thermal test_Final.pdf

7/30/2019 thermal test_Final.pdf

1/9

Calculations

1. Average resistance

TimeResistance

R() Y() B() Average()

0 min 7.1 7.1 7.1 7.1

10 min 8.1 8 8 8.03

20 min 8.4 8 8.5 8.330 min 8.8 8.7 8.8 8.77

40 min 8.8 8.9 8.7 8.8

50 min 9 9 9 9

60 min 9.1 9.1 9.1 9.1

2. Corresponding temperatures and temperature rise.

For copper windings t2 temperature at a time t2= (R2/R1) (t1+235) 235

Since winding resistance R1 at 0min is the resistance at ambient temperature, temperature rise is t2t1

Observed ambient temperature = 31 C

Time t2the winding temperature t2-t1the temperature rise

0 min 31 C 0

10 min 65.84 C 34.84

20 min 75.96 C 44.96

30 min 93.57 C 62.57

40 min 94.69 C 63.69

50 min 102.18 C 71.18

60 min 105.93 C 74.93

3. Thermal time constant

From the graph m = 76C

Gradient at t=0 is 3.91343 using the approximated curve a*(1-(exp(-x/b))) for Math lab curve fitting

Where a = 76.9 and b = 19.65

Then


2/9


3/9

Theory

Temperature rise is cause by energy losses inside the machine which produce heat. The increase of

temperature can be measured by resistance measuring methods. Due to cooling effects the temperature most

times reaches to steady value, we are interested to find this maximum reach value of temperature and

compare it with machine ratings.

Resistance variation with temperature for conductor

From the graph

(R2-R0)/(T2-T0) = Ro = temperature coefficient of resistance for particular metal

R1 + R1T2 = R2 +R2T1

T2= (R2/R1) (T1+235) 235 for copper =1/235 C-1

If we can find resistance at two different temperatures for copper with one temperature is known, we can use

above equation to find unknown temperature and hence the increase or decrease of temperature.

And for an electrical machine

Heat absorbed by the machine=(Heat generated by the machine)-(Heat dissipate to the surrounding )

whd = (p1 dt)(p2dt)(1)..where w= weight of machine active parts , h=specific heat constant

p2= b A.. (2)where = temperature rise, b= coefficient of heat transfer, A=cooling surface

combining these two together

By solving above

( )


4/9

Laboratory practice VI EE4092

Temperature rise of an Electrical Machine

Name: A.R.Devinda

Index No: 090103B

Group: G4Date of Performance: 27.05.2013

Date of submission: 10.06.2013

Instructed By: Mr. Hasala Dharmawardena

Group Members: D.M.S.Disanayake

C.L.Fernando

M.H.M.Fernando


5/9

Discussion

1. Discuss the various methods of measuring the temperature of the windings.Electrical machines which consists windings in there interior will produce thermal heat as they are energized

with electricity. This thermal energy which consider as a loss in the field of electrical machines will rise the

temperature of machine parts and dissipate excess to the air or particular coolant. The properties of the

machine parts are different and they have their own capabilities with limitations and during different

operating conditions the machine should withstand the changes of physical quantities like temperature, flux,

voltage etc. Other than that we should operate an electrical machine in such a way that it will not experience

any abnormal conditions.

The rise of temperature beyond the safe limits will reduce the life time of machine, cause initiation of another

type of fault or even break the machine permanently. So we are interested to find maximum temperature rise

of the windings of electrical machine during its operation. But the problem is if the machine is assembled, even

more installed and even more if it is operating how to find the temperature rise of its windings.

Various methods of measuring the temperature of the windings

1. Temperature Rise in Copper Windings from Resistance Measurements.A rise in temperature increases the amount of molecular agitation in a conductor obstructing the

movement of charge through the same conductor. To an observer, the measured resistance of the

conductor has increased with the temperature change.

Since the variation of resistance for particular temperature range is nearly constant. We can evaluate

temperature rise if we know the ambient temperature, resistance at ambient temperature and

temperature coefficient of resistance for particular metal.

(R2-R0)/(T2-T0) = Ro

R1 + R1T2 = R2 +R2T1

T2= (R2/R1) (T1+235) 235

In this method resistance can be measured

Current voltage method of measurement of winding resistance Bridge Method of measurement of winding resistance


6/9

2. Thermal measurements using optiacal methods.

An infrared thermal imaging camera uses a special lens that is focused mainly of infrared light emitted by

all the objects that it can see. The infrared light emitted is processed by an array of infrared detectors,

which then creates a temperature pattern that is also known as thermogram. The entire process from

obtaining the infrared light to making a thermogram takes approximately 0.033 milliseconds. After the

thermogram is created, it is then made into an electrical impulse.

The electric impulse that was made from the thermogram is then sent to a single processing unit or a

circuit board that is mainly focused on transmitting the signal into a data for display. Once the data has

been sent to the display, people are be able to see various colors depending on the intensity of the

emission of the infrared light. Through the different combinations that came from the impulses made by

different objects, an infrared thermal image is then created.


7/9

2. What are the importances of the class of insulation?When operating an electrical machine we match it with our load requirements. For the rated values

the machine should operate continuously. The thermal insulation or the coolant of an electrical

machine which effects the cost of the machine, do not same for every application. Depending on the

losses at rated conditions and the capabilities of the insulations they are chosen for particular

application. During operation we need to ensure that the temperature inside the motor windings does

not rise to a level that exceeds the critical temperature. The critical temperature is the temperature inwhich the (stator and rotor) winding insulation can withstand without permanent damage. Insulation

damage can shorten the useful life of the motor and eventually results in electrical faults.

IEC 60034-1 classifies the temperature rise limits of insulation materials and specifies the maximum

permissible temperatures that the various classes of insulation materials could withstand. The

temperature rise of the electrical machine is the permissible increase in temperature, above this

maximum ambient, to allow for the losses in the motor when running at full load.

Maximum ambient is 40 C

The importance of the class of insulation is it provide a norm to engineers all over world. Those who

design electrical machines can choose level of insulation according to the capacity and cost, while the

users and maintainers have a sense of thermal withstand capacity of the machine

3. Compare the obtained results with machine ratings.Product catalogue states about the level of temperature standards of their products. We are having

lafert st 90s c43 phase induction motor. Which has class F insulation given in the name plate. And

the maximum temperature reaches by machine at rated conditions, obtained by us using calculation is

76C.

So the motor is safe to operate at rated conditions.


8/9

4. Why it is necessary to know about the parameters estimated in this document.Thermal time constant

Thermal time constant is a constant representing the degree of thermal responsiveness. Since all the

operations of the infrared camera are thermal phenomena, it is invariably accompanied by a time lag.

For example, when the measurement object suddenly changes from 100C toC, the temperature of

the device does not change suddenly but gradually to 0C. Thermal time constant represents the time

required for the device to transition from the state maintained at a certain temperature to a differenttemperature in this manner, and a standard value of 63.2% is used as the rate of change generally.

The Thermal time constant of FSV-1200 series is 16sec. In other words, when the measurement

object suddenly changes from 100C toC, the device recognises the measurement object at 36.8C

after 16 msec. Therefore when the temperature of the measurement object changes suddenly and

measurement is done immediately, it may be affected by the temperature of the previous

measurement object and may cause after images.

Maximum temperature rise

This is a design factor of electrical machine. Any influence to normal operating conditions can cause

inefficiency in the machine and hence losses. In such cases are present it is worth to know about the

maximum temperature rise of the machine. But this is not normally mention at the name plate so

using a correct testing approach it should be measured and check whether it matches with the

insulation class of the machine.

5. Discuss about various methods of cooling general purpose machine.

The energy transfer in rotating electrical machines cause energy losses as heat, that heat rise the

temperature of the machine and finally heat is dissipated to surrounding by conduction, convection

and radiation. Cooling is the mechanism of heat exchanging to the outer mediums from inside of the

machine. Depending of the machine various types of coolant mediums are present and sometimes one

machine may use different layer s coolants.

Mechanism of cooling

Primary coolant is at lower temperature than the machine and secondary coolant is even lower

temperature than the primary coolant. Heat exchanger layers separates two coolants while allowing

the passage of heat.

Methods of cooling

Indirect air coolingWinding conductors of electrical machine is not directly contact with the cooling air because of the

presence of electrical insulation on the windings. There are two methods of indirect air cooling. One is

open ventilated way in this method air is not recalculated it is continuously drawn from the

environment. Other method is recirculating air by using heat exchanger. The heat takes by coolant is

cooled by air water heat exchanger or using other coolant. Hydro generators about 50MW use

recirculating air cooling.

Indirect hydrogen coolingFor machines above 50MW air cooling is unstable. Since hydrogen molecules are small they result in

low windage losses and better transfer of air. Hence the efficiency goes up when using hydrogen


9/9

cooling. Other advantages are increase in life span, elimination of fire hazards, smaller size of cooler

and lesser noise.

Direct coolingThis is the conventional cooling method. The types of mediums use for direct cooling are hydrogen,

water and oil. The coolant is circulating inside the machine via conductors closed to windings and cool

the inside of the machine. The advantages of this method are increase in rating, machine losses are

reduced.

Documents

thermal test_Final.pdf