A Guide to Inverter Selection and Fan Speed Control

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.

 

AC Motors

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. This prevents the electro-magnetically-generated noise and additional self-heating of the motor 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, otherwise referred to as a variable frequency drive (VFD).

 

EC Motors

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 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.

 

Inverter Selection

An inverter 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. Where an application does not require the motor to run at full speed an inverter can be used to run a motor at the required RPM. The vast majority of inverters have three phase outputs but can be supplied for single or three phase input. This makes speed control possible where there is only single phase supply.

Three inverter invertek motor fan control speed energy savingphase external rotor and induction motors can be speed controlled off a three phase supply via an inverter. Where there is a single phase supply the inverter can convert the single-phase supply to three-phase power. Single -three phase inverters will take a 230V single phase supply and convert it to a 230V three phase supply to power a three phase fan.

The correct selection of an inverter depends on the motor kW rating of the fan, the inverter has to be the same kW or higher. For example a 0.75kW fan could use a 0.75kw inverter or higher.

An inverter can vary the motor speed with no loss of efficiency (98% efficiency at full load), the resulting energy savings usually pay for the inverter in a relatively short period of time. In addition, the motor only works as hard as it needs to, so helping to prolong the life of the motor. We know that installing an inverter to an older, struggling system may even help to extend its service life.

In most cases, fans are sized and installed to match the maximum ventilation requirements. Installing an inverter allows the user to control the speed of the fan motor to match the actual ventilation needs of the system. Reducing the speed of the motor will reduce the amount of energy needed to power it, sometimes by as much as eight times the reduction in speed.

Many companies have started to address the issue of efficiency in line with improving their green credentials and energy consumption and recognise that inverters can play a key role. Reducing the amount of time that a motor runs at full speed will not only save energy, it will also reduce the strain on the motor and any related components. This means lower maintenance while the improved control provided also makes it easier to stop the system; another factor that can impact on the wear and tear of the mechanical components.

 

Case study

The solution to an inconvenient problem was put into action when Axair supplied a number of fans to a site in Manchester who use fans to extract fumes from safety cabinets. These included S20-2L-1-B and S20-2M-1-B fans.

The site reported having problems with fans and stated that fans had stopped working altogether. It was usually a case of fitting a new motorised impeller but during visits from Axair and later inspections of the equipment back in our workshop, it was discovered that the motors either had a capacitor go down or the bearings were a little noisy.

The motors appeared to be running hot which could have caused the capacitor to fail and also cause the motor bearings to make a slight noise. Axair’s technical team discovered that the electrical supply to the plant room had a direct effect on the electrical equipment. The supply was generated by combined heat and power which can cause power surges or even drops in power.

A three phase motorised impeller and an inverter was fitted that was 230/1 in and 230/3 out to the fan motor. The motor was wired in delta and after setting the inverter to the correct settings there have been no reported problems since.

 

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.

 

Axair Fans offer a range of inverters for accurate speed control and efficiency. For more information or to request datasheet or wiring downloads speak to one of our Sales Engineers today. Call 01782 349 430 or alternatively you can reach one of our technical advisors live on the chat.

 

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