Core components of frequency converter: bus capacitor
The Leadway HARSVERT series frequency converter is a voltage source type cascaded frequency converter. The core component of the equipment is the capacitor, which serves as the DC energy storage link and undertakes important functions such as decoupling, filtering, and providing reactive power. If the capacitor experiences a significant drop in capacitance or an increase in ripple resistance, it will lead to large fluctuations in motor current, unstable speed, and severe capacitor overheating. Therefore, Leadway has put a lot of thought into the selection of capacitors.
Currently, the most common capacitors used in DC bus systems are aluminum electrolytic capacitors. The most common faults, such as capacitance degradation and overheating, are mostly caused by capacitor aging. To prevent equipment failure due to capacitor failure, DC bus capacitors typically need to be inspected and replaced approximately every 7-10 years.
Figure 1 Capacitor
(1) Aluminum electrolytic capacitors
As the most common type of capacitor, aluminum electrolytic capacitors use aluminum foil as the anode foil, with an aluminum oxide film formed through chemical deposition on the surface serving as the dielectric and providing insulation. The electrolyte acts as the actual cathode and also repairs the oxide film, while the cathode foil serves as the cathode lead. Electrolytic capacitors increase their capacitance by etching the electrodes to roughen their surfaces.
Since the insulating medium is an aluminum oxide film on the surface of the aluminum foil, it is difficult to control the thickness and uniformity of the oxide layer and achieve high withstand voltage. At different stages, the following situations may occur:
• Early stage of electrolytic capacitor operation
During operation under load, the electrolyte continuously repairs and thickens the anodic oxide film in aluminum electrolytic capacitors (the shape-compensating effect), leading to a decrease in capacitance.
• Later stage of electrolytic capacitor operation
Due to factors such as high capacitor temperature and electrolyte leakage caused by poor sealing performance, significant electrolyte loss occurs, leading to a thicker solution and increased resistivity. This, in turn, increases the equivalent series resistance of the electrolyte, further increasing losses and ultimately resulting in increased heat generation and temperature rise, creating a vicious cycle.
At this point, the electrolyte, due to its viscosity, has difficulty fully contacting the uneven oxide film layer on the aluminum foil surface, reducing the effective plate area of the capacitor and thus decreasing its capacitance. Therefore, the structure of aluminum electrolytic capacitors limits their lifespan. Are there better alternatives to aluminum electrolytic capacitors?
(2) Metal film capacitor
To reduce the maintenance costs of frequency converters throughout their lifespan, it would be better to eliminate the need to replace capacitors throughout their entire lifespan. Therefore, metal film capacitors have come under consideration.
Metal film capacitors have only been around for about 10 years. Early production was limited, leading to high manufacturing costs. However, with increased production volume, manufacturing costs have significantly decreased, making large-scale application possible. Because the operating principle of metal film capacitors is completely different from that of aluminum electrolytic capacitors, the problems associated with aluminum electrolytic capacitors can be easily solved with metal film capacitors.
Metal film capacitors utilize a polymer film substrate and a metal film, eliminating electrolyte issues and allowing for thickness adjustments to achieve higher voltage ratings. They also boast a lifespan exceeding 15 years, significantly reducing operating costs throughout the product's lifecycle. These advantages, making them suitable for inverter applications, have attracted the attention of Leadway.
Simply attracting attention isn't enough; to be qualified for a core component position in a Leadway inverter, one needs exceptional qualifications. In this "battle for positions" among core inverter components, aluminum electrolytic capacitors and metal film capacitors have shown their strengths, as illustrated in Table 1.
Table 1 Comparison of Aluminum Electrolytic Capacitors and Metal Film Capacitors
The numerical results clearly demonstrate that metal film capacitors exhibit significant advantages in various aspects, including service life, withstand voltage, surge voltage resistance, low-temperature resistance, vibration resistance, long-term storage capability, and ease of installation. Applying metal film capacitors can substantially improve the reliability of frequency converters and significantly reduce the operating and maintenance costs associated with capacitors; therefore, this "position" is undoubtedly theirs!
(3) New changes in catalytic frequency conversion
After taking on an important role in Leadway inverters, metal film capacitors, as a new generation of components, offer a rated voltage of 1200V thanks to their high voltage withstand and self-healing properties. This eliminates the need for resistor voltage equalization circuits, avoiding overvoltage damage caused by voltage equalization failure, and bringing more positive changes to the design and operation of inverters.
• Changes: Superior design (see Figure 2)
• Change: Stronger Qualities
Figure 2 Comparison of Design Schemes
When the voltage at the capacitor terminals exceeds the withstand voltage limit, the metal film capacitor will experience breakdown and evaporation of the metal coating, and will recover on its own as the overvoltage disappears, but will lose that portion of the capacitance; overvoltage in electrolytic capacitors will cause the oxide film to break down and the electrolyte to evaporate, and may even lead to capacitor explosion.
Using metal film capacitors as DC bus capacitors can further improve the reliability and stability of the entire high-voltage frequency converter, and provide self-healing capability after overvoltage breakdown, significantly reducing the customer's operating costs throughout the frequency converter's lifecycle. Metal film capacitors as DC support capacitors for medium and high-voltage frequency converters have gradually become an industry consensus and an inevitable trend.
Inverter core components
Insulated Gate Bipolar Transistor (IGBT)
Leadway's frequency converters utilize IGBTs (Insulated Gate Bipolar Transistors) as their core component. The IGBT's controllable switching capability allows the frequency converter to control its output voltage and frequency via PWM mode. Its characteristics determine the frequency converter's input and output voltage, current, and output dv/dt performance; therefore, the selection of the IGBT is crucial to the overall product design and quality. How does Leadway select such a critical component?
Figure 3 IGBT Module
(1) Selection of IGBT rated current
• Heat dissipation efficiency
Heat dissipation efficiency refers to the speed and ability of heat sinks to transfer heat. Leadway uses a new type of die-cast one-piece heat sink, which achieves efficient heat dissipation through fins.
• Overload requirements
Leadway frequency converters are all designed to allow 120% overload and 1 minute overload every 10 minutes, fully considering the overload conditions that may occur in field use, ensuring continuous and stable operation of the equipment;
• Core temperature rise limit
The allowable temperature rise of an IGBT determines the allowable output current. In addition to heat dissipation design, the higher the core temperature tolerance of the IGBT, the greater the allowable output current and the better the robustness; the new IGBT core temperature tolerance can reach 150℃.
(2) Selection of IGBT rated voltage
• Power grid overvoltage
IGBTs need to be able to withstand voltage rises caused by grid fluctuations, typically 110% to 130% of the grid's rated voltage;
• Turn off overvoltage
IGBTs need to be able to withstand voltage spikes caused by inductive loads and stray inductance in the circuit when turned off;
• Fault overvoltage
IGBTs must also be able to withstand overvoltage caused by overcurrent in the line and reliably shut down.
(3) Packaging updates to adapt to frequency development
Because the packaging of a frequency converter affects the layout design of the control and main circuits, and further influences the electrical performance of the circuit, the packaging of IGBTs must constantly evolve to meet the needs of frequency converter development. Therefore, Leadway has also adopted innovative packaging designs for IGBTs.
• Higher current density
With technological advancements, the current density of IGBT chips has gradually increased. The latest low-power IGBT modules can integrate rectification and inverter circuits into a single module, achieving a complete high-voltage frequency conversion topology. The power consumption of this integrated module is consistent with discrete devices without exceeding that of traditional three-module designs. This packaging significantly simplifies equipment structure and size, reduces cable connections and potential failure points, and improves equipment stability.
• More powerful rectifier diode chip
Using rectifier diode chips (integrated modules) can further increase the withstand voltage of the rectifier bridge to 2000~2200V (compared to the traditional 1800V withstand voltage), and the surge current withstand capacity (I2t, IFSM) is also further enhanced, which can effectively prevent failure caused by grid voltage fluctuations and inrush currents.
Figure 4. Employing a more powerful rectifier diode.
• Solderable lead package
Figure 5 Soldered pin IGBT
In this type of package, the control circuit (or main circuit) of the IGBT is brought out through soldered pins. As shown in Figure 5, it can be soldered onto the circuit board, using a multilayer bus design to reduce stray inductance of the bus, lower the turn-off spike voltage on the IGBT, and extend the lifespan of the IGBT.
(4) Built-in temperature measurement for precise core protection
The core of IGBT protection is temperature protection, so measuring the core temperature of the IGBT is crucial. Traditional temperature measurement solutions require installing temperature sensing components on the heatsink next to the IGBT, but this approach may introduce several problems:
• Located on the radiator, far from the heat source;
• Large deviation in test results;
• Slow temperature detection response and large time constant;
• The relay output cannot read the actual temperature value.
To avoid the impact of the aforementioned problems, Leadway adopted a new IGBT module-integrated temperature detection solution. Temperature sensing components are installed inside the IGBT module, enabling the temperature sensor to collect actual temperature values and send them to the main control system, thereby achieving the following functions:
• Integrated inside the IGBT, close to the heat source;
• More accurate test results;
• High timeliness and fast response in temperature detection;
• Real-time temperature display.
This new type of IGBT can further improve the reliability of the entire high-voltage frequency converter and effectively shorten the testing and maintenance time, reducing users' maintenance costs. By adopting new IGBTs represented by All-in-one and EconoDual packages, Leadway has brought a new trend to frequency converter design. The newly launched seventh-generation high-voltage frequency converter product is poised for a new era of power, with a future that promises to be "seven" years ahead!
