Maintenance and fault diagnosis of VFD (Variable-frequency Drive)

1 Maintenance of VFD (Variable-frequency Drive)

The maintenance of VFD (Variable-frequency Drive) includes daily inspection and regular inspection. If any fault is found, it must be diagnosed and repaired in time, so as to keep the VFD (Variable-frequency Drive) in good condition.

1.1 Common sense of VFD (Variable-frequency Drive) operation inspection

VFD (Variable-frequency Drive) can be checked together with the motor and mechanical equipment during operation. Usually include the following.

1) Monitor whether the working current exceeds the allowable value of VFD (Variable-frequency Drive) and motor. The current value marked on the VFD (Variable-frequency Drive) nameplate refers to the rated current value, which refers to the allowable current value of the VFD (Variable-frequency Drive) when the indoor air temperature or ambient air temperature is 35°C. The current value is reduced, because the insulation value of VFD (Variable-frequency Drive) will decrease when the ambient temperature exceeds 35 ℃. According to experience, the rated current should be reduced by 5% for every 5% increase in ambient air. If the ambient air temperature is lower than 35°C, the rated current of the VFD (Variable-frequency Drive) will increase by 8% at about 30°C.

2) Monitor whether the three-phase current is balanced. Whether the three-phase current of VFD (Variable-frequency Drive) is balanced or not depends on the motor. Therefore, monitoring the balance of the three-phase current should be considered comprehensively. If some optional accessories (such as reactors, etc.) are added, careful and detailed analysis is required. Under normal circumstances, the maximum value of any phase current does not exceed the rated value, and the percentage tolerance of the unequal current of each phase can reach 10%. Or adjust the phase sequence of the input power to make it the minimum.

The most serious unbalanced three-phase current is the blown fuse of one phase of the power supply. In this case, the current of the other two phases is very large, which is extremely unfavorable to VFD (Variable-frequency Drive) and motor, so in VFD In the (Variable-frequency Drive) circuit, it is repeatedly emphasized that a circuit breaker should be used instead of a fuse.

3) Monitor changes in power supply voltage. The change of power supply voltage is not only one of the reasons for the heating of the motor, but also affects the frequency conversion of VFD (Variable-frequency Drive) and the speed of the motor. In actual work, the stability of the power supply voltage has a floating range, so the variation margin of the power supply voltage is generally allowed to be within 10%. If a power regulator is used to power VFD (Variable-frequency Drive), the power supply voltage is basically stable, and the effect is of course better, but it will increase the cost. The taps of the power transformer can also be adjusted manually.

4) Monitor the heating of VFD (Variable-frequency Drive). It is very important to monitor the heating of VFD (Variable-frequency Drive) and motor during operation, because the main circuit damage of many VFD (Variable-frequency Drive) and motor winding insulation damage are mostly caused by excessive temperature. There are many methods for measuring the maximum allowable temperature of the motor coil, such as resistance measurement, thermocouple measurement, thermometer measurement, and color-changing temperature-measuring patch test. To monitor the heating condition of VFD (Variable-frequency Drive), you can use thermometer measurement method or color-changing temperature measurement patch method.

The color-changing temperature-measuring patch is a product that can change color as the temperature of the object changes, and thus can measure the temperature change of the object. It is especially suitable for parts that are difficult to measure with ordinary thermometers, external surfaces, and objects in high-risk environments, such as VFD (Variable-frequency Drive) main circuit thyristor, high-power rectifier, etc.

The color-changing temperature measuring patch can be divided into single color change or multi-color change, reversible or irreversible according to the color change. For example, liquid crystal color-changing products can change colors such as red, green, and blue within a specified small temperature range. Different colors correspond to different temperatures. As the temperature recovers, the color also recovers. This belongs to the reversible multi-color series. . Only one color change occurs is the monochrome series. Color irreversible is an irreversible series. In most cases, if there is no one monitoring the temperature measurement site, but to be observed by inspectors, an irreversible color-changing temperature measurement patch is required. After the temperature is overheated, its color will not return, that is, the overheated color will be retained. Because it can remember the occurrence process and hidden dangers of over-temperature accidents, it is favored by users.

In order to facilitate the selection of suitable VFD (Variable-frequency Drive) measurement, the color-changing temperature measurement patch has products with various temperature levels such as 55 ohms, 60 ohms, 65 ohms, 70 ohms, 75 ohms, 80 ohms, and 85 ohms. In the relevant parts of VFD (Variable-frequency Drive) where the detection standard has been clearly defined, a temperature measuring chip that changes from white to red at a single temperature measurement point can be used, which is more intuitive and obvious during detection. If the observation distance is long and the light is poor, you can choose a large-size 2 cm × 2 cm or 3 cm × 3 cm temperature measurement patch, or you can choose a round temperature measurement patch.

The available colors are not only white to red, but also white to yellow, white to green, white to black, etc.

If you want a wider temperature measurement range, or if you have not determined the temperature measurement standard for a while, you can choose temperature measurement patches of several temperature levels at the same time. In order to distinguish different temperatures from each other, temperature measuring patches of different sizes, colors or shapes can be pasted and arranged to represent the difference in discoloration temperature. For example, use 60 Yibai to yellow, 70 Yibai to green, 80 Yibai to red three kinds of patches