VFD Cable

[mull982]

I've seen and heard alot of cases where a VFD shielded type cable was used between a vfd and the motor. I've heard some people state that this type of cable was not necessary while others have stated that it is. I'd like to get a better understanding of why and when to use VFD cable between the drive and motor and have the following questions. (All questions are based on 480V VFD applications)

What is the purpose for using a VFD shielded type cable? Any references or theory explaining the reasons would be helpful. I've always understood that it was to prevent coupling VFD noise as a result of the PWM waveform onto other equipment and cables?

Is it o.k. to run cable connected to a vfd in the same conduit as other cable on non-vfd applications? If it were a case where they had to be run together would it help ot use a VFD type cable for the cable connected to a vfd?

If a cable was connected between a vfd and a motor in and the cable was in a conduit by itself and the motor was in a location by itself, then is there a need to use a vfd cable?

 

[StephenSDH]

What is the purpose for using a VFD shielded type cable? Any references or theory explaining the reasons would be helpful. I've always understood that it was to prevent coupling VFD noise as a result of the PWM waveform onto other equipment and cables?

Your right. The output of the drive is pulses of dc voltage. The cables between the drive and the motor are an antenna. The vfd cable both twists the phase conductors and surrounds the conductors with a grounded shield. A large reason I prefer to use the vfd cable is because the wires are shielded all the way to the vfd terminals. I have had head aches with high speed counters, when thhn was used inside control cabinet, even though it was carefully kept away from control wires.

Is it o.k. to run cable connected to a vfd in the same conduit as other cable on non-vfd applications? If it were a case where they had to be run together would it help ot use a VFD type cable for the cable connected to a vfd?

I would only do this if you don't mind noise showing up on the cables. The vfd cable will make a huge difference.

If a cable was connected between a vfd and a motor in and the cable was in a conduit by itself and the motor was in a location by itself, then is there a need to use a vfd cable?

No the cable isn't really needed then. The conduit will effectively contain all noise between the drive and motor.

 

[TxEngr]

The fast switching frequencies of a PWM based drive can combine with the cable inpedences to produce a standing wave reflection on the cable that can create double the voltage at the motor terminals. This could place 1350V at the motor terminals on a 480V drive which can kill a motor not rated for this. This is affected by the type of cable, length of the cable and several other factors. The special drive cable has three symmetrically arranged conductors with three symmetrically arranged ground wires surrounded by a shield and/or typically with a continuous armor (CLX). This design should reduce the probability of producing this reflected wave as well as reduce noise to the outside caused by the high switching frequencies.

When installing new drives, I always use the new style cable. It?s slightly more expensive and a lot harder to install, but you won?t be troubleshooting the drive installation for the next 6 months. I?ve had some experience with this. And make sure you're using an inverter duty rated motor. Any IEEE 841 compliant motor will work.

If you use the drive rated cable, it?s fully shielded so running it near other wire shouldn?t cause a problem. However, I?d still provide some separation as a general practice. You?re not going to pull the continuous armor stuff in conduit anyway, so it?ll be tray.

Allen Bradley has a ?Drives Engineering Handbook? available on their website that?s a great reference on this. I think ABB has some of the same information as well. Just do a google search for ?Allen Bradley Drives Engineering Handbook? and you?ll find it.