# Can I Use This Wire? Version 2



## Str8jacket (Oct 17, 2015)

I was going to add into the other thread but seeing as though it was all about control wire i didnt want to confuse things.

Im wiring up the box for my vfd and have the brake resistor box mounted outside underneath the main vfd box as  think its going to be out of the way not collect as much dust etc,  have come to running the wires from vfd to the resistor and am not sure where to route them. 

Im using 2.5mm2 1000v 105*C rated wire to the terminals on the resistor enclosure but its not shielded..... 

Can I run this out of the top of the drive and down inside the box to come out the bottom? All other wiring is on the opposite side of the vfd and I'm using shielded wire for the rest of the control circuits.


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## mksj (Oct 17, 2015)

The Brake resistor is not part of the control wiring and will have no affect on where or how you mount it/connecting wires relative to surrounding wires. So would do what is easiest.  Please note that there can be very high voltage on the brake wiring, on the order of close to 400V. The wire you are using is more than adequate. 

Most of the installations I have done, I mount the brake resistor vertically on the side of the VFD housing (or cabinet), so it gets natural conduction upwards. If mounted to metal, then one can directly mount it. If PVC or fiber box, would probably use a small standoffs. My experience on the lathe with lots of start/stops, my 500W brake resistor is at worst gets only slightly warm, so no big worries.


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## Str8jacket (Oct 18, 2015)

Awesome, makes life easier. The manual says DC volts max to resistor is 780V. I read somewhere that the brake circuit can make a lot of noise when active so I was worried about that, the manual also says to have it come out the top of VFD and straight out the enclosure.

I may be overly complicating it for my application on a lathe as when the brakes are on its for a few seconds then the motor is stopped, not like a conveyer belt or something.

What formula do I use to work out the actual current draw?

power / volts gives me                 600W / 780V = .76Amps
Volts / Resistance gives me        780V / 60 ohms = 13Amps

I need to do a degree.....


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## mksj (Oct 18, 2015)

Not sure where the 780V came from, this may just be the voltage rating which is about 2X higher than the actual voltage encountered. The factory VFD default maximum braking voltage is somewhere around 360-380V for a 240V 3 phase motor.  V=I*R is the formula to calculate the voltage drop, Power (Watts)  = V(V) × I(A), but you cannot base it on the resistor and voltage (this is the rating for the resistor). Also since the dissipation is over a few seconds, wire can sustain much higher short term current than the rating which is continuous current carrying capacity. Your 2.5mm or 14G wire is definitely up to the job, if not overkill. The short circuit wire capacity is relative to time, 14G wire can sustain over 100 amps for over  5 seconds. I wouldn't worry about it.


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## JimDawson (Oct 18, 2015)

I've never run into a problem with any wire routing on a breaking resistor, even when the control wiring is close to the resistor wires.  It's just normally not an issue.  The wire size and type that you propose is more than adequate for the application, just don't bundle the resistor wires tight to the control wires.  An inch or so of separation is enough.

The only time I have ever seen a breaking resistor get warm is in a high speed cutoff saw application where there are up to 100 starts and stops per minute on a 25KW system.  In a normal machine tool application, the resistor will never even get warm.  On my mill, I have the resistor mounted on an aluminum bracket just next to the VFD in open air, but it could be mounted in the VFD enclosure if there is room.
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## Str8jacket (Oct 18, 2015)

Hi Mksj, I have a 400V drive, thats probably caused the confusion. When I looked at the results of my calcs it didnt look right.

Jim, thanks for backing up what Mksj has said, I do tend to go overkill. Probably from reading to much stuff written by professionals who are trying to meet a code or have alot more funds. The only brake resistors i have experience with are on 300t elec drive dump trucks! They get hot... 

manual specs are my only real reference as there is no one local who has any knowledge on them. 

The trick is to know where to scale back without blowing myself up!

Going to hook in today and hopefully get aome action. Thanks guys.


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## Str8jacket (Oct 24, 2015)

Well got it going, and you guys are right, full noise to a complete stop in less a than second and it doesn't get warm☺i will need to find the setting thst shows what volts that circuit pulls when active.
Cheers


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## mksj (Oct 25, 2015)

This would be B096 setting or Dynamic Braking Coontrol parameter, range is 660-760V, the factory setting is 720V on the 400V version of the WJ200. Not sure of your VFD.   If one gets an overvoltage message during braking, on can increase the default voltage a bit up. Mine is set about 20V above the factory default. Usually an overvoltage error is because the braking time is too short for the rotational momentum, also not having a braking resistor to dissipate the energy created when braking.


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## Str8jacket (Oct 25, 2015)

Hi mksj, I have control techniques unidrive sp. The set up parameters must be different to the wj200. For the brake resistor circuit there is a formula for max on period and the dwell between cycles required by the resistor to cool down before another max brake application.

With my 600watt resistor it worked out to be .06 sec full power and it is rated at 10% duty cycle so i set the dwell to .6 sec. Actually i originally left it at 2 seconds and it tripped a few times so i set it to what the equation worked out to be. After reading yor post and thinking about it for a bit i realised id actually reduced its power rating by extending the time between. It hasnt tripped since. 

Hope that made sense??? New to this


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