Does VFD Cable Really Eliminate dv/dt Problems?

If you are running a VFD/VSD I would have to say you should probably use VFD/VSD Cable. There is a reason special cable is used for VFD's.

I guess I was really wondering if VFD cable protects the motor insulation by eliminating high voltage spikes caused by dv/dt (rise time) of the output IGBT circuit. I know that shielded VFD cable reduces EMI/RFI noise and common mode problems like bearing currents, but does it impact or effect the cause of high voltages present at the motor terminals?

I would assume not, but I'm no expert in this field, maybe someone who is will come along with a definite answer for you.

What is VFD cable.

VFD - Variable Frequency Drive. (also known as a VSD, or Variable Speed Drive)

VFD Cable is a shielded cable specifically designed for VFD drives. The cable is shielded from any outside electrical interferences which may cause issues with the drive. They are also rated and matched to the drive they will supply power for.

It is a cable with shield.Please note the length makes a big factor.If the distance is too long between the motor and the VFD refelcted voltages can build up to a dangerous level.dv/dt as far as I know is a design part of VFD and devices used.The motors are built to withstand high dv/dt rise.Special motors are now available for use with VFD

The dv/dt depends on the carrier frequency selected in VFD. The higher the carrier frequency the higher the dv/dt. The cable can't eliminate dv/dt. The rate of rise of the voltage can be reduced by installing the sinus filter at the output of the VFD. The bearing current depends on the followings.

1. Rise time of the output PWM wave- Can be reduced if filter is installed at the output of the VFD.
2. Also by installing the VFD near to the motor reduce the standing wave voltage.By reducing the cable length dv/dt can be reduced.
3. Common Mode voltage : The bearing current can be reduced by selecting the VFD of 12,24,36 pulse. The NDE side bearing must be insulated to break the path of bearing current.
4. By selecting the lower carrier frequency

Read More: Why V/F ratio is kept constant in VFD

Thanks all for the help. I'm looking into the TCI filter now.

No CE-marked drive needs a special cable.

Do you mean a special cable called "VFD Cable"?

It’s not something that has a definitive “one size fits all” answer. But first, understand that depending on where you are, there are different rules and regulations in this regard, so some of the previous responses are along those lines. I’m going to assume from your syntax and references that you are in North America, where we have no regulatory imperatives about this issue, it is strictly voluntary. Outside of NA most people have no choice but to use VFD cable, so pondering the consequences is for them, pointless.

From a cabling issue, there is a major difference between NA and most of the rest of the world in that we use a lot more conduit, whereas a lot of others use the cables directly, similarly to what we would call “tray cable” or the Canadians call “Tech Cable”. When we use STEEL conduit (and properly ground it) on the output of a VFD, the conduit acts as the EMI/RFI shield. But without that, you need to do something else, hence the shielding requirement. So if you use PVC conduit, cable tray, wire duct or you are going to mix VFD output cables with other conductors in the same wire way, you absolutely MUST use shielded cable, aka “VFD cable”. There is nothing yet in the NEC (and CEC) compelling you to, but they about fire safety, not equipment longevity.

Cable capacitance is an issue that has virtually no significance in short runs, but increases with distance. Where that distance lies is the subject of much debate and also depends on several technical factors, but 25’ (7.5m) is generally considered the only “safe” distance. But some VFD mfrs have developed special transistor firing algorithms that reduce the formation of reflected waves and can go upwards of 400’ (122m) with no special treatment of the output circuit. Either way however, carrier frequency makes a huge difference in this regard so any time you increase your CF above 4kHZ, you are rapidly DECREASING the cable length at which you will have problems with reflected waves and thereby, dV/dt issues at the motor.

What “VFD cable” does in this regard is to assure a consistent conductor geometry; the bundling of the conductors remains constant, so the capacitance between conductors is even and predictable, which means some of the flux differences between conductors negate each other consistently. This is compared to individual conductors pulled randomly in conduit where those differences are random so the negation effects are inconsistent, helping to lead to the standing waves of higher voltages that then lead to dV/dt damage in the windings. Capable capacitance is a small effect to be sure, but again one that increases with distance between the drive and motor. This is the aspect that the VFD cable mfrs are referring to when they say that the cable itself will reduce dV/dt problems. Reduce? Yes. Eliminate? No. You need other measures to effectively eliminate it, such as the dV/dt filters.

But remember, for much of the world they are REQUIRED to use shielded VFD cable, and statistically those installations do not see the same amount of dV/dt damage to motor windings as we do here in NA, so empirical data appears to back up that claim. My take on this however, based on almost 40 years of applying VFDs in the US, is that most of the motor problems are due to a total disregard to ALL issues on how to properly install VFDs for success. Most electricians have had little to no training in this regard and just hook them up like any other motor starter. Then when someone tells them to use VFD cable, they look at the cost and blow it off as a “ripoff”. But if they were to actually READ the installation manuals of their drives, it’s always in there.

RTFM* is the first rule to success with VFD installations.

* Read The “Factory” Manual ...