EE4092: Laboratory practice VII

TEMPERATURE RISE OF AN ELECTRICAL MACHINE

Instructed by: Dr. Udayanga Hemapala

Group members:

100116L Ekanayake E.M.H.A.

100255K Kirinde W.M.C.N.S.

100342B Munasinghe D.T.

100627E Muthuransi L.W.N.

Name : S.B.N.S.Senanayake

Index No : 100494P

Group : G-11

Field : Electrical Engineering

Date of performance : 20.08.2014

Date of submission : 03.09.2014

OBSERVATION SHEET

NAME : S.B.N.S.Senanayake

INDEX NO : 100494P

PRACTICAL : Temperature Rise of an Electrical Machine

GROUP NO : G11

DATE OF PER. : 20/08/2014

INSTRUCTED BY : Dr. Udayanga Hemapala

Rated Current of the motor : 2.8A

Time (minutes) Winding resistance ( Ω)

0 7.1

5 7.9

10 8.2

15 8.4

20 8.6

25 8.6

30 8.6

THEORY

a) Temperature rise

The relationship between resistance and temperature can be denoted by,

R1=R0 (1+αθ1) -------------(1)

Where, R1 is the winding resistance at ambient temperature of θ1

R0 is the winding resistance at 00C

α is the temperature coefficient of resistance for copper

Similarly for R2 resistance at θ2

R2=R0 (1+αθ2 )---------------(2)

(1)/(2) and using the graph,

R2

R1=

(1+αθ 2 )(1+αθ1)

=(235+θ2 )(235+θ1 )

R2 (235+θ1)=(235+θ2 )R1

θ2=(235+θ1 )R2

R1−235

------(3)

b)Temperature rise Vs time curve

Consider about small time interval Δt and temperature rise at that time interval is Δθ then

Heat produce = p(Δt)

Heat dissipated = hAθ(Δt) (from Newton’s law of cooling)

Heat Stored from temperature rise = mc(Δθ)

Where, h is the heat transfer coefficient

A is area

m is mass

c is specific heat capacity

From law of energy conservation,

Heat produce = Heat dissipated + Heat Stored from temperature rise

p(Δt) = hAθ(Δt) + mc(Δθ)

take lim (Δt) = 0

p(dt) = hAθ(dt) + mc(dθ)

mc dθdt

+hAθ=p

When t tends to infinity, θ will approach to a constant valueθm and therefore,

dθdt

=0

mc × 0+hA θm=p

hA θm=p

θm= phA

Define time constant τ=mcha

dθdt

+1τ

θ=1τ

θm

Integrating factor=e∫e1τ

dt

=etτ

Multiply the equation by integrating factor

e t / τ dθdt

+e t / τ 1τ

θ=e t / τ 1τ

θm

d (θ etτ )

dt =et / τ 1τ θm

By integrating the equation,

∫ d (θ etτ )=¿∫ e t /τ 1

τθm dt ¿

θ etτ=et / τ θm+C Where ‘C’ is a constant

θ=θm+C e−tτ

By substituting the initial conditions, t = 0, θ=θ0 ,where θ0 is the initial temperature rise

of the motor

θ=θm(1−e−tτ )+θo e

−tτ ----------(4)

c) Estimation of Thermal Time Constant ()

By differentiation

equation (4) with respect to time, at t = 0,

τ=θm−θo

( dθdt

)t=0

For the machine started at the ambient temperature

θo=0

Therefore,

τ=θm

( dθdt

)t=0

CALCULATIONS

Sample calculation:

When Time = 5 min,

Winding Resistance (R2) = 7.9Ω

Ambient temperature (t1) = 300C

Winding Resistance at ambient temperature (R1) =7.1Ω

Using equation (3),θ2=(235+θ1 )

R2

R1−235

θ2=(235+30)×7 .97 .1

−235

2 = 59.9 0 C

Temperature rise = 2 - 1 = 59.9-30 = 29.9 0 C

Time (min) Winding resistance( Ω ) Temperature (°C) Temperature rise

(°C)

0 7.1 30.0 0.05 7.9 59.9 29.910 8.2 71.1 41.115 8.4 78.5 48.520 8.6 86.0 56.025 8.6 86.0 56.0

30 8.6 86.0 56.0

Finding θm and τ from the graph;

θm = 560C

( dθdt

)t=0=152 . 5

=60 C /mi

τ=θm

( dθdt

)t=0=

566

= 9.33 min

GRAPHS

b) Temperature rise Vs time curve

Table 1: Temperature Rise Vs Time

Time (min) Temperature rise (°C)

0 0.0

5 29.9

10 41.1

15 48.5

20 56.0

25 56.0

30 56.0

0 5 10 15 20 25 30 350

5

10

15

20

25

30

35

40

45

50

55

60Temperature rise Vs Time

Time(minutes)

Tem

pera

ture

ris

e (°

C)

DISCUSSION

1. The various methods of measuring the temperature rise of the windings.

The temperature rise is defined as the average temperature rise of the windings above

the ambient temperature.

The temperatures of windings can be determined by various methods.

The change of resistance method

Resistance of the winding increases with the temperature of the winding. Therefore measuring

winding resistance can be used as a method for measuring temperature of the winding.

Winding temperature can be calculated using the following equation.

θ2=(235+θ1 )R2

R1−235

Where,

R1 = Winding resistance at ambient temperature

R2 = Winding temperature after the operation of ‘T’ time

θ1 = Ambient temperature

θ2 = Winding temperature after the operation of ‘T’ time

Using Temperature sensors

In this method temperature sensors are inserted in to the winding core and in between the

windings of the machine. Thermocouples and resistance temperature detectors are most

commonly used sensors and among those two thermocouples are used most frequently.

When using thermocouples, it is important to attach the thermocouple tightly to the

device. Gluing or soldering the thermocouple is the recommended attaching method. In

this method approximate temperature of the winding can be directly measured.

Stator current method

In this method, the stator current is used to predict the winding temperature. A point on

the motor thermal damage curve which is provided by the manufacture represents a

thermal time limit. Thermal time limit gives the idea of how long a motor can withstand

the corresponding level of stator current without exceeding the thermal boundary

specified by the motor manufacturer.

Optical method (Infrared thermography method)

Figure 1: Infrared Thermography method

Thermal imager is a non-contact temperature measurement technique.IR cameras are

often used to get a quick view to locate the hotspots. The temperature at each point can be

identified by the different colors of the thermal image. The accuracy will be poor, due to

the different emissivity of different components. Also this method only provides the

facility to read the temperature on the outer surface of the machines. Therefore this

method can only be used to take the approximate value of temperature is sufficient.

2. The importance of the class of insulation

Insulation material characteristics undergo heavy changes when the temperature varies.

Use of high temperatures than the recommended values generally reduces the lifetime of

the electrical insulating materials. So an upper temperature limit is set for insulation to

operate reliably over long duration.

Electrical insulation materials are categorized into certain basic thermal classes which

show the maximum temperature in degrees of Celsius which should be used for that

insulation material. Excess temperature to the insulation will result in premature failure

due to reduced winding life.

Insulation classes are rated by standard NEMA (National Electrical

Manufacturers Association) classifications according to maximum allowable operating

temperatures as shown in figure 2. Allowable temperature rises are based upon a

reference ambient temperature of 40oC.

Figure 2; NEMA Insulation Classes

A motor should not operate with temperatures above the maximum.

Each 10 oC rise above the rating reduces the motor lifetime by one half as shown in figure

3.

Figure 3: Variation of Lifetime of motor with Temperature

3. Comparison of the obtained results with the machine ratings

The insulation class of the three phase induction motor which was used for the

practical is ‘Class F’. According to the NEMA insulation classes the maximum allowable

operation temperature of the machine is 1550C and allowable temperature rise is 1050C

when ambient temperature is 40oC. In our practical the maximum temperature rise

obtained was 560C. The maximum operating temperature was 860C and we assumed that

the laboratory operating environment is at ambient temperature of 30oC.

Those obtained values are well below the maximum allowable limits specified by

the NEMA insulation classes. Therefore the motor operates safely in the laboratory

environment.

4. The necessary to know about the parameters estimated in this experiment Maximum temperature rise

The maximum temperature rise gives maximum temperature difference with

respect to its ambient temperature when the machine is continuously operating. The

temperature of both windings and the core rises due to copper loss and hysteresis loss.

This temperature rise of the windings is directly affected to winding insulation. If it

exceeds the nominal value, final insulation failure may also happen. Therefore it is

important to know the final machine temperature. Due to the condition of the installed

environment of the motor Ex: enclosures, cooling mechanism, the final temperature rise

can be changed than the deigned values. Also it is important to the maximum temperature

rise, when designing nearby other equipment and the enclosure boxes because they

should be able to withstand this temperature.

Thermal time constant

This is the measure of the rate of change of the temperature of the motor. This is a

measure of both thermal mass and thermal resistance of the machine. It gives information

regarding how motor is affected by heat. Thermal time constant become very important

parameter, when motor running in overload condition for long time.

5. The various methods of cooling general purpose machines

Natural cooling

This method is used in small machines. Heat is transferred to the atmosphere by

natural convection and this process is accelerated by wind.

Forced air cooling

In this method air is circulated internally and externally by one or more bi-

directional type fans mounted on the rotor shaft. In addition, the external frames of the

motor are usually provided with cooling ribs to increase the surface area for heat

radiation.

Direct cooling

In this method, cooling ducts are formed through the stator core of the machine.

In medium size synchronous generators, water is normally used as a cooling medium and

the water is pumped though stainless steel cooling tubes. Air or H2 gas is used as cooling

medium, in large electric machines which are operated at higher power ratings.