A frequency converter is a power converter which consists of alternating voltage a in the frequency and amplitude variable AC voltage for the direct supply of electric machines such as three-phase motors generated. Setpoint values for the frequency and amplitude of the output AC voltage are based on the requirements of the electrical machine and its current load. Some frequency converters have additional sensor inputs in order to record state parameters of the electrical machine such as speed or the current angular position of the rotor . Depending on the type of electrical machine, frequency converters can work with both single-phase AC voltage and three-phase AC voltage and can also generate three-phase AC voltage from single-phase AC voltage for supplying three-phase motors.
Converters have a similar electrical structure, but are not used to control and supply an electric motor, but usually work with a fixed frequency and voltage amplitude at the output to supply several different consumers.
Frequency converters and converters are electronic devices without mechanically moving components. In contrast to this, a converter is a rotating electrical machine, as it is used, among other things, in railway power converter plants.
Basic structure and functionality
In principle, the indirect, static frequency converter consists of a rectifier , which feeds a DC voltage intermediate circuit, and an inverter fed from this intermediate circuit . The intermediate circuit consists of a capacitor to smooth the DC voltage and inductances to suppress interference. Both uncontrolled and controlled bridges are used as rectifiers. When using a controlled bridge, the DC link can also be supplied with an active power factor correction (PFC).
Several inverters can be connected to the intermediate circuit, which is why this type of frequency converter is very often found in machine tools . An intermediate circuit which works with direct current and a smoothing choke as a storage element can also be implemented. The storage element in the intermediate circuit, in the case of a DC voltage circuit the capacitor and in the case of direct current the smoothing reactor, bridges the gaps in the energy supply.
There are also direct converters , also known as matrix converters, which do not require an intermediate circuit. Matrix converters require a seamless energy supply; these converters are usually designed for three-phase alternating current .
The inverter works with power electronic switches (controlled bridges). These can be power transistors , such as
- Metal-Oxide-Semiconductor Field Effect Transistor , (MOSFET)
- Insulated Gate Bipolar Transistor , (IGBT)
- IGC thyristors , (IGCT).
It generates a variable voltage through pulse width modulation (PWM). The level of the resulting output voltage and its frequency can be regulated within wide limits. In addition to pulse width modulation, there are also sinusoidal frequency converters with a self-oscillating, variable clock frequency; these generate a purely sinusoidal voltage at the output.
In order to be able to brake, simple frequency converters have a so-called brake chopper , which conducts the excess energy from the intermediate circuit into a braking resistor and converts it into heat. Otherwise the intermediate circuit voltage would rise and destroy the capacitors. The chopper (breaker) is pulse-width-controlled for quasi-analog control of the heating output.
For braking powers from 1 kW - the limit is fluid - more complex, regenerative frequency converters are used. From a financial point of view, their advantage lies less in the reduction of energy costs, but in the saving of the braking resistor and its cooling. Its input circuit in front of the intermediate circuit is very similar to the output circuit, only the frequency is specified on the network side and with reactive power minimization.
A direct converter with thyristors can only generate output frequencies lower than the input frequency. DC link converters and direct converters with IGBTs, on the other hand, can also generate output frequencies that are above the input frequency (up to several hundred Hertz).
Direct converters are always capable of energy recovery. Another advantage of direct converters is that they work practically loss-free with the same input and output frequency without switching processes (bridge operation). Therefore, they are particularly suitable as a heavy or soft start circuit for drives that otherwise run smoothly ( e.g. elevators ).
If asynchronous motors are operated directly on the AC voltage network, they have a fixed speed, the nominal speed, which depends on the number of pole pairs and the network frequency. When starting, high current peaks occur and the torque is low. This is counteracted conventionally with various means. This includes star-delta connection , starting transformer and thyristor starter with phase control . In this way, however, no higher torque below the nominal speed can be achieved, and operation above the nominal speed is also not possible.
Extended speed range
In contrast, frequency converters enable infinitely variable speeds from almost zero to the nominal speed without the torque dropping (basic setting range). The motor can also be operated above the nominal rotational frequency (field weakening range), but then the output torque drops because the operating voltage can no longer be adapted to the increased frequency (see V / f operation). Due to these properties, frequency converters are widespread in industry and allow the use of inexpensive standard asynchronous motors in an extended speed range.
The basic setting range can for engines of a type plate of their phase voltage Δ / Y: 230 V / 400 V to a 400 V inverter to 87 Hz is set , and are thereby operated at a higher speed at the rated torque when in delta configuration is connected. (This also applies to other mains voltages .) A. That the internal fan is a higher load and the iron losses (depending on the frequency) increase, so that the motor can be thermally or mechanically overloaded.
The slip speed (= synchronous speed minus asynchronous speed at the rated torque) and the number of poles of the electrical machine are decisive for the lowest lower speed (or lower limit frequency). The slip frequency in the rotor is calculated from the relationship: slip speed times pole number divided by 60: .
The slip frequency must be exceeded for safe operation (rule of thumb: double the slip frequency for a suitable lowest speed), otherwise the motor will block when it is stationary. In modern converters, this restriction is circumvented by active slip compensation.
High torque start
By programming a frequency ramp for start-up, difficult start-up conditions can also be managed without strong overcurrent peaks. Braking is also possible with a descending frequency ramp. Many frequency converters can themselves monitor whether the motor is still running within a permissible slip and thus prevent the rotating field from breaking off . Converters with space vector control allow the torque and speed of an asynchronous motor to be controlled separately by tracking the actual frequency based on the registered feedback effects of the motor.
Use and restrictions
Frequency converters are used in particular on three-phase motors to improve or expand their start-up and speed behavior. Frequency converters are now also available for single or two-phase AC motors such as B. capacitor motors to regulate their speed. The frequency converter takes over the provision of the second phase previously generated by the capacitor.
There are also single-phase frequency converters where no changes need to be made to the single-phase motor with capacitor. This is particularly interesting for existing drives such as pumps, fans, bench drills or drives for conveyor belts. Shaded pole motors can also be operated with such frequency converters with restrictions . The devices first run up the capacitor motor at the nominal frequency and then reduce the frequency according to the desired speed. This is necessary because the capacitor can only generate the auxiliary phase required for starting at the nominal frequency. Because of this, such frequency converters cannot increase the starting torque.
Frequency converters generate strong electrical interference signals on the motor lead, which can not only interfere with other consumers, but also lead to increased insulation loads in the motor. The motor supply cable must often be shielded to avoid radiated interference. A so-called sine filter between the converter and the motor can help. Such sine filters differ from a line filter in that they have a lower cutoff frequency and a higher load capacity.
When operating above the nominal speed, increased eddy current and hysteresis losses occur in the motor, but this is often compensated for by the fan wheel, which is also rotating faster. The motor must be approved for the frequency for continuous operation. Slowly rotating motors up to 3 Hz, as is often used in industry, are cooled by external fans, the speed of which depends on a so-called external network, i.e. three-phase current of 50 or 60 Hz.
For these reasons, frequency converters require professional installation.
With frequency converters a distinction is made between several main areas of application, which also decide which type, i.e. with which characteristics, is used:
Frequency converters are used in modern electric railways under the name traction converters to generate the three-phase current for the continuously variable three-phase drive motors from the respective traction current system of the overhead line or the conductor rail .
The traction converter typically consists of a four-quadrant controller (4QS), an intermediate circuit ( ZK) operated with DC voltage , a pulse-controlled inverter (PWR) and, in DC networks, possibly a braking unit (BST). The 4QS is not required for operation under a DC voltage network.
Pump and fan applications
At the beginning (from 0 Hz) almost no torque is required here, since the air resistance is 0 at the beginning. However, the torque increases approximately quadratically. The rated speed corresponds to the rated torque.
The drive torque decreases as the square of the speed, so the required drive torque drops to 25% when the volume flow is halved. Since the mechanical drive power is calculated as M × 2 × π × n, the drive power is now only one eighth of the nominal power. (M ⇔ torque, n ⇔ revolutions per second) Possible losses of the converter are not taken into account.
Lifting and moving applications
A high breakaway torque is required at the beginning (from 0 Hz), which far exceeds the rated torque (approx. 125–200% depending on the application). Since the rotor of the motor then turns or accelerates evenly, the required torque remains constant. This torque is usually a little below the torque curve of the motor.
A soft start of the system can also be implemented here via the converter.
A servo drive is an electronically controlled drive with position, speed or torque control (or a combination of these) for applications in production machines and automation solutions with high to very high demands on the dynamics, the setting ranges and / or the accuracy of the movement. Servo drives are often used in machine tools , printing machines , packaging machines or industrial robots .
Their use is characterized by the fact that they can often be operated with strong changes in speed and torque as well as briefly with high overload . Servomotors can generate their nominal torque as holding torque for an unlimited period of time, even when they are at a standstill.
In addition to the power connections, frequency converters usually have digital and / or analog inputs and outputs. At an analog input z. B. a potentiometer can be connected to set the output frequency.
For example, the standard signal levels 0–10 V, 0–20 mA or 4–20 mA are used for analog control .
Connections for fieldbuses or Industrial Ethernet are also available for most frequency converters . Examples of such interfaces are CAN with CANopen or DeviceNet protocols, Profibus with PROFIdrive , Interbus or the Ethernet-based solutions EtherNet / IP with CIP Motion, Profinet with PROFIdrive, Ethernet POWERLINK , EtherCAT or one of the three SERCOS versions. Drive profiles have been defined so that these frequency converters from different manufacturers behave in the same way on these different fieldbuses. Four of these drive profiles have been specified worldwide in the international standard IEC 61800-7 .
By parameterizing, converters can be adapted to the motor to be driven in order to operate and protect it optimally. Nowadays this is rarely done using potentiometers and DIP switches , but rather using a suitable keyboard / display unit that is located on the converter and allows navigation in a menu structure. Particularly complex converters enable programming in their own programming language or using a corresponding graphic program on the PC. Finished data sets are then loaded into the converter via an interface.
Nowadays, the finished parameter set is often saved on a storage medium (e.g. chip and flash cards), which is then inserted into the frequency converter.
Some models can measure the drive properties themselves (often referred to as autotune) and set their control parameters themselves during commissioning. Sometimes they can also process programmed traversing movements independently ( motion control ).
Energy recovery and four-quadrant operation
Since the intermediate circuit can only store a certain amount of non-destructive energy due to its structure, measures must be taken to reduce the stored energy.
A variant that is mostly used with inexpensive converters is the conversion of electrical energy into thermal energy with the so-called brake chopper, a resistor that is switched on by an electronic switch controlled by the intermediate circuit voltage. With larger amounts of energy, however, this is an inefficient process for both ecological and economic reasons. There are regenerative converters for these applications . You can transfer the energy from the intermediate circuit back into the network by reversing it to the network frequency.
All types of motors with regenerative converters can thus be operated as generators even at changing speeds. This is particularly interesting for vehicles and other drives that have to brake cyclically, such as B. Centrifuges in sugar factories , drives for elevators and cranes and load devices on engine test stands. In the case of locomotives or other vehicles, the braking energy can be used ( regenerative braking ). Hybrid cars feed into their batteries.
Energy recovery devices can also be retrofitted. For this purpose, they are connected to one or more frequency converters instead of the brake chopper.
U / f operation
This is the simplest way of operating a frequency converter. The converter regulates the motor voltage and the frequency in a constant ratio. Frequency and voltage are proportional to each other. Due to the inductive behavior of the motor, this leads to a constant torque over a wide range without overloading the motor. At very low speeds, however, this operating mode results in a lower torque due to the ohmic resistance of the winding. To remedy this, a voltage increase (boost) can often be set in the lower frequency range (I × R compensation).
In V / f operation, the speed of the connected motor varies depending on its load.
A constant speed feedback can either be achieved with a control using a speed sensor or by means of slip compensation, which makes it possible to achieve a constant speed without speed feedback. U / f operation is therefore only sufficient for low speed constancy requirements and without heavy starting .
The above characteristic shows that the magnetic flux remains constant up to the nominal frequency. If a three-phase asynchronous motor is operated with a frequency converter above the nominal frequency, the motor is in the field weakening range, since the core is saturated. The output voltage of the converter reaches its maximum value at this point and the torque drops.
The vector control or also field-oriented control consists of a speed controller based on a subordinate current controller. The instantaneous reactive and active current components are regulated. The motor parameters are stored in a motor model electronically stored in the converter or, if necessary, even automatically determined and adapted. This has the advantage that there is no need for separate speed measurement and feedback to control speed and torque. Rather, the returned variable used for regulation is exclusively the instantaneous current. Based on its magnitude and phase relation to the voltage, all necessary motor states (speed, slip, torque and even the thermal power loss) can be determined.
In this way, very high speed and torque setting ranges are possible. Typical are control ranges for the speed of 1: 120 (with an additional speed sensor even up to 1: 2000). The torque ranges from zero to four times the nominal motor torque.
Similar to the commutator in DC machines, commutation is the control of the power supply to the motor windings by the semiconductor switch in the frequency converter. The procedures correspond to those used for commutation of brushless DC motors . A distinction is made between the following types of commutation:
- Space vector modulation , also known as sine commutated pulse width modulation ( English Space Vector PWM )
- Block commutated pulse width modulation , also known as block commutation. (for synchronous machines)
- Less common: hysteresis controller ( two-point controller )
With block commutation, exactly 2 out of 3 three-phase windings are always energized. The third winding is unused and is used by some frequency converters to measure the voltage induced on the rotor in order to determine the current position angle of the rotor. This means that permanently excited machines can be commutated by the frequency converter without the additional sensors such as absolute encoders otherwise required , but must be blindly commutated during the start phase due to the low speed. Due to the constant magnetic flow, there are hardly any disadvantages in terms of torque ripple or efficiency compared to sinusoidal commutation. In analogy to a stepper motor, this operating mode is also referred to as 6-step operation.
Sinus commutation by the frequency converter is common for operating asynchronous machines (sinus converter; the pulse widths are modulated sinusoidally). Exactly 3 out of 6 semiconductor switches are always switched on. The switching signals are usually generated by microcontrollers, which are available in versions with 6 PWM outputs especially for motor applications.
The six transistors and their anti-parallel diodes in the adjacent inverter circuit are numbered in the order in which they begin with block commutation. A transistor is considered switched on if it or its diode conducts.
The following 8 switching states are possible:
|number||Switched on||Switching status of the bridge arms|
|0||V2, V4, V6||000|
|1||V1, V2, V3||110|
|2||V2, V3, V4||010|
|3||V3, V4, V5||011|
|4th||V4, V5, V6||001|
|5||V5, V6, V1||101|
|6th||V6, V1, V2||100|
|7th||V1, V3, V5||111|
The states 1 to 6 form phase-shifted star voltages on a symmetrical load with the instantaneous values + Uo / 3, + 2Uo / 3, + Uo / 3, -Uo / 3, −2Uo / 3, -Uo / 3, + Uo / 3, ... etc. Their basic vibrations correspond to a three-phase system.
States 0 and 7 de-energize the load. They are used to reduce the output voltages in the short term. A sinusoidal current is now achieved through a time-weighted switchover between the 8 states.
In order to reduce switching processes and the associated switching losses , the states are combined in their order in a meaningful way. Let us assume that the voltage on the load should change in small steps with reduced voltage from switching state 1 (V1V2V3) to state 2 (V2V3V4), i.e. H. the connected machine can be rotated electrically by 60 °. The switching sequence is ideal for this
- … V1V2V3, V1V3V5, V2V3V4, V2V4V6, V2V3V4, V1V3V5, V1V2V3… etc. on.
The individual master times result from the control algorithm used and the level of the required parameter (voltage, current, torque).
With this sequence of switching states, only one switching process takes place with each commutation. Usual PWM frequencies in drive technology are between 2 kHz and approx. 20 kHz. As the switching frequency increases , the sine is more closely approximated, the switching losses in the converter increase and the losses in the motor decrease due to the better sinusoidal current curve.
Optimization by superimposing harmonics
For a further optimization of the sinus commutation, the superposition of the third harmonic at the desired output frequency is common.
A normal sine is shown in blue . The factor 100 should symbolize a modulation with PWM from 0 to 100%. The third harmonic is shown in green . The frequency is exactly 3 times as high and the phase position is the same as the fundamental . The amplitude with 15% pulse width was initially chosen arbitrarily. The curve shown in black now shows an addition of the two sinusoidal frequencies . The result is a signal with a smaller amplitude, which is more like a square wave than a sinusoid. In addition, the maximum value of the amplitude is smaller than the originally undistorted sinusoid because the 3rd harmonic frequency at the maximum value of the fundamental oscillation always has its reverse maximum. If both sinusoidal oscillations are now generated in a microcontroller via a table, no computing power is required for the addition and the pulse width modulator is only used to around 85% of its possible maximum working range. The remaining 15% can be used to increase the performance of the frequency converter.
However, it is now extremely problematic to operate various three-phase current consumers (including asynchronous motors) with curve shapes other than sine waves. Therefore it will be shown in the following that the commutation with the third harmonic has no influence on the sinusoidal shape at the output of the frequency converter.
- Is shown in blue
- Is shown in green
- Is shown in red
That gives the usual picture of 3 three-phase phases. If, instead of a star connection, a consumer is connected between the phases (delta connection), the differential voltage between any 2 phases results in a sinusoidal voltage with increased amplitude (shown in yellow). So if the difference is a sinusoidal result, this remains valid if the same function is added to and in each case. If there is a shift between the three-phase phases, the third harmonic (shown in orange) is completely identical to the next phase, since it is shifted by exactly one full period. When commutating with the 3rd harmonic, there are again sinusoidal voltages and currents at the output of the frequency converter, regardless of whether the consumer is connected in star or delta.
The resulting increase in performance of the frequency converter and a drive connected to it is usually around 15%. This corresponds roughly to the amplitude loss of a 6-pulse three-phase rectifier circuit, which supplies around 86% of the peak voltage in the intermediate circuit.
Frequency converters work with steep switching edges in order to minimize power loss and achieve a high level of efficiency. In IGBT frequency converters for 400 V mains operation (560 V intermediate circuit voltage ), the IGBTs switch within around 200 ns. This leads to a voltage rise on the motor cables of around 3 kV / µs. A typical low-capacitance motor cable is a mutually shielded multi-core cable with a capacitance per unit length of approx. 200 pF / m. The steepness of the edge leads to recharging currents of around 0.6 A / m. In the case of long motor cables, this adds up to recharging currents of up to 20 A, which also flow with low-power devices and place a considerable load on the inverter. With long motor cables, the currents do not increase any further due to the wave propagation (5 ns / m).
If a motor cable is improperly connected, e.g. B. Connection of a possible cable shield as a twisted cable end ( pigtail ) or no support of the shield at all, then the recharging currents flow via cables and circuit parts z. B. control terminals or field bus systems and can cause massive interference there. In this context, one speaks of galvanic coupling.
A suitable connection of the motor cable (e.g. a large area of possible cable shielding) is essential for good EMC behavior of the frequency converter. Favorable EMC behavior can be achieved through suitable filtering in the converter, in that the filter offers the interference current a short path to the intermediate circuit, preferably in the vicinity of the interference sources, the IGBTs. Since all filters work against ground, a TN-S network system is absolutely necessary. It must be ensured that the filter elements do not lead to excessive blurring of the switching edges, since slow switching times can quickly lead to the IGBTs overheating. This effect can also be counteracted by suitable cooling.
A simple frequency converter consists of an uncontrolled rectifier and a DC voltage intermediate circuit with electrolytic capacitors as energy storage and for smoothing the intermediate circuit voltage.
The network (voltage source with low impedance) and the intermediate circuit (capacitors) are connected to one another with the aid of the rectifier diodes. This leads to impulse-like charging currents (low current flow angle) that load the network. The system perturbations can be reduced by connecting line reactors upstream. The connection of frequency converters directly to the mains leads to a heavy load on the network and intermediate circuit due to an increased effective current and may reduce the service life of the frequency converter.
There are now frequency converters that rectify and convert at the same time directly from the three-phase network via synchronously controlled power semiconductors without an intermediate circuit capacitor (matrix converter). The resulting high-frequency power consumption can be filtered much more easily (with small capacitors and chokes) than the pulse-shaped power consumption with uncontrolled rectifiers. However, the disadvantage here is a slightly reduced maximum output voltage, since no peak value rectification takes place.
Another variant is the upstream connection of a PFC stage for charging the intermediate circuit capacitor, which can also be capable of being fed back. This allows operation with almost no interference from the network without torque fluctuations.
System perturbations for frequency converters are specified in EN 61000-3. Individual standards of this series cover frequency converters up to a connected load (current equivalent) of 75 A per phase. Systems with larger connected loads (> 75 A per phase) can significantly influence neighboring systems and possibly an entire low-voltage network. Therefore, in these cases, an individual assessment is generally required in accordance with current technical regulations.
Effects on the electric motor
Due to the fact that partial discharges (PD) with a high rate of voltage change d u / d t and high voltage peaks occur in converter operation, there are significantly higher loads on the winding insulation compared to normal operation. With long cable lengths (e.g.> 25 m) and other unfavorable circumstances caused by reflections and transients, d u / d t values of 10 to 50 kV / µs and voltage peaks up to three times the intermediate circuit voltage can be achieved. The insulation of the motor windings experiences loads similar to the traveling waves, as they occur with lightning discharges, however with the difference to the single event that they occur here as permanent load. The voltage on the winding changes so quickly that there is a different potential between the beginning and the end of a strand. In the most unfavorable cases (with parallel coil groups and random winding) the insulation between two wires in contact is stressed with the full peak voltage.
In addition, there is a build-up of tension on the motor shaft and an undesired flow of current from the motor shaft via the bearings to the housing. This parasitic flow of current causes electrical erosion on the sliding surfaces of the bearings and thus premature wear. In addition to specialist work at various technical universities, the British working group REMA / GAMBICA has dealt intensively with this problem. In addition to selecting the motor suitable for converter operation (see DIN VDE 0530-25), suitable precautions must be taken for the technical design and installation. This involves, on the one hand, the most efficient containment of the interference level (e.g. by means of a sinusoidal filter, if possible in the converter or immediately after the output terminals), EMC-compliant cabling and precautions for the electromechanical structure, either through solid grounding and cross-connection of the motor and the unit to be driven or electrically isolating coupling between the motor shaft and the unit to be driven (which is only feasible in very few applications).
For EMC-compliant cabling, especially over longer distances, specially optimized, double-shielded and low-capacitance cables with internal YK stranding with 3 + 3 cores are very suitable. The protective conductor PE is symmetrically divided into 3 individual PEs and arranged in the interstices of the power wires, e.g. B. 2YSLCYK-JB 3 × 35 + 3 × 6 mm² from a well-known German manufacturer. In addition, EMC-compliant EMC screw connections that are matched to the cable diameter are useful for each input into a housing.
Particular care is recommended when planning if frequency converters are to be retrofitted to systems that have been in existence for many years. It may be necessary to renew the motor windings (with one with a suitable dielectric strength) or to replace individual motors. Systems or machines with electronic controls and borderline EMC design can end up in a very fault-prone and unstable position due to the installation of frequency converters and may require the EMC measures to be adapted to current standards.
When using a PWM clock frequency in the listening area, annoying noises are often created. These are caused by mechanical vibrations in the motor windings, which magnetically attract and repel each other at the pulse frequency. To avoid this effect, the pulse frequency is increased to> 16 kHz if possible, which, however, increases the power dissipation of the frequency converter. The usual pulse frequency for small converters is in the audible range and is usually adjustable from 3 to 16 kHz. With modern control methods, this pulse frequency affecting the noise can also be cyclically changed ( wobble ) around an average value during operation . The subjective noise behavior is thereby significantly improved. If used correctly, the wobbling of the pulse frequency has no influence on the operating behavior.
For some applications, variable speed turbo couplings are preferred instead of frequency converters . Due to the principle, the power is transmitted through a fluid, leaks cannot be ruled out in the long term. Typically, variable speed turbo clutches are used where mechanical drives are already available and control of the same is not possible. With regular maintenance, variable speed turbo clutches are characterized by a service life that is many times longer. The operator thus saves maintenance and repair costs.
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