Frequency inverters for energy efficient fresh air supply
A 'frequency inverter‘, also known as a 'Variable Frequency Drive or VFD‘, is an electronic device that controls the speed and torque of an electric AC motor by varying the voltage and frequency of its power supply. It allows precise adjustment of motor speed, making it ideal for applications like fan speed control where airflow needs to be regulated efficiently. Using a frequency inverter not only improves performance but also reduces energy consumption and wear on mechanical components. In particular, the combination of a frequency controller with HVAC sensors offers many possibilities to increase the energy efficiency of a ventilation system by applying demand-controlled ventilation. With demand-controlled ventilation, the fan speed is continuously optimized to always have just enough fresh air supply. As soon as the HVAC sensors indicate that the air quality is decreasing, the fan speed will be increased to supply more fresh air. When the air quality is good enough, the fan speed will be reduced again. In this way, the ventilation system can save energy while continuously providing sufficient fresh air.
Electric motors convert electrical energy into motionBefore discussing a variable frequency drive in detail, some information about an electric motor is first needed. An electric motor is a machine that converts electrical energy into motion (also called kinetic energy). The motor primarily converts electrical energy into rotational motion of the motor shaft. A motor shaft is the part of an electric motor that spins when the motor is running. You can think of it like the axle of a wheel — it's the part that transfers the motor’s turning power to whatever it's driving, like a fan blade or a pump.
The motor speed can be regulated using a speed controller. The number of different motor types is countless, but roughly we can distinguish between AC motors and EC motors. EC motors always have a built-in speed controller, but for AC motors, an external speed controller can be provided. There are several types of speed controllers: transformer controllers, TRIAC controllers, and frequency inverters. Each type uses different technology to control motor speed. Each type has its own advantages and disadvantages. Since a frequency controller is used to control an AC motor, first some additional information about this.
AC motor: electric currents and magnetism
Electric motors work based on the interaction between magnetism and electric currents. In addition to electrical energy, magnetism is also required. Synchronous AC motors use permanent magnets, while asynchronous AC motors generate their own magnetic fields using induction (also an interplay between magnetism and electricity).
In the stator – the part of the motor that doesn’t move – a coil is installed. When alternating current flows through this coil, a magnetic field is generated. Since the current is alternating, the polarity of the magnetic field constantly changes. It appears as if this magnetic field continuously rotates in circles. This happens at the same rate as the frequency of the alternating voltage. The rotor – the rotating part of the motor – follows this changing magnetic field. The rotor of synchronous motors consists of permanent magnets. The rotor of asynchronous motors have a squirrel-cage design – it looks a bit like a metal wheel with thick bars running along its length, connected at both ends by rings — kind of like a hamster wheel made of metal. When this squirrel-caged rotor is placed in a moving magnetic field, an electric current is induced, which in turn creates a magnetic field.
All these magnetic fields and electric currents generate heat (energy that is lost in heat). Heat is therefore the biggest enemy of the robust AC motor. In case of overheating, there is a risk of damage due to internal short circuit. Detecting overheating in time is therefore very important for an AC motor. Some versions are equipped with temperature sensors inside the motor (TK or PTC). These can be read by some speed controllers to stop the motor in time in case of overheating and prevent motor damage.
Technical details of the AC motor
In order to select the correct type of frequency converter for a particular motor, it is necessary to know the following data:
- Supply voltage - The electricity that an AC motor needs to operate is called the supply voltage. It is expressed in [VAC]. It can be supplied in either single-phase or three-phase form. The following options are available via the public electricity grid: 1-phase 230 VAC / 3-phase 230 VAC / 3-phase 400 VAC.
- Current consumption - The amount of energy that is consumed by the motor. The amount of electrical current used by the motor is expressed in Ampère or [A]. The amount of current drawn increases as the motor speed increases or as the load increases (such as larger fan blades). The maximum drawn current is usually listed on the motor’s technical label.
- Motor power - The combination of the supplied voltage, the (maximum) drawn current and the efficiency of the motor is referred to as the motor power. This is expressed in Watts or kilowatts. This is also typically listed on the motor’s technical label.
Next to these essential information, there is usually more information available on the technical label of the motor. The rotational speed of the motor shaft is expressed in revolutions per minute [rpm]. The torque, or the force that the motor shaft can deliver, is expressed in newton meters [Nm]. In a practical example: a tractor has a motor with low speed but high torque. That’s why a tractor moves slowly but can pull enormous loads across a field.
A Formula One car has a motor with very high speed (many rpm) but lower torque than a tractor. This is ideal, since the race car is very light and therefore requires relatively little force.
Different types of speed controllers
As already mentioned, there are several types of speed controllers. Each type uses different technology with corresponding advantages and disadvantages. However, the frequency inverter stands out because it is more advanced. Simple speed controllers only reduce the motor voltage. A frequency inverter, however, does much more...
Transformer controllers and TRIAC controllers reduce the motor speed by lowering the voltage sent to the motor. Lower motor voltage results in lower speed. Transformer controllers reduce voltage in steps (typically 5 steps). Electronic speed controllers offer variable speed control. The big advantage of both types of speed controllers is their simplicity in wiring and commissioning. Once the motor is connected, the controller can be used immediately. No configuration is needed.
A frequency inverter also regulates the motor speed continuously (just like the TRIAC controller). However, its regulation is more complex than that of a TRIAC controller (more on this shortly). This more advanced regulation requires additional configuration. After connecting, some settings generally need to be adjusted in the frequency inverter before it can be used. Thanks to additional inputs and outputs on the device, many extra logic functions and features can be added.
Frequency inverter vs TRIAC controller
So how does a frequency inverter differ from a TRIAC controller? A frequency inverter not only changes the motor voltage but also the frequency! A TRIAC controller chops segments out of the supplied voltage, but does not change the frequency – it remains at 50 Hz. This results in lower torque (less force). The motor still tries to run at maximum speed, because the frequency is still at its maximum. The frequency creates the rotating magnetic field responsible for motor speed. Lowering the motor voltage without changing the frequency creates a risk that the motor will stall. When you lower the voltage to a motor without changing the frequency, the motor gets weaker because it produces less torque — which is the force that keeps it turning. If the torque drops too much, the motor may not be able to overcome the resistance from the load, like the blades of a fan, and it can stall or stop spinning. That’s why simply reducing voltage (as TRIAC controllers do) can be risky, especially at low speeds, compared to frequency inverters that adjust both voltage and frequency together to keep the motor running smoothly.
A frequency inverter keeps the ratio between voltage and frequency constant (U/f = constant). This ensures the motor always receives an optimized voltage (less energy consumption!). When the motor voltage is reduced, the frequency is also reduced. This causes the motor to rotate more slowly while still maintaining nearly its full torque. When motor speed is controlled by a frequency inverter, the motor remains powerful even at lower speeds. The risk of stalling at low speeds is significantly lower here.
Due to the non-perfectly sinusoidal motor voltage sent to the motor by TRIAC controllers, the motor can be noisy, especially at lower speeds. The frequency inverter creates an almost perfect sinusoidal voltage thanks to the PWM technology, which allows both the inverter and the motor to operate completely silently.
How does a frequency inverter work?
From a technical standpoint, frequency inverters can be divided into three functional blocks:
- The Rectifier – This is where the supplied alternating voltage (single-phase or three-phase AC) is converted into direct current (DC).
- The DC Bus – This module acts as an energy reservoir. The DC bus can be viewed as a large internal battery within the frequency inverter.
- The Inverter Stage – Here, the DC voltage is converted back into alternating voltage (single-phase or three-phase). The technology used for this conversion is PWM, or Pulse Width Modulation. IGBTs (insulated-gate bipolar transistors) allow current to flow momentarily in rapid succession (you can think of them like light switches that turn on and off extremely quickly). The combination of all these short pulses produces an almost perfect sine wave voltage. IGBTs are much faster than TRIACs and can switch much higher currents. However, they are also more expensive than TRIACs.
The big difference between a frequency inverter and a transformer controller and TRIAC controller is the fact that the frequency inverter first converts the supplied energy to DC voltage and then converts it back to alternating voltage. TRIAC and transformer controllers only reduce the supplied AC voltage.
Electro Magnetic Compatibility or EMC
EMC stands for Electromagnetic Compatibility. Every frequency inverter uses IGBT’s (the high-speed electronic switches) to regulate motor speed. While these switches are highly efficient, they also generate electrical noise—also known as ElectroMagnetic Interference (EMI)—which can travel back into the building’s power grid. TRIAC controllers and transformer controllers produce much less EMI than a frequency controller because they switch at a much slower rate. That’s why the EMC filter plays a crucial role in keeping your building’s electrical environment stable when frequency inverters are installed in the building.
EMI interference doesn’t cause any physical noise you can hear, but it can disrupt the operation of other sensitive electronic devices in the building. Systems like fire alarms, lighting controls, communication networks, and office equipment can all be affected by this invisible disturbance. That’s where the EMC filter comes in. The EMC filter acts as a protective barrier, filtering out the electrical noise generated by the inverter and preventing it from spreading through the power supply. In essence, the EMC filter ensures that the inverter operates without disturbing other equipment in the building. Installing an EMC filter isn’t just a good idea—it’s often a requirement. In commercial, industrial, or multi-use buildings, regulations typically mandate the use of EMC filters when frequency inverters are installed. This helps ensure compliance with electrical safety standards while also maintaining the reliability of all other electronic systems in the facility.
Product range of frequency inverters
Sentera is a distributor of Invertek frequency controllers for HVAC applications. The Optidrive E3 series are known for their ease of use, excellent quality and standard settings that have already been optimized for HVAC applications. This simplifies commissioning and configuration. All devices are equipped with built-in EMC filter category C1 according to EN61800-3:2004. Our product range of frequency inverters consists of three variants:
- Frequency inverters -E2 for installation in an electrical cabinet with terminal strips for connecting external control signals. These frequency inverters are equipped with the standard operating panel (5 push buttons and a 7-segment LED display). External start-stop commands and 0-10 Volt speed reference signals can be connected via the terminal block. The frequency inverter uses these external control signals to know how to control the motor.
The housing of the -E2 devices offers an IP20 protection degree against ingress of moisture and dust. We strongly recommend to install these devices in an electrical cabinet with sufficient ventilation and cooling to guarantee a good dissipation of heat. - Frequency inverters -E6-19 for outdoor installation with terminal strips for connecting external control signals. These frequency inverters are equipped with the standard operating panel (5 push buttons and a 7-segment LED display). External start-stop commands and 0-10 Volt speed reference signals can be connected via the terminal block. The frequency inverter uses these external control signals to know how to control the motor.
The housing of the -E6-19 devices offers an IP66 protection degree against ingress of water and dirt. Thanks to this sturdy housing, they can simply be installed outdoors nearby the motor. They are dust tight and ready for washdown duty thanks to the sealed ABS enclosure and corrosion resistant heatsink. The tough polycarbonate plastic enclosure is designed to withstand degradation by ultra violet (UV), greases, oils and acids. Also robust enough not to be brittle at -20°C. It is recommended to protect the device from direct rain and sunshine. - Frequency inverters -E6-19 for outdoor installation with built-in control buttons. These frequency inverters are equipped with a built in potentiometer for speed adjustment, a 3-position switch for Run reverse – OFF – Run forward command and a lockable mains disconnect switch.
The housing of the -E6-19 devices offers an IP66 protection degree against ingress of water and dirt. Thanks to this sturdy housing, they can simply be installed outdoors nearby the motor. They are dust tight and ready for washdown duty thanks to the sealed ABS enclosure and corrosion resistant heatsink. The tough polycarbonate plastic enclosure is designed to withstand degradation by ultra violet (UV), greases, oils and acids. Also robust enough not to be brittle at -20°C. It is recommended to protect the device from direct rain and sunshine.
with terminal strips for connecting external control signals. These frequency inverters are equipped with the standard operating panel (5 push buttons and a 7-segment LED display). External start-stop commands and 0-10 Volt speed reference signals can be connected via the terminal block. The frequency inverter uses these external control signals to know how to control the motor.
These frequency inverters are equipped with the standard operating panel (5 push buttons and a 7-segment LED display). External start-stop commands and 0-10 Volt speed reference signals can be connected via the terminal block. The frequency inverter uses these external control signals to know how to control the motor.
These frequency inverters are equipped with a built in potentiometer for speed adjustment, a 3-position switch for Run reverse – OFF – Run forward command and a lockable mains disconnect switch. How to select the correct frequency inverter for your application?
After the above choice has been made, a selection must also be made based on the motor technical side. To select the correct device for your application, you will need the following information:
- What is the available supply voltage on site?
Typical available supply voltages are: 1ph 230 Volt supply, 3ph 230 Volt supply or 3ph 400 Volt supply. - What voltage does the motor require? (This information can be found on the technical label of the motor.)
AC motors are typically available in following voltages: 1ph 230 VAC, 3 ph 230 VAC or 3ph 400 VAC - What is the motor current? This information is also stated on the technical label of the motor and is expressed in [A]
The current that the frequency inverter can deliver, must be higher than the motor current. In case multiple motors are controlled with one frequency inverter, the combined sum of all motor currents (plus some margin) must be lower than the maximum current of the frequency inverter.
Usually the current [A] and power [kW] indications on the motor and the frequency inverter will match. In case of doubt, it is advisable to choose a type of frequency inverter that can supply more current than the maximum motor current.