Speed control is often overlooked when specifying fans, however significant savings can be had by the use of the correct speed controller and fan combination. These are not just direct savings in energy consumed by the fans but indirect savings from ancillary equipment. The addition of a fan speed controller can add extra functionality to products enhancing their usability and safety.
It is often desirable where applications have varying output requirements, and control of the motor speed is more efficient than mechanically limiting the process output with devices like throttling valves or dampers.
Simple voltage control can be applied to some fans more successfully than others. The control of both AC and EC motors are discussed below.
In the case of a transformer, the resultant voltage output to the fan motor is sinusoidal (has the form of a sine curve) and as ‘clean’ as the mains input. The motor does not then suffer from electro-magnetically-generated noise or additional self-heating due to waveform harmonics. The down-side to this method is the cost, weight, and space required for a transformer.
Where electronic voltage controllers are concerned, these rely on devices that switch the voltage waveform ‘on’ at a particular ‘conduction angle’ that is varied by the turning of a potentiometer. The advantages are low cost, low weight and less space required.
External rotor motors have high resistance rotors, which are necessary to enable voltage control. Most will work with electronic voltage controllers because the extra heat is easily lost to the airstream. It is worth noting that it is only in larger sizes of external rotor motor that transformer control can become necessary.
Standard (internal rotor) industrial motors are not designed for voltage control and, depending on the type of fan, are likely to overheat once the voltage drops out of the normal tolerance band. For example, a normal 230V means 216.2V to 253V in the UK. Three phase voltage control by transformer is extremely expensive because there is a transformer for each phase.
This is normally accomplished with an inverter drive. This device electronically converts a fixed mains frequency sinusoidal input into a variable frequency and voltage output. Both must be varied in proportion to maintain motor flux density.
The vast majority of inverters have three phase outputs, but can be supplied for single or three phase input. This makes speed control possible using standard (internal rotor) motors and is available where there is only single phase supply.
Whilst some electrical systems and motors combine to be extremely troublesome, inverters are, on the whole, successfully applied and provide worthwhile benefits in terms of precise, multi-speed, continuously variable, and fully automatic fan control. The benefits are in energy saving, low noise, low maintenance, and low relocation costs, with over-current protection as a standard feature.
EC motors have several benefits as they do not waste power as the electronics control the stator; they provide better performance and controllability, and they run cooler than induction motors. Read our guide on EC Fans and EC Technology for more information.
EC motors are brushless DC motors controlled by external electronics; either an electronic circuit board or variable frequency drive. The rotor contains permanent magnets and the stator has a set of fixed windings. The mechanical commutation is performed by the electronic circuitry. The circuit board switches the phases in the fixed windings to keep the motor turning. This supplies the right amount of armature current. When the current is delivered in the right direction at the precise time, the higher the accuracy is achieved.
For EC motors, the commutation circuits accept inputs with pulse-width modulation of 4 to 20 mA and 0 to 10 V. This controls the speed in a range of 10% to 100%. Monitoring of EC motors is simple with the integrated circuit, and can be easily accessed by the designer to provide feedback.
This means that, with a fairly low cost 0-10v potentiometer, the speed of the fan is easily controlled either manually or by receiving a 0–10v signal from an external measurement device such as a pressure or temperature sensor.
Whether choosing to opt for voltage variation or frequency control, there are tangible energy savings to be had from combining the correct fan speed control method for the AC or EC fan and its application.
To discuss your application with one of our team of engineers, please contact us.