Inverter application notes
1. The signal line and control line should be shielded to prevent interference. When the distance is long, for example, over 100 meters, the wire cross-section should be increased. Avoid placing signal wires and control wires in the same cable trench or bridge as power cables to minimize mutual interference. It is best to place them inside a conduit for better protection.
2. In most applications, current signals are preferred for transmission because they are less prone to attenuation and interference. Although sensors typically output voltage signals, these can be converted into current signals using a converter for more reliable performance.
3. Closed-loop control of inverters is generally positive, meaning that a larger input signal results in a larger output (e.g., when a central air conditioner regulates pressure, flow, or temperature). However, it can also be negative, where a large input leads to a smaller output (e.g., during heating with a water pump). A diagram illustrating closed-loop control is shown in Figure 1.
4. If possible, use pressure signals instead of flow signals in closed-loop control. Pressure sensors are cheaper, easier to install, and simpler to debug. However, if precise flow ratio control is required, a flow controller must be used, along with an appropriate flow meter such as electromagnetic, target, vortex, or orifice plate types, depending on the process conditions.
5. The built-in PLC and PID functions of inverters are suitable for systems with small signal fluctuations and stable operation. However, since these functions only adjust time constants during operation, they may not meet all transient requirements, making debugging time-consuming. They are not very intelligent, so external intelligent PID regulators, like Japan’s Fuji PXD series or Xiamen Anton, are often used. These offer features like SV (upper limit value) and PV (running value) indication, ensuring smooth transitions and optimal performance. For PLCs, choose based on the nature of control points, digital or analog signals, and processing needs—popular options include Siemens S7-400, S7-300, and S7-200.
6. Signal converters are commonly used in inverter peripheral circuits, usually composed of Hall elements and electronic circuits. They can convert voltage to current, current to voltage, DC to AC, AC to DC, voltage to frequency, current to frequency, and support multiple inputs and outputs. Products like Shenzhen St. CE-T series power isolation sensors/transmitters are widely used and easily integrated into various systems.
7. Inverters often require peripheral circuits for proper operation. Common methods include:
- Logic circuits made with relays;
- Purchasing pre-made external circuits (e.g., from Mitsubishi);
- Using simple programmable controllers like LOGO;
- Selecting function cards for specific inverter models;
- Choosing small to medium-sized programmable logic controllers.
8. There are two common schemes for frequency conversion technology in parallel pump constant pressure water supply systems (e.g., clean water pumps, large water stations, hot water centers).
- Scheme 1 saves initial investment but has poor energy efficiency. It starts the inverter at 50 Hz, then switches to power frequency, causing turbulence and loss.
- Scheme 2 requires higher investment but is about 20% more energy-efficient, with no turbulence loss and better performance. Diagrams showing both schemes are included.
9. When using multiple pumps in parallel for constant pressure water supply, a single sensor in signal series mode offers several advantages:
- Cost savings by using one set of sensors and PIDs;
- Consistent output frequency and pressure, reducing turbulence loss;
- PLC controls the number of pumps based on flow changes, typically starting 1, 2, or 3 pumps depending on demand;
- Even with different pump frequencies, synchronized operation ensures consistent pressure and minimal losses, resulting in optimal energy savings.
10. Reducing the base frequency is the most effective way to increase starting torque. By lowering the base frequency, the motor can start more easily, especially for equipment like extruders, washing machines, mixers, fans, and pumps. This method improves torque without increasing the starting frequency. The base frequency should not be reduced below 30 Hz to avoid damage to IGBTs due to high du/dt. It is safe and effective to reduce the base frequency from 50 Hz to 30 Hz, and the output frequency can still reach 50 Hz during normal operation.
11. The relationship between dynamic pressure, static pressure, and total pressure is as follows:
- Static pressure is the head required to lift water to the highest point, approximately 1 kg per 10 meters of water column.
- Dynamic pressure is the pressure drop caused by resistance from valves, pipes, and flow variations.
- Total pressure equals static pressure plus dynamic pressure, typically 1.2 times the static pressure.
- Pumps should be set to a lower limit frequency of around 30 Hz to prevent air pockets and potential overpressure issues.
12. Empirical and economic values for energy savings using frequency converters have been proven through many successful applications.
- The empirical value is conservative, allowing for flexibility and potential for further optimization. It should be adjusted based on site conditions to ensure normal operation while achieving energy savings.
- The economic value balances system requirements and efficiency, aiming for maximum energy savings. Inverters are highly efficient, with typical efficiencies between 97% and 98%, though they do have a small loss of 2% to 3%.
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