Frequency Inverters
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About frequency converters
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What is a frequency converter?
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How does a frequency converter work?
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What are the advantages of a frequency converter?
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What types of frequency converters are available?
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Buying criteria for frequency converters – what do you have to pay attention to?
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The most frequently asked questions about frequency converters
What is a frequency converter?
A frequency converter converts the specified alternating voltage from the local power grid into an alternating voltage with variable amplitude and frequency. In this way, voltages can be generated that are precisely tailored to the connected device.
Frequency converters are mainly used to improve the start-up and speed behavior of three-phase motors. Without frequency converters, such motors would not be infinitely variable, but could only be operated at the constant speed specified by the power grid.
Frequency converters can be used for two different operating modes:
- In single-phase operation, the converter is connected to the conventional AC grid. The name of the operating mode refers to the supply voltage, which in this case is single-phase.
- In three-phase operation, the converter is supplied with three-phase current. Here, too, the name of the operating mode refers to the supply voltage, which is three-phase for three-phase current.
A modern frequency converter in a compact design.
Frequency converters can be used to continuously adjust speeds up to the rated speed of the motor without the torque of the motor falling. If the rated speed, where the motor reaches the maximum power under full load, is exceeded, the output torque decreases.
How does a frequency converter work?
In order for the speed of a motor to be individually controlled in the drive technology, the supply voltage of the three-phase motor must have a variable frequency. It must be decoupled from the input frequency. For this purpose, the input voltage of the mains, which has a fixed frequency, is converted into a DC voltage. This DC voltage is then output at the output in rapid succession. The following diagram illustrates the structure:
Structure and modules
Principle circuit diagram of a frequency converter.
Rectification (1)
The bridge rectifier (diode D1 – D6) "folds up" the negative half-waves of the sinusoidal line AC voltages. The negative components of the alternating voltage are thus used to supply the power to the subsequent circuit. In practice, this results in a DC voltage with a certain ripple. In the case of rectification, the required EMC filters ( E lectro m agnetic V tolerable) are also integrated. This ensures that no mains faults are entering the frequency converter or that no faults are emitted into the mains.
DC link (2)
A capacitor (C) is installed in the DC link to smooth the remaining ripple of the voltage generated by the rectifier. Coils (L) are also integrated in the DC link to attenuated significant current fluctuations. However, the coils are less relevant to illustrate the function.
The brake chopper (chopper), which is also located in the DC link, is much more important. This consists of an electronic switch (transistor T7) and a braking resistor (R). This converts the excess energy generated by the motor during deceleration into heat and effectively brakes the motor.
Inverter (3)
The inverter controls the motor . In principle, one can imagine the inverter as an arrangement of six electronic switches (power transistor T1 – T6). Two transistors are always connected in series, whereby a motor connection cable is routed outwards between the transistors. The transistors are controlled via a complex control electronics.
Electronic control (4)
The control electronics control the power transistors according to the respective requirements. For this purpose, the processor-controlled electronics have the necessary inputs for a controller potentiometer or corresponding bus systems such as EtherCAT, POWERLINK or PROFIBUS-DP. Alternatively, the required settings can also be made via a keypad with display. Extensive protective circuits as well as outputs for status and error messages round off the scope of the electronics.
Sine filter (5)
In drive technology, massive interference signals are generated by the switching of very high currents. This places a heavy load on the motor and especially on the insulation. This is why sine-wave filters are used to suppress all faults that occur and to forward a clean sine-wave voltage to the motor (M).
How the inverter works
The ratio between pulse width and pause results in the output signal.
In order to generate a sine output voltage, the voltage would have to drop from the value "0" to the respective maximum and then back to "0". However, only a DC voltage with a constant height is available in the DC link.
This is why pulse-wide modulation (PWM) is used. This means that the high voltage from the DC link is switched on and switched off again immediately . After a short pause, the voltage is switched on and off again. This switching rhythm is repeated continuously.
The attached circuit diagram illustrates the effect of the pulse width modulated control pulses. Although the switching pulses of the same value do not look like a sine, they have a sine-like effect in relation to the motor current . Narrow switch-on pulses with long pauses between the pulses generate a low current. Wide pulses with short breaks generate a high motor current.
For a better overview, only a few switching pulses are shown in the diagram. In practice, however, several thousand switching pulses per second are generated. With the corresponding constellation, the frequency and amplitude of the motor current can be individually changed.
What are the advantages of a frequency converter?
In many applications it is necessary that motion sequences start slowly and can be controlled specifically in the speed.
This does not only apply to industrial plants or machines. Even the elevator in the office building should be gently driven and brake equally gently.
In addition to the more comfortable use, a soft-start control also has a very positive effect on the wear of all moving mechanical parts and gears.
Previously, speed-controlled drives were only possible with a thyristor control and a cost-intensive and service-intensive DC motor.
With a frequency converter, low-cost asynchronous motors can be used, which also require only minimal maintenance.
Low-cost motors can be used thanks to frequency converters.
What types of frequency converters are available?
Indirect frequency converter for 0.75 kW motors.
With the indirect frequency converter described above, the incoming alternating voltage flows into a rectifier, which then feeds a DC link.
In the DC link, the current is smoothed by buffer capacitors and suppressed by inductive coils. The DC link in turn supplies an inverter, which then outputs an output voltage in the desired amplitude and frequency for the device to be supplied.
In contrast, there is no DC link for the direct converter (also called matrix converter). Instead, the frequency direction is executed in a single, rather complicated circuit with different paths.
A major advantage of direct converters is that they are regenerative and that they operate almost without loss at the same input and output frequency. However, they require a continuous energy supply (in the form of three-phase current). In addition, they provide a comparatively low maximum output voltage.
Feedback
With regenerative converters, the conversion goes in both directions: You can also transfer energy that becomes free during a braking process back to the mains supply. In other words, the converter can also be operated as a generator! This mechanism is particularly useful when drives often need to be braked, for example in centrifuges, elevators or electric locomotives.
Good to know: Choose suitable motors
As partial discharges occur in the converter with large voltage changes and high current peaks, the insulation of the motor windings is comparatively heavy. Therefore, only certain motor types are suitable for operation with frequency converters (see DIN VDE 0530-25). EMC-compliant cabling of the motor is also essential.
Buying criteria for frequency converters – what do you have to pay attention to?
The following technical parameters of a frequency converter must comply with the requirements of the application you are planning:
- What input voltage is available to you? Depending on the model, frequency converters can process 230 V, 400 V, 480 V or 580 V alternating or three-phase current.
- The output voltage and the maximum output power that the converter can provide must match your application. Typical output power ranges from 0.1 to around 20 kW. The value kW stands for kilowatts, where 1000 watts corresponds to the value of one kW.
- The frequency range of the converter must also contain the values you require. Some devices only generate frequencies that are close to the standard input frequency of 50 Hz (for example, 48 to 62 Hz). Other converters cover the full range from 0 Hz to 650 Hz.
- Most frequency converters can be connected to field buses or Ethernet, for example via a Profibus interface. Drive profiles defined in the standard IEC 61800-7, which are binding, ensure that the converters of different manufacturers behave the same on the field buses.
- In addition to the power connections, converters can also have additional inputs and outputs. A potentiometer for setting the output frequency, for example, can be connected via this. Most modern frequency converters, however, have a keyboard unit and a digital display to ensure a comfortable adjustment to the respective connected motor. Some converters can be configured individually using their own programming language. The finished code is then loaded into the converter via another interface.
The most frequently asked questions about frequency converters
What about the frequency ramp?
The frequency ramp is a setting mode in which the converter continuously starts the frequency of the output voltage from zero to the desired value. This prevents overcurrent peaks when starting the motor. Braking of the converter also works best with a (descending) frequency ramp.
Can each IEC standard motor be operated with a frequency converter?
In principle already, but because the motors are subjected to higher thermal loads with frequency converters, phase insulations should always be installed in the windings. In addition, the number of poles of the motor and thus the speed of the rotor must be precisely selected so that a practical control range can be achieved even under load.
What is a (rotating) converter?
In a converter, rotating electrical components (such as an electric motor and a generator) are used to generate variable output voltages. Unlike the electrical machine, frequency converters do not contain any mechanically moving components. A converter thus performs a function similar to a frequency converter, but the voltage is adjusted by electro-mechanical means.
What is the difference between a frequency converter and a transformer?
Even with a transformer, a variable output voltage can be produced from a fixed input voltage. However, transformers can only change the amplitude of the voltage, not its frequency. For complex AC and three-phase applications, transformers alone are not sufficient. However, they are often used as a component within frequency converters, for example to generate different voltage levels.