The supplier of the frequency converter braking unit reminds you that both current type and voltage type frequency converters belong to AC-DC-AC frequency converters, consisting of a rectifier and an inverter.
Due to the fact that loads are generally inductive, there must be reactive power transfer between their power sources. Therefore, in the intermediate DC link, there is a need for components to buffer reactive power.
If a large capacitor is used to buffer reactive power, it constitutes a voltage source type frequency converter; If a large reactor is used to buffer reactive power, it constitutes a current source type frequency converter.
The difference between voltage type frequency converters and current type frequency converters lies only in the form of the intermediate DC link filter. However, this results in significant performance differences between the two types of frequency converters, as shown in the following comparison list:
1. Energy storage components: voltage type frequency converter - capacitor; Current type - reactor.
2. Characteristics of output waveform: Voltage waveform is a rectangular wave, current waveform is approximately a sine wave; The current type frequency converter has a rectangular waveform for current and an approximate sine waveform for voltage
3. The characteristics of the circuit composition include a feedback diode DC power supply in parallel with a large capacity capacitor (low impedance voltage source) as the voltage type; The current type non feedback diode DC power supply in series with a large inductance (high impedance current source) makes it easy for the motor to operate in four quadrants.
4. In terms of characteristics, the voltage type generates overcurrent when the load is short circuited, and open-loop motors may also operate stably; The current type can suppress overcurrent when the load is short circuited, and feedback control is required for unstable operation of the motor.
Current source inverters use naturally commutated thyristors as power switches, which have expensive DC side inductance and are used in doubly fed speed regulation. They require commutation circuits at over synchronous speeds and have poor performance at low slip frequencies.
Structural characteristics of frequency converter
The DC link of the current type frequency converter is named after the use of inductive components, which has the advantage of four quadrant operation capability and can easily achieve the braking function of the motor. The disadvantage is that it requires forced commutation of the inverter bridge, and the device structure is complex, making adjustment difficult. In addition, due to the use of thyristor phase-shifting rectification on the power grid side, the input current harmonics are relatively large, which will have a certain impact on the power grid when the capacity is large.
2. The voltage type frequency converter is named after the use of capacitive components in the DC link of the frequency converter. Its characteristic is that it cannot operate in four quadrants. When the load motor needs to brake, a separate braking circuit needs to be installed. When the power is high, a sine wave filter needs to be added to the output.
3. High current frequency converter uses GTO, SCR or IGCT components in series to achieve direct high-voltage frequency conversion, with a current voltage of up to 10KV. Due to the use of inductive components in the DC link, it is not sensitive enough to current, making it less prone to overcurrent faults. The inverter is also reliable in operation and has good protection performance. The input side adopts thyristor phase controlled rectification, and the input current harmonics are relatively large. When the capacity of the frequency converter is large, the pollution to the power grid and the interference to communication electronic equipment should be considered. The voltage equalization and buffering circuit is technically complex and costly. Due to the large number of components and device volume, adjustment and maintenance are relatively difficult. The inverter bridge adopts forced commutation and generates a large amount of heat, which requires solving the heat dissipation problem of the components. Its advantage lies in its ability to operate in four quadrants and brake. It should be noted that this type of frequency converter requires the installation of high-voltage self-healing capacitors on its input and output sides due to its low input power factor and high input and output harmonics.
4. The circuit structure of the high-voltage inverter adopts IGBT direct series technology, also known as direct device series high-voltage inverter. It uses high-voltage capacitors for filtering and energy storage in the DC link, with an output voltage of up to 6KV. Its advantage is that it can use lower voltage resistant power devices, and all IGBTs on the series bridge arm have the same function, enabling mutual backup or redundant design. The disadvantage is that the number of levels is relatively low, only two levels, and the output voltage dV/dt is also large, requiring the use of special motors or high-voltage sine wave filters, which will increase the cost significantly. It does not have a four quadrant operation function, and a separate braking unit needs to be installed during braking. This type of frequency converter also needs to solve the problem of device voltage equalization, which generally requires special design of drive circuits and buffer circuits. There are also extremely strict requirements for the delay of IGBT drive circuits. Once the turn-on and turn off times of IGBT are inconsistent, or the slopes of the rising and falling edges are too different, it will cause damage to power devices.
There are many types of high-voltage inverters, and their classification methods are also diverse. According to whether there is a DC part in the intermediate link, it can be divided into AC/AC frequency converters and AC-DC-AC frequency converters; According to the properties of the DC component, it can be divided into current type and voltage type frequency converters.
Current type frequency converter
Named after the use of inductive components in the DC link of the frequency converter, it has the advantage of four quadrant operation capability and can easily achieve the braking function of the motor. The disadvantage is that it requires forced commutation of the inverter bridge, and the device structure is complex, making adjustment difficult. In addition, due to the use of thyristor phase-shifting rectification on the power grid side, the input current harmonics are relatively large, which will have a certain impact on the power grid when the capacity is large
Voltage type frequency converter
Named after the use of capacitive components in the DC link of the frequency converter, its characteristic is that it cannot operate in four quadrants. When the load motor needs to be braked, a separate braking circuit needs to be installed. When the power is high, a sine wave filter needs to be added to the output.
1. What is the difference between voltage type and current type?
The main circuit of a frequency converter can be roughly divided into two categories: voltage type is a frequency converter that converts the DC of the voltage source into AC, and the filtering of the DC circuit is a capacitor; Current type is a frequency converter that converts the DC of a current source into AC, and its DC circuit filter is an inductor.
2. Why does the voltage and current of a frequency converter change proportionally?
The torque of an asynchronous motor is generated by the interaction between the magnetic flux of the motor and the current flowing through the rotor. At the rated frequency, if the voltage is constant and only the frequency is reduced, the magnetic flux will be too large, the magnetic circuit will saturate, and in severe cases, the motor will be burned out. Therefore, the frequency and voltage should be changed proportionally, that is, while changing the frequency, the output voltage of the frequency converter should be controlled to maintain a certain magnetic flux of the motor and avoid the occurrence of weak magnetism and magnetic saturation phenomena. This control method is commonly used for energy-saving frequency converters in fans and pumps.