Although the phenomenon of frequency converter damage motor more and more attention, but people do not know the mechanism of this phenomenon, let alone how to prevent.
1. The damage of inverter to motor
The damage of inverter to motor includes two aspects: stator winding damage and bearing damage. The damage usually occurs within a few weeks to a dozen months, the specific time and frequency converter brand, motor brand, motor power, frequency converter carrier frequency, between the inverter and motor cable length, environmental temperature and many other factors. The early accidental damage of motor brings great economic loss to the production of enterprise. This loss is not only the cost of motor maintenance and replacement, but also the economic loss caused by accidental shutdown. Therefore, when using frequency converter to drive motor, we must pay enough attention to the problem of motor damage.
2. The difference between inverter drive and power frequency drive
To understand the power frequency motor in the drive conditions more vulnerable to damage mechanism, first of all to understand the inverter drive motor voltage and power frequency voltage difference. Then understand how this difference has a negative impact on the motor.
The basic structure of inverter, including rectifier circuit and inverter circuit two parts. The rectifying circuit is a DC voltage output circuit composed of a common diode and a filter capacitor. The inverter transforms the DC voltage into a PWM voltage waveform. Therefore, the voltage waveform of the inverter-driven motor is a pulse waveform with varying pulse width, not a sine wave voltage waveform. Using pulse voltage to drive motor is the basic reason that leads to motor damage easily.
3. The mechanism of frequency converter damage motor stator winding
When a pulse voltage is transmitted over a cable, if the impedance of the cable does not match the impedance of the load, reflection will occur at the load end. As a result of the reflection, the incident wave is superposed with the reflected wave to form a higher voltage, which can be as large as 2 times the DC bus voltage and about 3 times the input voltage of the inverter. Too High Peak voltage is applied to the stator windings of the motor, resulting in voltage impact on the windings, frequent overvoltage impact will lead to premature motor failure.
After the motor driven by inverter is impacted by the peak voltage, its actual life is related to many factors, including temperature, pollution, vibration, voltage, carrier frequency and coil insulation technology.
The higher the carrier frequency of the inverter, the closer the output current waveform is to sine wave, which will reduce the operating temperature of the motor, thus extending the insulation life. However, higher carrier frequencies mean more spikes per second and more shocks to the motor. Figure 4 shows the insulation life as a function of cable length and carrier frequency. For a 200-foot cable, when the carrier frequency increases from 3 khz to 12 khz (a four-fold change) , the life of the insulation is reduced from about 80,000 hours to 20,000 hours (a difference of 4 times) .
The higher the motor temperature, the shorter the insulation life, when the temperature rises to 75 ° C, the motor life is only 50% . The motor driven by inverter, because the PWM voltage contains more high-frequency components, motor temperature will be far higher than the frequency voltage-driven situation.
4. The damage mechanism of inverter to motor bearing
The reason why the inverter damages the motor bearing is that there is current flowing through the bearing, and this current is in the state of discontinuous connection.
There are two main reasons that lead to the over current in the bearing of AC motor, first, the induction voltage caused by the imbalance of internal electromagnetic field, second, the high frequency current path caused by stray capacitance.
The magnetic field inside an ideal AC induction motor is symmetric. When the current in the three-phase winding is equal and the phase difference is 120, no voltage is induced on the shaft of the motor. When the PWM voltage output by the inverter causes the magnetic field inside the motor to be asymmetric, the voltage will be induced on the shaft rod, the voltage range is 10 ~ 30V, this is related to the driving voltage, the higher the driving voltage, the higher the voltage on the shaft.
A current path is formed when the value of this voltage exceeds the insulation strength of the oil in the bearing. At some point during the shaft rotation, the oil insulation blocks the current. This process is similar to the mechanical switch on and off process, this process will produce arc, ablation shaft, ball, shaft bowl surface, the formation of pits. If there is no external vibration, small pits will not have a large impact, but if there is external vibration, there will be grooves, which has a great impact on the operation of the motor.
In addition, the experiment shows that the voltage on the shaft is related to the fundamental frequency of the output voltage. The lower the fundamental frequency, the higher the voltage on the shaft, the more serious the bearing damage.
At the beginning of the motor operation, when the oil temperature is low, the current range of 5 -200 Ma, such a small current will not produce any damage to the bearing. However, when the motor runs for a period of time, as the oil temperature increases, the peak current will reach 5 -10 a, which will produce arc, the surface of the bearing parts in the formation of small pits.
5, motor stator winding protection
When the length of the cable more than 30 meters, the modern inverter will inevitably produce peak voltage in the motor end, shorten the life of the motor. To prevent motor damage, there are two ideas, one is to use winding insulation resistance higher motor (commonly known as frequency conversion motor) , the other is to take measures to reduce the peak voltage. The former is suitable for new construction projects, and the latter is suitable for retrofitting existing motors.
At present, there are four methods of motor protection in common use as follows:
1) install reactor at the output end of frequency converter: this measure is most commonly used, but it should be noted that this method for shorter cables (30 meters below) has some effect, but sometimes the effect is not ideal.
2) install DV/DT filter at the output end of the inverter: this measure is suitable for the situation where the cable length is less than 300 meters, the price is slightly higher than the reactor, but the effect has been improved obviously.
3) to install the sine wave filter at the output end of the inverter: this measure is the most ideal. Because here, the PWM pulse voltage into a sinusoidal voltage, is the motor to work with the same frequency voltage conditions, the peak voltage problem has been completely resolved (cable again long, there will not be a peak voltage) .
4) installing peak voltage absorber at the interface of cable and motor: the disadvantages of the previous measures are that when the motor power is large, the volume and weight of reactor or filter are very large, and the price is high. In addition, both reactor and filter can cause a certain voltage drop, which affects the output torque of motor.
Source: internet (for study only, intrusion and deletion)
