# Installing a dynamic braking resistor on an AB 10HP VFD



## mikey383 (May 21, 2012)

I have a Powerflex 70 15HP ND/10HP HD drive that I'm getting ready to install a Post Glover braking resistor on. The resistor is 2160 watts 45 ohms. The motor is a 10HP. 

My question is: What type of wire should I install? I planning on running 8 awg to it, but should it be 1000v rated? 

I talked to my boss about this, and he thinks THHN will be fine, but I'm on the iffy side. Since the DC bus voltage is roughly 650V, I'm hesitant about using THHN.


----------



## Jhellwig (Jun 18, 2014)

All the ones I have seen are close enough to the drive that the leads on the resistor are long enough to go to the terminals and have high temp wire.

600 volts rms is way more than 650 volts dc.


----------



## ponyboy (Nov 18, 2012)

Jhellwig said:


> 600 volts rms is way more than 650 volts dc.



I think it's around 850


----------



## mikey383 (May 21, 2012)

```

```



Jhellwig said:


> All the ones I have seen are close enough to the drive that the leads on the resistor are long enough to go to the terminals and have high temp wire.
> 
> 600 volts rms is way more than 650 volts dc.


This resistor has no leads. It's simply a box with wire wound resistors. I have to supply the wire, which is why I was asking.


----------



## xpertpc (Oct 11, 2012)

That should be an external air ventilated resistor with stud connections, standard thhn wire with a non insulated terminal should work fine, as said above 600 vac rms is about 850 Peak to Peak (DC) but don't think they rate it that way.

I just looked at one of my servo braking resistors, the manual made no mention of wire type beyond the internal high temp wires.


----------



## JRaef (Mar 23, 2009)

As a general rule, you should not use THHN on the output side of a VFD anyway, it is now considered a poor practice because THHN, although fine for sine wave power, uses PVC insulation which has bubbles in it, and the bubbles allow for easier corona discharge to take place in the presence of high speed DC pulses, which is what you get on the output of transistors in a PWM VFD. Corona discharge is typically something only thought of with regard to medium voltage cabling, but it can happen on PWM outputs because of the high speed switching of the transistors used and the fast rise time of those pulses. Although the output of the braking transistor is thought of as DC, it s still PWM from a transistor and can exhibit the same problems. 

This is not to say everyone should rush out and replace all the THHN they have installed on the outputs of VFDs, but when the cables fail, replace them with XLP insulated conductors. XLP is Cross Linked Polyethylene and because it is manufactured as a form of heat shrink tubing, it has no voids (bubbles) in it and is much more resistant to corona discharge. RHH/RHW cable uses XLP insulation. It will come in 600V, 1000V and 2000V versions depending on the mfr, I recommend using the 1000V for motor leads on 480V drives, but because the distance is likely very short to your brake resistors, the 600V is fine. 

Just don't use THHN...


----------



## ibew415 (Mar 23, 2014)

JRaef said:


> As a general rule, you should not use THHN on the output side of a VFD anyway, it is now considered a poor practice because THHN, although fine for sine wave power, uses PVC insulation which has bubbles in it, and the bubbles allow for easier corona discharge to take place in the presence of high speed DC pulses, which is what you get on the output of transistors in a PWM VFD. Corona discharge is typically something only thought of with regard to medium voltage cabling, but it can happen on PWM outputs because of the high speed switching of the transistors used and the fast rise time of those pulses. Although the output of the braking transistor is thought of as DC, it s still PWM from a transistor and can exhibit the same problems.
> 
> This is not to say everyone should rush out and replace all the THHN they have installed on the outputs of VFDs, but when the cables fail, replace them with XLP insulated conductors. XLP is Cross Linked Polyethylene and because it is manufactured as a form of heat shrink tubing, it has no voids (bubbles) in it and is much more resistant to corona discharge. RHH/RHW cable uses XLP insulation. It will come in 600V, 1000V and 2000V versions depending on the mfr, I recommend using the 1000V for motor leads on 480V drives, but because the distance is likely very short to your brake resistors, the 600V is fine.
> 
> Just don't use THHN...



So the primary downsize to corona discharge in THHN would be insulation damage?


----------



## JRaef (Mar 23, 2009)

ibew415 said:


> So the primary downsize to corona discharge in THHN would be insulation damage?


Yes. For the past few years when people complain to me about "drive failures" due to nuisance tripping I first have them meg the cables again and they often find insulation failure after a few years of use. About 2 weeks ago one of them pulled the 150' THHN/THWN cables and on all three legs, there were tiny burn-through marks at increments of almost exactly the same 16' spacing in all 3 cables. That must have been the wave length of the reflected wave generated by the PWM and cable capacitance. The motor seemed fine, it was an inverter duty motor so it had the higher winding insulation rating. But the problem just moved into the cables. That wasn't the first time I've seen it, just the first time it was so obvious.

This is probably one of the better articles I've seen on this issue.
http://www.motioncontroltips.com/2015/02/14/problems-vfds-cause-cable-types-help-solve/

In there, you'll see them say that technically, THHN with at least 20mil insulation thickness should work too, it's just that 20mil makes it a lot stiffer and undesirable for regular use, so nobody stocks it.


----------



## justinbc6 (Mar 13, 2015)

what type of wire to run from the drive to the resistor? there's a high heat cable out there, but i cant remember what its called. I install a lot of drives and resistors on cranes and i use the high heat cable when its available, but ive used thhn also, and i dont think any of them have melted yet. 

It really probably all depends on the environment and how often its being used.


----------

