Many people have already discovered the damage of the inverter to the motor. For example, in a water pump factory, in the past two years, his users frequently reported that the pump was damaged during the warranty period. In the past, the quality of this pump factory was very reliable. After investigation, it was found that these damaged pumps were driven by inverters.
The emergence of frequency converters has brought innovation to industrial automation control and motor energy saving. In industrial production, the inverter is almost inseparable. Even in daily life, elevators and inverter air conditioners have become indispensable parts. The inverters have begun to penetrate into all corners of production and life. However, the frequency converter has also brought many unprecedented problems, and the damage motor is one of the most typical phenomena.
Many people have already discovered the damage of the inverter to the motor. For example, in a water pump factory, in the past two years, his users frequently reported that the pump was damaged during the warranty period. In the past, the quality of this pump factory was very reliable. After investigation, it was found that these damaged pumps were driven by inverters.
Although the phenomenon that the inverter damages the motor is getting more and more attention, people are still unclear about the mechanism that causes this phenomenon, and they do not know how to prevent it. The purpose of this article is to address 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 in a few weeks to ten months, the specific time and the brand of the inverter, the brand of the motor, the power of the motor, the carrier frequency of the inverter, the cable length between the inverter and the motor, the ambient temperature, etc. Many factors are related. The early accidental damage of the motor brings huge economic losses to the production of the enterprise. This loss is not only the cost of motor repair and replacement, but also the economic loss caused by accidental shutdown. Therefore, when using an inverter to drive a motor, it is necessary to pay sufficient attention to the problem of motor damage.
The difference between inverter drive and power frequency drive
To understand the mechanism that the power frequency motor is more susceptible to damage under the driving conditions of the inverter, first understand the difference between the voltage of the inverter drive motor and the power frequency voltage. Then understand how this difference has an adverse effect on the motor.
The basic structure of the inverter is shown in Figure 2, including the rectifier circuit and the inverter circuit. The rectifier circuit is a DC voltage output circuit composed of a common diode and a filter capacitor, and the inverter circuit converts the DC voltage into a pulse width modulated voltage waveform (PWM voltage). Therefore, the voltage waveform of the inverter drive motor is a pulse waveform with a pulse width change instead of a sine wave voltage waveform. Driving the motor with a pulsed voltage is the root cause of the motor's easy damage.
Mechanism of damage to stator winding of motor by inverter
When the pulse voltage is transmitted over the cable, if the impedance of the cable does not match the impedance of the load, reflections will occur at the load end. As a result of the reflection, the incident wave and the reflected wave are superimposed to form a higher voltage, and its amplitude can be up to twice the DC bus voltage, which is approximately three times the input voltage of the inverter, as shown in FIG. Excessive spike voltage is applied to the coil of the motor stator, causing a voltage surge to the coil. Frequent overvoltage surges can cause premature motor failure.
After the inverter-driven motor is subjected to a spike voltage, its actual life is related to many factors, including temperature, pollution, vibration, voltage, carrier frequency, and coil insulation.
The higher the carrier frequency of the inverter, the closer the output current waveform is to the sine wave, which will reduce the operating temperature of the motor and thus extend the life of the insulation. However, a higher carrier frequency means more spikes per second and more impact on the motor. Figure 4 shows the insulation life as a function of cable length and carrier frequency. As can be seen from the figure, for a 200-foot cable, when the carrier frequency is increased from 3 kHz to 12 kHz (a change of 4 times), the life of the insulation is reduced from about 80,000 hours to 20,000 hours (a difference of 4 times).
The effect of carrier frequency on insulation
The higher the temperature of the motor, the shorter the insulation life, as shown in Figure 5, when the temperature rises to 75? At C, the life of the motor is only 50%. The motor driven by the inverter, because the PWM voltage contains more high-frequency components, the motor temperature will be much higher than the power frequency voltage drive.
Mechanism of damage to motor bearings by inverter
The reason why the inverter damages the motor bearing is that there is a current flowing through the bearing, and the current is in an intermittent communication state, the circuit that is intermittently connected generates an arc, and the arc burns the bearing.
There are two main reasons for the current flowing through the bearings of the AC motor. First, the induced voltage generated by the internal electromagnetic field imbalance, and the second, the high-frequency current path caused by the 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 phase difference is 120? When the voltage is not induced on the shaft of the motor. When the PWM voltage outputted by the inverter causes the magnetic field inside the motor to be asymmetrical, the voltage is induced on the shaft. The amplitude of the voltage 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 insulation strength of the lubricating oil in the bearing, a current path is formed. During the rotation of the shaft, at some point, the insulation of the lubricating oil blocks the current. This process is similar to the on-off process of a mechanical switch, in which an arc is generated, ablation of the shaft, balls, and the surface of the shaft bowl to form a dimple. If there is no external vibration, the small pit will not have an excessive influence, but if there is external vibration, a groove will be generated, 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 inverter output voltage. The lower the fundamental frequency, the higher the voltage on the shaft and the more serious the bearing damage.
At the beginning of the motor operation, when the temperature of the lubricating oil is low, the current amplitude is 5-200 mA, and such a small current will not cause any damage to the bearing. However, when the motor is running for a period of time, as the temperature of the lubricating oil rises, the peak current will reach 5-10A, which will cause arcing and form small pits on the surface of the bearing component.
Motor stator winding protection
When the length of the cable exceeds 30 meters, the modern inverter will inevitably generate a spike voltage at the motor end, shortening the life of the motor. To prevent damage to the motor, there are two ideas. One is to use a motor with higher insulation resistance than the winding insulation (generally called a variable frequency motor), and the other is to take measures to reduce the peak voltage. The former measure is suitable for new projects, and the latter measure is suitable for retrofitting existing motors.
At present, there are four commonly used motor protection methods:
1) Install a reactor at the output of the inverter: This measure is most commonly used, but it should be noted that this method has a certain effect on a shorter cable (less than 30 meters), but sometimes the effect is not ideal, as shown in Figure 6 (c ) shown.
2) Install the dv/dt filter at the output of the inverter: This measure is suitable for the case where the cable length is less than 300 meters, and the price is slightly higher than the reactor, but the effect is obviously improved, as shown in Figure 6(d). .
3) Install a sine wave filter at the output of the inverter: this measure is ideal. Because here, the PWM pulse voltage is changed to a sine wave voltage, and the motor operates under the same conditions as the power frequency voltage. The problem of the spike voltage is completely solved (the cable is long and the peak voltage does not occur) .
4) Install the peak voltage absorber at the position of the cable and motor interface: 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 high. In addition, the reactor Both the filter and the filter will cause a certain voltage drop, affecting the output torque of the motor, and the inverter spike absorber can overcome these shortcomings. The SVA peak voltage absorber developed by the Aerospace Science and Industry Group Second Hospital 706 is an ideal device for solving motor damage by using advanced power electronics technology and intelligent control technology. In addition, the SVA spike absorber protects the bearings of the motor.
The spike voltage absorber is a new type of motor protection device, as shown in Figure 7 (the Aerospace Science and Industry Group's SVA model). Connect the power input of the motor in parallel.
The block diagram of the SVA spike voltage absorber is shown in Figure 8. Its working process is as follows :
1) The peak voltage detecting circuit detects the voltage amplitude of the motor power line in real time;
2) controlling the spike energy buffer circuit to absorb the energy of the spike voltage when the magnitude of the detected voltage exceeds a set threshold;
3) When the energy of the spike voltage is filled with the peak energy buffer, the peak energy absorption control valve is opened, and the peak energy in the buffer is discharged to the peak energy absorber to convert the electrical energy into heat energy;
4) The temperature monitor monitors the temperature of the peak energy absorber. When the temperature is too high, properly close the peak energy absorption control valve to reduce energy absorption (on the premise of ensuring the motor is protected), to avoid overheating of the peak voltage absorber. damage;
5) The function of the bearing current absorbing circuit is to absorb the bearing current and protect the motor bearing.
Compared with the motor protection methods such as the du/dt filter and the sine wave filter described above, the peak absorber has the 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 outstanding. In addition, since it is installed in parallel, there is no voltage drop, and there is a certain voltage drop on the du/dt filter and the sine wave filter. 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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