Sentera is specialist in HVAC control solutions. A control solution is a way to control a device (for example, a fan). These control systems come in various forms, ranging from very simple to more complex. In this article, we will explain the difference between the two most commonly used control systems in the ventilation industry.
The simplest control system is known and used by everyone: ON-OFF control. An ON-OFF control works like a classic light switch: the system has only two positions, fully on or fully off. In the context of ventilation, this means that either plenty of fresh air is supplied, or none at all. As a result, you often get fluctuations: it gets a bit stuffy before the system starts up, and then it suddenly blows quite hard.
A more sophisticated control system is proportional control. A proportional control system works more like a dimmer or a gas pedal in a car or a water tap. If you want to fill a small glass, you turn the tap on a little. When you want to fill a whole bucket, you turn it on all the way. Instead of just on or off, the system adjusts gradually according to demand. In a ventilation system, proportional control is implemented as follows: If the air is slightly polluted, the system ventilates gently. If more people enter the room and the air quality deteriorates, the system automatically increases the ventilation level. This keeps the indoor climate much more constant and comfortable, without major fluctuations.
In short: an ON-OFF control does all or nothing, whereas a proportional control does exactly what is needed, at the right moment.
ON-OFF control
A major advantage of an ON-OFF control system is that it is very simple and reliable. The system operates according to a clear principle: as soon as a certain limit is reached, it switches on, and when that limit is no longer exceeded, it switches off again. As a result, there is little chance of errors and the operation is easy to understand for both users and installers. In the world of ventilation, on usually means high ventilation speed and off means low or minimal ventilation speed. Ventilation systems are often never completely switched off.
In addition, an ON-OFF control system is cheaper to purchase and install. No complex controllers, frequency inverters, or advanced sensors are required. This makes the system accessible and attractive for smaller installations or applications where the budget is limited.
An ON-OFF system also scores well in terms of maintenance and susceptibility to malfunctions. Because there are fewer components and less complex electronics, the chance of defects is smaller, and any repairs are easier and faster to carry out. Should a malfunction occur nonetheless, it is usually easy to find thanks to the simplicity of the control system.
Furthermore, an ON-OFF control system is more than sufficient in many situations. In applications where the ventilation requirement is clearly present or absent—such as in a toilet, a garage, or a sporadically used room—it is perfectly sufficient for the system to simply switch on when needed and then switch off again. In such cases, simplicity often outweighs the benefits of a more sophisticated control system.
Finally, an ON-OFF control system also offers a certain robustness. It requires no fine-tuning or complex settings and generally continues to function well without much need for adjustment. This makes it particularly suitable for situations where ease of use and reliability are more important than highly precise control.
A typical application is the detection of carbon monoxide (CO) in parking garages. As soon as a dangerous CO limit is exceeded, it is advisable to switch the ventilation on at full power immediately to neutralize the dangerous situation as quickly as possible by supplying a large amount of fresh air. When the CO concentration is back below the alarm level, the ventilation can switch back to minimum speed.
Another example is to prevent frost damage to the filter system of outdoor swimming pools. When the temperature drops below 5 °C, a circulation pump can be activated to prevent frost damage. As long as there is a risk of frost damage, the pump continues to operate. When the temperature rises back above the set threshold value (5 °C), the pump stops.
ON-OFF control for Motors
Electric motors consume a lot of energy. The more powerful they are, the more energy they consume and the more airflow they can generate. With AC motors, the motor's energy (electrical current) is supplied by a fan speed controller or a frequency inverter. EC motors have built-in speed control. In both cases, a distinction must be made between electric power and electrical control signals. The control signal is the start/stop command received by the speed controller (or the electronics of the EC motor). Electrical power is the energy a motor needs to rotate. The motor converts this electrical energy into mechanical energy. Electronic control signals are small electrical currents used to control the motor. They indicate when the motor should start or stop, how fast the motor should rotate, etc.
An electric motor contains a coil made of windings. In technical terms, this is an inductive load. One of the basic laws of physics states that an electric current flowing through a winding always wants to maintain itself. Interrupting an electric current in an inductive load is therefore not so easy. Especially with larger electric motors (larger electrical currents), special switching equipment is required for this. This equipment switches the motor currents - the large amount of energy that the motor needs to operate. This switching equipment is controlled by small electronic control signals.
Switching a motor on or off therefore requires a control signal that can be high (motor on) or low (motor off). Switching the motor on and off is therefore done via a digital control signal, often a potential-free contact or a digital input of the fan speed controller. When this contact is closed, the speed controller receives a start command and the fan starts rotating. When the contact is opened, the motor enters stop or standby mode. There are also fan speed controllers where the digital input is used to switch between high and low fan speed.
The following fan speed controllers feature a digital input:
- Transformer fan speed controllers for 5-step speed control. Following fan speed controllers feature a digital input to switch the motor remotely ON-OFF:
- STRA series - the allround fan speed controller! STRA1 series control 1ph motors and STRA4 series control 3ph motors. These fan speed controllers have a digital input, an alarm output and an unregulated output. When the digital input is on, the motor starts running. The unregulated output is active when the motor runs. In case motor overheating is detected, the alarm output is activated. After a power failure, the motor will automatically restart.
- SFPR series - the ideal fan speed controller for industrial kitchen hoods! SFPR1 series control 1ph motors and SFPR4 series control 3ph motors. They feature an extra output to control the gas valve. The fan is activated via the thermostat input; when the temperature setpoint is exceeded, the fan is activated. An air flow sensor (pressure relay) is required to detect the airflow. The gas valve output is activated simultaneously with the fan. In case air flow is not detected within 60 seconds after the motor is started, the gas valve output is deactivated for security reasons. After a power failure, the motor will automatically restart.
- STTA4 series are fan speed controllers with thermomagnetic circuit breaker. This provides protection against overload. They control 3ph motors. The digital input can be used for remote start-stop commands. After a power failure, the motor will automatically restart.
- Transformer fan speed controllers for 5-step speed control. Following fan speed controllers feature a digital input to switch between Low and High speed:
- SC2-1 series have a digital input to switch between two motor speeds. Both speeds can be selected via two knobs on the front panel. SC2-1 series control 1ph motors.
- SC2A series have a digital input to switch between two motor speeds. Both speeds can be selected via two knobs on the front panel. On top of that, they also have a TK input to monitor motor temperature. In case motor overheating is detected, the system is stopped and the alarm is activated. SC2A1 series control 1ph motors, SC2A4 series control 3ph motors.
- SER-1 series have an emergency button to activate smoke extraction (full speed). They control 1ph motors. The smoke extraction can also be activated via a digital input.
- Electronic fan speed controllers for variable speed control. Following fan speed controllers feature a digital input to switch the motor remotely ON-OFF:
- ITRS9 series are variable fan speed controllers for 1ph motors. The motor accelerates in kick start or soft start mode. The motor can be enabled via the integrated ON-OFF switch or via the digital input. The TK monitoring function deactivates the motor in case motor overheating is detected. In that case, the alarm is activated to indicate motor problems.
- ITRS9 series are variable fan speed controllers for 1ph motors. The motor accelerates in kick start or soft start mode. The motor can be enabled via the integrated ON-OFF switch or via the digital input. The TK monitoring function deactivates the motor in case motor overheating is detected. In that case, the alarm is activated to indicate motor problems.
- Frequency inverters for variable speed control.
- They feature multiple digital inputs and many control options. A simple start-stop command can be given via input 1. Models for wall-mounting and for DIN-rail mounting are available. FI-E11 series control 1ph motors, FI-E44 series control 3 ph motors.
Proportional Control
The main advantage of proportional control is that it continuously adapts to actual demand. Instead of simply switching completely on or off, the system adjusts gradually. This means that exactly the amount of ventilation needed at that moment is provided at all times. As a result, air quality remains much more consistent, and you avoid large fluctuations such as suddenly feeling stuffy or experiencing excessive drafts.
Proportional control ensures a higher level of comfort. Because the system does not switch on or off abruptly, but rather adjusts smoothly, users experience less disturbance from noise or air currents. The ventilation operates in the background without really being noticeable, while the room remains pleasant.
An additional benefit is energy efficiency. Because the system only works harder when truly necessary, less energy is consumed than with a system that regularly runs at full power. This applies to both the electricity consumption of the fan and the reduction of heat loss due to excessive ventilation. In the long term, this can result in noticeable savings. Furthermore, proportional control contributes to a longer lifespan of the installation. Because the system has to switch abruptly between on and off less frequently, components such as motors and switches are subjected to less stress. This reduces wear and lowers the risk of defects.
Proportional control is therefore particularly suitable for environments where conditions change constantly, such as offices, classrooms, or homes with fluctuating occupancy. In such situations, it ensures that the system automatically adapts to usage without the need for human intervention. As a result, it combines comfort, efficiency, and ease of use in a single solution.
A typical application is measuring the CO2 concentration in the room using a CO2 sensor with an analogue output signal. With this analogue output signal, the ventilation system can be controlled proportionally. As more people share an enclosed space, the CO2 concentration will increase rapidly. This will cause the analogue output signal of the sensor to rise. The ventilation speed will subsequently increase. Thanks to the extra fresh air supply, the CO2 level in the room will drop again, and a balance will be established, ensuring that the air quality remains good.
A similar application is controlling the ventilation with a relative humidity sensor. If a room sensor cannot be used, an air duct sensor can be employed. This sensor is best installed in the exhaust duct, as it allows for the measurement of air quality within the room. As the relative humidity measurement increases, the sensor's analogue signal will rise and the fan speed will increase. The additional ventilation will cause the relative humidity in the room to decrease until a balance is reached.
Proportional control for motors
Just like with the ON-OFF control, switching the motor on and off is done via an ON-OFF signal connected to the digital input of the fan speed controller. When this contact is closed, the speed controller receives a start command and the fan starts rotating. When the contact is opened, the motor enters stop or standby mode. This is the start/stop control described in the previous chapter.
Now we take it a step further. When the fan is running, it can rotate slowly or quickly. Additionally, the speed can also be controlled. Therefore, in addition to the start/stop command, an extra control signal will be required. In this case, a proportional or analogue control signal. This analogue signal will indicate to the fan speed controller (or the EC motor) how fast it should make the motor rotate. An analogue signal is a continuous electrical signal that represents varying levels between its minimum and maximum value. Analogue signals are used to transmit a value from one device to another. You can think of it as a kind of communication language between two devices.
There are many types of analogue control signals: a 0–10 Volt signal, 0-20 mA, a PWM signal, or a communication protocol such as Modbus RTU are often used to set the motor speed. They all have their own advantages and disadvantages. Here is an example of a 0-10 Volt control signal: When the control signal is 0 Volts, the motor will run at minimum speed; when the control signal is 10 Volts, the motor will run at maximum speed. The minimum and maximum speeds can usually be defined in the speed controller. The analogue control signal tells the speed controller how fast the motor should run. In addition to the analogue control signal, a start-stop command is usually provided via a separate digital control signal. In this way, the motor can be started, its speed regulated, and stopped.
We make a distinction between power and control signals. The supply voltage (the power) therefore remains continuously connected to the speed controller or EC motor. The digital control signal determines whether power is sent to the motor. The analogue control signal determines how much power is sent to the motor and how fast the motor will rotate.
The following fan speed controllers require an analogue control signal:
- Transformer fan speed controllers for 5-step speed control:
- STVS series are 5-step fan speed controllers with analogue input. The 0-10 Volt control signal activates one of the five steps. When the control signal is lower than 2 Volt, the motor stops. STVS1 series control 1ph motors and STVS4 series control 3ph motors.
- STVS series are 5-step fan speed controllers with analogue input. The 0-10 Volt control signal activates one of the five steps. When the control signal is lower than 2 Volt, the motor stops. STVS1 series control 1ph motors and STVS4 series control 3ph motors.
- Electronic fan speed controllers for variable speed control:
- EVS-1 series are variable speed controllers for 1ph motors. They feature a digital input to remotely start and stop the motor. Via the analogue input, fan speed can be adjusted. The enclosure is designed for wall-mounting.
- EVSS1 series are variable speed controllers for 1ph motors. Just like EVS1- series, they feature a digital input to remotely start and stop the motor. Via the analogue input, fan speed can be adjusted. On top of that, they also have a TK input to monitor motor temperature. In case motor overheating is detected, the system is stopped and the alarm is activated. The enclosure is designed for wall-mounting.
- MVS-1 series are variable speed controllers for 1ph motors. They feature a digital input to remotely start and stop the motor. Via the analogue input, fan speed can be adjusted. The enclosure is designed for DIN-rail mounting in an electrical cabinet.
- MVSS1 series are variable speed controllers for 1ph motors. Just like EVS1- series, they feature a digital input to remotely start and stop the motor. Via the analogue input, fan speed can be adjusted. On top of that, they also have a TK input to monitor motor temperature. In case motor overheating is detected, the system is stopped and the alarm is activated. The enclosure is designed for DIN-rail mounting in an electrical cabinet.
- TVSS5 series are variable speed controllers for 3ph motors. They feature a digital input to remotely start and stop the motor. Via the analogue input, fan speed can be adjusted. On top of that, they also have a TK input to monitor motor temperature. In case motor overheating is detected, the system is stopped and the alarm is activated. The enclosure is designed for DIN-rail mounting in an electrical cabinet.
- Frequency inverters
- Frequency inverters are variable speed controllers. They feature multiple digital inputs and many control options. Via the analogue input, motor speed can be adjusted. Models for wall-mounting and for DIN-rail mounting are available. FI-E11 series control 1ph motors, FI-E44 series control 3 ph motors.
How to create a control signal?
A digital control signal can be created by a simple switch or a relay. A switch can be used to manually switch a device on or off. A relay is a switch that is controlled by electricity instead of your hand. When a small electrical signal is sent to the relay, it turns another circuit on or off. A sensor with a relay output will switch the relay when a certain value (e.g. temperature, CO value, etc.) is reached. In this way, the sensor can, for example, start a fan when a certain CO level is reached. The fan will continue to run at maximum speed until the CO levels have normalized again. When the CO concentration drops back below the threshold value, the relay will switch off and the fan will stop.
An analogue control signal is used to set the fan speed. It is more than just on or off. With an analogue signal, you can define a number or value. An analogue signal usually comes from a sensor or from a potentiometer. A potentiometer can be used to manually adjust the fan speed. By turning the knob, you create a 0-10 Volt signal to control the fan speed. Sensors typically measure temperature, relative humidity, CO2, or air quality. A sensor converts the measurement into an analogue signal - by default a 0-10 Volt signal.
The combination of a sensor and a fan speed controller makes it possible to regulate fan speed demand-based. Demand-based ventilation control adjusts the amount of fresh air supplied to a space according to actual needs, such as the number of people present or the level of air pollution. Its main advantage is energy efficiency, because the system only increases ventilation when necessary instead of running at full capacity all the time. This leads to lower operating costs, since fans, heating, and cooling systems do not have to work as much during periods of low demand. At the same time, it improves indoor air quality by providing more fresh air when pollutant or carbon dioxide levels rise. It also helps extend the lifespan of the equipment, as the system is subjected to less continuous strain. Overall, demand-based ventilation control creates a more efficient, cost-effective, and comfortable indoor environment by responding to real conditions rather than operating at a constant level.
The following sensors have an analogue output and can be used to control fan speed:
How to do ON-OFF control with a 0-10 Volt signal?
This question may arise when a sensor with a 0-10 Volt output is installed and the fan needs to be controlled ON-OFF. In this instance, the ARM series offer the solution. The ARM module converts an analogue signal into a relay output. Hence the name: Analogue to Relay Module. The analogue signal from a sensor can be connected to the ARM module. The switching point can be set. As soon as this switching point is reached, the relay output of the ARM module switches.
An example: the minimum temperature that can be measured is 0 °C, the maximum temperature is 50 °C. The analogue output signal of the temperature sensor will vary between 0 and 10 Volts, where 0 Volts corresponds to 0 °C and 10 Volts to 50 °C. When the measured temperature is 25 °C, the analogue output signal will therefore be 5 Volts. In short, the analogue signal follows a linear curve; it increases proportionally as the temperature rises.
A typical application is switching on the cooling. In this example, cooling is only required when the temperature rises above 25 °C. This means that the cooling must be switched on the moment the temperature reaches 25 °C. If the switching point of the ARM module is set to 5 Volts (25 °C), the relay contact will switch when this temperature is reached. The cooling can be switched on via the relay contact.