Heating Element Wattage

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keyturbocars
keyturbocars Solar Expert Posts: 375 ✭✭
in Wind Power Generation #1
I am not the best at electrical calculations, and I am wondering if there is an easy way for me to calculate the wattage of a heating element based on the measured resistance?

My HY-2000 wind turbine system has a 3 phase AC dump load made up of air heating elements. I am curious if there is a way for me to verify the wattage of these heating elements by taking resistance measurements across the 3 phase on the dump load heater box. I have the 3 connections coming into it which dumps the 3 phase AC and there is also another connection on the heater box for ground.

Thanks for any tips.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #2
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    Re: Heating Element Wattage

    Here is some information on Nichrome Wire resistance heater calculations:

    http://www.wiretron.com/design.html

    If you know the resistance, you can use the normal electrical calculations to estimate the current and power usage:
    • V = I*R
    • I = V/R
    • P = I^2 * R = V^2 / R
    The resistance of Nicrhome wire does increase slightly as it gets hot (6% at 2,000 degrees F)... So the cold resistance will be be lower than operational resistance when hot.

    You may have problems measuring the resistance of the wire with a standard DMM... They are not too accurate when measuring resistance in the single digit, or less, Ohm range.

    If you need accurate measurements--you might try using a 12 volt battery to drive the heating element and measure the voltage/current to get the resistance.

    There is a table in the above link that gives you, for a particular wire diameter the estimated current to take it to a specific temperature (for coiled wire, ~1/2 the current is required). If you know the diameter of the wire (assuming it is Nichrome) then you can estimate the amperage for a given operational temperature.

    If the wire glows red, then you can use a color temperature charge to estimate the temperature:

    http://physics.info/color/

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • keyturbocars
    keyturbocars Solar Expert Posts: 375 ✭✭
    #3
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    Re: Heating Element Wattage

    Thanks Bill for the great information. You are very knowledgeable!

    If I were to try to measure the resistance of the 3 phase heater box (dump load), then would I measure each phase relative to ground?

    Not sure if it make sense to measure between phase connections. I suspect that it would be better to measure from each phase to ground.
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  • NewYorkMan
    NewYorkMan Registered Users Posts: 19 ✭
    #4
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    Re: Heating Element Wattage

    Yes, do as BB said. It is pretty easy. Here is a sample calculation.

    First, measure the resistance of the heating element by using a rms mulitmeter like a Fluke. We will make up a number of 5 ohms.

    Second, approximate the voltage of your battery bank when it dumps. You can look this up in your manual but let's approximate it at 60 volts.

    Now dump wattage: Power = (volts^2)/resistance

    which gives Power = ((60 volts)^2)/5 Ohms = 720 watts dumped by dump load

    That is an example. Your exact situation is obviously different. Also, does your dump load connect directly to your turbine or do they dump off of the battery? If the charge controller disconnects the turbine from the battery and connects it directly to the dump load, then the calculation is different. It would depend on the varying voltage of the turbine.

    Now, my system dumps through the battery. When my batteries hit full charge, the dump load is turned on. But, the dump load dumps from the battery and the wind turbine continues to charge the battery. This is not a problem because my dump load is set to consume about 25% more energy than the max power of the turbine. So there is no risk of overcharging the battery.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #5
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    Re: Heating Element Wattage

    I would guess if this is a three phase load, just measure the resistance between each pair of terminals. You should have three measurements and resistance should be the same (A-B, B-C, A-C if you name each terminal A, B, and C). The power dissipated (assuming equal current in each leg) will simply be the some of the Watts for each of the three connections.

    Three phase power is very heavy into math and system configuration issues... If you need more information, you may need to look for websites like:

    www.3phasepower.org

    to find the information you need (or somebody really familiar with 3 phase power--been too many decades for me since I last did the math:cry:).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • keyturbocars
    keyturbocars Solar Expert Posts: 375 ✭✭
    #6
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    Re: Heating Element Wattage

    Thanks for the info NewYorkMan. The HY-2000 wind turbine controller actually dumps the 3 phase wild AC directly to the heating elements. It dumps the turbine output directly to the dump load.

    I do have another dump load set up that diverts off the batteries to heat hot water, but that diverts DC power off the battey bank. I have 2 dump loads in my system. Primary is the DC diversion that heats hot water. Then as a back up, the HY-2000 controller dumps the 3 phase AC power to an air heating box.

    I did just try to measure resistance with an older Fluke, but as BB said it's hard to take a good reading. When I measure between each 3 phase connection relative to ground, the numbers were varying a lot. One time, the numbers settled down on 1.4 Ohms. I checked the next phase connection and it was around 1.5 Ohms. Then when I tried to check the 3rd phase connection relative to ground, the Fluke was just doing the "dance" and didn't want to settle on anything. Or maybe it was just that I got tired of holding my arms up in the air and didn't wait long enough! :) I have my 3 phase dump load bolted up on a concrete wall in the garage to keep the heat away from my other electronics.

    I'll attach a picture of it here. I am trying to determine if this is adequate to load down my HY-2000 wind turbine. I would like to have a dump load that is around 3.5kW to match the peak outputs I've seen from my HY-2000. I want to set up a way to manually dump the 3 phase output during high wind events to slow down the turbine before shorting it out for full electromagnetic braking. In other words, I am trying to come up with a safer way to shut down the turbine than just shorting it out. Last night, my weather station showed peak winds of 57mph. Trying to shut down a wind turbine in a strong wind can be a scary experience. I had one situation with another wind turbine where I tried to shut down and at least 1 phase (or more) of the stator burned out and I had a runaway, freewheeling turbine. It finally melted down completely. NOT a fun experience! I am trying to learn my lessons and set this wind turbine up so I can shut it down more safely.

    Last night, I was trying to time it so I shut down (shorted out) when there was a slight lull in the wind and also when the HY-2000 controller was dumping the 3 phase output. It would be better if I could manually dump the 3 phase AC output and then wait a moment before shorting the 3 phase output all together. It's hard enough to find a lull in the winds (never know if it's going to keep getting stronger or calm down). Wait too long for those lulls, and you could end up in a windier situation! All that to say, I'm trying to determine if the 3 phase AC air heater box is even properly matched to the 3.5kW peak output of the HY-2000 turbine.

    Edward
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #7
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    Re: Heating Element Wattage

    Don't measure it to ground (if you have a ground connection--I would be looking for a short circuit in the box).

    There is no good reason for the three phase alternator output to be referenced to ground--Unless there is one 3 phase load box terminal tied to ground for safety (and the load is only connected to the alternator and never to the battery bank (aka transfer switch like setup). But, in general, a grounded 3-T terminal in the load box would be problematic.

    Only measure from 3 phase terminal to 3 phase terminal. The difference of 1.4 or 1.5 Ohms is close enough for this work (double check by shorting your two meter leads together and if, for example, you get 0.2 Ohms, then subtract that from your above readings--or 1.2 to 1.5 Ohms).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • LucMan
    LucMan Solar Expert Posts: 223 ✭✭✭
    #8
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    Re: Heating Element Wattage
    keyturbocars wrote: »
    I did just try to measure resistance with an older Fluke, but as BB said it's hard to take a good reading. When I measure between each 3 phase connection relative to ground, the numbers were varying a lot. One time, the numbers settled down on 1.4 Ohms. I checked the next phase connection and it was around 1.5 Ohms. Then when I tried to check the 3rd phase connection relative to ground, the Fluke was just doing the "dance" and didn't want to settle on anything.
    Edward


    That's why I just can't part with my old Simpson 260 analog meter.
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  • keyturbocars
    keyturbocars Solar Expert Posts: 375 ✭✭
    #9
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    Re: Heating Element Wattage

    Thanks for the 3 phase info Bill. Was busy today fixing my roof damage and didn't get much more thinking done on the wind turbine.
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