The influence of the carrier frequency of the inverter and the setting standard

  Most of the inverters are in the form of PWM modulation for the inverter. That is to say, the voltage output from the inverter is actually a series of pulses, and the width and interval of the pulses are not equal. Its size depends on the intersection of the modulated wave and the carrier, which is the switching frequency. The higher the switching frequency, the greater the number of pulses in a cycle, the better the smoothness of the current waveform, but the greater the interference to other devices. The lower the carrier frequency or the poor setting, the motor will make an unpleasant noise. By adjusting the switching frequency, the noise of the system can be minimized, the smoothness of the waveform is best, and interference can be minimized. 1. Overview of low frequency inverter carrier frequency

For inverters with voltage ≤500V, almost all of the main circuits of AC-DC-AC are used today. The control mode is sinusoidal pulse width modulation (SPWM). Its carrier frequency is adjustable, generally from 1~15kHz, which is convenient. Make artificial choices. However, in actual use, many users only follow the original set value of the inverter manufacturing unit, and do not adjust according to the actual situation on site. As a result, the carrier frequency value is improperly selected, which affects the correct effective working state. How to correctly select the carrier frequency value of the inverter during the use of the inverter is also important. This article provides considerations for the following aspects and as a basis for the correct selection of carrier frequency values.

2, carrier frequency and inverter power consumption

The power loss of the power module IGBT is related to the carrier frequency, and the carrier frequency is increased, the power loss is increased, such that the efficiency is reduced, and the second is that the power module generates heat, which is disadvantageous for long-term operation, of course, the inverter The higher the operating voltage, the greater the impact on power loss.

The larger the carrier frequency, the greater the loss of the inverter and the smaller the output power. If the ambient temperature is high, the dead zone of the upper and lower inverter tubes of the inverter bridge will become smaller during the alternate conduction process. In severe cases, the bridge arm may be short-circuited and damage the inverter.

3, carrier frequency and ambient temperature

When the frequency converter requires high carrier frequency and the ambient temperature is high, it is very unfavorable for the power module. At this time, the frequency of the carrier frequency of the different power inverters and the ambient temperature are used. The size and the allowable constant output current of the inverter should be appropriately reduced to ensure safe, reliable and long-term operation of the power module IGBT.

4, carrier frequency and motor power

If the motor power is large, the relative carrier frequency is lower. The purpose is to reduce the interference (the influence on the use of other devices) and reduce the power consumption and heat generation. Generally, this principle is followed, but the specific values ​​of different manufacturers are different.

5. The relationship between the carrier frequency and the length of the secondary output (U, V, W) of the inverter.

Generally, as the output line grows, the leakage current of the inverter will increase. Therefore, if the output line is long, the carrier frequency should be appropriately reduced.

Carrier frequency: 15kHz 10kHz 5kHz 1kHz

Line length: <50m>50~100m>100~150m>150~200m

Air compressor frequency conversion modification notes and advantages

In the current operation of many air compressors, a large part of the time is carried out under non-full load conditions, so that waste of energy is generated by conventional control, resulting in inefficiency. But now there is a new generation of air compressor products, because it passes through the speed controller, so that it has the opportunity to save energy and improve efficiency when running at full load. This air compressor has a built-in speed controller VSD as part of the overall machine equipment.

In this article, we will mainly introduce the basic situation of the air compressor using the speed controller (especially the inverter), and briefly explain some small points after the air compressor uses the inverter. When the air compressor is running and the power frequency is controlled, the system is loaded for pressure control, and the motor is always in the power frequency running state. After the inverter device is modified, the pipe network pressure can be controlled under any constant pressure, and the frequency conversion system automatically controls the motor speed regulation.

The pressure sensor device is used to detect the pressure of the pipe network, and an analog signal is given to the frequency converter through the PID adjustment. The frequency converter gives the appropriate voltage and frequency to the air compressor motor, adjusts the motor speed and output power, thus forming a closed loop feedback. The system maintains a constant pressure on the pipe network and can be automatically adjusted by the internal PLC.

In the actual application, the required supply power is different depending on the usage time, day and night, depending on the amount of gas supplied. The traditional adjustment method is to load through the adjustment of the pressure valve.

It is relatively simple to install the inverter when the air compressor is installed. Remove the motor from the original AC, install it into the inverter cabinet, and take the cable from the power supply open side to the inverter cabinet. The external sampling signal is connected to the control AC to automatically adjust the motor speed to achieve constant voltage control (PID adjustment). The frequency converter itself has various protections (overvoltage, overcurrent, overload, overheating and undervoltage protection).

The advantages of using frequency conversion: easy to start without impact current, automatic adjustment, constant pressure, low noise, less wear and tear on the machine, and stable operation of the machine.

1) Startup method:

The air compressor is a large moment of inertia load. This kind of starting characteristic can easily cause the inverter of V/F control mode to skip flow protection during startup. It is recommended to use a speed sensorless vector inverter with high starting torque. To ensure the continuity of constant pressure gas supply, and to ensure reliable and stable operation of the equipment.

The air compressor is not allowed to run at low frequencies for a long time. When the speed of the air compressor is too low, the working stability of the air compressor will be deteriorated on the one hand, and the lubrication of the cylinder will be deteriorated on the other hand, which will accelerate wear. Therefore, the lower limit of the working frequency should be no less than 25Hz.

The start and stop of the motor with frequency converter speed regulation cannot be directly operated by the circuit breaker and the contactor, and the control terminal of the frequency converter is used for operation. Otherwise, the frequency converter may be out of control and may cause serious consequences.

When using the inverter to control the motor speed, the temperature rise and noise of the motor will be higher than when using the grid power (power frequency); at low speed, due to the low speed of the motor blades, attention should be paid to ventilation cooling and proper load reduction to avoid motor temperature. Rise exceeds the allowable value.

The start signal of the inverter is controlled by the delta connection AC contactor KM1, that is, the inverter does not start without output when the star type; the setting time of the air compressor time relay JS is greater than or equal to the startup time of the inverter, which ensures that the inverter is empty. The inverter starts.

The designed system is preferably equipped with two sets of control loops of frequency conversion and power frequency to ensure that the inverter has abnormal jump protection without affecting production.

Before the motor is used for the first time or after being placed for a long time before being connected to the inverter, the insulation resistance measurement of the motor must be performed (using a 500V or 1000V megohmmeter, the measured value should be not less than 5M ohm). If the insulation resistance is too low, the inverter will be damaged. When the inverter is connected to the motor, it is not allowed to measure the insulation resistance of the motor with a megohmmeter. Otherwise, the high voltage output of the megohmmeter will damage the inverter.

2) Air compressor modification inverter selection and installation environment

It is recommended to use a general-purpose inverter with a higher power than the air compressor or a special-purpose inverter of the same type as the southern Lixin air compressor, so as to avoid the frequency trip when the air compressor starts.

If the inverter is not used for more than one year, the capacitor must be recharged for one hour per year.

The frequency converter should be installed vertically, leaving room for ventilation and controlling the ambient temperature not to exceed 40 °C.

3) Air compressor frequency conversion transformation wiring installation:

When carrying out the air compressor frequency conversion transformation, care should be taken to maintain the integrity of the original equipment main circuit and control circuit as much as possible, and the less changes to its circuit are better; this is beneficial to the air pressure in the event of fault or maintenance of the inverter. The machine can be easily changed back to the original control mode, which ensures that the air compressor can be operated under both variable frequency and power frequency conditions, and the PLC program can be rewritten without modification.

The lower limit operating frequency of the inverter is set at 25-30HZ. If the HZ number is too low, it may cause the separator to be unable to effectively separate the oil and gas, resulting in oil leakage of the air compressor. However, it is necessary to consider the setting of the lower limit according to the actual situation, because the mechanical wear and the efficiency of different air compressors are not the same, and the lower limit frequency of the oil leakage is not necessarily the same.

The impedance of the power supply line should not be too small. The inverter is connected to the voltage grid. When the capacity of the distribution transformer is greater than 500KVA or the capacity of the distribution transformer is greater than 10 times the capacity of the inverter, or when the inverter is connected close to the distribution transformer, due to the loop impedance. Small, the input moment will cause a large surge to the inverter, which will damage the rectifier components of the inverter. When the line impedance is too small, an AC reactor should be installed between the grid and the inverter.

When the three-phase voltage unbalance rate of the power grid is greater than 3%, the peak value of the input current of the inverter is very large, which may cause the inverter and the connection to overheat or damage the electronic components. At this time, an AC reactor is also required. Especially when the transformer is V-shaped, it is more serious. In addition to installing a reactor on the AC side, it is necessary to install a DC reactor on the DC side.

It is not possible to install an excessively large capacitor on the incoming side because of increasing the power factor, and it is no longer possible to install a capacitor between the motor and the inverter. Otherwise, the line impedance will drop and overcurrent will be generated to damage the inverter.

It is generally not advisable to install an AC contactor between the inverter and the motor to avoid overvoltage generated in the current interruption and damage the inverter. If it needs to be installed, the output contactor should be closed before the inverter runs.

The sensor is installed on the main air supply pipe. Some gas supply systems use a check valve at the air outlet of the air compressor. The position of the sensor should be after the check valve, that is, away from the end of the air outlet. Otherwise, the pressure will drop suddenly when the air compressor is unloaded, and the frequency converter will oscillate frequently when loading and unloading frequently.

Pay attention to the thermal protection of the motor. If the motor is adapted to the drive capacity, the thermal protection inside the drive can effectively protect the motor. If the capacity of the two does not match, the protection value must be adjusted or other protective measures should be taken to ensure the safe operation of the motor. The electronic thermal protection value of the inverter (motor overload detection) can be set within the range of 25%-105% of the rated current of the inverter.

4) How to deal with interference from air compressor frequency conversion?

Some air compressor controllers have high requirements on system grounding. If the system is not reliably grounded, it will inevitably cause interference to the detection signal during frequency conversion operation. I have encountered it several times. The control line must be shielded and grounded reliably. It is best to separate it from the output cable. The grounding of the inverter cabinet must be reliable. It is best to ground independently and the air compressor should be grounded reliably. This ensures that the interference is effectively suppressed. It can also prevent the inverter from causing electric shock due to leakage.

In order to effectively filter out the higher harmonic components in the inverter output current and reduce the electromagnetic interference caused by higher harmonics, it is recommended to use the output AC reactor to reduce the motor running noise and temperature rise and improve the stability of the motor. Sex.

5) Use of braking resistor

For the case of lifting the load and frequent start and stop, there will be a large torque, and the appropriate braking resistor needs to be selected. Otherwise, the inverter will often trip due to overcurrent or overvoltage fault.

For the case where the inverter drives the ordinary motor for constant torque operation, long-term low-speed operation should be avoided as much as possible, otherwise the heat dissipation effect of the motor will be worse and the heat will be severe. If you need to run at low speed and constant torque for a long time, you must use a variable frequency motor. When the motor has another brake, the inverter should work in the free stop mode, and the brake action signal will be issued after the inverter issues a stop command. The braking resistor connected to the inverter must not be less than the braking resistor required by the inverter. Under the premise of meeting the braking requirements, the braking resistor should be larger. Do not short-circuit the terminals that should be connected to the braking resistor. Otherwise, a short-circuit accident will occur through the switch when braking.

Correctly handle the speed increase and deceleration problems. The acceleration and deceleration time set by the inverter is too short, and it is easy to be affected by “electric shock” and may damage the inverter. Therefore, when using the inverter, the acceleration and deceleration time should be extended as much as possible under the premise of the load device. If the load is heavy, the acceleration and deceleration time should be increased; otherwise, the acceleration and deceleration time can be appropriately reduced. If the load equipment needs to be added or decelerated in a short time, it must be considered to increase the capacity of the inverter to avoid too much current and exceed the rated current of the inverter. If the load equipment requires a short acceleration and deceleration time (such as within 1S), then consider using a brake system on the inverter. Generally, larger inverters are equipped with a brake system.

Summary: After the air compressor is modified by using the AC inverter, it is not only easy to operate, simple, and has a small amount of maintenance, but also has significant energy-saving effects and high-automatic adjustment to achieve high energy-saving requirements. The air compressor constant pressure air supply uses the frequency converter and pressure control to form a closed-loop control system, which reduces the pressure fluctuation to 1.5%, reduces noise, reduces vibration and ensures stable operation of the equipment. After using the frequency converter, the air compressor can be started freely under any pressure, breaking the rule that the pressure start is not allowed before, and the starting current is greatly reduced compared with the previous one. After using the frequency converter, the power saving rate is generally about 20%, which is lower than that of the fan and the pump type. However, the motor power is relatively large, and its power saving value is large, and the economic benefits are relatively significant.

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