Explanation What is a frequency converter?

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FAULHABER Drive Systems

Wuxi InfiMotion Technology Co., Ltd.

Without frequency converters, there are no controlled motors. No wonder, then, that frequency converters are extremely widespread in almost all industrial sectors. But what exactly is a frequency converter, and what are its basic components? How does a frequency converter work? Where are frequency converters used, and what does this mean, for example, for the lifespan of motors and the energy efficiency of industrial plants?

A frequency converter is an electronic device that converts the frequency and alternating voltage of an electrical power supply to precisely regulate the speed of electric motors with a variable, precisely controllable alternating voltage. The most common applications for frequency converters include drives for fans, pumps, conveyors, and three-phase drives of industrial machines.

What are the main components of a frequency converter?

Essentially, frequency converters consist of an input filter, a rectifier, an inverter, and a control or regulation circuit.

• The input filter allows for minimizing disturbances or voltage spikes from the power grid.
• The rectifier consists of electrical components such as switching thyristors or transistors as so-called controlled or uncontrolled bridge circuits.
• The intermediate circuit mainly consists of transistors and coils.
• The inverter is used to convert direct current into variable alternating current and to regulate the output frequency as well as the level of the output voltage.
• The control or regulation circuit usually consists of a control card or a fieldbus interface.

Fundamentally, frequency converters are composed of these components, although specific frequency converters do not require an intermediate circuit as they need a continuous power supply.

How does a frequency converter work?

Based on the aforementioned system components, the operation of frequency converters is based on the conversion of direct current (DC) into alternating current (AC) with variable frequency. The conversion takes place in three steps:

Rectification: The frequency converter transforms the incoming alternating current into direct current. This is done with a rectifier that uses controlled or uncontrolled bridge circuits to convert the alternating voltage into a constant direct voltage.

Intermediate circuit: In the intermediate circuit, the direct current is stored and smoothed for the next conversion step using capacitors or coils. The intermediate circuit is thus a kind of buffer that feeds the smoothed direct voltage into the inverter.

Inverter: The inverter converts the direct current from the intermediate circuit into an alternating voltage with variable frequency and amplitude or voltage. The frequency of the alternating voltage matches the connected motor or the desired operating point, allowing its speed and torque to be controlled.

Control and regulation electronics: In addition to monitoring and controlling system parameters such as voltage, frequency, and current, the electronics allow for seamless speed control and torque management of a motor or the synchronization control of multiple electric motors connected to a single converter.

The most common operating modes of frequency converters in this context include V/f operation and vector control. In V/f operation, the voltage and frequency are regulated in a constant ratio to deliver constant torque over a wide range. Sometimes a voltage boost is required to compensate for power losses at low speeds.

In vector control, the torque and speed are more precisely controlled by considering the rotor position in the electric motor. This mode of operation is often found in more demanding drive applications.

Where are frequency converters used?

Frequency converters are predominantly found in drives for pumps, fans, and compressors. It is estimated that about two-thirds of all frequency converters worldwide are used in such applications to adjust the speed of electric motors and for energy savings. However, frequency converters are also indispensable in logistics and manufacturing for controlling the speed and direction of movement of conveyors and transport systems.

While in industrial production, frequency converters take over the precise control of tools, for instance in metal or wood processing, they ensure smooth starting and precise positioning in cranes and handling systems for the safe movement and transport of loads. In many areas of mobility, frequency converters regulate the speeds of electric vehicles. And the list of examples could go on endlessly at this point.

What safety measures are integrated into modern frequency converters?

Modern frequency converters are equipped with a variety of functions to ensure the safety of machines and personnel. Here are some examples:

Safe Stop 1: This function allows a motor to stop in a controlled manner, coming to rest in a safe position. This function is especially important in applications where a sudden motor stop could lead to dangerous situations.

Emergency Safe Stop: This function immediately stops a motor in an emergency to ensure operator safety in critical situations.

Safe Torque Off: An unexpected start-up of a motor can be dangerous. Therefore, this function ensures that a motor is set to a torque-free operating state. This function, which increases the operational safety of a machine, can be found in many converters.

Safe Limited Speed: In this case, the motor cannot exceed a predefined maximum speed. This protects the motor itself and any connected machinery from overload and damage.

Many frequency converters also integrate safety logic, which allows the implementation and execution of various safety functions without external devices. One advantage here is the reduced wiring effort as well as the simpler installation and commissioning of a motor.

Additional diagnostic functions also ensure that the condition of a motor and the corresponding safety functions are continuously monitored. This allows for early detection of potential problems and timely implementation of targeted measures.

What are the advantages of frequency converters?

One of the most important functions of frequency converters is also their central advantage: they can adjust and seamlessly control the speed of electric motors over a wide range as needed. This guarantees a high level of operational flexibility under very different operating conditions.

By adjusting motor speeds according to demand, converters offer many opportunities for significant energy savings. For example, studies on the use of frequency converters in compressors show that a 20 percent reduction in motor speed can lead to energy savings of about 50 percent.

Since frequency converters significantly reduce the starting current, the load on the power grid is decreased.

Through a soft start and demand-specific speed control, as well as the avoidance of current peaks, the mechanical wear of machines or other system components can be minimized. This leads to a longer service life of machines and systems and also reduces maintenance costs.

Frequency converters are also generally easy to parameterize, thus facilitating integration into existing systems, which, among other things, promotes the transition to modern drive technology.

Most frequency converters have advanced functions such as feedback and control mechanisms. Coupled with corresponding sensors, operating parameters such as pressure or temperature can be monitored, thereby allowing motor performance to be adjusted accordingly. This intelligent control ultimately improves the efficiency and safety of systems.

How do frequency converters affect the energy efficiency of industrial systems?

A key aspect already mentioned among the advantages of frequency converters is significant energy savings through application-oriented control of motor speeds. By adjusting frequency and voltage to the actual required load, power losses in partial load operation can also be reduced, thereby increasing the efficiency of a drive system and reducing the overall energy consumption of a plant. Moreover, frequency converters ensure an even distribution of energy consumption, thus avoiding peak loads.

By being able to find the optimal operating point of systems or machines, frequency converters maximize the balance between performance and energy efficiency. The high-precision control ensures that systems and machines operate in their most efficient range.

Of course, frequency converters can also be integrated into intelligent networks, enabling continuous monitoring and analysis of energy consumption.

Ultimately, frequency converters contribute to the reduction of CO2 emissions through the aforementioned characteristics—a particularly important topic for many regarding sustainable manufacturing and continuously rising energy costs.

Providers of frequency converters (excerpt not exhaustive)

• ABB
• ABM Greiffenberger
• AC-Motoren
• Baumüller
• B&R
• Blemo
• Bonfiglioli
• Danfoss
• DEMAG
• Eaton
• Geovent
• HAINZL Motion & Drives
• Hitachi
• KEB
• Lenze
• Leybold
• Maxon Motor
• Meister Automation
• Mitsubishi Electric
• Nord Drivesystems
• RS Components
• SEW Eurodrive
• Siba
• Siemens
• Unitronics
• VTDRIVE
• Yaskawa
• Ziehl-Abegg

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