There are also some problems in the process of the inverter. For example, vibration and noise are the problems that often occur when the inverter is working. What is the cause of vibration and noise in the inverter? What are the problems? The solution is to introduce the causes and treatment methods of vibration and noise generated by the inverter.

The reason for vibration and noise is that when the inverter is working, the magnetic field caused by the higher harmonics in the output waveform generates electromagnetic force for many mechanical components. The frequency of the power can always be close to or coincide with the natural frequency of some mechanical components. Causes resonance.

The higher harmonics that have a large influence on the vibration are mainly lower harmonic components, and have a greater influence in the PAM (Pulse Amplitude Modulation), the pulse amplitude modulation method and the square wave PWM mode. However, when the sinusoidal PWM method is used, the low-order harmonic components are small and the influence is small. When the inverter is used to drive the motor, since the output voltage and current contain higher harmonic components, the higher harmonic flux of the air gap increases, so the noise increases. The electromagnetic noise instrumentation world network gives the following characteristics: Since the low-order harmonic component in the inverter output resonates with the rotor’s inherent mechanical frequency, the noise near the natural frequency of the rotor increases. The higher harmonic components in the inverter output resonate with the core housing and the like, and the noise near the respective natural frequencies of these components increases. The noise generated by the drive motor of the inverter, especially the harsh noise, is related to the switching frequency of the PWM control, especially in the low frequency region. Using frequency converter speed regulation will generate noise and vibration, which is the influence of high harmonic components in the output waveform of the inverter. As the operating frequency changes, the fundamental component and the higher harmonic components all vary over a wide range, which is likely to cause resonance with various parts of the motor.

Vibration and noise processing methods to reduce or eliminate vibration, you can connect the AC reactor on the output side of the inverter to absorb the higher harmonic current components in the inverter output current.

When using the PAM mode or the square wave PWM mode inverter, the sine wave PWM mode inverter can be used instead to reduce the ripple torque. In order to prevent the vibration of the mechanical system connected to the motor and the load, the entire system must not resonate with the electromagnetic force generated by the motor. The following measures are generally used to suppress and reduce noise: an AC reactor is connected to the output side of the inverter. If the electromagnetic torque has a margin, the U/f can be made smaller. When using a special motor, such as when the noise level of the lower frequency is more serious, check the resonance with the natural frequency of the shaft system (including the load).

Soft starters and frequency converters are two completely different applications. The frequency converter is used for speed regulation. The output not only changes the voltage but also changes the frequency. The soft starter is actually a voltage regulator. When the motor starts, the output only changes the voltage and does not change the frequency. The frequency converter has all the soft starter functions, but it is much more expensive than the soft starter and the structure is much more complicated.
One

The main difference between the two is:

1, soft start is to try to cut and destroy the sinusoidal waveform, it is easy, just as water flows down to the bottom;

2, the frequency converter is to do everything possible to obtain a sinusoidal waveform with adjustable frequency, it is difficult! Just as water is going to flow as high as it is difficult.
two

The characteristics of the inverter are:

1. About the inverter, it is a device that changes the frequency of three-phase sinusoidal alternating current;

2. It is an irreversible process, or impossible, because it is necessary to change the direct current into a sinusoidal AC waveform. Only the PWM widened carrier whose average voltage waveform is a sine wave can realize the frequency conversion purpose of the sinusoidal waveform;

3. The average voltage waveform is a sinusoidal PWM widened carrier, which is realized by the switching action of the full control device IGBT. The actual output of the inverter is a periodically varying rectangular wave with different widths;

4. The asynchronous motor filters out the sinusoidal waveform from the PWM widened carrier and generates a constant rotating magnetic field required for electric power;

5, and change the synchronous speed of the asynchronous motor by changing the frequency to achieve the purpose of frequency control of the AC motor
three

Differences in technical indicators:

1. The thyristor AC voltage regulating circuit is a circuit for adjusting the thyristor alternating current waveform by adjusting the conduction angle of the thyristor to obtain an alternating current with an adjustable average voltage;

2. The AC voltage regulating circuit is applied to an automatic heating temperature control circuit, a stage dimming circuit, and the like, and the load circuit related to the average voltage is independent of the AC waveform;

3, but some people and even so-called experts put this thyristor AC voltage regulator circuit for the start of asynchronous AC motor, and its name is soft start;

4. Because the asynchronous AC motor works under the three-phase symmetrical sinusoidal AC waveform, there is a constant rotating magnetic field;

5. If the alternating current of the waveform of the sinusoidal alternating current is adjusted for the conduction angle of the thyristor by using the conduction angle of the thyristor, a constant rotating magnetic field cannot be generated;

6. This AC waveform is used for the asynchronous motor to completely destroy the starting performance of the asynchronous motor. The starting torque is low and the starting current is large. There is no soft start function that everyone imagines;

7. All soft-start motors are started when they are close to full pressure. The start-up time is long, the starting current is large, and the impact on the power grid is greater;

8, asynchronous motor self-twisting step-down start, star-delta start, water resistance step-down start, etc., step-down does not change the sinusoidal waveform, are good effective starting devices, are better than soft start;

The frequency converter is mainly composed of rectification (AC to DC), filtering, re-rectification (DC to AC), braking unit, drive unit, and detection unit micro-processing unit.

1. Why is the rotational speed of the motor freely changeable?
Motor rotation speed unit: r / min rotations per minute, can also be expressed as rpm.
For example: 2-pole motor 50Hz 3000 [r/min]; 4-pole motor 50Hz 1500 [r/min]
Conclusion: The rotational speed of the motor is proportional to the frequency

The rotational speed of an inductive AC motor (hereinafter simply referred to as a motor) is approximately determined by the number of poles and frequency of the motor. The number of poles of the motor is fixed by the working principle of the motor. Since the pole value is not a continuous value (a multiple of 2, such as a pole number of 2, 4, 6), it is generally uncomfortable and the speed of the motor is adjusted by changing the value.
In addition, the frequency can be supplied to the motor after being adjusted outside the motor, so that the rotational speed of the motor can be freely controlled.
Therefore, the inverter for the purpose of controlling the frequency is the preferred device for the motor speed control device.
n = 60f/p
n: synchronization speed
f: power frequency
p: motor pole pairs
Conclusion: Changing frequency and voltage is the optimal motor control method

If the frequency is changed only without changing the voltage, the frequency will decrease and the motor will be over-voltage (overexcitation), causing the motor to be burned out. Therefore, the inverter must change the voltage at the same time while changing the frequency. When the output frequency is above the rated frequency, the voltage cannot continue to increase, and the maximum can only be equal to the rated voltage of the motor.

For example, in order to reduce the rotational speed of the motor by half, the output frequency of the inverter is changed from 50 Hz to 25 Hz, and the output voltage of the inverter needs to be changed from 400V to about 200V.

2. What is the output torque when the motor’s rotational speed (frequency) changes?
The starting torque and maximum torque when the inverter is driven are less than that of the direct-frequency power supply. When the motor is powered by the commercial power supply, the starting and acceleration shocks are large, and when the inverter is used for power supply, these impacts are weaker. A direct start of the power frequency produces a large starting current. When the inverter is used, the output voltage and frequency of the inverter are gradually added to the motor, so the starting current and impact of the motor are smaller.

Generally, the torque produced by the motor is reduced as the frequency decreases (the speed decreases). The reduced actual data is given in some of the drive manuals.

By using a flux vector controlled inverter, the torque of the motor at low speeds is improved, and even in the low speed range, the motor can output sufficient torque.

3. When the inverter is adjusted to a frequency greater than 50Hz, the output torque of the motor will decrease.
The usual motor is designed and manufactured at a voltage of 50 Hz, and its rated torque is also given in this voltage range. Therefore, the speed regulation below the rated frequency is called constant torque speed regulation. (T=Te, P<=Pe)
When the output frequency of the inverter is greater than 50Hz, the torque generated by the motor should decrease in a linear relationship inversely proportional to the frequency.

When the motor is running at a frequency greater than 50 Hz, the size of the motor load must be considered to prevent the motor from outputting insufficient torque.

For example, the torque generated by the motor at 100 Hz is reduced to approximately 1/2 of the torque at 50 Hz.

Therefore, the speed regulation above the rated frequency is called constant power speed regulation. (P=Ue*Ie)

4. Application of inverter above 50Hz
As you know, for a particular motor, its rated voltage and current rating are constant.

If the inverter and motor are rated: 15kW/380V/30A, the motor can work above 50Hz.

When the speed is 50Hz, the output voltage of the inverter is 380V, and the current is 30A. At this time, if the output frequency is increased to 60Hz, the maximum output voltage of the inverter can only be 380V/30A. Obviously, the output power is unchanged. So we call it constant power speed regulation.

What is the torque situation at this time?
Because P = wT (w: angular velocity, T: torque). Since P does not change, w increases, so the torque will decrease accordingly.

We can also look at another angle:

The stator voltage of the motor U = E + I*R (I is the current, R is the electronic resistance, and E is the induced potential)

It can be seen that when U, I are unchanged, E does not change.

And E = k*f*X, (k: constant, f: frequency, X: flux), so when f is from 50–>60Hz, X will decrease accordingly.

For the motor, T = K * I * X, (K: constant, I: current, X: flux), so the torque T will decrease as the flux X decreases.

Meanwhile, when it is less than 50 Hz, since I*R is small, the magnetic flux (X) is constant when U/f=E/f is constant. The torque T is proportional to the current. This is why the overcurrent capability of the frequency converter is often used to describe its overload (torque) capability. Also known as constant torque speed regulation (rated current is not changed -> maximum torque is unchanged)

Conclusion: When the inverter output frequency increases from above 50 Hz, the output torque of the motor will decrease.

5. Other factors related to output torque
The heat and heat dissipation capacity determine the output current capability of the inverter, which affects the output torque capability of the inverter.

Carrier frequency: Generally, the rated current of the inverter is the highest carrier frequency, and the value of continuous output can be guaranteed at the highest ambient temperature. When the carrier frequency is reduced, the current of the motor will not be affected. However, the heat of the components will decrease.

Ambient temperature: It is not like increasing the protection current value of the inverter because it detects that the ambient temperature is low.

Altitude: Increased altitude, which has an impact on heat dissipation and insulation performance. Generally less than 1000m can be ignored. It is enough to derate 5% per 1000 meters above.

(1) The overvoltage of the breaking transformer is based on the theory of intercepting overvoltage. When the transformer is disconnected, the current in the transformer inductance cannot be abrupt, and the magnetic field energy stored therein oscillates between the transformer magnetizing inductance and the ground capacitance. An overvoltage has occurred.

(2) Overvoltage generated by transformer with load closing In the actual test, the combined no-load transformer has detected several times the overvoltage of the power supply voltage. The physical principle is that the no-load transformer can still be equivalent to a magnetizing inductance and Parallel connection of the equivalent capacitance of the transformer itself. If the neutral point of the transformer is not grounded, the switch is aperiodic closing (one phase or two phase first), due to the inductance of the feeder, the capacitance of the transformer to the ground, the longitudinal capacitance and the inductance of the transformer. As a result, a higher overvoltage is generated, especially in the transformer neutral point overvoltage. Although the transformer is basically loaded with a load, the transformer will also generate an overvoltage when it is loaded with a load, but it is smaller when it is relatively empty. There is a relatively large capacitance in the real load, because the storage of the capacitor does not suddenly increase, and the transmission cable has a distributed capacitance to the ground when transmitting a high frequency oscillating voltage, and these capacitors have an absorption effect on the overvoltage. The combination of the two causes the overvoltage of the transformer during the closing process to be suppressed, but sometimes the value is still high and may even be higher than the withstand voltage of the component, which is very dangerous.

(3) The commutation overvoltage rectifying element of the rectifying element is high in steering, and therefore the steering overvoltage is also high. This not only damages components, but also creates electromagnetic interference.

Frequency converter overvoltage processing method

(1) For the breaking overvoltage of the phase shifting transformer of the frequency converter, the overvoltage absorption circuit is formed by the RC absorption network and the zinc oxide arrester, and good results are obtained.

(2) For the overvoltage generated by the transformer with load closing, a switch with good cycle performance can be selected (the switch will have different periods after long-term operation); a good resistance-capacitance absorption circuit or active suppressor technical solution is adopted; The shielded transformer can also effectively suppress the closing overvoltage. However, the difficulty of making a high-power transformer in the formation of an electrostatic shielding layer will be considerable.

(3) For the overvoltage generated by the commutation of the rectifying element, the point of attention is: the reverse withstand voltage of the rectifying element is sufficient, and the second is that the absorption circuit and the freewheeling circuit must be properly controlled. Otherwise, the rectifier device may be broken down by an overvoltage. (4) Since the overvoltage during the operation of the inverter is basically generated when the transformer is opened, it is necessary to start from the transformer to find a way to suppress the overvoltage of the inverter. Can be used:

1 Increase the magnetizing inductance of the transformer and the capacitance to the ground, increase the excitation inductance to reduce the no-load current, which will increase the cost of the transformer.

2 increase the capacitance of the transformer to the ground: in principle, it is easy to analyze, but in fact, due to the structure and material limitations of the transformer itself, it is impossible to make a transformer with any insulation method or high insulation level, so it is necessary to increase it greatly. The capacitance to ground C of the transformer is also quite difficult.

+Advantages of  Frequency inverters

The frequency converter is often used in electrical systems, but the service life of the frequency converter will also be shortened due to overvoltage and overcurrent, and once the inverter fails, it will cause serious failures in the entire electrical system. Therefore, the electrician is gradually paying attention to and paying attention to the maintenance and repair skills of the inverter. After all, anyone who wants to buy the inverter can live longer, so how should the inverter “longevity”?

The inverter includes a rectifier circuit and an inverter circuit. The input AC power is converted into a DC voltage through the rectifier circuit and the flat circuit, and then the DC voltage is converted into pulses of different widths through the inverter. Voltage (called pulse width modulation voltage, PWM). By using this PWM voltage to drive the motor, the motor torque and speed can be adjusted.
This working principle leads to the following three kinds of electromagnetic interference: