Inverter - is this right?

Discussion in 'Motorhome Chat' started by maz, Mar 19, 2013.

  1. maz

    maz Funster

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    I have a 200w pure sine wave inverter that I use for inductive chargers such as the electric toothbrush. I am thinking that it might be quite nice to also use it to power the electric blanket when off hook-up. Blanket is rated at 85w and would be used for preheating only, maybe 15 mins.

    Now I wouldn't have thought that would be a problem. I know the controllers of modern electric blankets can have problems with modified sine wave inverters, but as mine is pure sine wave that's not an issue.

    However researching the issue I came across this:

    I would like to run an electric blanket and a small tv in my caravan, total watts about 400. What size inverter would you suggest and suit my needs?

    Appliances that have coils that produce heat typically require 3 to 7 times the listed power. Therefore, if the blanket is rated at 100Watts continuous, you may require a 1200W Inverter just for the blanket.

    on this website: http://performanceshop.com.au/power-inverter-faq.html#applianceruninverter

    This doesn't seem right. Ok, so there are wires running up and down the electric blanket but surely that's not the same as a coil? So are they right - or just trying to sell a much larger inverter than necessary? :RollEyes:
     
    Last edited: Mar 19, 2013
  2. Gooney

    Gooney Funster

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    Are you sure your not thinking of wiring up your wheel rims Maz?:Wink:
     
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  3. maz

    maz Funster

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    Now you've given the game away. :BigGrin:
     
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  4. Reallyretired

    Reallyretired Funster

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    Electrical resistance increases with temperature, therefore a cold electric blanket will draw more current than when it has warmed up. I would imagine that this is what they are talking about.

    From Wikipaedia

    Temperature dependence
    Main article: Electrical resistivity and conductivity#Temperature dependence

    Near room temperature, the resistivity of metals typically increases as temperature is increased, while the resistivity of semiconductors typically decreases as temperature is increased. The resistivity of insulators and electrolytes may increase or decrease depending on the system. For the detailed behavior and explanation, see Electrical resistivity and conductivity.

    As a consequence, the resistance of wires, resistors, and other components often change with temperature. This effect may be undesired, causing an electronic circuit to malfunction at extreme temperatures. In some cases, however, the effect is put to good use. When temperature-dependent resistance of a component is used purposefully, the component is called a resistance thermometer or thermistor. (A resistance thermometer is made of metal, usually platinum, while a thermistor is made of ceramic or polymer.)

    Resistance thermometers and thermistors are generally used in two ways. First, they can be used as thermometers: By measuring the resistance, the temperature of the environment can be inferred. Second, they can be used in conjunction with Joule heating (also called self-heating): If a large current is running through the resistor, the resistor's temperature rises and therefore its resistance changes. Therefore, these components can be used in a circuit-protection role similar to fuses, or for feedback in circuits, or for many other purposes. In general, self-heating can turn a resistor into a nonlinear and hysteretic circuit element. For more details see Thermistor#Self-heating effects.

    If the temperature T does not vary too much, a linear approximation is typically used:

    R(T) = R_0[1+\alpha (T - T_0)]

    where \alpha is called the temperature coefficient of resistance, T_0 is a fixed reference temperature (usually room temperature), and R_0 is the resistance at temperature T_0. The parameter \alpha is an empirical parameter fitted from measurement data. Because the linear approximation is only an approximation, \alpha is different for different reference temperatures. For this reason it is usual to specify the temperature that \alpha was measured at with a suffix, such as \alpha_{15}, and the relationship only holds in a range of temperatures around the reference.[9]

    The temperature coefficient \alpha is typically +3×10−3 K−1 to +6×10−3 K−1 for metals near room temperature. It is usually negative for semiconductors and insulators, with highly variable magnitude.[10]
     
  5. pappajohn

    pappajohn Funster Life Member

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    I think where they refer to 'coils' they are talking electrical windings...IE: leccy motors.

    a motor will draw a far higher current when starting to turn than when it is actually turning.

    your leccy blanket is nothing more than a light bulb without glass....just a resistive loop.
     
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  6. Gromett

    Gromett Funster

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    Beat me to it. Also things like power factor come into play when talking about inductive loads which can play havoc with an inverter.
     
  7. maz

    maz Funster

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    So basically they have it wrong on their website. It's not relevant at all to electric blankets. :RollEyes:
     
  8. Reallyretired

    Reallyretired Funster

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    It is relevant, see my post above.
    A resistive load that produces a change in temperature will have a lower resistance, i.e. use more current, when its cold. If you have a 200w inverter and the continouos rating of your electric blanket is 85w then you should have sufficient spare capacity though.
     
  9. Jaws

    Jaws Funster Life Member

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    Maz, I think more to the point is how big is the bank of batteries you have ?
    A 200w blanket running through an inverter is going to draw BIG amps, at a very rough guess as inverter efficiency is unknown, 20 amps plus.. Thats one hellova draw !
     
  10. maz

    maz Funster

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    No it's an 85w blanket; inverter is 200w. It'll be on for about 15mins. Battery bank is 330Ah.
     
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  11. Gromett

    Gromett Funster

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    Going back to the original question I would agree, any heating to be done should not be done with batteries...

    When I had a proper bed I would pull the covers back and have the heating up for 5 minutes.

    If off hookup I would resort to a couple of hot water bottles..
     
  12. maz

    maz Funster

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    But they're talking about 3-7 times the given power rating of the blanket. That's a huge difference!
     
  13. pappajohn

    pappajohn Funster Life Member

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    you must have a very efficient blanket.

    ours, new this winter, is rated at 79watts (0.3amps @230v) with three heat settings and on full it takes around 30 minutes to produce any real warmth.

    with 20% inverter losses that amounts to around 8 amps/hour so not too heavy on 12v power....1/2hr = 4amp....1/4hr = 2amps
     
  14. maz

    maz Funster

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    I use it every night on hook-up and 15 mins is pleasantly warm. :Smile:
     
  15. pappajohn

    pappajohn Funster Life Member

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    we obviously have differing ideas of pleasantly warm....:Laughing:
     
  16. Jaws

    Jaws Funster Life Member

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    Well I aint gonna cuddle you !!!!
     
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  17. Snowbird

    Snowbird Funster Life Member

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    I find a nice warm bottom to put my cold feet on is not only more energy efficient than an electric blanket, but saves my battery power for more important things like keeping the beer fridge cold. But then again am just a man and therefore more practical :Cool:.
     
  18. sedge

    sedge Funster

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    You seem to have that, if I may be so bold? - arse about feet in your bed, Mo ! :Rofl1:

    Pete exists (as did my first husband) primarily as a heat source for me in bed. I thought that was actually what they were designed for?

    However last night was weird, cos for some reason I was roasting and he was cooling me off with his front against my back. Lovely!
     
  19. canopus

    canopus Funster Life Member

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    Its a question of whether you're wearing wyncyette or birthday suit I would say :Rofl1::Rofl1::Rofl1:
     
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