×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

DIssapating 50HP (~38kW) of DC
2

DIssapating 50HP (~38kW) of DC

DIssapating 50HP (~38kW) of DC

(OP)
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

RE: DIssapating 50HP (~38kW) of DC

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.   

View Clyde's profile on LinkedIn

RE: DIssapating 50HP (~38kW) of DC

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

RE: DIssapating 50HP (~38kW) of DC

(OP)
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

RE: DIssapating 50HP (~38kW) of DC

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

RE: DIssapating 50HP (~38kW) of DC

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.

 

RE: DIssapating 50HP (~38kW) of DC

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.
  

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

RE: DIssapating 50HP (~38kW) of DC

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.

RE: DIssapating 50HP (~38kW) of DC

(OP)
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

RE: DIssapating 50HP (~38kW) of DC

(OP)
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

RE: DIssapating 50HP (~38kW) of DC

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.
  

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

RE: DIssapating 50HP (~38kW) of DC

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.

RE: DIssapating 50HP (~38kW) of DC

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

RE: DIssapating 50HP (~38kW) of DC

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

RE: DIssapating 50HP (~38kW) of DC

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.

RE: DIssapating 50HP (~38kW) of DC

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.

RE: DIssapating 50HP (~38kW) of DC

(OP)
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

 

RE: DIssapating 50HP (~38kW) of DC

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

RE: DIssapating 50HP (~38kW) of DC

(OP)
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

RE: DIssapating 50HP (~38kW) of DC

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

RE: DIssapating 50HP (~38kW) of DC

What Gunnar says. But the terminal voltage of a DC machine is almost proportional to the speed. You don't have any problems braking with a resistor but the number of heaters may have to be increased for full current at lower speeds. Regeneration becomes more difficult as the voltage drops.

These issues depend on the use of the dyno. It will be pretty easy to test line powered induction motors, but it may be challenging testing VFD driven motors at slower speeds.

For examples sake, assume that you are testing a motor and at 100% speed the generator is producing 100 Amps at 100 Volts. You are dissipating this in a load bank with 1 Ohm resistance. Your electrical load is 10 kW.
Now at 50% speed you want to maintain 100 Amps but your voltage is now about 50 Volts. You must now drop the resistance to 0.5 Ohms to draw 100 Amps at 50 Volts.
At 10% speed you may need 10 times the number of heating elements to draw 100 Amps.

One good alternative may be the liquid rheostat. The resistance may be varied by moving the electrodes closer together. One type of liquid rheostat used to start large wound rotor motors uses two circular plates in a vertical plastic (non-conducting) tank. Initially the electrodes may be about two feet apart but as the motor is accelerating the upper electrode is being dropped until it is just a few inches from the lower electrode. The current carrying capacity is mostly determined by the area of the plates and the resistance is easily varied.

Another option may be an induction motor and a regenerating VFD.

The greater the speed range that you want to test over the more challenging your application becomes.

Gunnar pretty well answered your last question, but may I add that at higher speeds power may be returned to the supply but as the voltage drops there comes a point where the returned energy must be wasted in resistors, internal or external.

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

RE: DIssapating 50HP (~38kW) of DC

As to the construction of a liquid rheostat;
For temporary use such as one time load testing a generator for a week, I have used plastic barrels with discarded leaf springs from a truck for electrodes.
For a permanent installation you will probably want to use stainless steel and consider a circulating pump.
Thanks to Gunnar and itsmoked for the liquid rheostat suggestions.

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

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources