VFD cable voltage ratings 1

This is a most excellent question and one I've wondered about for years. I've seen ringing and reflected pulses totaling 1400+ volts. How does 600VAC rated motor wires cope with that?

As a practical matter, I would think the motor winding insulation would fail from overvoltage before the 600 V insulation in the wire in most cases.



David Castor

There are several issues here.

The 600/1000 V rating is a (usually) 60 seconds rating. In any case, it is a lot longer than the microseconds that ringing exists. There is also a lot of safety margin in the 600/1000 V rating. Cable insulation seldom breaks down even at 5 kV.

Then, there is the effect of the capacitive current that the ringing cause. Very close to the insulation, and where cable cores are tight, there is such a high field-strength that partial discharge aka corona is active. P.D. produces ozone and that ozone is trapped inside the cable where it starts reacting with ('eating') organic matter. There is no other organic matter than the insulation, so it starts to degrade and gets brittle.

The effect is seldom seen in 400 V installations, but not unknown of in 690 V (EU) installations.

Several cable manufacturers have cables that can take this kind of stress. I think that Belden was one of the pioniers.

Then, you have the EMI and PE impedance issues. But those stories will not be told here and now.


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

PD on a 690V system? That's interesting - PD is a problem I associate with MV gear in the UK, usually 11kV and upward. Are you aware of any good references for PD in LV cables (before I go and Google it)?


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If we learn from our mistakes I'm getting a great education!

I think Belden wrote about it when they introduced their VFD-cables.

It is very real. Had a case in a paper mill where cable more or less fell apart when moved on the tray.

I made a presentation on that many years ago. See attached clip.

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

" Lower capacitance, resulting in:
– Longer cable runs
– Reduced peak motor terminal voltage for extended motor life
– Reduced likelihood of corona discharge
– Reduced magnitude of standing waves
– Increased efficiency of power transfer"


The reduced capacitance means less HF current in air between cores, which translates into lower field strength.

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

So, it seems that the simple summary to answer my question is that the 600V rated cable has very generous safety margins and the ringing voltages rarely cause any problem, at least at 460/575V.

Thanks to everyone who responded.

An alternate way of stating it would be that the motor is a weaker link than the cable in the insulation system leading from drive output to shaft. It is also important to note that different type of cables can make the ringing problem worse. And create and environment encouraging or discouraging common mode currents.

Neil

Thanks Gunnar. LPS for that.


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If we learn from our mistakes I'm getting a great education!