Determining Black Box Heater Connections

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Pitt123

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We are looking to wire up a heater that we know we bring 480V (we specified system voltage) however the heater elements are inside a box and all we have is 6 terminals. I am thinking that these 6 terminals are the end of each of the (3) heater elements (phases) and we connect our feeders and jumper them to either wire them in wye or delta.

If we know the total heater wattage is there a way to determine weather to connect these in wye or delta? Obviously delta gives us 480V across each element, and wye gives us 277V. I dont think you can measure resistance across each coil though in order to use V^2 / R because the resistance will change when the heaters increase in temperature.

Will measuring the resistance work or is there another way?
 
This almost sounds like a homework problem. :)

If the product gives you the option to connect it in delta or wye, how would measuring the resistance tell you which was is correct to wire?

Now, if you're telling me you don't know which way to wire it (under each case) then just measure the resistance and find the pairs of terminals that are connected through an element....the rest should be open.

Or, just contact the manufacturer and get a wiring diagram. :D
 
lakee911 said:
This almost sounds like a homework problem. :)

If the product gives you the option to connect it in delta or wye, how would measuring the resistance tell you which was is correct to wire?

Now, if you're telling me you don't know which way to wire it (under each case) then just measure the resistance and find the pairs of terminals that are connected through an element....the rest should be open.

Or, just contact the manufacturer and get a wiring diagram. :D
Click to expand...

This is not a homework problem.

If I knew the resistance across each element, and then I know the voltage across each element as being 480V for delta, or 277V for wye connected, then by using the equation P=V^2 / R I could find the power across each element. Once I found the power across each element I could multiply this by 3 to determine the heaters total 3-phase power. I could then compare this to heater wattage rating to see if it matched.

However like I stated I dont know if measuring resistance while the elements are cold will be and accurate way of determining operating resistance.
 
Pitt123 said:
... However like I stated I dont know if measuring resistance while the elements are cold will be and accurate way of determining operating resistance.
Click to expand...
I suspect close enough to tell which one you want. Heating elements are generally pretty low temperature coeficient.

But if your answers are indeterminate, hook it up Wye and check the current. If the current (power) is low, switch to Delta.

cf
 
Do you know the operating temperature of the unit? Do you know the type of heater elements?

You are quite correct that resistance of the elements will change with temperature. The amount of this change depends upon the temperature change, so if you know the operating temperature and heater material, you can calculate what the room temperature resistance should be.

-Jon
 
Nichrome is pretty flat. You are only looking at maybe 10% in 1900F delta t.

For example, 15kw total or 5kw/element
At temperature (say 2000F), each element is:

Delta R = 480^2/5000 = 46 ohms

Wye R = 277^2/5000 = 15 ohms

At Room Temp R will drop about 10% (yes, I am aware this is not mathematically rigorous - but well within OP's requirements)

Delta R(rt) = 46 - 5 = 41 ohms

Wye R(rt) = 15 - 2 = 13 ohms

Pitt -
I think that doing the calc will work fine.

cf
 
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lakee911 said:
Or, just contact the manufacturer and get a wiring diagram.
Click to expand...
That's what I'd do before hooking this up either way. If you were able to specify voltage, then it's a new item and should have come with documentation. I wouldn't want to have to pay for a new one.
 
I would try to get information from manufacturer/supplier.

If not able to do this try wye connection first each element only sees 277. Then check amperage to see if it is what you expect it to be.

If you try the delta connection first and the elements are rated for 277 you will supply them with 480 and they probably will not take that very well.
 
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