Why does VFD cable data sheet show Inductance, Capacitance?

AI Thread Summary
VFD-rated cables include inductance and capacitance data because these parameters affect the cable's impedance, which varies with frequency, particularly in variable frequency drive (VFD) applications. The high-frequency switching of transistors in VFDs generates a PWM waveform that can create voltage spikes as it travels along the cable, potentially damaging motor insulation and causing electromagnetic interference (EMI). Understanding these characteristics helps in predicting issues like ringing voltage and ensuring the proper selection of cables to mitigate problems. The rise times of square waves can reach significant levels, necessitating careful management of cable length and motor limitations. Properly accounting for these factors is essential for reliable VFD operation.
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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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