Temperature increase from amperage loss in wire

[scott_alexsk]

Hello,

I was wondering if anyone knew a calculation or method to determine the amount of current moving through a given material, knowing the resistance of the material, the specific heat and the temperature (while the material is actually being heated by current).

Thanks,
-scott

 

[NoTime]

Perhaps the easiest is to just get a meter that can measure the current.

Or
If you know the resistance then you can measure the voltage at the feedpoints to find the current.

Current = Voltage/Resistance.

Note that resistance of most materials is dependent on temperature so you need the temperature to calculate
the corrected resistance.

If you are trying to figure out just how hot an object will get, then this is largely determined by the physical construction.
Airflow and surface area are two major factors.

 

[scott_alexsk]

Sorry for responding back so late. I have been kind of busy. I also just noticed that I posted this in the Systems Design section as opposed to the electrical engineering section...

Anyways I do not have a meter to measure either voltage or current, and it is just useful, at least I think, for me to know how to do these calculations. Say I have a material at room temperature and I heat for a very short amount of time to achieve the effect I am looking for. Assume my instruments are precise enough to measure the highest point of the temperature change. Now from the heat I get from the resistance of the material can I calculate how many amps had gone through the wire assuming I now the resistance and specific heat of the material? Since I am dealing with rather low temperatures and little temperature change do I still need to take resistance correction into account? So in other words what is the relation between ohms and temperature according to the amount of amps I have going through the material? Sorry if my semantics are messed up, I know little of the topic.

Thanks,
-scott

 

[Gokul43201]

Not enough information.

As mentioned by NT, the steady-state temperature depends on the surroundings (temperature, flow, pressure) and the surface conditions (insulation, coating, emissivity, heat transfer coefficient). If you know all these things, you can calculate the current in the wire, given its electrical resistivity, geometry, heat capacity and thermal conductivity.

 

[scott_alexsk]

The equations I have looked at involving the heat transfer coefficient do not really seem to take into acount the amount of time that is involved in the transfer of heat. For those few seconds that current is flowing through the Nitinol I want to determine how much of the heat that is generated, is lost to following out of the wire. Do I really need to take pressure into account since I am dealing with standard pressure?

-scott

 

[berkeman]

It sounds like you are working on a transient situation, Scott, not a steady-state thermal transfer. Right? The short current pulse through your conductive object will deposit some heat in the object, and that heat will then flow out into whatever is surrounding your sample. If you have a calorimeter arrangement, then you can calculate how much heat was deposited in the sample, as long as you wait long enough for thermal equilibrium to be obtained. Is that what you are trying to do?

 

[scott_alexsk]

Yes exactly, the current may be on only for a few fractions of a second, depending on the precision of the delay time switch I get. I was able to put together a circuit which converts the DC current into a wave with varying duty cycles, depending how I set the pentometer, in order to slow down the transfer of heat so I can better document the reaction time in accordance to the temperature of the wire. I am afriad my arrangement does not allow for me to put this in some sort of bomb calorimeter since I am using a spring balance to measure the amount of force the wire pulls with when turning from its marsenite to its austenite state.

Thanks,
-scott

 

[Cyrus]

Since the time is very small, I am going to ignore radiation and convection.

Then:

$$I(t)R^2 = mc_p \frac {dT}{dt}$$

solve that, you will get:

$$\int I(t)R^2 dt = mc_P (T-T_i)$$

But I don't really understand your setup.

 

[Bystander]

http://www.google.com/search?hl=en&q="transient+hot+wire+thermal+conductivity"&btnG=Google+Search

 

[NoTime]

Nitinol also know as muscle wire.
Not sure what you intend by
"reaction time in accordance to the temperature of the wire"
so its difficult to make an suggestions.

 

[scott_alexsk]

Ok my setup is that I have a short section of nitinol wire stretched between a spring balance and a fixed point.

The wire can stretch at most 10% of its length in its room temperature marsenite state. The spring will stretch out the nitinol to varying lenghts according to how far I fix the opposite end. When the nitinol is heated to a certain point it will transform into its Austenite State and attempt to reconfigure itself into its preset shape which in this case is simply a shorter length wire.

I will have an electronic thermometer hooked up to the wire, a pulse wave modulation circuit (modulated DC), with variable duty cycles, and my time delay switch to turn that cicuit on and off. Setting the time-delay switch for different time spans with the duty cycle being set 10% on, 90% off I will be able to find the amount of energy needed to reach the Nitinols highest pull back force. I will use the temperature of the wire when the circuit is turned off to determine how much energy the nitinol actually gained.

Thanks,
-scott

 

[NoTime]

Why are you concerned about input energy?
It's the temperature of the wire that controls its state.

It seems to me that environmental effects are going to distort input energy to final temp.
You might get a figure for the environmental losses by determining how long it takes the wire to cool.
Lag time in the thermometer will be an issue as well.

You could use capacitive discharge to dump a specific amount of energy into the wire.
This could eliminate some of the problem, but even then inertial effects might come into play.

I don't see any advantage to using pulse mode, with or without variable duty cycle, over DC with a timer.

 

[scott_alexsk]

Using a the circuit I can modulate the DC current to be at most 90 off, 10 on. This more gently heats the wire, allowing for a more accurate reflection of the maximium force it can pull with, because the reaction is slower, besides increasing the life of the material considerably. The timer is used to eliminate human error for turning the wire off. From experiments I can see, for the conditions I have, how long the circuit has to be turned on to get the maximium affect without overheating the wire.

The calculations for the loss of energy from the wire are not critical for this experiment but later I am going to be calculating the efficiency of using the nitinol wire where it is exposed to moving air, there I have to determine how effeciently the wire uses energy compared to other methods. There I need to determine how much energy actually enters the wire before it transforms into its austenite state.

Thanks,
-scott

 

[Gokul43201]

I have to determine how effeciently the wire uses energy compared to other methods.

Can you describe this quantity better? What do you mean by "uses"? Could you write down a mathematical relation or show us an example?

Calculating dissipative power loss for a non-ohmic, hysteretic material like nitinol is not straightforward (the I-V characteristics are strongly dependent on applied strains).

 

[scott_alexsk]

Alright, my total research goal is to look at nitinol uses in aircraft. At some point I will be constructing a wing using the material. In order to contrast the effeciency of the current method of hydrolics and the nitinol I need to know for one how well each solution moves air but also how much energy both methods use. But this is subject to change. Coming up with a 'standard' to measure the nitinol wing agaisnt will be a real annoying task if I do it. Please do not put too much time into this. My teacher contacted a university nearby today and they should be able to help me work out this problem, so really just general observations of potential flaws or ideas are welcome.

Thanks,
-scott

 

[Mech_Engineer]

Alright, my total research goal is to look at nitinol uses in aircraft. At some point I will be constructing a wing using the material. In order to contrast the effeciency of the current method of hydrolics and the nitinol I need to know for one how well each solution moves air but also how much energy both methods use. But this is subject to change. Coming up with a 'standard' to measure the nitinol wing agaisnt will be a real annoying task if I do it. Please do not put too much time into this. My teacher contacted a university nearby today and they should be able to help me work out this problem, so really just general observations of potential flaws or ideas are welcome.

Thanks,
-scott

Off the top of my head:

The main problem I can think of is the fact that nitinol needs to be heated to fairly high temps to return to it's memory shape, and the atmosphere is very cold at cruising altitude. This would mean the aircraft would have sluggish control surfaces at altitude, or they might not work at all. They would also not work in very cold environments.

You would be limited by the amount of travel available in a piece of Nitinol. Since any Nitinol wires I have heard of can only contract about 10% of their length or even less, you would need a very long wire to gain the same amount of travel as a hydraulic piston, or would have to use a complicated lever system.

Additionally, a nitinol "piston" would have to be very large, meaning it wouldn't be able to heat and cool very rapidly, further degrading your control surfaces' reaction time.

Nitinol wires are only able to pull, and as a result it would be difficult to fit the necessary levers needed for a pulling wire to "push" into a wing for a control surface to move in the correct direction (take for example the air brakes on the top of an airliner's wing). This also means you would need a nitinol piston for movement in each direction, where as a piston can pull AND push. It's obvious that this would be quite difficult to implement in the limited space of a wing.

Finally, nitinol wires are not very efficient at converting electrical energy to mechanical work compared to a hydraulic system. Far too much energy is being lost to heat.

Unfortunately, it sounds like nitinol would not be a very good replacement for hydraulics...

 

[NoTime]

Hmmm,
I've seen some stuff related to this concept.
The idea seems to have the shape changing material be the control surface as opposed to activating a standard control surface.

As you note temp control would be an issue.

 

[scott_alexsk]

Yes Nitinol has already been used as the control surface as opposed to the piston. Several groups have already tested nitinol applications including Nasa which concluded that a curved wing surface from nitinol 'trained' to transform into that shape is about 10% more effecient at moving air, whatever that means. Mech Engineer, Nitinol does not just pull, it simply returns to its predeformed shape. So if I squish it, when heated it will exert force to push back out, to its trained shape. On the issue of temperature control, with the literature I have read suggests using insulators. Also using a heating element of some sort, like Tungsten filaments, has been shown to be more efficient than electrical heating.

Thanks,
-scott

 

[Gokul43201]

Off the top of my head:

The main problem I can think of is the fact that nitinol needs to be heated to fairly high temps to return to it's memory shape, and the atmosphere is very cold at cruising altitude.

Nitinol can be engineered to have a transformation temperature anywhere from -100C to +100C. I've got a piece of wire in one of my desk drawers with a trans. temp. of about 0C (32F).

 

[scott_alexsk]

Also by subsituting titaninium with platnium, the transformation temperatures have be shown to go up to 1000 C (ie for high speed applications).

-scott

 

[Mech_Engineer]

A Tungsten filament would still be resistive heating, unless you are planning on inductive applications... I fail to see how a Tungsten filament would be more efficient at heating a piece of Nitiniol, compared to just passing an current through the piece itself, since heating the Nitinol directly could allow more uniform heat generation if the current density is carefully managed. You would also need far more current to pass through the Tungsten given it's excellent electrical resistivity (or lack thereof comapred to Nitinol).

It would be quite a feat to design a system that would be able to heat something quick enough to allow for control of an aircraft, especially in adverse weather conditions where reaction times are needed to be far less than one second... This system would also have to be very efficient since the whole point is to come up with a more efficient method of controlling an aircraft. Of course, it might be possible to use such a method for something that does not need to react very quickly, such as flaps.

Hmm, I didn't realize they were considering making an entire control surface out of the material. I worry about the time constant for heating an entire surface in order to make it return to its memory shape; and then to bend it back again you would need to heat an opposite side or opposing surface...

Just mentioning problems that I am seeing... by all means you should give it your best shot.

 

[scott_alexsk]

According to what I have seen from information on just heating small wires, depending on the methods you use, you can get transformations in the millisecond range.

Perhaps unevenness in heating could be dealt with by putting a grid of some sort of a heating element over the surface of the material. Also from what I have read uneveness in heating is fairly minimal, even with heating from just one corner of a sheet is at most 1 degrees C difference, within several seconds. The heat also spreads very quickly, so it will not be an uneven warping, or atleast, nothing noticable to the human eye, (which may be a problem with higher speeds) However I am only going to deal with, at least this year, low speed applications.

I will have to find flexible, thin, and light insulators which can minimize the heat loss that will form the actual surface of the aircraft. I think that is one of my biggest problems, along with constructing the wing to measure the 'effeciency' of the nitinol wing agaisnt.

Thanks,
-scott