Frequency inverters
A frequency inverter, also known as a variable frequency drive (VFD) or adjustable speed drive, is a device that controls the speed of an electric motor by varying the frequency and voltage of the power supplied to it. In simpler terms, it acts like a sophisticated knob for your motor, allowing you to adjust how fast or slow it operates.
The primary advantage of a frequency inverter lies in its ability to enhance energy efficiency and optimize motor performance. By dynamically adjusting the motor's speed to match the actual requirements of a task, a frequency inverter prevents unnecessary energy consumption. This is particularly beneficial in applications where the load fluctuates or when a constant speed is not essential, such as in heating, ventilation, and air conditioning systems.
Moreover, frequency inverters extend the lifespan of motors by reducing wear and tear associated with abrupt starts and stops. They also provide precise control, improving the overall accuracy and quality of processes. In industrial settings, where electric motors play a crucial role, integrating frequency inverters contributes to cost savings, environmental sustainability, and more reliable operation of machinery. In essence, a frequency inverter is like a smart accelerator for electric motors, offering flexibility, efficiency, and longevity.
The major difference between a frequency converter and a fan speed controller is not easy to explain in non-technical terminology. Here's an attempt anyway. A frequency converter offers optimal motor control because it can regulate not only the voltage, but also the frequency. This has the advantage that the motor can be controlled much more efficiently and accurately. Disadvantages of this technology are its complexity and price. A fan speed controller can only regulate the motor voltage. This is done simply by cutting away parts of the supplied voltage with TRIAC technology (phase angle control). The advantage of this is that fewer expensive electronic components are required and that the device can be put into use immediately. A frequency controller must first be configured before it can be put into use.
Pulse Width Modulation for optimal motor control
Frequency inverters, also called Variable Speed Drives, provide infinitely variable speed control for AC fans. The careful reader knows that the electronic fan speed controllers discussed above do too. So what is the difference? A frequency inverter uses Pulse Width Modulation (PWM), IGBT technology, to adjust the motor voltage and frequency. With PWM, there is an exceptionally quiet motor control in all circumstances and motor voltage that is almost perfectly sinusoidal. Depending on its settings, the frequency inverter itself can also operate very silently.
Because a frequency inverter switches from direct current to alternate current frequently, it can cause EMC pollution in other devices that are connected to the same power grid. To reduce this EMC pollution, (expensive) special filters were developed. A frequency inverter also has a high configuration cost relative to other fan speed controllers because of its inherent complexity. In a nutshell, frequency inverters are more expensive than electronic fan speed controllers, more complicated to configure, and may require some additional tools, but they offer very precise motor control. These speed controllers are very energy efficient and capable of controlling high motor currents.
The requested motor speed can be adjusted via the controls that are integrated onto the device itself (potentiometer or push buttons). It is also possible to adjust the motor speed remotely via Modbus RTU or an analogue control signal (e.g. 0-10 Volt signal).
Electromagnetic pollution
Electromagnetic pollution caused by frequency inverters refers to the unintended electromagnetic interference they may generate, potentially affecting nearby electronic devices and communication systems. This interference can manifest as disruptions, glitches, or malfunctions in radios, TVs, and other sensitive equipment. The risks associated with electromagnetic pollution include compromised performance and reliability of nearby electronic devices, which may be critical in residential or industrial settings.
To avoid these issues, it is crucial to implement mitigation measures. The electromagnetic compatibility (EMC) filters that are standard integrated in our frequency inverters, help to suppress electromagnetic interference, preventing it from radiating into the surrounding environment. Proper grounding and shielding of cables also play a role in minimizing electromagnetic pollution. Installation practices, such as maintaining appropriate distances between sensitive equipment (e.g. data cables, analogue signals, communication cables, etc.) and potential sources of electromagnetic pollution (e.g. power cables, electric motors, frequency inverters, etc.) can further reduce the risk of interference.
What is an electric motor?
An electric motor is a machine that converts electrical energy into mechanical energy. The interaction of a magnetic field and electric current in a coil (motor winding) generates a force (torque) on the motor shaft. A motor has a moving part, the rotor, and a stationary part, the stator. In most classic AC motors, the motor windings (coils) are integrated into the stator. Electric alternating current running through the stator windings generates a rotating magnetic field. The magnetic field of the rotor follows the rotating magnetic stator field. This principle makes an electric motor rotate.
Asynchronous motors usually have a squirrel-cage rotor. The magnetic stator field induces currents in the rotor windings (Faraday’s law of induction). These electric currents in the rotor windings generate the magnetic field of the rotor. Synchronous motors usually have a rotor with permanent magnets. In this case the permanent magnets follow the rotating stator field.
Thermal protection for AC motors
An AC motor is a robust device with a long service life. However, operating an AC motor at low speed for a longer period of time is not without risks. At low speed, the motor cools itself less. This can cause overheating of the motor windings, which can cause degradation of its insulation. This can cause electric leakages, short circuits, and eventually, motor failure. To prevent motor failure, it is important to prevent the motor from being overheated. For this purpose, many AC motors are equipped with thermal contacts, also called TK. These thermal contacts measure the temperature in the motor windings. In case of the motor overheating, the TK contacts open. Some fan speed controllers provide extra protection against overheating via their TK monitoring function, which deactivates the motor in case of overheating to prevent motor damage. At the same time, the alarm output will be enabled to indicate a motor problem.
Why do we need to control fan speed?
A motor at full speed is noisy, consumes much energy, costs money, and exacerbates heat losses. If we decrease fan speed, the motor will make less noise, will consume less energy, and this will, in turn, reduce the operational costs of the ventilation system. All this serves to increase the comfort of residents. Why would we not simply buy a smaller motor if that were the case? A motor needs to be at full capacity, like when there is a large crowd of people in a single room. A motor will also need to run faster when the temperature or relative humidity differs too greatly from the outdoors. In other words, to regulate the Indoor Air Quality, the motor and fan speeds need to be adjusted.
Energy savings - Another advantage of fan speed control is energy savings. If we would not control the fan speed, but instead let the motor run at full speed, there would certainly be a sufficient fresh air supply. But even a slight reduction in fan speed has a major impact on the electrical energy consumption of the fan. A typical HVAC fan follows a quadratic torque curve. Depending on the motor type, a reduction of 25 % air volume flow corresponds with 50 % less energy consumption. In addition, a lower air volume flow rate also results in a quieter operation.
Extended service life - Air filters last longer when reducing the air volume flow rate. This is logical; the more air that passes through the filters, the higher the risk of contamination of the filters. A reduced air volume flow rate also has a positive effect on the service life of the mechanical parts of the fan. These prolonged service intervals reduce the operational costs and the total lifetime cost.
Minimise heat losses - In colder and moderate climates, extracted warm indoor air is replaced by fresh air that can be much colder. That means that if we ventilate, we would need to spend more energy on heating. Modern ventilation systems are equipped with a heat exchanger to minimize such heat losses. Nevertheless, additional energy can be saved by reducing the fan speed when possible. By measuring the air quality of the indoor air, the fan speed can be continuously optimised while the indoor air quality is guaranteed.