I retired from a large campus that had over 500 VFD'S. Never wasted money on shielded cable on there 480 volt output side of any drives. Used plain Jane THWN copper wire for every 6 & 18 pulse drives. Most motors were plain Jane NEMA motors & the rest high efficiency motors. Had some 40 year old motors that were on drives. Even had two 4,160 volt 1,750 HP chiller motors on drives with regular medium voltage cable. Most drives were less then 40' from motors.
Considering the cable lengths were mostly under 40' how many cents do you reckon you saved skimping on the cable? As a service tech doing warranty inspections and repairs for an HVAC manufacturer (northern Europe) I've replaced bearings (or the whole motor) on hundreds of under 2 year old 1-20kW VFD driven motors. The bearing grease is usually burnt into a black tar or a crumbling solid and the races are pitted or cloudy. Many have been wired with an MCMK cable with 3 phase conductors and a shield where the shielding copper foil and strands have been twisted at the ends into a single PE and EMC conductor. A version with 3 phase wires, a designated PE of the same gauge or thicker and a separate EMC shield would have cost between 5 to 10 cents more per meter. Some have had a proper cable but the grounding contact of the shield inside the metal glands hadn't been done properly. Often a safety switch between the VFD and the motor had been the culprit. Bearing replacement on a large motor can cost a 1000€ while the unit is out of service for the day. I think VFD motor drive cables should be the last thing to skimp on. I tell that to all our customers and recommend improvements on new sites. Many times I've known right away that unless the cabling is corrected we'll be back in a year or two.
this is a brilliant presentation, thank you. The only thing I would add is that conductors need to be SIZED TO PASS THE PEAK CURRENT density, not the average value. This helps transfer the dV/dT spikes into the motor without dissipating energy in the serial impedance of an undersized sized cable. (ppl talk about reflecting or bouncing waves when energy can't go through!) This is is true with any Pulsed Modulation: "pass the peak not the average"! 🤞
Great information, but I note Belden does not recommend the use of 360 degree shield terminating cable glands. With a shieled terminating gland, significant shield currents can be released, for example at enclosure ingress, exposing sensitive equipment to noise currents that would be contained if an isolating cable gland such as a Crouse CGB, ADE or CMP TC or A2 Gland. Pete C
I don't know where you read that, but 360 bonding of cable screens at BOTH ends to the equipment chassis is almost always best for EMI. The old advice to bond only at one end was typically to avoid the potential for excessive currents to flow through the screen and damage them, but there are ways to address this concern.
Geee, build the vfd power pack on the motor back-end or very close to the motor, then no more long cables... sorry belden...:) or go back to the one common line shaft or adjustable gearbox
very informative, but one question - doesn't the motor act as as smoothing inductor (reactor) and smooth out the square wave at least somewhat in the entire circuit?
You're asking, will the motor’s inductances do the same job as an output load reactor? The answer is absolutely not. The output load reactor is placed next to the drive. This slows down the rise time before it even gets to the cable. This helps in many ways and if you'd like to know more, I can tell you. You might not believe this but smaller motors have more problems with pulse width modulation then larger motors when you are running long cables. If you'd like to know, just ask.
