What is a potentiometer?
The word potentiometer is on the one hand the name of an electronic component. On the other hand it is also the name for a speed controller for EC motors. In such a speed controller the electronic component is usually used. In both cases, the word 'potentiometer' indicates that something can be adjusted infinitely variable.
Speed controller for EC motors
An EC motor can be seen as the combination of an AC motor with a built-in speed controller (see also the article AC versus EC motors). This built-in speed controller needs information about the desired motor speed. A potentiometer is one of the possibilities to communicate the desired motor speed to the EC motor. That is why a potentiometer is sometimes also described as a speed controller for EC motors. The real speed controller is in fact integrated into the EC motor, while the potentiometer is the device with which the desired speed can be manually set. Using a potentiometer, the speed of an EC motor can be adjusted infinitely variable.
But how does that work? How can the potentiometer inform the EC motor how fast it should turn? Simple: via an electrical signal. In technical jargon, this is called an analogue signal. This means that this electrical signal can be set continuously variable between the minimum and maximum value. The most commonly used analogue signal is 0 to 10 Volt. It can vary between 0 Volt and 10 Volt.
In other words, the potentiometer is a device that translates the position of the rotary knob into an analogue signal (e.g. 0-10 Volt). This analogue signal can be used to control another device. The number of examples is endless, but in the HVAC world analogue signals are widely used to control EC motors, variable speed controllers, valve blade positioning, setting the desired temperature, etc. We will keep on using the example of controlling the EC motor in this article. In this example, the fan would stand still if the control signal is 0 Volt. When the control signal (infinitely variable) increases to 10 Volt, the fan will accelerate (infinitely variable) to the maximum speed, which is reached at 10 Volt.
Different types of analogue signals
In practice, there are many different types of analogue signals, each with their own advantages and disadvantages. The device that is to be controlled by the analogue signal determines which signal type is required. In some cases, there are multiple options.
Here we list the most commonly used analogue signals:
- Voltage signals (e.g. 0-10 Volt): These analogue signals use a different voltage or potential to transmit the information. The EC motor will detect the voltage level of the analogue signal and determine the desired motor speed based on that. This form of analogue signal is very popular because the value of the signal can easily be measured with a Voltmeter. This makes the trouble shooting much easier.
The disadvantage is that the cable length must remain limited. Due to the electrical resistance of cables, there will be a voltage drop with longer cable lengths (10 Volt at the beginning of the cable will no longer be 10 Volt at the end of the cable). This results in lower accuracy. In the example of the EC motor, it will be impossible to reach the maximum fan speed if the analogue signal cable between potentiometer and EC motor is too long. The reason is that the analogue control signal cannot reach it's maximum value of 10 Volt due to the voltage drop in the long cable. - Electrical resistance (e.g. 0 to 10 kΩ): This is the most well-known way to communicate a value in the world of electronics. By the way, a potentiometer is also an electronic component with variable resistance value - more about this later in this article. Back to our example with the EC motor. The EC motor will determine the desired motor speed based on the resistance value of the analogue signal. Here too, a longer cable length between the potentiometer and the EC motor will result in reduced accuracy due to the increasing electrical resistance of the cable. If the cable length between both devices can be kept short, this is a simple and cost-effective solution.
- Current signals (e.g. 4-20 mA): Analogue signals that vary the electrical current to communicate a value. The EC motor will determine the motor speed based on the current of the analogue signal. The more mA detected, the higher the motor speed. In this example, 20 mA corresponds with the maximum motor speed.
The big advantage here is that no accuracy is lost in case of increasing cable length. The increased electrical resistance of the cable will be compensated by the analog signal and the desired current will be achieved. A cable break can also be detected (0 mA can only occur in case of a cable break, since the minimum value of the analogue signal is 4 mA). Detecting possible errors is more complex because current is more difficult to measure than voltage. - Frequency signals (e.g. Pulse-Width Modulation or PWM): This type of analogue signal is also called a pulse train. It is a constant series of pulses with identical amplitude (voltage). The difference is in the frequency and width of the pulses. The EC motor receives a constant series of electrical pulses. The motor speed is determined based on the frequency and duration of the pulses. This form of analog signal is not sensitive to increasing electrical resistance or voltage drops due to longer cable lengths. More advanced electronics are needed to correctly interpret the pulse train and detecting possible errors is also less easy.
In the end, all these analogue signals do the same: they transmit or communicate a certain value between different devices. The difference between these analogue signal types can be seen as communicating the same message in a different language.
In a summary: Voltage signals and electrical resistance are simple and suitable for shorter distances, current and frequency signals are more complex and more suitable for longer distances.
The electronic component 'Potentiometer'
A potentiometer is a three-terminal electronic component that acts as a variable resistor or voltage divider. It consists of a resistive element, a sliding or rotating contact (called a wiper), and three terminals: Two fixed terminals are connected to the ends of the resistive element. One variable terminal (the wiper) slides or rotates along the resistive element to vary the resistance and, consequently, the voltage output.
When a voltage is applied across the two fixed terminals, the wiper divides the voltage based on its position along the resistive element. Moving the wiper changes the resistance in one segment of the circuit while simultaneously altering the resistance in the other segment. This adjusts the voltage between the wiper and one of the fixed terminals.
A potentiometer is often used in an electronic circuit to allow the user to easily adjust a certain value. For example, to set the volume of the radio.
The Sentera product range of potentiometers and control switches
EC fan controllers for continuously variable fan speed control
A potentiometer is typically used to control the speed of EC motors in the HVAC business. That is why it is also referred to as EC fan speed controller or EC fan controllers. The potentiometer generates a control signal (typically 0-10 Volt). This control signal provides information to another device (e.g. fan speed controller). In this example, the potentiometer ‘informs’ the fan speed controller about the requested fan speed via the control signal. An analogue signal can represent a certain value (For example: 8 Volt = 80%). This value lies within a range (0-10 Volt or 0 - 100%). Potentiometers or EC fan speed controllers generate a continuous variable control signal that can be used to define the requested fan speed.
The Sentera product range includes three groups of EC fan speed controllers. These groups are divided according to the supply voltage that the potentiometer needs to function:
1. Low supply voltage
These potentiometers are extremely suitable for combination with EC motors that provide a supply voltage of 10 Volt DC (or similar). It offers the possibility to connect both the supply voltage and the analogue control signal via one cable.
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- SDP-E0US series require a supply voltage in the range of 5 to 24 VDC. The output type can be adjusted by changing the position of a jumper. The minimum and maximum value of the analogue output signal can be adjusted via two trimmers. The jumper and both trimmers can be found behind the front panel of the potentiometer, where the wires are connected.
- SDP-M010 series require a supply voltage of 24 VDC. Via the knob on the front panel, the analogue output signal can be set. If necessary, this knob can be overruled by the Modbus RTU communication. If overruling via Modbus RTU communication is active, the analogue output signal will follow the information in the corresponding Modbus holding register. The knob on the front panel is deactivated during the overruling. Next to adjusting the analogue output signal, all potentiometer settings can be adjusted via Modbus RTU communication. A typical application is to overrule the knob on the front panel during certain moments of the day. For example in a school building. EC fan speed can then be set remotely (via the BMS system or a central computer) while the knob on the front panel is disabled.
- MTP-D010 series require a supply voltage in the range of 3 to 15 VDC. These potentiometers still come in the classic enclosure type. The analogue output signal can be set between 10 % and 100 % of the supplied voltage. E.g. if this potentiometer is connected to a supply voltage of 10 VDC, the analogue output signal can be set in the range of 1 to 10 VDC. If the fan speed is too high at its maximum value, it can be reduced towards 1 to 8 Volt for example.
2. 230 VAC supply voltage
These potentiometers require a supply voltage of 230 VAC. The analogue signal can be connected via a separate cable. Power cables (230 VAC) and control signal cables must always be separated to prevent interference. These potentiometers were developed to generate an analogue signal for devices that do not provide a 10 Volt DC (or similar) supply voltage for the potentiometer.
3. Unpowered potentiometers 10 kOhm
These potentiometers do not require a power supply. They offer a variable resistance value in the range of 0 to 10 kilo Ohms (0 to 10,000 Ohms). This makes it possible to connect these potentiometers with a three wire cable. The only difference between the products in this group is their enclosure.
Control switches for EC motors or damper actuators
These devices regulate EC fan speed in steps. The potentiometers mentioned above generate a continuously variable signal. However, there are certain applications where the user wants to regulate fan speed in a few steps from minimum to maximum, not continuously variable. For these applications, Sentera control switches can be used. Control switches generate a control signal in 3 steps. They divide the analogue 0-10 Volt signal in three (adjustable) steps. This makes it possible to adjust the fan speed in three steps.
Control switches for AC motors with multiple windings
A very specific group of AC motors has a similar operation. These are 3-speed motors that are used in ceiling fans, for example. This group of control switches is designed to control AC motors with 3 separate motor windings. Each winding gives the motor a different speed. When winding one is energized, the motor starts to turn slowly. When winding two is energized, the motor turns a little faster. When the third winding is energized, the motor runs at full speed. To control these type of AC motors, a mechanical switch is needed that connects the 230 VAC supply voltage to one of the three motor windings. Just to be clear, this group of control switches has nothing to do with analogue signals.