Electric Heater Theory

[Ztrain1985]

Maybe its just me, but im having some issues wrapping my head around this.

I was told that in a resitive heater, if you decrease voltage, you increase current draw. However, I can not figure out how.

Say we have a 240 V heater, drawing 10 A, meaning its a 2400 Watt unit.

If we half the voltage, P=(1/2 * V)^2/R. R should be constant, so using V=IR, 240=10*R, R = 24 Ohms. Since Voltage is squared though, halving the voltage means reducing the power output to 1/4 its value. Or, 120^2/24 = 600 Watts. Again using the Power Equation, 600=120V*I meaning current is 5A.

How exactly is current supposed to increase with a voltage drop? Unless Power doesnt change when voltage does (or the resistance somehow changes), what am I missing?

 

[davidbeach]

At 1000W, 240V resistance heater will draw half as much current as a 1000W, 120V resistance heater. A 1000W, 240V resistance heater operated at 120V will draw 50% of the current it does when operated at 240V and produce 250W rather than 1000W.

 

[Ztrain1985]

So am I right to think that a heater WILL NOT draw more current if the voltage is decreased?

 

[davidbeach]

A given heater element will draw current proportional to voltage. A heater element is a constant impedance device and will always draw less current as voltage decreases.

Constant power devices, such as motors (over limited ranges of voltages), draw more current as voltage decreases.

 

[peebee]

You were told wrong. V=IR, R is a constant, so if V goes up then I goes up. If V goes down then R goes down. This is true for any constant-resistance load.

In contrast, motors do what you state, as motors are not constant-resistance, but rather constant-power. To maintain the constant P=IV power, if voltage goes down then current must go up. But that's just for motors, not heaters.

 

[itsmoked]

Depending on how hot the element runs you may not get a linear reduction in current with voltage but it will still be a reduction.

(Probably more info then you wanted.)

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Skogsgurra]

To say that motors are constant power devices is gross oversimplification. It is not so, and saying that without adding the necessary details an constraints only creates confusion.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[davidbeach]

That's why I included "over limited ranges of voltages". Generally considered close enough that for load flow studies (voltage [±]5% or less) collections of motors can be assigned to a block of constant power loads. I'd never dream to suggest that going from 240V to 120V a motor would be constant power.

 

[Ztrain1985]

Alright, thanks guys. Im gonna have to go back and ask him to show me how he calculated this.

 

[jghrist]

I think davidbeach had the right idea in his initial post. What the guy meant was that if you use a 240 volt heater instead of a 120 volt heater, you will draw less current to get the same heat. This is correct because now we're not talking about reducing the voltage on a constant resistance element. We're talking about using a higher resistance element to get the same power with a higher voltage.