Dynamic Brake Resistor - Exposed Live Parts

[123MB]

Hi All

First of all sorry for the silly question, I lack a bit of experience in this area

Background:

We are designing an outdoor enclosure for a dynamic brake resistor where a bit of water may enter the enclosure from time to time. I am trying to determine the practicability of insulating the live parts inside this section in order to reduce the possibility of the water causing short circuits and tripping the drive.

Detail question:
1) On a typical wirewound dynamic brake resistor, is the resistor wire typically a live exposed part? Or is it somehow insulated, i.e similar to a heating coil in a space heater?

2) If the resistor wire is a live exposed part, is an enamel coated brake resistor the only way to achieve an equivalent level of insulation?

3) If one had an enamel coated resistor, would it be practicable to require that connection lugs used for cable connection onto the resistor shall be protected with insulating heat shrink, thereby resulting in the entire resistor installation being insulated?

Thanks

 

[itsmoked]

On a typical wirewound dynamic brake resistor, is the resistor wire typically a live exposed part? Or is it somehow insulated, i.e similar to a heating coil in a space heater?

1) Some are some aren't. Most are but it depends, certainly the connections are NOT so it doesn't really matter.

2) If the resistor wire is a live exposed part, is an enamel coated brake resistor the only way to achieve an equivalent level of insulation?

2) Won't work either because of the terminations. See 1)

3) If one had an enamel coated resistor, would it be practicable to require that connection lugs used for cable connection onto the resistor shall be protected with insulating heat shrink, thereby resulting in the entire resistor installation being insulated?

3) No, because the shrink will still be fried.

You simply can't let water get to the resistor. At the very least splashed water could crack a solid in-use resistor.

The only solutions I can imagine are:
A) Put it all in a rain-tight enclosure that air can easily pass thru but falling and blown rain can not enter.

B) Make or get a resistor that is self supporting nichrome wire and run the terminations into a rain-tight enclosure where the un-insulated heat radiating terminations can remain cool enough.

C) Build an add-on/lean-to that prevents any water from landing on standard braking resistors.

D) Skip braking resistors entirely by switching to a drive that jams the waste energy back into the power mains where it can do some good instead heating air.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ScottyUK]

How big is this DBR - watts, kW, MW?

 

[MikeHalloran]

Locomotives have used braking resistors for a very long time.
They are typically equipped with fans, and with screens to prevent contact with personnel or tree branches, but I don't think the railroads bother protecting the resistors from rain.
Inquiries about construction and/or suppliers of same might save you some aggravation.



Mike Halloran
Pembroke Pines, FL, USA

 

[123MB]

Thanks all

The resistor is 12kW continuous rated and will be used at a duty cycle of 50%.

 

[123MB]

All

Thanks again for your responses, but I am still unsure about this application and hopefully you can bare with a few more silly questions.

The amount of water we are talking about is small, for example, a few drops of water per hour.

itsmoked, I'm not sure what you mean in the quotes below

splashed water could crack a solid in-use resistor

Sorry I am not sure what you mean by 'solid in use resistor'

the shrink will still be fried

Do you mean the insulation rating of the shrink will not stand up to this?

Thanks

 

[itsmoked]

Hi 123;
I was referring to ceramic based resistors of any kind. They'll crack if hot and splashed. And, yes, trying to include insulation up to and including the terminals on the resistor will cause materials like shrink tubing to fail quickly.

6kW consistently would make some form of regeneration worth considering!

Why would occasional drops hit the resistor? Tell us more about the situation.



Keith Cress
kcress -

http://www.flaminsystems.com

 

[flexoprinting]

Good morning

I think what itsmoked was referring to is a Dynamic brake resistor depending on the application can get very hot, sudden
cooling with water will cause a lot of stress in the materials associated with it.

Chuck

 

[123MB]

..thanks itsmoked and all.

The braking resistor is installed into it's own section of a three-section MCC. The other two sections house a tuned line filter and the third section contains the actual drive and controls (PLC, etc).

The braking resistor section is in the corner of the MCC and includes a part of the top of the enclosure and one side. The top of the enclosure is fitted with a sun shield.

In order to maximize cooling the top of the braking resistor section is open to the atmosphere and practically consists of a square cut out of the top of the brake resistor section. A mesh screen has been specified to cover this opening and a right angle 'guard' has been specified around the opening to minimise potential for rain to get to the mesh. The height of the right angle guard is such that it doesn't go right up to the sunshield, in order to prevent heat collecting under the sunshield. The sunshield does not come down and around the guard to form a 'top hat' as it is feared the heat will collect too much under the hood.

Practically the fear is that a more standard top hat arrangement - i.e one where a 'top hat' is provided over, around and down over the opening, will not dissipate enough heat.

Following from the above I am concerned that atleast a little rain may reach the mesh (i.e. with a bit of sideways wind and nasty weather) and get into the resistor section. Hence I am trying to put together prescriptions for the design of the panel to minimise the probability for this water to cause issues.

See attached drawing for the proposal.

 

[jraef]

In my opinion, this is a poor engineering decision as presented and your concerns are valid. There is no chance that this will last any appreciable length of time as designed in that drawing. Assuming you cannot change the VFD to be one that uses Line Regenerative Braking instead of Dynamic Braking Resistors as keith suggested (i.e. you ARE the VFD supplier and your company doesn't offer that), the approach to making this work reliably is to do it correctly and not the cheapest way out. Fan forced ventilation pulling cooling air in from the bottom through baffles, then with downward directed exhaust and intake vent hoods. Nothing open on the top of the enclosure. NOTHING. That is a recipe for failure.

NOT an endorsement of this product, I just pulled this image as an example of the way it is done.

" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[itsmoked]

Thanks 123 for the detailed explanation!

I concur with jraef.

Given what you may already have in front of you I'd switch to fan forced with the outlet screened and pointed down. An over temperature switch set to detect cooling failure via a clogged/blocked airway or a fan failure should shut down the system.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[123MB]

Thanks All

I have recommended to install an IP44 (NEMA 3R) vent hood on the roof in lieu of the opening and to make provision for the future installation of a fan sucking cold air from the bottom of the resistor section and exhausting out of the vent hood. Let's see how we go.

Just coming back to something that was mentioned in an above post:

Locomotives have used braking resistors for a very long time.
They are typically equipped with fans, and with screens to prevent contact with personnel or tree branches, but I don't think the railroads bother protecting the resistors from rain.

This is something that I haven't been able to reconcile in my research.

How is it that it seems that locomotive brake resistors are exposed to the rain. They must be constructed so that the live parts are totally insulated? But is this typically the case for smaller scale dynamic brake resistors?

Thankyou.

 

[itsmoked]

I looked into that also. I'm not so sure they are exposed to rain. Further, they're metal plates so actually getting water on them would be different than a ceramic base. If you look at locomotives the dynamic braking units are usually not directly exposed from above, they're mounted in side cab projections and have the air sucked into them up from sort of downward, and it exhausts out the top. When you lay on a signal platform that spans the tracks and look down from a couple of feet on the top of an engine passing below you see several big axial fans, (well, until you close your eyes in terror), the fans are there for the horizontal ceiling mounted engine radiators which don't care a bit if they get rained on.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[LionelHutz]

It's seems wrong to expect a 12kW dynamic braking resistor to be ceramic. I've seen many resistors rated for outdoor use and none were fully insulated or ceramic coated or any of that crap. Just plain stainless steel with ceramic insulation/mounting. The elements had lots of space around them so water would do nothing to affect the operation. Basically, mount the resistor assembly on insulators suitable to the task so it's floating in air with lots of clearance and put a drip loop in the wires going to it and why does it matter if it gets wet?

 

[stevenal]

Why future fan capability? Seems like you'll need to dissipate 12 kW now as well as later.

 

[Compositepro]

High voltage power lines are resistors too. I do not understand your excessive concern about drops of water being near live wires. Condensation in enclosures happens quite often. You certainly do not want water constantly dripping on or immersing the resistor but the resistor is hot. It will be the driest component in the cabinet and will dry the cabinet itself at that power level.

Why would you use a fan to draw cold air out of the bottom of the cabinet? Hot air rises and this will behave as a fan due to natural convection. Do not fight it with a fan.

 

[LionelHutz]

I'm pretty sure 6kW of heat inside an enclosure would require fans. But, again why mount the resistor inside another enclosure at all? You can buy 3R rated resistor grids that you can simply mount outside the main enclosure.

 

[stevenal]

Compositepro,
I think the main concern is with water dripping on a hot ceramic core causing it to break. Other core materials might be suitable. By your name, perhaps you can recommend a material. I think those suggesting fans above said exactly what you did: In at the bottom and out the top.

 

[jraef]

Your water heater or spa has heating elements directly IN the water you are in contact with. That's not really a technical problem, it's more of an economic one. Coated resistors used as immersion heaters will be a LOT more expensive, and because they are sized for a HEATING rating, not RESISTANCE values, you can end up with resistors that have SIGNIFICANTLY larger dimensions when using them for DB, compared to DB resistors. I have used DB resistors that are powder coated and rated NEMA 3R, then you use a sealing gland on the wires going back into the drive enclosure. That works fine, but I found the added cost to be higher than just putting them in the box and using fan forced ventilation (plus, they are a fire hazard in any area where there are combustible materials present).


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[LionelHutz]

They'd be a fire hazard inside a box with fans too.