

Synchronous reluctance motors (SynRMs), for instance, can achieve the specific fan powers, whilst being controlled by a variable speed drive (VSD). The user benefits from VSDs including improved harmonic mitigation, ability to a catch a spinning load, adequate power drip ride through, fieldbus communications and built-in control features to enhance the application.

If a controller cannot catch a spinning load (usually after a power outage), then it has to bring the motor to a very abrupt stop before accelerating the motor back to the desired speed. This abrupt stop is likely to severely damage the mechanics of the fan, the couplings and its mechanics. The overall stop and re-start cycle takes a lot longer to perform which is not great for time critical applications.

Furthermore, if a system cannot ride though a power dip, then the controller will need resetting to bring it back to operation. Resetting takes time, and during that time the HVAC system is not operating.  If the controller is not on fieldbus, then the reset has to be performed manually taking even more time.

EC fans have external rotors where fan blades are integrated. In most cases, EC pumps mount the motor directly to pump. Therefore, the EC motors are not sized in traditional IEC dimensions. They are application-specific and customised. This also can require an entire fan or pump to be replaced if the motor fails – where traditional motors are easily replaced separately and are normally stocked locally.

SynRM is built into a standard IEC motor frame (unlike PM and ECM) and does not generate unwanted dangerous voltages when operating on a wind-milling fan (unlike PM). Being synchronous, the motors are easy to control accurately, even above nominal speeds where improved fan efficiencies can be realised.

Great care must be taken to ensure correct specifications are applied that support the eventual owner/operator of the facility, including lifetime considerations, when specifying motor technologies.

To discover the best motor technology for your HVAC application, download ABB's Top Tip eBook entitled 'Electronically Commutated Motors' by clicking here.

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