EE4092: Laboratory practice VII
TEMPERATURE RISE OF AN ELECTRICAL MACHINE
Instructed by: Dr. Udayanga Hemapala
Name
: S.B.N.S.Senanayake
Group members:
Index No
: 100494P
100116L Ekanayake E.M.H.A.
Group
: G-11
100255K Kirinde W.M.C.N.S.
Field
: Electrical Engineering
100342B Munasinghe D.T.
Date of performance : 20.08.2014
100627E Muthuransi L.W.N.
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 1 )
-------------(1)
R1 is the winding resistance at ambient temperature of 1
Where,
R0 is the winding resistance at 00C α is the temperature coefficient of resistance for copper Similarly for R2 resistance at 2
R2 R0 (1 2 )
---------------(2)
(1)/(2) and using the graph, R2 (1 2 ) (235 2 ) R1 (1 1 ) (235 1 )
R2 (235 1 ) (235 2 ) R1
2 ( 235 1 )
R2 235 R1
------(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)
When t tends to infinity, θ will approach to a constant value
and therefore,
Define time constant
∫
Multiply the equation by integrating factor
By integrating the equation, ∫ (
)
∫ Where ‘C’ is a constant
By substituting the initial conditions, t = 0,
,where
is the initial temperature
rise of the motor ----------(4) c) Estimation of Thermal Time Constant ()
o
m By differentiation equation (4) with respect to time, at t = 0, d
(
For the machine started at the ambient temperature
Therefore,
(
m d dt
) t 0
dt
) 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),
2 (235 1 )
R2 235 R1
2 (235 30)
7.9 235 7.1
2 = 59.9 0C Temperature rise = 2 - 1 = 59.9-30 = 29.90C
Time (min)
Winding resistance (Ω)
Temperature (°C)
Temperature rise (°C)
0
7.1
30.0
0.0
5
7.9
59.9
29.9
10
8.2
71.1
41.1
15
8.4
78.5
48.5
20
8.6
86.0
56.0
25
8.6
86.0
56.0
30
8.6
86.0
56.0
Finding m and from the graph; θm = 560C
(
d 15 ) t 0 6 0 C / mi dt 2.5
(
m d dt
) t 0
=
= 9.33 min
GRAPHS b) Temperature rise Vs time curve
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
Table 1: Temperature Rise Vs Time
Temperature rise Vs Time 60
55
50
45
40
Temperature rise (°C)
35
30
25
20
15
10
5
0 0
5
10
15
20
Time(minutes)
25
30
35
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.
2 ( 235 1 )
R2 235 R1
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.
