How to improve the reliability of variable frequency speed regulation system

1 How to improve the reliability of variable frequency speed regulation system will be discussed from three aspects.

(1) Improve the quality and reliability of the inverter itself.

How inverter manufacturers can produce products with stable performance, reliable operation and reasonable price to meet market demand based on users' high reliability requirements is also the key to the sustainable development of this product or this industry.

The specific requirements are as follows:

① The circuit structure of the inverter should be simple and reliable

a. It is preferred to use a true high-to-high (AC-DC-AC) direct frequency conversion main circuit.

b. The fewer power units, the better. Try to avoid connecting power units and power electronic devices in series. Because the reliability of the series system decreases (N times), while the failure rate of the series system increases (N times). Although the parallel system is conducive to improving reliability, reducing failure rate, and increasing the average life of the device, the current of the power electronic devices can reach 3KA--4KA, so they can be used without parallel connection. The main problem at present is that after the withstand voltage level of power electronic devices is further improved, power electronic devices can be used without series connection or parallel connection, which will greatly improve the reliability of the inverter.

② The selection of power electronic modules in the inverter - the withstand voltage and current ratings must have sufficient margin. A foreign inverter manufacturer once said: "Fully derating the power module can be exchanged for the reliability of the device." On the contrary, if the selected power module parameters are close to the calculated values, there is no certain margin, and there is a fluke mentality, the result will cause the inverter power module to burn out.

The reasons are nothing more than two aspects:

(1) The rated voltage and current of the power module are too small.

(2) Are the purchased power modules genuine? Are they products from famous brands? At present, domestic IGBT power modules cannot meet the requirements of high-voltage inverters, but the quality of IGBT purchased from abroad is not satisfactory. First, it is impossible to purchase military supplies (military products) and first-class products (authentic products). Most of them are products that have been screened out because of 1-2 unqualified indicators. Second, China generally does not have strict dynamic testing equipment. Those power modules that have not been selected have not passed the electrothermal aging treatment. It is reasonable for unqualified power modules to malfunction or even burn out when installed on the whole machine.

③ The power electronic module in the inverter should have sufficient ventilation and cooling measures to ensure operation within the allowable junction temperature. The lower the allowable junction temperature, the higher the reliability of the inverter.

The power module can be cooled by air cooling, water cooling or advanced heat pipe technology. Regardless of the method, the heat generated by the rectifier transformer and converter device loss must be removed to maintain the allowable junction temperature.

The manufacturer specifies the required ventilation volume based on the capacity and losses of the device.

[Note] 1) Fresh cold air is supplied from the bottom of the inverter cabinet and discharged from the top of the cabinet. Air flow in the reverse direction (from top to bottom) is not allowed.

2) A filter should be installed at the air inlet at the bottom of the inverter cabinet to prevent impurities such as dust and oil mist from entering the cabinet.

The purpose of ensuring the required ventilation volume (m3/min or CFM) of the inverter is to dissipate heat and cool the power module to work normally without exceeding the allowable junction temperature. In addition, some industrial and mining areas are located in coastal areas, rivers and lakes or in salt fog, humid and corrosive gas environments, and moisture-proof and anti-corrosion measures should also be taken. For example, in order to prevent the inverter cabinet from getting damp and the insulation from decreasing after being out of service for a few days or weeks, affecting smooth driving, a low-voltage electric heater (electric heating wire heating, infrared electric heating or far-infrared electric heating device) is installed in the inverter cabinet. When the inverter is out of service, the electric heater is automatically put into operation to ensure dehumidification and drying in the box. ④ The inverter must be well-made and securely connected.

Specific requirements:

a. Use fewer connectors, avoid plug-in methods (easy to loosen, unreliable), use welding instead of wiring terminals, and use potentiometers as little as possible.

b. Use a large area of ​​whole printed circuit board.

c. Use power electronic power modules with unique wiring-free structure to improve product reliability.

d. Reasonable layout. For example: thermal insulation and anti-electromagnetic interference measures between the rectifier transformer and the converter; photoelectric isolation and optical cable transmission between high voltage and low voltage.

⑤ The inverter should be strictly tested with load and 48-72 hours performance assessment before leaving the factory. An inverter is assembled from a large number of electrical components. In addition to selecting brand-name and genuine components that meet technical requirements, and undergoing testing and screening to remove unreliable components, the inverter must be tested for live operation before leaving the factory. Generally, a 48-72 hour continuous operation test is required, and a 7-day (24*7=168 hour) test is required for particularly reliable occasions. The assessment conditions are:

a. Load test (not no-load or light-load)

b. Have the temperature and relative humidity required for actual application.

Any inverter that has been tested and found to meet technical specifications and passed long-term assessment will have high reliability after leaving the factory and being put into operation.

2 According to the production machine load requirements and motor specifications, correctly select the inverter type and capacity matching

If the inverter itself has high reliability, but the inverter selection and capacity are not properly matched, the variable frequency speed regulation system cannot achieve high reliability and may even fail to operate.

First, choose the correct inverter type according to the load characteristics. The general principle is to match the inverter with the characteristics of the load.

(1) Constant torque production equipment - within the speed range, the load torque is basically constant. A frequency converter with constant torque performance should be selected. Its overload capacity is 150% of the rated current for 1 minute.

(2) Square torque production equipment - within the speed range, the load torque is proportional to the square of the speed, that is, M∝n2. Centrifugal fans and water pumps are typical representatives of this. The inverter with M∝n2 characteristics has a small overload capacity, 110%-120% rated current overload for 1 minute.

(3) Constant power load production equipment - within the speed range, the torque is large at low speed and small at high speed, i.e. M•NC (constant). Typical equipment include machine tools and winding mechanisms.

Of course, some inverter manufacturers do not distinguish between constant torque and square torque loads, and their products are universal. Both loads can be selected. The constant power load characteristic is achieved by relying on the V/F ratio, and there is no inverter with constant power performance.

In summary, the inverter model should be adapted to the load torque. A inverter with constant torque characteristics can be used for fan and water pump loads, while a inverter with square torque characteristics must not be used for constant torque loads.

Secondly, match the inverter capacity according to the rated parameters of the motor brand.

Usually matching principle: PEINV≥PEmotor(kW)
IEINV≥1.1-1.2IEmotor(A)

Pay attention to the parameter of current, because the power consumption of the power electronic module is IX△U (the product of current and tube voltage drop), which has no direct relationship with the output voltage of the inverter. The output power of the inverter is proportional to the product of output voltage and output current. In practice, it often happens that the output current has exceeded, but the output power has not exceeded, resulting in the failure of the power electronic power module burning. Therefore, the current index should be considered mainly.

There are various application forms of frequency conversion speed regulation main circuit structure, how to choose and match it?

(1) One-to-one single motor variable frequency speed regulation method.

Because the inverter has soft start (low voltage and low frequency starting - gradually increase voltage, frequency and speed), there is no impact current phenomenon.

Therefore, select IEINV≥(1.1-1.2)IE motor. According to the determined IEINV current value, check the product catalog to find a suitable inverter.

(2) Multi-motor variable frequency speed regulation (multiple motors share one large inverter)

For example, there are N motors with the same parameters, and K motors are started synchronously. The maximum current condition is when (NK) motors have been started and are in high-frequency and high-voltage operation, and finally K motors are started directly (the direct starting current is very large, 5-7 times that of asynchronous motors and 10-14 times that of permanent magnet synchronous motors, set as IQ motor).

The sufficient and necessary conditions for selecting a frequency converter are:
a) IEINV a ≥ (1.1-1.2) [N•IE motor]
b) IEINV b ≥ (1.1-1.2) [(NK)•IE motor + K•IQ motor]

Select a larger current value for IEINV and IE INV, and then check the product catalog to determine the specifications of the inverter.

(3) Multi-inverter multi-motor variable frequency speed control mode with shared DC power supply (the inverter and motor are still in a 1-to-1 mode)

With the advancement of frequency conversion technology, there are small inverter multi-motor solutions (essentially similar to 1-to-1 frequency conversion speed regulation) and shared DC power supply solutions (multiple inverters share a set of DC power supplies, and one inverter drives one motor.)

Common DC power supply current calculation formula: IE con ≥ (1.1-1.2) [IE motor1 + ... + IE motorn] or (1.1-1.2) • N • IE motor

3 The inverter should have a good operating environment and maintenance

Even if high-quality inverters are selected and the variable frequency drive system is matched correctly, if you want to achieve long-term safe and stable operation, you should also have a good operating environment and good equipment maintenance.

(1) The operating environment of the inverter

Almost all inverter manufacturers say that they can work in an environment with a temperature of 0-40℃ and a relative humidity of RH≤95% (no condensation). However, in order to operate the inverter more effectively, it is recommended that the inverter be placed in an air-conditioned environment with a temperature controlled at 25±3℃ and a relative humidity of RH≤70%-75%. Practice has shown that the failure rate of an inverter placed in an air-conditioned environment is much lower than that of an inverter without air-conditioning, and the reliability of the system is greatly increased.

In addition, it is best to use an independent dedicated air conditioner for the inverter, and avoid using workshop air conditioning or central air conditioning, because central air conditioning and workshop space will bring oil, dust and corrosive gases in the air into the inverter cabinet, causing damage to the inverter power electronics and microelectronics (IC integrated circuit ) components.

(2) Strengthen daily inspections and regular maintenance.

① Daily operation inspection items mainly check for abnormal phenomena, such as cooling system abnormalities, overheating, discoloration, odor, abnormal sound and abnormal vibration. Regularly record the input and output electrical parameters of the inverter to see if they are normal.

② Regular maintenance - power off and perform maintenance after the inverter is out of service. In addition to cleaning and tightening the terminals, the key maintenance items (i.e. the weak links of the inverter) are:

a. Check whether the dielectric capacitor is deformed and leaks dielectric fluid; whether it corrodes the printed circuit board, causing the insulation resistance to drop and causing IC soft failure. Foreign manufacturers stipulate that the dielectric capacitor should be replaced compulsorily after 3-5 years of operation .

b. Clean the cooling fan and filter. After 2-3 years of operation, the cooling fan should also be replaced compulsorily.

c. Check if the printed circuit board is corroded or damaged. It is best to spray the film to resist corrosion and enhance insulation performance. A chemical fiber factory in our company cleaned, dried and sprayed the printed circuit boards of Yaskawa inverters, repaired dozens of damaged inverters, and promoted this spraying technology on more than ten Toshiba inverters, achieving good results - no similar failures occurred after the repair. Of course, when spraying the film, special attention should be paid to protecting the various connector ports, and do not let the film protective agent spray in to avoid poor contact. Specifically, the connector ports can be covered with a masking agent or plastic tape before spraying the film.

4 Conclusion

The purpose of using AC variable frequency speed regulation is: first, to meet production requirements, to improve output and quality; second, to save energy by speed regulation of fans and pumps. It is very important to improve the reliability of the variable frequency speed regulation system. On the one hand, it is necessary to improve the product quality of the inverter itself, and to control the quality of design, component selection, production, installation, testing and other links. On the other hand, how to match the inverter and do a good job of daily maintenance should not be ignored.