DIssapating 50HP (~38kW) of DC 2

[p27182]

Hey experts,

I want to ask for some advice.
(skip to the bottom to jump to the point)

I'm currently tasked with finding a load bank for a dynamometer whose mechanical load is a 50hp DC generator. (The dyno is to be used testing big VFDs and whatnot).

On a dyno we've already got hooked up and running, we have been adjusting the field voltage on the generator while it's being driven by a motor hooked up to a VFD in order to provide a variable mechanical load. The output of the generator is currently hooked up to a big air cooled resistor bank to dissapate the heat.

Since we don't have another big bank like this, I've been asked to find out how to acquire another one, build one, smaller if possible (water cooled?).

So long story short:
What would you guys reccomend for dissapating about 38kW of DC power for a lengthy amount of time (hours)?
and where would I look for resistor banks capable of dissapating this much juice? I'm not getting nearly as many reasonable google results as I expected.

thanks for any info/suggestions!

-pat

 

[Clyde38]

I don't know if this helps, but in the past, I have used heating elements from hot water heaters submerged in flowing water in a utility sink.

[link

http://www.linkedin.com/in/clydehancock

]

[/url]

 

[Skogsgurra]

You could use a bath tub (or oak barrel) and electrodes made from stainless steel to build a water resistor. I do not say that it is a minor undertaking, but it isn't a major one either. Somewhere in-between.

And you can use just about any scrap and crap you can find. See

http://en.wikipedia.org/wiki/Liquid_rheostat

for general information and warnings.



Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[p27182]

Thanks for the ideas and suggestions!

I'm looking into them now.

I have another related question- I found heater cores here that are capable of dissapating 15kW, however they are 3phase...

Couldn't I replace the DC generator with a 3phase 480v induction motor and use that as a generator hoked up to these heater cores? as long as I don't push it past 60hz?

thanks again!

-pat

 

[Skogsgurra]

Induction motors need reactive power. They usually get that from the grid (hence low P.F.) and will not generate much voltage if you connect directly to resistive load. Caoacitors *could* help, but doubt if it works in this case.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[LionelHutz]

I think you could use a 50hp DC drive to regen the energy back into the utility instead of wasting it as heat.

Another option is a 3-phase AC induction motor and a VFD with regen capabilities or a seperate VFD and regen controller. We are using 3 seperate VFD's hooked to one big regen controller here for test loads.

You can check with power resistor companies such as Post Glover or I-Guard and they will build you a resistor to suit.

Depending on the voltage of this DC generator, you could also just use heating elements such as electric stove elements or water heater elements.

 

[ScottyUK]

3-phase resistor heaters don't give a damn about 3-phase, single-phase, or DC. Bank the phases in parallel and connect up to the DC. Ohm's Law will be obeyed.


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If we learn from our mistakes I'm getting a great education!

 

[btrueblood]

Years back we had a load bank using standard 100-watt and 500-watt light bulbs; we could switch them on and off with switches, or screw them in/out. It does get warm in the room with that much power being dissipated.

 

[p27182]

Thanks again for the info and ideas!

So a resistor set up is gonna be 4g+ for just the bank, and on top of that the idea of just burning up the energy is inefficient.

I'm going to try to propose the regen idea but I dont know if my supervisor is going to allow me the necessary amount of time to research and implement the appropriate drive and try it out.

thanks again!
-pat

 

[p27182]

Lionel-

Just got off the phone with Baldor asking about your idea, I dont think the guy really understood what I was trying to say, so let me ask you-

with a DC regen drive, as long as the drive is juicing up the field, any rotation of the shaft will spit juice back on the mains power right? what would determine the mechanical resistance of the shaft on the motor? just the field voltage?

and also- another question- the field power consumptions does not vary with motor output does it? just wanna make sure...


thanks again!

-pat

 

[ScottyUK]

Field power should be a small fraction of the main output of the drive, so I wouldn't worry about it. Depending on the drive internal design it will probably be met by the regenerating load. You will need to specify a regenerative drive becasue not all DC drives are regenerative. Expect to pay a premium for the feature.


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If we learn from our mistakes I'm getting a great education!

 

[sreid]

Two things I have used;

Electric stove top elements.

Quartz infrared heating elements.

They change resistance a lot from cold to hot but they are cheap, high wattage resistors.

 

[itsmoked]

The cheapest is the salt water rheostat. But the salt and the 'live' water are serious negatives.

Domestic electric water heaters have some really small, really large wattage elements. Typically they are around 3 to 5kW and pretty inexpensive. You could buy a handful of them and mount them in a metal barrel. Boil away.

You do need to understand your DC voltage to wire things correctly.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Compositepro]

I think Keith's got the best solution, safe and cheap. $300 or$400 for parts including a drum.

 

[danw2]

I was an antique tractor show where the dyno load was the water heater electric elements suggested above mounted in an open barrel of water.

 

[zeusfaber]

Obvious, but worth saying anyway:- If you're going to use immersion heater elements as loads, be sure to bypass the built-in thermostat: In a dyno application it, like an emergency stop button, is not your friend.

A.

 

[p27182]

Hey guys thanks again for all the ideas and suggestions!

A new quote from resistorsonline.com came up to 1600 bones, so that'll likely be the route, unless I can get this GE regen DC motor drive working...

I will certainly pitch the water heater idea as well, but I don't know how they'll feel about banking water.

and zeus- thx for the reminder.

-pat

 

[waross]

RE the DC motor/generator regeneration. With a constant field, the voltage of a DC machine will be proportional to the speed. At lower speeds you will have enough voltage for regeneration into a resistor bank but not enough voltage to regenerate into DC mains.
A stronger field will increase the voltage at lower speeds but there is a limit to how much you can increase the field strength.

At full load you may be boiling about one quart per minute. You may want to consider a make-up tank and a circulating pump.
If the water level drops below a heating element for even a few minutes that element will probably burn out.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[p27182]

waross,

so at slower speeds, would a regen drive just be dissapating the unusable power through internal resistors? or would it be dissapated through the process of just trying to spit it back onto the mains?

thanks for the info
-pat

 

[Skogsgurra]

DC machines work all the way down to 0 RPM and reverse, if needed. That means that you can brake with full torque also at speeds near and including zero RPM. That is what cranes do.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...