is this true? 2

Voltage reflection, which can destroy a motor, may occur when the motor cable is longer than or equal to the critical length; lead lengths as short as 25 ft are sufficient for such an event . Thus, voltage reflection is a function of the lead length of the power cable between the drive and motor.

We can predict voltage reflection using the following equation:

Eq. 1: Lcritical=Vcable × tr
where, Lcritical is the critical cable length in feet.

Ive used more than 100 meters cables between VFDs and Motors !

 

[electricpete]

From wave behavior there can be amplification of a wavefront when it hits transition between motor impedance and cable impedance. Worst case Zmotor>>Zcable, the wave hits something similar to an open circuit and the magnitude doubles.

For short cables this wave behavior does not apply. As cable gets longer we get into the region where it applies.

Here is a discussion "Riding the Reflected Wave"

http://www.reliance.com/pdf/drives/whitepapers/D7722.pdf

The figure on top of page 6 shows this effect where rise time and cable length are independent variables. Motor impedance, cable type and shielding also affect the reflection/magnification.

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[Skogsgurra]

Have a look at thread237-262036

At the end of this thread, there is a discussion on overvoltage caused by VFD. There is also a link to a paper showing how cable length influences overvoltage. It increases first and then decreases as cables get above a certain length.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[electricpete]

It has been mentioned in this thread that insulation stress increases with cable length. That is true, up to a certain limit. The stress is then reduced as cable length increases.

The reason is that, as long as the voltage front is being built up (not reached its maximum value), the reflected voltage at the motor terminals "steps on" a lower voltage than the maximum and the combined voltage (incoming plus reflected) will not reach its maximum possible value.

It is when the cable length corresponds to rise-time times wave velocity (usually 60 - 70 percent of speed of light) that you get the maximum voltage rise. It is usually something like 1.8 - 1.9 of the DC link voltage.

When cable gets longer, the voltage front gets dispersed and therefore does not cause the same high voltage. That means that voltage stress decreases when you get above a certain cable length. I think that would be an interesting fact to add to the thread.

http://www.rema.uk.com/pdfs/vsd1.pdf

page 4 has a graph (MOTOR PULSE WITHSTAND CHARACTERISTIC CURVES
PEAK VOLTAGE/RISE TIME) showing this for a set of risetime, peak voltage, cable lengths and other parameters. Good read!

I have heard of the disperson phenonenon in other contexts. If different frequencies travel at a slightly different speed in the media (characteristic of a "dispersive media" then a sharp rising edge will spread out into a smoother curve. To what extent it applies to vfd pulses in typical copper cable I don't have any ability to judge, but I take your word for it.

The curve on page 4 of the link shows a flattening of the max stress at high cable lengths, not a lessening... correct?

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[Skogsgurra]

No, not quite.

The curves are withstand requirements.

The actual voltages measured are represented by the circles, squares and triangles. The do, as you can see, fall when cable length increases.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[electricpete]

Thanks Gunnar. I see you are correct the graph does show decreasing pulse height with length.

Now another thing is confusing me about that graph - it shows 2 different axes. Rise time across bottom for the envelope lines and cable length across top for the test result measured voltage squares, triangles circles. There is really no basis for plotting these on same graph, right? i.e. no specific correlation between a given cable length and given rise time.... so magnitude of circles and squares in relation to line has no meaning unless someone tells us rise time of those circles and squares so we can locate appropriate portion of the limit line (?)

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[electricpete]

OK, I see the cable length scale is irregular. I suspect it was adjusted so that for the pulses we can see both associated rise time and cable length. But if that's the case than variation of rise time with cable length would be identical for all three voltages. Hmm.

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[electricpete]

At any rate, I gather Gunnar's piont is you can get out of the "danger zone" either by making cable short enough or long enough. In the middle is where the peaks get closest to exceeding the spec requirements in terms of rise time and magnitude. Makes sense.

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[Drivesrock]

Here is another paper.

http://www.gambica.org.uk/web_image...tion_Voltage_Stresses_Report1_3rd Edition.pdf