Common causes and countermeasures of inverter failures

Causes of inverter undervoltage failure:

1. Power supply phase loss

Cause: When the inverter power supply phase is lost, the three-phase rectification becomes two-phase rectification. After the load is applied, the DC voltage after rectification is low, causing undervoltage failure.

Countermeasure: Check whether the circuit breaker or contactor contacts of the inverter power supply are in good contact, whether the contact resistance is too large, whether the input voltage is normal, etc.

2. The current limiting resistor of the DC circuit inside the inverter or the thyristor of the short-circuit current limiting resistor is damaged

Cause: When the current limiting resistor or the thyristor of the short-circuit current limiting resistor is damaged, the filter capacitor inside the inverter cannot be charged, causing undervoltage failure.

Countermeasure: Find the cause of the damage to the resistor or thyristor (such as frequent motor starting, small inverter capacity and motor mismatch, etc.), and replace the current limiting resistor or thyristor.

3. Too many inverters working or starting at the same time

Cause: When multiple inverters start or work at the same time, the grid voltage will drop briefly. When the voltage drop lasts longer than the time allowed by the inverter (generally, the inverter has a minimum allowable voltage drop time), it will cause an undervoltage fault of the inverter.

Countermeasure: Try to reduce the number of inverters started or working at the same time, install AC reactors on the input side of the inverter, and increase the capacity of the power supply transformer if it doesn't work.

4. Interference from the outside world or between inverters

Cause: Interference from the outside world or mutual interference between inverters may cause abnormal operation of the inverter detection electronic circuit, resulting in false alarm of the inverter.

Countermeasure: Enhance the anti-interference ability of the inverter. For details, see "Effective Anti-interference Measures for Inverters".

Causes of inverter overvoltage failure:

1. For variable frequency speed regulation systems without braking resistors and braking units, overvoltage may occur during shutdown

Cause: The main reason is that the deceleration time is set too short, causing the motor speed to be greater than the speed at shutdown.

Countermeasure: Increase the deceleration time or install a braking resistor or braking unit.

2. For variable frequency speed regulation systems with braking resistors and braking units, overvoltage occurs during braking

Cause: The braking current is set too large or the braking time is too short, or the braking is added too early.

Countermeasure: Reduce the braking current or extend the braking time, and reduce the frequency when adding braking (add braking when the frequency drops to a lower level).

3. When the compensation capacitor is added in the substation or power supply line, the inverter will have an overvoltage failure

Cause: When the compensation capacitor is added, it will cause a spike voltage in the power grid, resulting in an inverter overvoltage failure.

Countermeasure: Install an AC reactor on the input side of the inverter.

4. Braking or deceleration time is too short

Cause: When the braking or deceleration time is too short, a large amount of energy generated by the motor feedback will accumulate on the filter capacitor, causing the inverter to overvoltage.

Countermeasure: Under the condition of meeting the control requirements, appropriately increase or extend the braking time or deceleration time.

5. Lightning overvoltage

Cause: When lightning occurs, it will cause the power grid to generate high voltage, impacting the inverter and causing overvoltage failure.

Countermeasure: As above, install an AC reactor on the input side of the inverter to enhance the inverter's ability to resist voltage changes.

6. Power supply overvoltage

Cause: Generally, the input voltage of the inverter allows a certain degree of overvoltage, but this allowed overvoltage lasts for a certain time limit. When the overvoltage lasts for a certain period of time, the inverter will alarm for overvoltage.

Countermeasure: The DC voltage upper limit of the inverter is generally set at a voltage of more than 700V, which is equivalent to an input AC power voltage of about 500V, which is more than 30% higher than 380V. This situation rarely occurs. Short-term power supply overvoltage can be prevented by installing an AC reactor.

Causes of inverter overheating failure:

1. The ambient temperature is too high

Cause: The inverter is composed of countless electronic devices, which will generate a lot of heat when working, especially when the IGBT works in a high-frequency state, the heat generated will be more. If the ambient temperature is too high, it will also cause the temperature of the internal components of the inverter to be too high. In order to protect the internal circuit of the inverter, the inverter will report a high temperature fault and shut down.

Countermeasures: Reduce the temperature of the place where the inverter is located, such as installing forced cooling measures such as air conditioning or fans.

2. Poor ventilation of the inverter

Cause: If the air duct of the inverter itself is blocked or the air duct of the control cabinet is blocked, it will affect the heat dissipation inside the inverter, causing the inverter to overheat and alarm.

Countermeasures: Regularly inspect the inverter, remove the garbage in its air duct, and smooth the air duct.

3. Fan is blocked or damaged

Cause: When the inverter fan is broken, a large amount of heat accumulates inside the inverter and cannot be dissipated.

Countermeasure: Replace the fan.

4. Overload

Cause: When the inverter is overloaded (a small horse pulling a big cart), it will generate too much current and a lot of heat. Sometimes the inverter will also overheat and alarm.

Countermeasure: Reduce the load or increase the capacity of the inverter.

Causes of inverter overcurrent:

1. The power supply voltage is too high

2. The inverter output is short-circuited

3. The V/F characteristic voltage is too high

Reason: If the V/F voltage is increased too much, the inverter output frequency is already relatively high, and the motor speed is still relatively low (that is, the change in motor speed lags behind the change in inverter frequency), it will cause a stall fault, resulting in an inverter overcurrent fault.

Countermeasure: The low-speed voltage increase should be repeatedly tested in practice, and should not be set too large, otherwise it will cause an overcurrent fault when the inverter starts.

4. The carrier frequency is set too high

Reason: When the inverter carrier frequency is set relatively high, the switching rate of the switch tube is relatively high, and the heat generation increases. At this time, the inverter's ability to resist changes in load current decreases. When the load current increases, the inverter may trip due to overcurrent. Therefore, when the inverter carrier frequency is increased, the inverter load current should also be appropriately reduced.

Countermeasure: Under the premise of meeting the speed regulation requirements, reduce the inverter carrier frequency.

5. The starting acceleration time is too short

Reason: The change in the inverter output frequency far exceeds the change in motor speed (stall), causing an overcurrent fault.

Countermeasure: Extend the inverter acceleration time.

6. Sudden increase in load

Reason: When the load suddenly increases, the current will also increase. When the current exceeds the overcurrent value set by the inverter, in order to protect the internal components of the inverter, it will report an "overcurrent" fault trip.

Countermeasure: Analyze the cause of the sudden load change. If possible, the capacity of the inverter can be appropriately increased.

7. The mechanical inertia of the transmission mechanism is too large and the capacity of the motor is relatively small

Reason: When the mechanical inertia of the transmission is large and the motor capacity is relatively small, the phenomenon of "a small horse pulling a big cart" will occur (especially at the beginning of startup), causing the motor current to be too large, resulting in the inverter overcurrent tripping.

Countermeasure: For large inertia loads, under the premise of ensuring the matching of the motor and the load, the voltage boost of the inverter at low speed startup can be appropriately increased, and the acceleration time of the inverter can be extended to prevent the occurrence of inverter overcurrent faults.

8. When reaching a certain speed, overcurrent suddenly occurs:

(1) Interference causes overvoltage and overcurrent

(2) Mechanical resonance

9. The inverter and motor capacity do not match

10. The rectifier or inverter side components in the inverter are damaged.

Reason: If the circuit breaker and the fast fuse do not respond, it is likely that the inverter tube (IGBT) is damaged. When the internal components of the inverter are damaged or the detection and control circuits fail, it often manifests as an "overcurrent" trip as soon as the inverter is powered on.

Countermeasure: Replace the components.

11. Phase loss on the power supply side of the inverter, output disconnection, internal motor fault and ground fault

Countermeasure: Check the power supply and inverter output circuit, and measure the insulation resistance between the motor phases and relative to the ground.

12. Failure of the internal detection circuit of the inverter

Cause: The detection circuit is damaged, causing the inverter to display an overcurrent alarm. For example, the Hall sensor that detects the current is affected by environmental factors such as temperature and humidity, and the working point is prone to drift, resulting in an overcurrent alarm.

Countermeasure: Replace the detection component.

Reasons for mechanical vibration of the motor when the inverter controls the motor:

1. The fixing screws of the mechanical equipment are loose, changing the original inherent oscillation frequency

Cause: Since the inverter output contains a large component of high-order harmonics, when the fixing screws of the mechanical equipment are loose, it may cause vibration of the mechanical equipment.

Countermeasure: Fix the screws.

2. The inverter is not set to "avoid frequency"

Reason: Generally, mechanical equipment has a fixed vibration frequency. For this reason, the inverter generally has a parameter called "avoid frequency" to avoid this frequency.

Countermeasure: Set the "avoid frequency" according to the output frequency of the inverter when the motor vibrates.

3. The distance between the inverter and the motor is too far

Reason: When the distance between the inverter and the motor is far, and the carrier frequency is high, the influence of the distributed capacitance between the cable and the earth increases, causing the motor to resonate.

Countermeasure: Install an output reactor to reduce the carrier frequency.

4. The operating frequency of the inverter without feedback vector control is too low. When the operating frequency is lower than 6Hz, resonance will occur due to unstable operation

5. The three-phase output voltage of the inverter is unbalanced

Reason: The three-phase voltage is unbalanced, which makes the rotating magnetic field generated by the stator winding become elliptical, causing torque imbalance and favoring motor resonance.

Countermeasure: There are many reasons for the unbalanced three-phase voltage of the inverter. For details, see "A Brief Discussion on Unbalanced Output of Inverter".

Effective anti-interference measures for frequency converters

In various industrial control systems, with the widespread use of power electronic devices such as frequency converters, the electromagnetic interference (EMI) of the system has become increasingly serious, and the corresponding anti-interference design technology (i.e. electromagnetic compatibility EMC) has become increasingly important.

The interference of the frequency converter system can sometimes directly cause damage to the hardware of the system. Sometimes, although it cannot damage the hardware of the system, it often causes the system program of the microprocessor to run out of control, resulting in control failure, thereby causing equipment and production accidents.

Therefore, how to improve the anti-interference ability and reliability of the system is an important content that cannot be ignored in the development and application of automation devices, and it is also one of the keys to the application and promotion of computer control technology. When it comes to the anti-interference problem of frequency converters, we must first understand the source and propagation mode of interference, and then take different measures against these interferences.

Sources of frequency converter interference

The first is interference from the external power grid. Harmonic interference in the power grid mainly interferes with the frequency converter through the power supply of the frequency converter. There are a large number of harmonic sources in the power grid, such as various rectifiers, AC/DC interchange equipment, electronic voltage adjustment equipment, nonlinear loads and lighting equipment. These loads cause waveform distortion of voltage and current in the power grid, thus causing harmful interference to other equipment in the power grid.

If the power supply of the inverter is disturbed by the polluted AC power grid and is not processed, the grid noise will interfere with the inverter through the power supply circuit of the power grid. The interference of the power supply to the inverter mainly includes (1) overvoltage, undervoltage, instantaneous power failure (2) surge and drop (3) peak voltage pulse (4) radio frequency interference.

1. Interference of thyristor commutation equipment on the inverter

When there is a large-capacity thyristor commutation equipment in the power supply network, since the thyristor is always turned on for part of the time in each half cycle of each phase, it is easy to cause a notch in the network voltage and serious waveform distortion. It makes it possible for the rectifier circuit on the input side of the inverter to be damaged due to the large reverse recovery voltage, which leads to the breakdown and burning of the input circuit.

2. Interference of power compensation capacitors on inverters

The power sector has certain requirements for the power factor of power users. For this reason, many users use centralized capacitor compensation in substations to improve the power factor. In the transient process of the compensation capacitor being put in or cut out, the network voltage may have a very high peak value, which may cause the inverter's rectifier diode to break down due to excessive reverse voltage.

The second is the inverter's own interference to the outside. The inverter's rectifier bridge is a nonlinear load for the power grid, and the harmonics it generates will cause harmonic interference to other electronic and electrical equipment in the same power grid. In addition, most inverters of the inverter use PWM technology. When working in switching mode and switching at high speed, a large amount of coupling noise is generated. Therefore, the inverter is an electromagnetic interference source for other electronic and electrical equipment in the system.

The input and output currents of the inverter contain many high-order harmonic components. In addition to the lower harmonics that can constitute the reactive power loss of the power supply, there are also many high-frequency harmonic components. They will spread their energy in various ways, forming interference signals to the inverter itself and other equipment.

(1) Input current waveform The input side of the inverter is a diode rectifier and capacitor filter circuit. Obviously, only when the line voltage UL of the power supply is greater than the DC voltage UD across the capacitor, there is a charging current in the rectifier bridge. Therefore, the charging current always appears near the amplitude value of the power supply voltage, in the form of a discontinuous shock wave. It has a strong high-order harmonic component. Relevant data show that the harmonic components of the 5th and 7th harmonics in the input current are the largest, which are 80% and 70% of the 50HZ fundamental wave respectively.

(2) Output voltage and current waveform The inverter bridge of most inverters adopts SPWM modulation, and its output voltage is a series of rectangular waves with a duty cycle distributed according to the sine law; due to the inductance of the motor stator winding, the stator current is very close to a sine wave. However, the harmonic component equal to the carrier frequency is still relatively large.

Propagation mode of interference signal

The inverter can generate high-power harmonics. Due to the high power, it has strong interference to other equipment in the system. Its interference path is consistent with the general electromagnetic interference path, which is mainly divided into conduction (i.e. circuit coupling), electromagnetic radiation, and inductive coupling. Specifically: first, it generates electromagnetic radiation to the surrounding electronic and electrical equipment; second, it generates electromagnetic noise to the directly driven motor, which increases the iron loss and copper loss of the motor; and conducts interference to the power supply, which is transmitted to other equipment in the system through the distribution network; finally, the inverter generates inductive coupling to other adjacent lines, inducing interference voltage or current. Similarly, the interference signal in the system interferes with the normal operation of the inverter through the same path.

(1) Circuit coupling mode, that is, propagation through the power supply network. Since the input current is a non-sinusoidal wave, when the capacity of the inverter is large, the network voltage will be distorted, affecting the operation of other equipment. At the same time, the conducted interference generated at the output end greatly increases the copper loss and iron loss of the directly driven motor, affecting the running characteristics of the motor. Obviously, this is the main propagation mode of the inverter input current interference signal.

(2) Inductive coupling method When the input circuit or output circuit of the inverter is very close to the circuit of other equipment, the high-order harmonic signal of the inverter will be coupled to other equipment by induction. There are two inductive methods:

a. Electromagnetic induction method, which is the main method of current interference signal;

b. Electrostatic induction method, which is the main method of voltage interference signal.

(3) Aerial radiation method, that is, radiating into the air in the form of electromagnetic waves, which is the main propagation method of high-frequency harmonic components.

Anti-interference countermeasures of variable frequency speed regulation system

According to the basic principle of electromagnetics, the formation of electromagnetic interference (EMI) must have three elements: electromagnetic interference source, electromagnetic interference path, and system sensitive to electromagnetic interference. To prevent interference, hardware anti-interference and software anti-interference can be used.

Among them, hardware anti-interference is the most basic and important anti-interference measure of the application system. Generally, it starts from the two aspects of resistance and prevention to suppress interference. Its general principle is to suppress and eliminate the interference source, cut off the coupling channel of interference to the system, and reduce the sensitivity of the system interference signal. Specific measures in engineering can be isolation, filtering, shielding, grounding and other methods.

1. The so-called interference isolation refers to isolating the interference source and the susceptible part from the circuit so that they do not have electrical contact. In the variable frequency speed drive system, an isolation transformer is usually used on the power line between the power supply and the amplifier circuit to avoid conducted interference, and the power isolation transformer can be applied to the noise isolation transformer.

2. The function of setting a filter in the system line is to suppress the interference signal from the inverter through the power line to the power supply from the motor. In order to reduce electromagnetic noise and loss, an output filter can be set on the output side of the inverter; in order to reduce interference to the power supply, an input filter can be set on the input side of the inverter. If there are sensitive electronic equipment in the line, a power supply noise filter can be set on the power line to avoid conducted interference.

In the input and output circuits of the inverter, in addition to the above-mentioned lower harmonic components, there are many high-frequency harmonic currents, which will transmit their energy in various ways to form interference signals to other equipment. The filter is the main means to weaken the higher-frequency harmonic components. According to the different places of use, it can be divided into:

(1) Input filter Usually there are two types:

a. Line filter Mainly composed of inductor coils. It weakens high-frequency harmonic currents by increasing the impedance of the line at high frequencies.

b. Radiation filter Mainly composed of high-frequency capacitors. It absorbs high-frequency harmonic components with radiation energy.

(2) Output filter Also composed of inductor coils. It can effectively weaken the high-order harmonic components in the output current. Not only does it play an anti-interference role, but it can also weaken the additional torque caused by high-order harmonic currents in the motor. For the anti-interference measures at the output end of the inverter, the following aspects must be noted:

a. The output end of the inverter is not allowed to be connected to a capacitor, so as to avoid generating a large peak charging (or discharging) current at the moment when the inverter tube is turned on (off), which will damage the inverter tube;

b. When the output filter is composed of an LC circuit, the side of the filter connected to the capacitor must be connected to the motor side.

3. Shielding the interference source is the most effective way to suppress interference. Usually the inverter itself is shielded with an iron shell to prevent electromagnetic interference from leaking; the output line is best shielded with a steel pipe. Especially when the inverter is controlled by an external signal, the signal line is required to be as short as possible (generally within 20m), and the signal line is shielded with a double core, and is completely separated from the main circuit line (AC380V) and the control line (AC220V). It must not be placed in the same piping or cable tray, and the surrounding electronic sensitive equipment lines are also required to be shielded. In order to make the shielding effective, the shielding cover must be reliably grounded.

4. Correct grounding can not only effectively suppress external interference in the system, but also reduce the interference of the equipment itself to the outside world. In the actual application system, due to the system power supply neutral line (neutral line), ground line (protective grounding, system grounding) is not separated, and the control system shielding ground (control signal shielding ground and main circuit wire shielding ground) is connected in a chaotic manner, which greatly reduces the stability and reliability of the system.

For the inverter, the correct grounding of the main circuit terminal PE (E, G) is an important means to improve the inverter's ability to suppress noise and reduce the inverter's interference, so it must be taken very seriously in actual applications. The cross-sectional area of ​​the inverter grounding wire should generally be no less than 2.5mm2, and the length should be controlled within 20m. It is recommended that the inverter grounding be separated from the grounding points of other power equipment and not share the same ground.

5. Use reactor

The proportion of low-frequency harmonic components (5th harmonic, 7th harmonic, 11th harmonic, 13th harmonic, etc.) in the inverter input current is very high. In addition to possibly interfering with the normal operation of other equipment, they also consume a large amount of reactive power, which greatly reduces the power factor of the line. Inserting a reactor in series in the input circuit is an effective way to suppress lower harmonic currents. Depending on the wiring position, there are mainly two types:

(1) Reactor is connected in series between the power supply and the input side of the inverter. Its main functions are:

a. By suppressing harmonic current, the power factor is increased to (0.75-0.85);

b. Weaken the impact of surge current in the input circuit on the inverter;

c. Weaken the impact of power supply voltage imbalance.

(2) DC reactor is connected in series between the rectifier bridge and the filter capacitor. Its function is relatively simple, which is to weaken the high-order harmonic components in the input current. However, it is more effective than AC reactor in improving the power factor, which can reach 0.95, and has the advantages of simple structure and small size.

6. Reasonable wiring

For interference signals transmitted by induction, they can be weakened by reasonable wiring. Specific methods are:

(1) The power line and signal line of the equipment should be kept away from the input and output lines of the inverter;

(2) The power line and signal line of other equipment should avoid being parallel to the input and output lines of the inverter;

Conclusion

Through the analysis of the source and propagation path of interference in the application of the inverter, practical countermeasures to solve these problems are proposed. With the continuous application of new technologies and new theories in the inverter, paying attention to the EMC requirements of the inverter has become a problem that must be faced in the design and application of the variable frequency speed regulation transmission system, and it is also one of the keys to the application and promotion of the inverter.

These problems of the inverter are expected to be solved through the function and compensation of the inverter itself. Industrial sites and social environments are placing increasing demands on inverters, and truly "green" inverters that meet actual needs will soon be available. We believe that the EMC problem of inverters will be effectively solved.

General inverter selection specification

The correct selection of the inverter is very critical for the normal operation of the control system. When selecting the inverter, you must fully understand the load characteristics driven by the inverter. In practice, people often divide production machinery into three types: constant torque load, constant power load and fan and pump load.

Constant torque load:

The load torque TL is independent of the speed n, and TL always remains constant or basically constant at any speed. For example, friction loads such as conveyor belts, mixers, extruders, and potential loads such as cranes and hoists are all constant torque loads.

When the inverter drags a load with constant torque properties, the torque at low speed must be large enough and have sufficient overload capacity. If it is necessary to run at a steady speed at a low speed, the heat dissipation capacity of the standard asynchronous motor should be considered to avoid excessive temperature rise of the motor.

Constant power load:

The torque required by the machine tool spindle and the winder and unwinder in the rolling mill, papermaking machine, and plastic film production line is generally inversely proportional to the speed, which is the so-called constant power load. The constant power nature of the load should be in terms of a certain speed change range. When the speed is very low, due to the limitation of mechanical strength, TL cannot increase infinitely and transform into constant torque property at low speed. The constant power area and constant torque area of ​​the load have a great influence on the selection of transmission scheme.

When the motor is in constant flux speed regulation, the maximum allowable output torque remains unchanged, which belongs to constant torque speed regulation; while in weak magnetic speed regulation, the maximum allowable output torque is inversely proportional to the speed, which belongs to constant power speed regulation. If the range of constant torque and constant power speed regulation of the motor is consistent with the constant torque and constant power range of the load, that is, the so-called "matching" case, the capacity of the motor and the capacity of the inverter are both minimum.

Fan and pump loads:

In various fans, water pumps, and oil pumps, as the impeller rotates, the resistance generated by air or liquid within a certain speed range is roughly proportional to the square of the speed n. As the speed decreases, the speed decreases by the square of the speed. The power required for this load is proportional to the cube of the speed. When the required air volume and flow rate decrease, using the inverter to adjust the air volume and flow rate by speed regulation can greatly save electricity. Since the power required at high speed increases too fast with the speed and is proportional to the cube of the speed, fans and pumps should not be operated at over-power frequency.

Siemens can provide different types of inverters, and users can choose different types of inverters according to their actual process requirements and application occasions. When selecting an inverter, pay attention to the following points:

1. Select the inverter according to the load characteristics. If the load is a constant torque load, you need to choose a siemens MMV/MDV inverter. If the load is a fan or pump, you should choose a siemens ECO inverter.

2. When selecting an inverter, the actual motor current value should be used as the basis for inverter selection, and the rated power of the motor can only be used as a reference. In addition, it should be fully considered that the output of the inverter contains high-order harmonics, which will cause the power factor and efficiency of the motor to deteriorate. Therefore, compared with the power supply of the power grid, the motor current increases by 10% and the temperature rise increases by about 20% when the inverter is used to power the motor. Therefore, when selecting motors and inverters, this situation should be taken into consideration and a proper margin should be left to prevent excessive temperature rise and affect the service life of the motor.

3. If the inverter is to run with a long cable, measures should be taken to suppress the influence of the long cable on the ground coupling capacitance to avoid insufficient output of the inverter. Therefore, the inverter should be selected by one gear or an output reactor should be installed at the output end of the inverter.

4. When the inverter is used to control several motors in parallel, it is necessary to consider that the total length of the cable from the inverter to the motor is within the allowable range of the inverter. If it exceeds the specified value, the inverter should be selected by one or two gears. In addition, in this case, the control mode of the inverter can only be V/F control mode, and the inverter cannot protect the motor from overcurrent and overload protection. At this time, a fuse needs to be added to each motor to achieve protection.

5. For some special applications, such as high ambient temperature, high switching frequency, high altitude, etc., this will cause the inverter to reduce capacity, and the inverter needs to be selected by one gear.

6. When using a frequency converter to control a high-speed motor, due to the small reactance of the high-speed motor, the high-order harmonics also increase the output current value. Therefore, when selecting a frequency converter for a high-speed motor, it should be slightly larger than the frequency converter for an ordinary motor.

7. When the frequency converter is used for a pole-changing motor, full attention should be paid to selecting the capacity of the frequency converter so that its maximum rated current is below the rated output current of the frequency converter. In addition, when the pole number conversion is performed during operation, the motor should be stopped first, otherwise the motor will run idle, and the frequency converter will be damaged in severe conditions.

8. When driving an explosion-proof motor, the frequency converter does not have an explosion-proof structure, and the frequency converter should be set outside the dangerous place.

9. When using a frequency converter to drive a gear reduction motor, the scope of use is restricted by the lubrication method of the gear rotating part. When lubricating with lubricating oil, there is no limit in the low speed range; in the high speed range exceeding the rated speed, there is a risk of running out of lubricating oil. Therefore, do not exceed the maximum allowable speed.

10. When the frequency converter drives a wound rotor asynchronous motor, most of them use existing motors. Compared with ordinary squirrel cage motors, the winding impedance of wound motors is small. Therefore, it is easy to cause overcurrent tripping due to ripple current, so a frequency converter with a slightly larger capacity than usual should be selected. Generally, wound motors are mostly used in occasions with large flywheel torque GD2, and more attention should be paid when setting the acceleration and deceleration time.

11. When the frequency converter drives a synchronous motor, the output capacity is reduced by 10% to 20% compared with the industrial frequency power supply, and the continuous output current of the frequency converter must be greater than the product of the rated current of the synchronous motor and the per-unit value of the synchronous pull-in current.

12. For loads with large torque fluctuations such as compressors and vibrators, and peak loads such as hydraulic pumps, if the frequency converter is selected according to the rated current or power value of the motor, the overcurrent protection may be activated due to the peak current. Therefore, the industrial frequency operation should be understood, and a frequency converter with a rated output current greater than its maximum current should be selected. When the frequency converter drives the submersible pump motor, because the rated current of the submersible pump motor is larger than that of the ordinary motor, when selecting the frequency converter, its rated current should be greater than the rated current of the submersible pump motor.

13. When the frequency converter controls the Roots blower, because its starting current is very large, when selecting the frequency converter, you must pay attention to whether the capacity of the frequency converter is large enough.

14. When selecting the frequency converter, you must pay attention to whether its protection level matches the situation on site. Otherwise, the dust and water vapor on site will affect the long-term operation of the frequency converter.

15. Single-phase motors are not suitable for frequency converter drive.