VFD (Variable-frequency Drive) the secret of motor damage

The emergence of VFD (Variable-frequency Drive) has brought innovations to industrial automation control and motor energy saving. Industrial production is almost inseparable from VFD (Variable-frequency Drive). Even in daily life, elevators and inverter air conditioners have become indispensable parts. VFD (Variable-frequency Drive) has begun to penetrate into every corner of production and life. However, VFD (Variable-frequency Drive) also brings many unprecedented troubles, among which damage to the motor is one of the most typical phenomena.

Many people have discovered that VFD (Variable-frequency Drive) can damage motors. 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 driven by VFD (Variable-frequency Drive).

Although the phenomenon of VFD (Variable-frequency Drive) damage to motors 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.

VFD (Variable-frequency Drive) damage to the motor

The damage of VFD (Variable-frequency Drive) 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 is related to many factors such as the brand of the VFD (Variable-frequency Drive), the brand of the motor, the power of the motor, the carrier frequency of the VFD (Variable-frequency Drive), the length of the cable between the VFD (Variable-frequency Drive) and the motor, and the ambient temperature. 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 more importantly, the economic loss caused by unexpected production stoppage. Therefore, when using a VFD (Variable-frequency Drive) to drive a motor, sufficient attention must be paid to the problem of motor damage.

Figure 1 VFD (Variable-frequency Drive) damage to the motor

The difference between VFD (Variable-frequency Drive) drive and industrial frequency drive

To understand the mechanism why power frequency motors are more likely to be damaged under VFD (Variable-frequency Drive) driving conditions, first understand the difference between the voltage of VFD (Variable-frequency Drive) driving motors and the power frequency voltage. Then learn how this difference can adversely affect the motor.

The basic structure of VFD (Variable-frequency Drive) is shown in Figure 2, including two parts: a rectifier circuit and an 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 VFD (Variable-frequency Drive) driving 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 VFD (Variable-frequency Drive) damage to 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 VFD (Variable-frequency Drive), 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.

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After the motor driven by VFD (Variable-frequency Drive) 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 VFD (Variable-frequency Drive), 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).

Fig. 4 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 motors driven by VFD (Variable-frequency Drive), since the PWM voltage contains more high-frequency components, the temperature of the motor will be much higher than that of a commercial-frequency voltage drive.

The mechanism of VFD (Variable-frequency Drive) damage to motor bearings

The reason why VFD (Variable-frequency Drive) 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 intermittently connected 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 VFD (Variable-frequency Drive) 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. 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 mechanical