The motor is broken fast, and the inverter is acting as a demon? Read the secret between the motor and the inverter in one article!
Many people have discovered the phenomenon of inverter damage to the motor. For example, in a water pump factory, in the past two years, its users frequently reported that the water pump was damaged during the warranty period. In the past, the quality of the pump factory’s products was very reliable. After investigation, it was found that these damaged water pumps were all driven by frequency converters.
The emergence of frequency converters has brought innovations to industrial automation control and motor energy saving. Industrial production is almost inseparable from frequency converters. Even in daily life, elevators and inverter air conditioners have become indispensable parts. Frequency converters have begun to penetrate into every corner of production and life. However, the frequency converter also brings many unprecedented troubles, among which damage to the motor is one of the most typical phenomena.
Many people have discovered the phenomenon of inverter damage to the motor. For example, in a water pump factory, in the past two years, its users frequently reported that the water pump was damaged during the warranty period. In the past, the quality of the pump factory’s products was very reliable. After investigation, it was found that these damaged water pumps were all driven by frequency converters.
Although the phenomenon that the frequency converter damages the motor has attracted more and more attention, people still don’t know the mechanism of this phenomenon, let alone how to prevent it. The purpose of this article is to resolve these confusions.
Inverter damage to the motor
The damage of the inverter to the motor includes two aspects, the damage of the stator winding and the damage of the bearing, as shown in Figure 1. This kind of damage generally occurs within a few weeks to ten months, and the specific time depends on the brand of the inverter, the brand of the motor, the power of the motor, the carrier frequency of the inverter, the length of the cable between the inverter and the motor, and the ambient temperature. Many factors are related. The early accidental damage of the motor brings huge economic losses to the production of the enterprise. This kind of loss is not only the cost of motor repair and replacement, but more importantly, the economic loss caused by unexpected production stoppage. Therefore, when using a frequency converter to drive a motor, sufficient attention must be paid to the problem of motor damage.
Inverter damage to the motor
The difference between inverter drive and industrial frequency drive
To understand the mechanism why power frequency motors are more likely to be damaged under the condition of inverter drive, first understand the difference between the voltage of the inverter driven motor and the power frequency voltage. Then learn how this difference can adversely affect the motor.
The basic structure of the frequency converter is shown in Figure 2, including two parts, the rectifier circuit and the inverter circuit. The rectifier circuit is a DC voltage output circuit composed of ordinary diodes and filter capacitors, and the inverter circuit converts the DC voltage into a pulse width modulated voltage waveform (PWM voltage). Therefore, the voltage waveform of the inverter-driven motor is a pulse waveform with varying pulse width, rather than a sine wave voltage waveform. Driving the motor with pulse voltage is the root cause of the motor’s easy damage.
The Mechanism of Inverter Damage Motor Stator Winding
When the pulse voltage is transmitted on the cable, if the impedance of the cable does not match the impedance of the load, reflection will occur at the load end. The result of the reflection is that the incident wave and the reflected wave are superimposed to form a higher voltage. Its amplitude can reach twice the DC bus voltage at most, which is about three times the input voltage of the inverter, as shown in Figure 3. Excessive peak voltage is added to the coil of the motor stator, causing a voltage shock to the coil, and frequent overvoltage shocks will cause the motor to fail prematurely.
After the motor driven by the frequency converter is impacted by the peak voltage, its actual life is related to many factors, including temperature, pollution, vibration, voltage, carrier frequency, and coil insulation process.
The higher the carrier frequency of the inverter, the closer the output current waveform is to a sine wave, which will reduce the operating temperature of the motor and prolong the life of the insulation. However, a higher carrier frequency means that the number of spike voltages generated per second is greater, and the number of shocks to the motor is greater. Figure 4 shows the insulation life as a function of cable length and carrier frequency. It can be seen from the figure that for a 200-foot cable, when the carrier frequency is increased from 3kHz to 12kHz (a change of 4 times), the life of the insulation decreases from about 80,000 hours to 20,000 hours (a difference of 4 times).
Influence of Carrier Frequency on Insulation
The higher the temperature of the motor, the shorter the life of the insulation, as shown in Figure 5, when the temperature rises to 75°C, the life of the motor is only 50%. For a motor driven by an inverter, since the PWM voltage contains more high-frequency components, the temperature of the motor will be much higher than that of a power frequency voltage drive.
Mechanism of Inverter Damage Motor Bearing
The reason why the frequency converter damages the motor bearing is that there is a current flowing through the bearing, and this current is in a state of intermittent connection. The intermittent connection circuit will generate an arc, and the arc will burn the bearing.
There are two main reasons for the current flowing in the bearings of the AC motor. First, the induced voltage generated by the imbalance of the internal electromagnetic field, and second, the high-frequency current path caused by stray capacitance.
The magnetic field inside the ideal AC induction motor is symmetrical. When the currents of the three-phase windings are equal and the phases differ by 120°, no voltage will be induced on the shaft of the motor. When the PWM voltage output by the inverter causes the magnetic field inside the motor to be asymmetrical, a voltage will be induced on the shaft. The voltage range is 10~30V, which is related to the driving voltage. The higher the driving voltage, the higher the voltage on the shaft. high. When the value of this voltage exceeds the dielectric strength of the lubricating oil in the bearing, a current path is formed. At some point during the rotation of the shaft, the insulation of the lubricating oil stops the current again. This process is similar to the on-off process of a mechanical switch. In this process, an arc will be generated, which will ablate the surface of the shaft, ball, and shaft bowl, forming pits. If there is no external vibration, small dimples will not have too much influence, but if there is external vibration, grooves will be produced, which has a great influence on the operation of the motor.
In addition, experiments have shown that the voltage on the shaft is also related to the fundamental frequency of the output voltage of the inverter. The lower the fundamental frequency, the higher the voltage on the shaft and the more serious the bearing damage.
In the early stage of motor operation, when the lubricating oil temperature is low, the current range is 5-200mA, such a small current will not cause any damage to the bearing. However, when the motor runs for a period of time, as the temperature of the lubricating oil increases, the peak current will reach 5-10A, which will cause flashover and form small pits on the surface of the bearing components.
Protection of motor stator windings
When the length of the cable exceeds 30 meters, modern frequency converters will inevitably generate voltage spikes at the motor end, shortening the life of the motor. There are two ideas to prevent damage to the motor. One is to use a motor with higher winding insulation and dielectric strength (generally called a variable frequency motor), and the other is to take measures to reduce the peak voltage. The former measure is suitable for newly-built projects, and the latter measure is suitable for transforming existing motors.
At present, the commonly used motor protection methods are as follows:
1) Install a reactor at the output end of the frequency converter: This measure is the most commonly used, but it should be noted that this method has a certain effect on shorter cables (below 30 meters), but sometimes the effect is not ideal, as shown in Figure 6(c ) shown.
2) Install a dv/dt filter at the output end of the frequency converter: This measure is suitable for occasions where the cable length is less than 300 meters, and the price is slightly higher than that of the reactor, but the effect has been significantly improved, as shown in Figure 6(d) .
3) Install a sine wave filter at the output of the frequency converter: this measure is the most ideal. Because here, the PWM pulse voltage is changed into a sine wave voltage, the motor works under the same conditions as the power frequency voltage, and the problem of peak voltage has been completely solved (no matter how long the cable is, there will be no peak voltage) .
4) Install a peak voltage absorber at the interface between the cable and the motor: the disadvantage of the previous measures is that when the power of the motor is large, the reactor or filter has a large volume and weight, and the price is relatively high. In addition, the reactor Both the filter and the filter will cause a certain voltage drop, which will affect the output torque of the motor. Using the inverter peak voltage absorber can overcome these shortcomings. The SVA spike voltage absorber developed by 706 of the Second Academy of Aerospace Science and Industry Corporation adopts advanced power electronics technology and intelligent control technology, and is an ideal device to solve motor damage. In addition, the SVA spike absorber protects the motor’s bearings.
Spike voltage absorber is a new type of motor protection device. Connect the power input terminals of the motor in parallel.
1) The peak voltage detection circuit detects the voltage amplitude on the motor power line in real time;
2) When the magnitude of the detected voltage exceeds the set threshold, control the peak energy buffer circuit to absorb the energy of the peak voltage;
3) When the energy of the peak voltage is full of the peak energy buffer, the peak energy absorption control valve is opened, so that the peak energy in the buffer is discharged into the peak energy absorber, and the electric energy is converted into heat energy;
4) The temperature monitor monitors the temperature of the peak energy absorber. When the temperature is too high, the peak energy absorption control valve is properly closed to reduce energy absorption (under the premise of ensuring that the motor is protected), so as to prevent the peak voltage absorber from overheating and causing damage. damage;
5) The function of the bearing current absorption circuit is to absorb the bearing current and protect the motor bearing.
Compared with the aforementioned du/dt filter, sine wave filter and other motor protection methods, the peak absorber has the biggest advantages of small size, low price, and easy installation (parallel installation). Especially in the case of high power, the advantages of the peak absorber in terms of price, volume, and weight are very prominent. In addition, since it is installed in parallel, there will be no voltage drop, and there will be a certain voltage drop on the du/dt filter and the sine wave filter, and the voltage drop of the sine wave filter is close to 10%, which will cause the torque of the motor reduce.
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