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Transformer for Heater Control

  1. Jun 14, 2012 #1
    I am looking to power a heater. According to the product specifications it will require 11.1V/26.4A to heat to 800C and 16.9V/33.1A to heat to 1000C.
    The manufacturer sells an expensive temperature controller which we would like to avoid using. It has been suggested to me that I use a variable transformer and step down transformer with the following specifications (single phase).

    Variable Transformer
    Current Secondary: 25 A
    Frequency: 50/60Hz
    Primary Voltage: 120VAC
    Secondary Voltage: 0-120/0-140 VAC
    Power, Rating: 3/3.5 kVA (Constant Current Load), 3.6 kVA (Constant Impedance Load)

    Step Down Transformer
    Input: 110/220 V
    Output: 8/16/32 V
    Power: 1.5 kVA

    I need to be able to control the voltage to the heater in order to control the temperature. My understanding is that I should connect a 120VAC 60Hz connection from the wall to the variable transformer and set to ~110V. This 110V connection then goes to the step down transformer which will be set to output 16V (or 32V if I need more than 1000C). From here I can then adjust the voltage in the variable transformer to change the voltage output at the step down transformer to the heater.

    I assume that I should not be hooking this up the opposite way (step down goes to variable transformer) because of the voltage outputs/inputs specified for each of these transformers.

    My first question is will this work? It seems that the power rating of the step down transformer is more than enough to power the heater and that the max power through the variable transformer will also not be exceeded.

    Assuming this does work I need to ensure that the user does not overload the heater causing it to burn out. If I were to install a fuse in the system where would be the best place to install it and how do I know what specifications this fuse should have (i.e maximum current) in order to protect the heater.

    Finally, when I hook this equipment up is there any guidelines I need to follow about what gauge wire I need to use and how these wires or how they should be insulated.

    I don't have any real experience with electrical design. Any help would be appreciated.
  2. jcsd
  3. Jun 14, 2012 #2
    I don't understand why you are using a variable transformer and then a step down transformer.

    Why not just set the variable transformer to give you the voltage you desire on its secondary?

    EDIT: Can you provide us with any information on the heater/transformers? (I.e. model, part number, datasheets)
  4. Jun 14, 2012 #3
    This is all the information I have available.

    This setup was suggested by the heater manufacturer. I believe the variable transformer is not used by itself because it can carry only 25A which is less than the heater will draw.
  5. Jun 14, 2012 #4
    Can you give us any information about the heater?

    I'm sure you can find a variac that will supply you with 30A+.
  6. Jun 14, 2012 #5

    jim hardy

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    That's a good plan, use a step-down transformer behind the variable to raise the current. Variable transformers are expensive and bulky. I note your heater is less than 600 watts (16.9 volts X 33.1 amps ) so a 10 amp variable should be plenty, and you might make a 5 work..

    The best way would be to measure its temperature and adjust the power accordingly.
    I'd wager that's what the 'expensive' controller does.

    "Ampacity" is the keyword to figure wire size.

    Wire insulation is designed for the temperature it is expected to encounter. Everyday plastic is not good for high temperature.

    Don't build a trap for somebody. I suggest you do two things:

    Take apart a household appliance like a range or toaster oven and study the wires it uses. You'll find braided glass insulation in better ones. Temperature rating is often printed on the wire.
    Also observe the backup safety device they all have - it interrupts power when the appliance becomes too hot.

    Spend a day on wire manufacturer's websites , like Belden or Alpha and learn about insulation. Myself i'd want that 1000 degree stuff they make for airplane engine compartments.

    old jim
    Last edited: Jun 14, 2012
  7. Jun 14, 2012 #6
    Can you provide a little more detail as to how this is going to work? It is still not entirely clear to me.

    The variac spits out 110V/25A on its secondary and passes this to the primary of the step down.

    Focusing now on the step down transformer,

    [tex]\frac{V_{p}}{V_{s}} = \frac{I_{s}}{I_{p}}[/tex]

    [tex]\Rightarrow I_{s} = \frac{(25*110)}{16} \approx 170A \text{(Assuming we select 16V tap on secondary of step down)} [/tex]

    We still need to adjust this voltage and current for the heater correct?

    Don't we need to know information regarding the impedance in order to design such a network on the secondary to give it the voltage and current it needs?
  8. Jun 14, 2012 #7
    Thanks a lo for your help Jim.

    You are correct that the expensive controller measures the temperature directly and has a feedback loop to control the heater.

    I know that I can figure out what position for the dial on the variable transformer corresponds to the maximum temperature and ensure that this will not be exceeded. However, I want to prevent any sort of operator error.

    My understanding is that a fuse will blow based on current and that the smaller fuses (smaller current) are cheaper. For this reason I would think that I should install a fuse before the variable transformer where the current corresponding to a given power will be lower. Does this make sense?

    Also, the heater has wires that are already thermally insulated. I was more referring to the wires that go between the transformers. It seems you are correct and I can predict the necessary gauge of wire from the current that will flow through them as you said.

    Thanks again.
  9. Jun 14, 2012 #8

    The purpose of the variable transformer is not to output 110V. It can output less than 110V if needed.

    For example if you set the variable transformer to 80V. The step down transformer is designed to go from 110V to 16V meaning it steps down by ratio of (0.14). If you input 80V the step down transformer will output 11.6V instead. This would be enough to power the heater at 800C. At least this is how I understand it, but I may be wrong.

    As for needing information about the impedance, I am not exactly sure what you mean.
  10. Jun 14, 2012 #9


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    I'm curious what your application is. How accurately do you need to have the temperature of what you are heating? If there are disturbances, will you be able to compensate manually to maintain your desired set-point?
  11. Jun 14, 2012 #10

    jim hardy

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    The information in op suggests it's a simple resistance heating element.

    11.6 V / 26.4 A = 0.44 ohm at 800 deg
    16.9 V / 33.1 A = 0.51 ohms at 1000 deg.

    That's .0008 ohms/ohm/degree , a bit higher than nichrome at room temperature but this isn't room temperature.

    That is an assumption on my part, though . I trust it's not an active electronic device at those temperatures.
    Last edited: Jun 14, 2012
  12. Jun 14, 2012 #11
    Jim is correct about the smaller variable transformer.

    PUT THE FUSE IN THE VARIABLE TRANSFORMER OUTPUT. If you put the fuse before the variable transformer, there is the possibility that you will burn out the variable transformer or the stepdown transformer.
    I know, we did.
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