Help: Make 60 Amp Load Bank To Run At 208,240, and 280 volts

Discussion in 'General Electronics Chat' started by machartm, Aug 7, 2014.

  1. machartm

    Thread Starter New Member

    Aug 7, 2014
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    My coworker wants me to come up with a design for a load bank to draw 60 amps at 208 volts, 240 volts, and 280 volts to test for harmonic distortion at high loads.

    I guess it's something for testing standards for UL and CSA... but I'm at a loss. I'm not very good with electronics since I am a Mechanical Engineering student, not an Electrical, nor have I dealt with that much energy before.

    How can I make something like this on the cheap?

    If it counts for anything the business I'm doing a co-op for makes Stoves and Ranges, so I suppose we have some high wattage heating elements available.
     
  2. inwo

    Well-Known Member

    Nov 7, 2013
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    You need a separate bank for each voltage.

    Unless you use a tapped transformer.

    Any type of current control will cause distortion.
     
  3. crutschow

    Expert

    Mar 14, 2008
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    You could use one bank but use a 60A switch to change part of the load for each voltage.

    60A @ 280V is 16.8kW so you will need some very high wattage resistors. You might consider a number of stove heater elements in series/parallel. These would need to be located in a well ventilated room.

    What device are you testing exactly? Is that the input or output current of the device?
     
  4. inwo

    Well-Known Member

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    What part of the world?
     
  5. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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  6. inwo

    Well-Known Member

    Nov 7, 2013
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    OMG that's huge.:eek:

    Did the math on this one.
    3 banks of 70 amps.

    It's only 7KW

    What duty cycle?
     
  7. inwo

    Well-Known Member

    Nov 7, 2013
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    Given the different voltages and currents I would look into a liquid rheostat.

    It would solve many problems.
    Adjustment cooling etc.

    I serviced them in the day before VFDs. ie. Load banks for wound rotor motors.

    Speed was controlled by salt water level.

    They ran 100% of the time and gave a little trouble, but not as much as you would expect.

    A transformer could be used to match impedance if needed.

    I don't know the V/A/W limits. I would think they could be scaled up pretty high.
     
  8. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Here where I work we needed to make a load back for life testing of some AC power relays. The devices switch on or off about every half second and switch 220V AC power into a load bank that if on continuously will dissipate 9,000 watts.

    To accomplish this safely and without overheating the building we started by renting a shipping container, plus some simple modifications such as a vent stack and a power port. I believe it runs us around $150 a month to have it sit just out back our building.

    Inside are something like 18 baseboard heating elements purchased from the home store (the orange one I believe) and screwed down onto a 2x4 wood frame.

    Been running perhaps 6 months now with no issue, but if you put your hand on the container even on a warm summer afternoon it is still warmer than the other containers out there.

    Your load back varies between 12,480 and 16,800 *continuous* dissipation.

    You should plan for a similar scheme.
     
  9. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    I've had 72KW being dissipated (for 4 hours) by our 2 Miller load banks in a 10 x 14ft room. (but open ceiling) and the room is inside a 3K sq ft building with AC/20ft ceilings..
    The room ambient rises a good 15-20 degrees locally and it feels like is sucks the moisture right out of your body.. I just chap-stick up and head in.. :)
    Air blowing out of those load banks is in the 250+ deg F range easily..
    Its TOASTY..
     
  10. machartm

    Thread Starter New Member

    Aug 7, 2014
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    Thanks for all the advice, but I'm a complete novice with circuitry. This is a lot of power and I don't want to destroy something, or myself... How would I wire something like this?
     
  11. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Why were you tasked with it in the first place? This isn't a project for a novice IMO...
    That IS quite a lot of power and you can really hurt someone/something,etc..
    IMO... its not just wiring it up.. Its more than that. You need to ensure parts don't burst into flames/melt,etc.. Ceramic insulators/wire insulation ratings/silicone sleeving, appropriately sized fans,switch ratings,etc.. all come into play here.

    In very basic terms a "resistive load" is just a resistor wired in series with your power supply. But getting the proper resistance (many in series or parallel,etc..) and sufficient wattage/cooling is where the difficulty comes in.

    I'd suggest you either rent an appropriate load bank or sub this out to a suitable engineer. If this is a one-time test for UL approval it WILL be cheaper to rent vs all the time/money/mistakes you could come across.
     
  12. nagromo

    New Member

    Aug 8, 2014
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    In my opinion (as an electrical engineer), this is a bigger mechanical/thermal problem than an electrical problem.

    It's also not a good project for a beginner, as (like you recognized) it can be quite dangerous dealing with so much heat and power. However, if you do need to do it for work, here's some advice to hopefully make it safer:

    I'm not sure how far you are in your mechanical engineering courses; have you taken thermodynamics or heat transfer?

    Have you taken the basic electrical course usually required for mechanical engineers, with Ohm's Law?

    First, you have to figure what heating elements you're using and how many you need. You need to know how many amps each heating element draws at full voltage (or the resistance of the heating element when hot). You may need electrical assistance for this, although it's all based on simple calculations from Ohm's Law.

    You could get a good estimate by dividing the total power desired (280V*60A = 16,800W) by the power rating of one heating element. Leave room for a few extra heating elements for safety margin.

    Then you need to mount the heating elements securely so that they won't move or touch anything that they could start on fire, and ensure that you have plenty of airflow to cool them off. You need to use only fireproof, high temperature materials. You also need to mount the heating elements using electrically insulating materials, such as ceramic or high-temperature plastic, so that you don't get an electrical short circuit.

    You also want at least 2-4 inches between heating elements and from heating elements to anything metal (other than the wiring right where it connects to a heating element). This is to avoid any short circuits and allow separation for the voltage.

    And because these heating elements will be very high temperature and high voltage, you want to prevent people from touching them (while still giving them plenty of airflow, probably from a fan).

    This is the most important portion of the project for safety; you want to mount everything securely so that they don't start a fire and have plenty of airflow for cooling.

    Wiring them together is relatively straightforward compared to mechanically mounting them. You need to use wire with high temperature insulation (likely silicone), and you likely need to use crimped bolt-on connections (again rated for the temperature) rather than soldering. And everything also needs to be rated for the current it will carry.

    Ideally, you could handle the mechanical/thermal portions yourself and get assistance from somebody else on the exact electrical design. If you have to do all the electrical portions as well, let us know. Also, since this is important to safety, you should definitely get someone who knows what they're doing to inspect it before use!
     
    Last edited: Aug 8, 2014
  13. machartm

    Thread Starter New Member

    Aug 7, 2014
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    have you taken thermodynamics or heat transfer?
    -Yes to both

    Have you taken the basic electrical course usually required for mechanical engineers, with Ohm's Law?
    -Yes, but it has been a few years.

    If I understand my Ohms law correctly... If I have heating elements that are rated fro 40 amps at 240 volts, should I connected two in series, and then connected that with a 3rd in parallel to the other two for a total of 60 amps?

    I figured I'd space the heating elements apart with other heating elements that would be off, depending on what voltage is currently selected with a switch.

    Speaking of which. Where would I look to find a switch that can handle this load?

    Otherwise I was thinking of just using a battery connector like this.
    http://upload.ecvv.com/upload/Product/200801/200631541130317741_Battery_Connector.jpg

    and then having each loop have the other respective end of the battery connector, and you just connect the two together depending on which load you want for whatever voltage you're using.

    [​IMG]
     
    Last edited: Aug 11, 2014
  14. machartm

    Thread Starter New Member

    Aug 7, 2014
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    I just used 40 amps because I know a single oven or cooktop is capable of drawing up to 40 amps at full load... I don't know hat the heating elements are rated for just yet.
     
  15. inwo

    Well-Known Member

    Nov 7, 2013
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    Silly question, if making a load for company to test similar loads to their electric ranges, why not use the actual device for testing?

    I'll bet they have them!:)
     
  16. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    glad you asked the question.. I didn't want to :p
     
  17. machartm

    Thread Starter New Member

    Aug 7, 2014
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    That's what I figured at first, but...

    A) They want to test the power supply at maximum load
    B) It needs to be constant and mostly steady. The units do "thermal cycling" to maintain steady temperatures in the cooking surfaces. In other words the power it draws varies greatly as it goes up and down to maintain these temperatures.
     
  18. inwo

    Well-Known Member

    Nov 7, 2013
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    Thanks, that makes sense.:D
     
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