Transformer fan speed controllers

09/12/2024 Yves Vinck

How does a transformer fan speed controller work

Transformer controllers regulate the speed of fans with AC motors in steps by reducing the motor voltage. This stepped speed control is accomplished by the electrical transformer they employ, hence the name 'transformer controller'. Transformer fan speed controllers are cost-effective and have proven to be very reliable and robust. They can also be used in situations where the power supply is unstable. Transformer speed controllers are typically used to regulate fan speed. Most customers are willing to accept the disadvantage of a slightly lower energy efficiency because the advantage of user-friendliness is more important to them. A transformer speed controller is one of the simplest methods to control the speed of an electric motor. Both connection and commissioning are particularly straightforward.

Quiet motor operation

These types of fan speed controllers are easy to install. They require no configuration and can be used immediately upon connection. Thanks to transformer technology, they generate a motor voltage with a perfect sinusoidal shape, resulting in exceptionally quiet motor operation and an extended service life. More detailed information about transformer technology is provided below. The perfectly sinusoidal motor voltage is the major advantage compared to electronic TRIAC controllers. A TRIAC controller cuts away pieces of the sinusoidal voltage, while a transformer speed controller maintains the sinusoidal shape but reduces it.

Humming noise of the electrical transformer

In a transformer, the alternating current creates a constantly changing magnetic field, causing the iron core to vibrate at a high frequency, which we perceive as a humming noise. These magnetic fields can cause small movements within the transformer itself. Loose coils, laminations (sheets) in the core, or even the transformer's casing can vibrate slightly, causing a humming sound. It is important to note that some humming is normal for transformers. However, an unusually loud hum can indicate a problem, such as loose parts, overloading, or malfunctioning components. Sentera transformers receive a special impregnated coating that reduces electrical noise from the transformers. Because of this humming noise, we recommend always installing the transformer fan speed controller in a technical room where this noise is disturbing.

Regulating fan speed by reducing the motor voltage

Transformer fan speed controllers regulate fan speed by reducing the motor voltage in steps. TRIAC or electronic fan speed controllers also regulate the motor speed by reducing the motor voltage. The difference is that transformer speed controllers do this in steps, while TRIAC controllers do this continuously. Both types of speed controllers are suitable only for voltage-controllable motors. These are electric motors where the speed can be controlled by lowering the supply voltage while the frequency remains constant. Both TRIAC and transformer fan speed controllers can be used in applications where the torque decreases with speed, such as fan speed control. Controlling the speed of fans with AC motors is one of the most common applications of transformer speed controllers. As mentioned above, the biggest advantages of a transformer fan speed controller are its simple operation and cost-effectiveness. No configuration is needed; once everything is connected, the fan can be controlled immediately. The construction, installation and commissioning of a transformer fan speed controller is much simpler than those of more complex speed controllers like frequency inverters, also translating into lower costs.

The transformer reduces the supply voltage referred to as the primary voltage. The reduced voltage that can be used to supply the motor is called the secondary voltage. The secondary voltage is reduced according to the ratio of the number of primary windings versus the number of secondary windings. For example, if the primary winding is twice the size of the secondary winding, the secondary voltage will be half the primary voltage. The principle diagram on the right shows an electrical transformer with only one secondary voltage. The transformers used in speed controllers offer five different secondary voltages. The motor speed is reduced by connecting the motor to one of these voltage taps (secondary voltages). This can be done by turning a knob, by an analogue input signal or by a command sent via Modbus RTU communication. Most Sentera transformer fan speed controllers allow five different motor speeds to be selected. Some models allow further reduction of the lowest speed by internally connecting the cable of the lowest speed to an even lower voltage tap on the transformer. However, this is not permitted for all motor types. If the starting voltage is too low, the motor may not be able to start, which can block the motor with the risk of burning.

The maximum current that the transformer can supply is determined by the thickness of the copper wires in the transformer coil winding. The maximum motor current determines the type of transformer that must be selected. For a motor with higher currents, a transformer with a thicker wire diameter must be selected. The maximum current capacity of the Sentera transformers is clearly shown on the website. The maximum current capcity means: the current consumption of the motor (expressed in Ampere) when the motor is running at full speed. The higher starting current that occurs briefly during motor start-up should not be taken into account. Sentera transformers have a constant wire thickness over the entire winding, guaranteeing better quality of the transformer. Many competitors offer cheaper transformers with variable wire thickness in the coil winding. As a result of the flow of electrical current, the copper wires will heat up. Thinner wires will heat up faster because they have a higher electrical resistance. When the heating becomes too strong, the insulation of the copper wires will melt, causing a short circuit and permanent damage. When this happens, the transformer must be replaced. Excessively high ambient temperatures, frequently restarting the motor or an installation method with insufficient cooling options can also cause this damage.

Autotransformers

Sentera transformer fan speed controllers are equipped with one or more autotransformers. An autotransformer employs a single winding (coil) that serves as both the primary and secondary winding. Different voltage taps are utilised to achieve varying output voltages. Unlike the autotransformer, the isolation transformer has two separate windings, the primary and secondary, providing electrical isolation between the input and output.

One winding means there is no galvanic separation of the primary winding from the secondary. The coils are connected directly, resulting in not only electromagnetic but also electrical connectivity. These qualities contribute greatly to higher efficiency, as only a part of the power is converted.

The operation of a transformer is based on two basic principles:

The time-varying electric current in the primary coil creates a time-varying electromagnetic field.
The electromagnetic field creates an alternating electric current via electromagnetic induction.

The single winding of an autotransformer yields a more compact and lightweight construction compared to conventional dual-winding transformers. This type of transformer is characterised by compact dimensions, high reliability and long service life. It is frequently used in various industries and production processes, as well as for ordinary household purposes when certain physical quantities need to be regulated.

How an electrical transformer works

In this chapter we explain in detail how an electrical transformer works.

A transformer is an electrical device that transfers electrical energy between two or more circuits through electromagnetic induction. Electromagnetic induction produces an electromotive force within a conductor exposed to time-varying magnetic fields. Transformers are used to increase or decrease the alternating voltages in electric power applications.

Alternating current is applied to the primary coil of the transformer. Current flowing through a coil generates a magnetic field. Since the current in the primary coil is alternating (changing direction constantly), the magnetic field also keeps changing its strength and direction. This "dancing" magnetic field is crucial for the next step.

The changing magnetic field acts like an invisible highway for electrical energy. It cuts through both the primary and secondary coils. In the secondary coil, this changing magnetic field creates a phenomenon called electromagnetic induction. This pushes the electrons in the secondary coil to move, generating a current. It works as follows: when the magnetic field around the conductor (secondary coil) changes, it nudges the electrons inside the conductor. This nudge creates a voltage (ElectroMotive Force or EMF) that pushes the electrons to flow in a particular direction, creating an electric current. The direction of the current depends on the direction of the change in the magnetic field, as explained by Lenz's law.

The voltage in the secondary coil depends on two factors:

Number of windings: The number of windings in each coil. If the secondary coil has more turns than the primary coil, the voltage will be higher. Conversely, fewer windings in the secondary coil will result in a lower voltage.
Strength of the Magnetic Field: The strength of the changing magnetic field. A stronger magnetic field will induce a larger voltage in the secondary coil.

A transformer speed controller is robust and easy to use. The disadvantage is the lower energy efficiency compared to more complex speed controllers. The efficiency of a transformer is the ratio of its output power to its input power. The lower energy efficiency of a transformer speed controller is due to:

Hysteresis Loss: When the magnetic field in the core reverses direction (which happens constantly in AC transformers), the material experiences a microscopic rearrangement of its internal structure. This back-and-forth process consumes a small amount of energy, appearing as heat loss.
Eddy Current Loss: The changing magnetic field also induces small circulating currents within the iron core itself. These eddy currents heat up the core, representing another form of no-load loss.
I²R Losses: This is the classic Joule heating effect. The current (I) flowing through the resistance (R) of the copper wires in the primary and secondary coils generates heat. As the load current increases, the I²R losses also increase proportionally.

Sentera employs various techniques to minimise these energy losses:

High-quality core materials: Using grain-oriented silicon steel with low hysteresis losses is crucial. This steel, also called electrical steel, is more expensive than other types of steel but offers better permeability to magnetic fields, resulting in fewer losses.
Lamination of the core: The core is made of extra thin metal sheets (laminations) to reduce eddy currents. These thin metal plates are perfectly aligned in the Sentera factory, fixed to each other, and then provided with a special impregnated coating. This method is time-consuming but provides a significant increase in energy efficiency.
Large conductor size: Using thicker wires in the windings reduces their resistance and lowers I²R losses. High-quality copper with a thick diameter has a lower resistance value, limiting losses at higher currents. Sentera transformers have a constant wire thickness over the entire winding, guaranteeing better quality of the transformer.

Why this basic technology remains interesting

Sentera transformer speed controllers are still widely used for fan speed control. Their ease of use, robust construction and attractive price are the main advantages. The fan speed can be adjusted in steps, and even at low speed the motor remains exceptionally quiet. Disadvantages of this technology are the lower energy efficiency and the noise that the speed controller generates. Sentera transformer speed controllers are designed to minimise these disadvantages as much as possible. Especially for fan applications that do not require continuous operation, a transformer speed controller is the perfect choice. Typical applications are extractor hoods, extraction fans, etc.

Product range of Sentera transformer fan speed controllers

Sentera is one of the leading manufacturers of fan speed controllers. For two decades, our transformer fan speed controllers have been the standard in the HVAC world. Quality and user-friendliness has always been our top priority. Due to the great success, many variants were created. As a result, it is not always easy to get an overview of this product range. The most important properties of the different series are briefly summarized below.

Sentera transformer fan speed controllers for single-phase motors with a maximum load up to (and including) 7,5 A have a high-quality plastic enclosure with metal cooling fins. This housing is manufactured in the Sentera plastics factory from flame-retardant ABS plastic. The cooling fin guarantees sufficient heat dissipation for controllers of this capacity. All other transformer fan speed controllers have a solid metal enclosure with sufficient capacity for heat dissipation.

Transformer fan speed controllers with build-in controls

A first group contains transformer fan speed controllers with build-in control switch(es) on the front panel. These speed controllers are easy to install and operate.

The entry-level The simplest transformer speed controllers have a rotary switch on the front panel that allows manual selection of fan speed. For single-phase 230 Volt motors there are the STR-1 series, for three-phase 230 Volt motors there are the STR-3 series and for three-phase 400 Volt motors there are the STR-4 series. These are the cheapest and simplest 5-step speed controllers in the Sentera range.
Motor overheating detection For single-phase and three-phase 400 Volt motors, the entry-level models are also available with an additional safety function to detect motor overheating. These are the STRS1 and STRS4 series respectively. Both series are interesting if the motor is equipped with TK temperature sensors (thermal contact) in the motor winding. These TK temperature sensors can be connected to the STRS1 and STRS4 series. If the motor temperature exceeds a critical value, the 5-step speed controller will shut down the motor to prevent permanent damage.
Emergency button for smoke evacuation For single-phase motors, the entry-level model is also available with an additional emergency button for smoke extraction. When the emergency button is pressed, the fan accelerates immediately towards maximum speed. After reset of the emergency button, the speed controller will function normally again. SER-1 series control single-phase motors.
Two separate 5-speed selector switches The SC2-1 series offer not one but two speed selector switches on the front panel. They control single-phase motors. One of both rotary switches is activated via a dry contact input (low or high). In many applications, an external time relay, a temperature switch or a differential pressure relay is connected to this dry contact input. In the case of the temperature switch, for example, the fan is controlled by switch 1 at low temperatures and by switch 2 at higher temperatures. This makes it possible to automatically switch between two different ventilation regimes, depending on the circumstances. It’s a simplified version of demand based ventilation.
Kitchen hood exhaust fan speed controllers SFPR1 and SFPR4 series are transformer fan speed controllers with an output to control a gas valve. An optional air flow sensor or pressure relay is required to detect the airflow. The 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. SFPR1 and SFPR4 series control single-phase or three-phase 400 Volt motors respectively. They restart automatically after a power failure and they feature motor overheating detection (TK motor contacts).

Remotely controllable transformer fan speed controllers

In some circumstances it is not desirable for the fan to operate continuously or not at the same speed continuously. Therefore, we offer transformer fan speed controllers that can be controlled remotely. There are variants where only the start signal can be given remotely as well as variants where the speed can be selected remotely.

Transformer fan speed controllers with dry contact inputs

Dry contact inputs can be activated by a digital signal (high or low). Typically, dry contact inputs are activated manually by use of a switch. They can also be activated automatically by use of a timer, a pressure relay, temperature switch, humidity switch, etc…

STRA1 and STRA4 series feature several additional dry contact inputs to start the motor remotely. The fact that different conditions can be combined makes these controllers universally applicable. The fan speed must be selected via the rotary switch on the front panel. STRA1 and STRA4 series control single-phase or three-phase 400 Volt motors respectively. They restart automatically after a power failure and they feature an alarm output and motor overheating detection (TK motor contacts).
SC2A1 and SC2A4 series have two speed selector switches on the front panel. These series also offer multiple additional dry contact inputs to start the motor remotely and to activate on of both speed selector switches. SC2A1 and SC2A4 series control single-phase or three-phase 400 Volt motors respectively. They restart automatically after a power failure and they feature an alarm output and motor overheating detection (TK motor contacts).
The RTR-1 series offers five dry contact inputs for activating one of the five available speed levels. This transformer fan speed controller can therefore be fully controlled remotely. Not only the starting signal, but also the desired fan speed can be set remotely. The RTR-1 series control single-phase motors.

Transformer fan speed controllers with analogue 0-10 Volt input

A 0-10 Volt control signal is connected to the transformer fan speed controller. This control signal determines which speed level is activated (at what speed the motor will run). A 0-10 Volt control signal can be generated manually via a potentiometer. Or it can be generated automatically by a sensor. E.g. the sensor transmits the measured CO2 level as a 0-10 Volt signal.

STVS1 and STVS4 series are transformer fan speed controllers with an analogue input. The 5 speed steps are selected via the analogue control signal (0-10 Volt). For example: when the analogue signal has a value of 3 Volt, speed 1 will be activated. when the analogue signal has a value of 5 Volt, speed 2 will be activated, etc. For demand-controlled ventilation, these speed controllers can be combined with one of the Sentera sensors with 0-10 Volt output signal. STVS1 and STVS4 series control single-phase or three-phase 400 Volt motors respectively. They restart automatically after a power failure and they feature motor overheating detection (TK motor contacts).

Transformer fan speed controllers with Modbus RTU communication

Modbus RTU (Remote Terminal Unit) is one of the most commonly used communication protocols in building and industrial automation. It is a serial communication method that allows multiple devices to be connected on a single communication line, facilitating efficient data exchange between controllers, sensors, fan speed regulators, actuators and other devices. Modbus RTU communication is many times more stable and reliable than classic 0-10 Volt signals.

RTVS8 and RTVS1 series of transformer fan speed controllers are controlled via Modbus RTU communication. The Modbus master of the network sends the requested speed level (1 - 5) to the corresponding Modbus holding register of the RTVS8 or RTVS1 slave device. Sentera sensors and potentiometers with Modbus communication can be combined with these speed controllers. They are also compatible with SenteraWeb cloud. This offers remote access, the possibility of receiving notifications, to use the day-week scheduler for different ventilation regimes, etc. RTVS1 series require a supply voltage of 230 VAC, while RTVS8 series can operate with a supply voltage in the range of 115 – 230 VAC. This makes them more universally applicable. Both series control single-phase motors. They restart automatically after a power failure and they feature an alarm output and motor overheating detection (TK motor contacts).

Transformer fan speed controllers with temperature sensor

Controlling the fan speed based on ambient temperature is widely used in the agricultural and horticultural sectors. The product ranges below are sold in large numbers in these industries. They have proven their quality and reliability for agricultural and horticultural applications.

GTH series of transformer fan speed controllers operate depending on the ambient temperature. In heating mode, the fan is activated when the measured temperature drops below the set temperature. When the measured temperature is higher than the selected temperature, the fan is deactivated. The unregulated output can control a water valve to regulate the flow of hot water or a relay to activate an electric heater. The unregulated output is activated simultaneously with the fan. When the fan runs, the heater is activated. In cooling mode, the functionality is inversed. Via a jumper, heating mode or cooling mode can be selected. An optional PT500 temperature probe is required to measure the ambient temperature. GTH series can be used to control single-phase motors.
The plug & play GTTE1 series are fully pre-wired. A supply and extraction fan can be plugged in via the Schuko sockets. When the ambient temperature becomes higher then the set temperature, fan speed will increase and the heater is deactivated. When the ambient temperature drops below the set temperature, the fans stop and the heater is activated. GTTE1 series control single-phase motors.