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Tuesday, June 28, 2011

Application of thermodynamics in electrical engineering

Thermodynamics is the study of energy conversion between heat and mechanical work, and subsequently the macroscopic variables such as temperature, volume and pressure. Thermodynamics is the branch of science or physics that studies various forms of energies and their conversion from one form to the other like electrical energy to mechanical energy, heat to electrical, chemical to mechanical, wind to electrical etc.

Historically, thermodynamics developed out of a need to increase the efficiency of early steam engines, particularly through the work of French physicist Nicolas Léonard Sadi Carnot (1824) who believed that engine efficiency was the key that could help France win the Napoleonic Wars. The first to give a concise definition of the subject was Scottish physicist William Thomson who in 1854 stated that:
Thermo-dynamics is the subject of the relation of heat to forces acting between contiguous parts of bodies, and the relation of heat to electrical agency.
The starting point for most thermodynamic considerations are the laws of thermodynamics, which postulate that energy can be exchanged between physical systems as heat or work. They also postulate the existence of a quantity named entropy, which can be defined for any isolated system that is in thermodynamic equilibrium.[8]
In thermodynamics, interactions between large ensembles of objects are studied and categorized. Central to this are the concepts of system and surroundings. A system is composed of particles, whose average motions define its properties, which in turn are related to one another through equations of state. Properties can be combined to express internal energy and thermodynamic potentials, which are useful for determining conditions for equilibrium and spontaneous processes.
With these tools, thermodynamics can be used to describe how systems respond to changes in their environment. This can be applied to a wide variety of topics in science and engineering, such as engines, phase transitions, chemical reactions, transport phenomena, and even black holes. The results of thermodynamics are essential for other fields of physics and for chemistry, chemical engineering, aerospace engineering, mechanical engineering, cell biology, biomedical engineering, materials science, and economics, to name a few.

Application of thermodynamics in electrical engineering-
1.Temperature measurement using NTC thermistors
2. Thermal considerations in using semiconductorsUse of heat sinks. Use of forced air.
3. Use of LM339 temperature sensitive diode, design of gain and offset circuitry to interface with analog to digital converter spanning range of 0.0 to 5.0 Volts, use of 4051 analog multiplexer under control of printer port to make up to eight measurements.
4. Running NTC thermistor in self heat mode to detect air movement and as fluid detector.
1. Temperature measurement using NTC thermistors
One of the easiest ways to measure temperature is to use a Wheatstone bridge (see drawing below). In a normal operation of the bridge, R3 is a variable resistor or a potentiometer. We will zero the bridge at 25°C using the potentiometer installed at R3 position. Next we will record the voltage versus temperature reading of the bridge. You will find that this is very linear, especially in the range of 0°C to 70°C. In this manner the output voltage would be a good indicator of temperature.
R 1 / R3 = R2 / Rt Þ condition of bridge being in balance @ 25°C
R1 = 250W, R2 =1000 W, R3 = 2500W, Rt = 10,000W.
The bridge voltage in circuit diagram (1) is now easily calculated using the formula below:
Vb = [ R3 / (R1 + R3) ] - [ Rt / (R2 + Rt) ] Vs eqn 3
1. Vs = Source Voltage = 7.55 volt DC Max
2. Vb = Bridge Voltage
Now: using the equation 3 (eqn 3) and the resistance versus temperature curve for NTC thermistor we can accurately predict the voltage at any temperature or vice versa.
This is very accurate and repeatable since the bridge aids linearize the NTC effect.
As an example:
(EX) Predict the voltage drop at temperature of 40°C
(A) At 40°C R t = 5282Ω, from Ametherm curves.
Using eqn 3, Vb = í[2500/(2500+250)] - [ 5282/(5282 + 1000)]ý (7.55v) = 0.5154
Refer to chart #1: T= 39°C Þ 0.519V and T= 38°C = 0.482V
Interpolating to the tenth of °C we will have (0.519) - (0.482) / 10 =0.0037
Therefore if we subtract 0.519-0.0037 = 0.5153 =38.9°C
Although our target was for 40°C, this was done to illustrate the effect of selecting a part, which has a tolerance of ± 2.2°C.

1 comment:

  1. this is application of thermodynamics in electrical engineering