Why does VFD cable data sheet show Inductance, Capacitance?

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Discussion Overview

The discussion revolves around the inclusion of inductance and capacitance values in VFD (Variable Frequency Drive) rated cable data sheets. Participants explore the implications of these electrical characteristics on the performance and behavior of VFD systems, particularly in relation to frequency changes and potential issues such as voltage spikes and electromagnetic interference.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant inquires about the significance of inductance and capacitance in VFD cables, questioning if there are specific calculations associated with these values.
  • Another participant explains that the impedance of the cable varies with frequency due to its inherent capacitance and inductance, which may induce phase shifts that could affect variable speed operations.
  • A different participant expresses interest in understanding issues like ringing voltage and mentions anecdotal experiences from colleagues regarding historical problems with VFDs, but lacks specific formulas or examples for predicting issues.
  • One participant discusses the effects of PWM (Pulse Width Modulation) on voltage spikes as the square wave pulses travel along the cable to the motor, highlighting concerns about insulation damage and electromagnetic interference due to the high frequency components of the square wave.

Areas of Agreement / Disagreement

Participants present multiple viewpoints regarding the implications of inductance and capacitance in VFD cables, and while there is some agreement on the potential issues caused by voltage spikes and impedance changes, no consensus is reached on specific calculations or solutions.

Contextual Notes

Participants mention limitations in their understanding of specific formulas or circuit examples related to VFD cable characteristics, indicating a reliance on anecdotal evidence rather than established calculations.

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Can someone point me to an understanding of why VFD rated cable include Inductance and Capacitance?
For example, Belden cable 29504 shows impedance, inductance, capacitance between conductors, capacitance between conductor and sheild, and DC resistance.
Is there a VFD motor calculation that is normally used with those values?
 
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As you probably know a VFD operates by changing frequency. Since the cable has an inherent capacitance and inductance the impedance of the cable will vary with frequency. Also the amount of capacitance and inductance will also induce some phase shift, which I'm not sure matters as far as variable speed is concerned. Another thing to consider is that the VFD's use transistors switching at high frequency (kHz range) to generate the AC waveform. This high frequency component will also encounter an impedance dependent on capacitance and inductance.
 
Thank you unix101os,
I was hoping someone might be familiar with ringing voltage or other examples of calculating possible problems.
I spoke with some of the silvered hairs at work and they were able to tell me about the time when vfds become popular in their areas, and the problems they had, but they were not familiar with specific formulas or circuit examples to predict problems other then voltage loss and to always use vfd rated cables
 
Mostly some from the Switching done to create the PWM waveform. A typical VFD is using PWM to use pulse of the full DC voltage in the drive to generate an approximate sine wave. But each pulse is a Voltage Square wave, each pulse travels along the cable to the motor, where the impedance changes dramatically,

As a traveling wave hits the motor - a large voltage spike can be created, the most common detrimental effect being damage to the insulation on the winding of the motor, but EMI / noise, is also a concern.

While the Switching Frequency may be 2 to 6 kHz in a common motor drive, the frequency components in a square wave are much higher, and the rise time of the square wave can be in the realm of 3 to 9 kv / uS - this wave, and the energy it represents, needs to be managed by considering the characteristic of the cable, length and the limitations of the motor.

http://www.belden.com/docs/upload/vfd_choosing_wp.pdf

E-> Sorry hit enter too quick...
 
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