Special wires for AC drives 3

[mkinnes]

After wiring an AC drive with THHN it was brought to my attention that there was a special wire used specifically for drive wiring. Having wired quite a few drives with THHN an experiencing zero problems this came as a bit of an eye opener. Im wondering if anyone else has run into a situation where this stuff is absolutely neccesary and could help point me to information that would expand my understanding.


Im guessing its more a power quality issue than anything but would like to understand this more in depth.

 

[fsmyth]

I am currently working on 800 kHz inverters driving
induction coils; have not seen mention of any special
wire (yet). Seems like that would be more demanding
than an AC drive. Keep us posted if you find out why.
<als>

 

[fsmyth]

Shoulda' added: also 400 HP D.C. drives; I'll check and see
if they use anything special tomorrow. (I know that the field
wiring is THHN, but the armatures are 300A. Have to check).
<als>

 

[Marke]

Hello Kinnes

One of the consequences of using todays IGBT based drives, is that they can generate a lot of radio frequency noise. This noise can interfere with communications and control equipment.
In Europe, there are regulations covering the noise generated by inverters, and in order to minimise the noise generated, it is critical to use a screened cable between the drive and the motor. There are various grades of screened cable and I suspect that you are refering to an "EMC" cable that is designed for connection between the drive and the motor.
The drive will function without screened cable, it will just be more noisy.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[Skogsgurra]

Yes,

EMI is one reason for not using standard cabling. There is another thing to consider; the life of the cable. The fast rise-times of a PWM wave produce reflected waves that cause overvoltage in the cable, which produces ozone (partial discharge between cores) that makes the insulation brittle. The effect is more pronounced in long cables.

The phenomenon is described in many articles like

http://www.ene.ttu.ee/elektriajamid/teadus/artiklid/Kreeka2/

and

http://www.ab.com/drives/techpapers/ieee/APEC_98.pdf

Belden has special cables for inverter drives. Have a look at their home page.

Gunnar Englund

http://www.gke.org

 

[dogleg43]

Belden provide good info, but we should always check with the manufacturer of the drive and the motor to find out if special cable is required. Also find out where the drive manufacturer recommends placing the external safety contactor. I've had some say line side others say load side of the VFD.

Speaking from experience (ie: mistakes, if there are problems, the first thing the customer will always do is call the manufacturer and they will pick your installation apart. It's best to start out on the right foot.

As a side note, if a shielded power cable is required, find out if the shield needs to be grounded on both ends. Again, some drive want both ends grounded, others only one end.

 

[alexit]

"Also find out where the drive manufacturer recommends placing the external safety contactor. I've had some say line side others say load side of the VFD."

Easy to tell for contactor: If VFD has ANY regen capability DO NOT place between VFD and mains! If VFD has braking resistors you may put either line or load side.

EMC cables should be used everywhere but expense is hard to prove, tell manager to take handy phone into plant, if he hears static noise you need more EMC cables.

 

[dpc]

General recommendation is for triplexed cable with multiple ground conductors spaced about the phase conductors.

One good solution is Okonite C-L-X metal-clad cable (or similar). They have recently made three interstitial ground wires standard on C-L-X for just this reason.

There are more specific recommendations in the IEC world than in the US for drive output cabling, I believe

 

[jraef]

Here is another good reference paper on this subject. One of the authors also worked on the AB technical paper refernced above.

http://bwcecom.belden.com/college/techpprs/capwmtp.htm

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[aolalde]

What an interesting subject. Thanks to Skogsgurra and Jraef for the comments and great references, both of you deserve a star.

 

[mkinnes]

Thanks for the replies.