Although the phenomenon of frequency converter damaging the motor is increasingly being paid attention to, people are still unclear about the mechanism causing this phenomenon, let alone how to prevent it.
Damage to the motor caused by the frequency converter
The damage to the motor caused by the frequency converter includes two aspects: damage to the stator winding and damage to the bearings. This kind of damage generally occurs within a few weeks to several months, and the specific time is related to many factors such as the brand of the frequency converter, the brand of the motor, the power of the motor, the carrier frequency of the frequency converter, the cable length between the frequency converter and the motor, and the ambient temperature. The early accidental damage to the motor brings huge economic losses to the enterprise's production.
This kind of loss is not only the cost of motor maintenance and replacement, but more importantly, the economic loss caused by unexpected production stoppage. Therefore, when using a frequency converter to drive the motor, it is necessary to pay sufficient attention to the issue of motor damage.
The difference between variable frequency drive and line frequency drive
To understand the mechanism by which AC motors are more prone to damage under variable frequency drive conditions, it is necessary to first understand the differences between the voltage of motors driven by variable frequency drives and the voltage of motors driven by the power frequency. Then, it is important to understand how this difference adversely affects the motor.
The basic structure of the frequency converter includes two parts: the rectifier circuit and the inverter circuit. The rectifier circuit is a DC voltage output circuit composed of ordinary diodes and filtering capacitors. The inverter circuit converts the DC voltage into a pulse width modulated voltage waveform (PWM voltage). Therefore, the voltage waveform that drives the motor with the frequency converter is a pulse waveform with varying pulse width, not a sinusoidal voltage waveform. Driving the motor with pulse voltage is the fundamental reason why the motor is prone to damage.
The mechanism of the inverter damaging the stator winding of the motor
When pulse voltage is transmitted through a cable, if the impedance of the cable does not match the impedance of the load, reflections will occur at the load end. The result of these reflections is the superposition of the incident wave and the reflected wave, resulting in a higher voltage. The amplitude of this voltage can reach up to twice the DC bus voltage, which is approximately three times the input voltage of the inverter. Excessive peak voltage applied to the coils of the motor stator can cause voltage shocks to the coils, and frequent overvoltage shocks can lead to premature motor failure.
The actual lifespan of a motor driven by a frequency converter is influenced by many factors, including temperature, pollution, vibration, voltage, carrier frequency, and the manufacturing process of coil insulation.
The higher the carrier frequency of the inverter, the closer the output current waveform is to a sine wave, which reduces the operating temperature of the motor and extends the life of the insulation. However, a higher carrier frequency means more peak voltages generated per second and more impacts on the motor. Figure 4 shows the variation of insulation life with cable length and carrier frequency. For a 200-foot cable, when the carrier frequency increases from 3kHz to 12kHz (a 4-fold change), the insulation life decreases from approximately 80,000 hours to 20,000 hours (a 4-fold difference).
The higher the temperature of the motor, the shorter the insulation life. When the temperature rises to 75°C, the motor's lifespan is only 50%. Motors driven by frequency converters, due to the presence of more high-frequency components in the PWM voltage, have much higher temperatures compared to motors driven by power frequency voltage.
The mechanism of how the frequency converter damages the motor bearings
The reason for the damage to the motor bearings by the frequency converter is that there is current flowing through the bearings, and this current is in an intermittent connected state. The intermittent connected circuit will generate an arc, which burns the bearings.
There are two main reasons for the current flowing through the bearings of the communication motor. First, the induced voltage generated by the imbalance of the internal electromagnetic field. Second, the high-frequency current path caused by stray capacitance.
The internal magnetic field of the ideal communication induction motor is symmetrical. When the currents of the three-phase windings are equal and have a phase difference of 120 degrees, no voltage will be induced on the motor's shaft. However, when the PWM voltage output from the inverter causes an imbalance in the internal magnetic field of the motor, a voltage will be induced on the shaft. The magnitude of the voltage ranges from 10 to 30V, depending on the driving voltage. The higher the driving voltage, the higher the voltage on the shaft.
When the voltage exceeds the insulation strength of the lubricating oil in the bearing, an electric current path is formed. During the rotation of the shaft, at a certain moment, the insulation of the lubricating oil interrupts the current. This process is similar to the on-off process of a mechanical switch, which generates an arc and burns the surface of the shaft, ball, and shaft bowl, forming craters. If there is no external vibration, small craters will not have a significant impact. However, if there is external vibration, grooves will be formed, which greatly affects 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 severe the bearing damage.
In the initial stage of motor operation, when the lubricating oil temperature is low, the current amplitude is between 5-200mA, such a small current will not cause any damage to the bearings. However, as the motor runs for a period of time and the lubricating oil temperature rises, the peak current can reach 5-10A, which will generate arcing 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 peak voltage at the motor end, shortening the motor's lifespan. To prevent damage to the motor, there are two approaches: one is to use a motor with higher insulation withstand voltage for the winding (generally referred to as a variable frequency motor), and the other is to take measures to reduce peak voltage. The former approach is suitable for newly constructed projects, while the latter approach is suitable for retrofitting existing motors.
Currently, there are four commonly used methods for motor protection:
1) Install a reactor at the output terminal of the frequency converter: This measure is commonly used, but it should be noted that this method has a certain effect on shorter cables (less than 30 meters), but sometimes the effect is not ideal.
2) Install a dv/dt filter at the output of the frequency converter: This measure is suitable for situations where the cable length is less than 300 meters. The price is slightly higher than that of a reactor, but the effect has been significantly improved.
3) Install a sinusoidal wave filter at the output of the inverter: This measure is the most ideal. Because here, the PWM pulse voltage is converted into a sinusoidal wave voltage, the motor works under the same conditions as the power frequency voltage, and the problem of peak voltage is completely solved (even if the cable is long, there will be no peak voltage).
4) Install surge voltage absorbers at the interface between the cable and the motor: The drawbacks of the previous measures are that when the motor has a high power, the volume and weight of the reactor or filter are large, the price is high. In addition, the reactor and filter will cause a certain voltage drop, affecting the output torque of the motor. By using a frequency converter surge voltage absorber, these drawbacks can be overcome.
