Shielded power cables for connecting motors to EMC TOFLEX frequency converters
Today, variable frequency drives with pulse-width modulation had a widespread. The drive includes a frequency converter and an electric power cable. The advantages of such system are high control accuracy, the ability to manage always the rolling torque and the engine speed, as well as energy savings. However, with the development of semiconductor power switches that are part of the frequency converter are having a negative impact on the insulation of the supply cable:
• Strong electromagnetic radiation power supply cable;
• Overvoltage in the supply cable;
• Leakage currents and interference
The high level of electromagnetic interference caused by very short intervals, switching operations of transistors and high frequency of the frequency converter (up to 20 kHz). These working conditions lead to a distortion of sinusoidal voltage at the output of the converter and a serious induction on the local electrical networks and equipment.
The mismatch of the characteristic impedance in the inverter system - cable - the engine has led to another problem - any overvoltage at the motor terminals (at the cable end) due to harmonics of reflection (eg, the effect of the reflected wave). This effect only occurs under the condition that the power cable is longer than the wavelength of the harmonic. If the cable length is less than the wavelength of harmonics, transients occur at the output of the inverter. As a result, there are surges in excess of the rated voltage of 2-3 times. Thus, on the cable insulation and the insulation system of the engine affects equally the electric and thermal aging.
The high frequency of the inverter output are the cause of the high capacity leakage currents. And cables with non-symmetrical construction (four-wire), when the two container nearly placed phase core relatively to the ground core significantly higher than the capacity of the third phase core, the phases capacitive leakage currents are not distributed evenly, which may lead to overheating of the cable.
To resolve the above problems of "Tomsk Cable Factory" offers solutions implemented in the supply TOFLEX EMC cable with optimum EMC properties:
Specially designed cable construction, based on the most modern materials, ensures a longer service life compared to general industrial cables.
• Combination screen provides protection from internal and external electromagnetic interference (GOST R 51524-2012) by 100% shielded cable with foil and minimize the electrical resistance of a wire braid across the length of the cable, ensuring the effective grounding of the screen along the length of the cable.
The symmetrical layout of the main phase conductor and split earth conductor and protective conductor aligns capacitive currents in phases, preventing overheating and distortion of the cable at high frequencies.
• The use of modern insulating materials with improved performance can reduce the insulation capacity and lower leakage currents. Thus, increasing the effective length of the cable, high-frequency noise is reduced guidance effect and increases the maximum current load.
• Flexible cable construction provides easiness of laying and installation.
• The outer shield allows to operate under the influence of lubricating oils.
• The cables insulated with ethylene propylene rubber are acceptable for use in hazardous zones of all classes.
• The cable is flame retardant while installing and corresponds to the class of fire hazard according to GOST 31565:
«ng (A)», «ng (A) -HL» - Fire Prevention Class I.1b.220.127.116.11;
«ng (A) -LS», «ng (A) -LS-CL" - fire prevention class I1b.18.104.22.168;
«ng (A) -HF», «ng (A) -HF-HL" - fire prevention class I.1b.22.214.171.124.
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