Electricity
Description
For too long, the importance of grounding to protect motor bearings has been underestimated. To minimize harmful currents and realize the full & quot;green& quot; potential of variable frequency drives, an economical, long-term method of shaft grounding is a must. Until all OEM motors marketed for use with variable frequency drives are truly & quot;inverter-ready& quot; retrofitting them with shaft grounding is the best approach.
In theory, because this grease contains conductive particles, it would provide a lower-impedance path through the bearing and would bleed off shaft current through the bearing without the damaging discharge. Unfortunately, the conductive particles in these lubricants increase mechanical wear to the bearing, rendering the lubricants ineffective and often causing premature failures. This method has been widely abandoned as a viable solution to bearing currents.
Line Termination Network (LTN) near Motor: The LTN is a NEMA 4X device mounted near the motor. The LTN theory of operation is based on transmission line analysis. The LTN passive network elements closely match the cable surge impedance so that voltage reflection is eliminated. A single LTN is possible for the entire hp range from 2 to 500 hp since bundled cable surge impedance only marginally changes from #18 awg to 500 MCM and motor surge impedance is always much greater than cable impedance. Motor terminal voltage is not sloped off but has the same risetime to the Vbus level as the VFD output risetime. However, peak motor terminal voltage is usually less than 1.2 pu as shown in Fig. 4. Terminator waveform plots in Fig. 5 and 6 show the reflected wave peak voltage minimized for a single PWM pulse sent from the VFD manufacturer. The LTN will limit peak voltage at 600 ft of cable to 780 Vpk on a 480 V system and 960 Vpk on a 575 V system. Both values are safe values within the motor 1,000 Vpk at 0.1 us capability
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Variable frequency drive
VFD drives
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China, 518000
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