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What is a VFD cable, cable design and its functions?

According to the working principle of various frequency converters, VFD cables should focus on solving the following problems:

(1) The cable body emits electromagnetic waves to the outside, suppressing the interference of higher-order harmonics through the cable to the outside.

In all kinds of factories, the motor power is large, and the length of  VFD cable connecting the variable frequency motor and the variable frequency power supply is long. When working, the cable is an effective carrier for the outward emission of high-frequency electromagnetic waves, which will interfere with the communication tools (such as cell phone) or amplitude modulation receivers (such as radio amplitude modulation band) in the surrounding areas. To achieve effective suppression of high-frequency interference, the shielding structure of  VFD cables is particularly important. The shielding structure is the best method to suppress external high-frequency interference, and the shielding structure is copper wire woven shielding. As the density of copper wire weaving increases and the thickness of copper tape increases, the shielding suppression coefficient also continuously increases, and the shielding effect becomes better;

(2) The impact of pulse voltage on insulation to prevent the impact of pulse voltage on VFD cable. it solves the problem of anti-interference ability and insulation safety and reliability from the perspective of cable structure design.

The frequency regulation range of variable frequency power supply is wide. Regardless of the frequency, it has a main frequency waveform profile, which contains many high-order harmonics. As a traveling wave, the amplitude superposition can reach several times of the working voltage after multiple reflections. The longer the VFD cable, the higher the amplitude. If the Factor of safety of cable insulation is not high, it may be broken down. Therefore, to ensure cable safety, we should start from the following three aspects:

(1) Increase the insulation thickness, improve the insulation voltage resistance, and select materials with better insulation performance.

(2) Add a semi conductive layer outside the conductor to homogenize the electric field and reduce tip discharge.

(3) The VFD cable adopts a symmetrical structure to achieve uniform electric field and phase balance.

3.Shielding layer grounding measures

A good grounding of the shielding layer is a necessary condition for suppressing the external emission of electromagnetic waves. The grounding method of copper wire braided shielding is relatively easy to solve, while the longitudinal wrapped copper strip corrugated shielding needs to be grounded with a special fixture. The contact surface between the fixture and the corrugated copper tube should match, and the grounding wire should be led out from the tail end of the fixture.

4. Additional tests for VFD cables

Generally, impulse voltage test is not required for low-voltage cables. For example, IEC 60502 standard only specifies impulse voltage test for cables of 3.6/6kV and above. The connection cable of the variable frequency motor is slightly different and needs to withstand high-frequency pulse voltage. The high-frequency wave amplitude can reach 1200~1900 V, and the ringing frequency is about 100~2000 kHz. Pulse voltage test (type test) is conducted on the cable to verify its insulation level. The test can refer to the IEC 60502 standard, which applies ten pulse voltage tests, positive and negative, and the test voltage can be considered as 40 kV.

Usage characteristics of VFD cables

1. The maximum allowable working temperature for cable conductors in the long term is 90 ℃.

2. During a short circuit (maximum duration not exceeding 5 seconds), the maximum temperature of the cable conductor shall not exceed 250 ℃.

3. The bending radius of wrapped shielded cable laying shall not be less than 15 times the outer diameter of the cable, and the bending radius of wrapped shielded cable laying shall not be less than 20 times the outer diameter of the cable.

Technical Characteristics of VFD Cables

1. Having small insulation medium loss and excellent thermal aging performance;

2. Has strong voltage impulse resistance and can withstand pulse voltage at high speed and frequent frequency conversion;

3. Has good shielding performance, with a shielding transmission impedance not exceeding 1 Ω/m within the range of 100MHz;

4. The cable structure is compact and round, with high electrical safety. The symmetrical three plus three cable structure design has more stable transmission performance than four core cables;

5. It can reduce the adverse effects of higher-order harmonics in the output of the frequency converter, reduce motor noise, and improve the torque efficiency of the motor;

6. It can improve the power factor of the motor input power supply and reduce its capacity;

7. Suppressed the increase in voltage at both ends of the capacitor caused by the instantaneous increase in power supply voltage, resulting in overvoltage protection for the frequency converter.

Development of 3.6/6-6/10 kV Medium VFD Cables

Due to the large-scale mechanical equipment, the motor capacity needs to be expanded, and the output current of the corresponding variable frequency power supply also needs to be increased. due to the limitations of high current variable frequency components, the development of further improving current capacity technology is limited. On the other hand, increasing the output voltage of the variable frequency power supply is relatively easy. After increasing the voltage, the power of the medium voltage variable frequency motor can be significantly increased, and the voltage level of the cable must also keep up. At present, 3.6/6-6/10kV medium voltage VFD cables have been put into use. From the perspective of insulation structure and electrical, mechanical and physical properties, they can be equivalent to power cables. Cross linked polyethylene is obviously the preferred insulation material. If it is required to be flexible when laying, Ethylene propylene rubber insulation has certain advantages. Due to the increase in working voltage, the emission ability of high-frequency electromagnetic waves is significantly enhanced, so the shielding structure is required to be more complete. Under the working conditions of VFD cables, coaxial cables are a suitable structure, so three main cores of the VFD cable adopt a coaxial structure, and the overall shielding structure is the same as that of the low-voltage VFD cable.

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