VFD feeder -dv/dt

[HamidEle]

Who can address the effect cable types may have in regard with dv/dt issue? Some VFD manufacturer was saying there is problem with shielded cables when the cable length is long.

 

[tommom]

I thought there was a problem with cable length, shielded or no. The length apparently depends upon the voltage first, then slightly by manufacturer.

 

[electricuwe]

There are more than one "dV/dt" issue:

- a drive may trip due to overcurrent
- transmission line effects may lead to increases voltage stress on motor isolation
- dV/dt at the motors terminals may lead to bearing currents, causing premature failure of the bearing

The first two topics are related to the cable used and it's length. Shielded cable has higher capacitance per unit lenght, reducing the cable length that does not cause concerns. On the other hand, omitting the shielded cable may lead to EMI problems elsewhere.

 

[Skogsgurra]

The simple answer to cable length is to use motor reactors. They reduce the dv/dt and capacitive loading of the VFD. But beware! They may cause resonance and get very hot. Damping resistors across the reactors (parallel to them) is an effective way to avoid resonance. Have a look at the AB, ABB and Siemens product pages for more information. Then, there are inverters with a sine output. They are not sensitive to cable length and need no screened cables or filters.

Gunnar Englund

http://www.gke.org

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