ISSN 0976 – 6545(Print)
ISSN 0976 – 6553(Online )
© I A E M E
E
Bhagirath Ahirwal
1 and Prof. Tarun Kumar Chatterjee
2
1
Senior Scientist, Flame & Explosion Lab., CSIR-Central Institute of Mining & Fuel Research, Barwa Road,
Dhanbad, (JH)-826015, India (Corresponding Author)
Phone: +91-326-2296025, +91-326-2296003
Fax: +91-326-2296019
E-mail: ahirwalcmri@yahoo.co.uk
2
Professor, Electrical Engineering Department, Indian School of Mines, Dhanbad, (JH)-826001, India
Phone: +91-326-2235437
Fax: +91-326-2296563
E-mail: tkcism@yahoo.com
Abstract
The increased safety motor is designed to use in the explosive atmospheres like petrochemical, refineries, oil mines, oil pipe lines and other classified hazardous areas. The main objective of the design of high tension (HT) increased safety (Ex e) motor is to control the arc/spark and temperature of any part of Ex e HT motor which cannot become a source of ignition in the explosive atmospheres. The time t
E
is the time taken by alternating current (AC) windings to reach the limiting temperature of winding insulation which is very important for increased safety motor and it depends on thermal withstanding characteristic of insulation system of motor winding. Thermal withstanding characteristic of insulation system depends on the starting time or blocking time in normal or specified abnormal condition during the application of drive. The time t
E
of rotor or stator winding can be affected by the change in some design parameters like mass of rotor winding, specific heat of winding material, heat dissipation factor, rotor copper loss, starting torque etc. The time t
E
is very critical for any HT Ex e motor and it should not be less than 5 seconds as per requirement of IS/IEC 60079-7:2007. In this paper the time t
E
is increased for Ex e HT motor by changing some design parameters. An effort has been made in this paper to study the effect of increased number of stator winding turns keeping the same copper mass winding on the time t
E
and the performance characteristic of motor. The Ex e high tension induction motors mentioned in this paper have been manufactured by Bharat Heavy
Electricals Ltd. (BHEL), Bhopal, India during the project as per the design and guidance of main author as project leader.
Key Words : Increased safety motor, starting torque, time t
E
, design parameters, hazardous area.
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International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print),
ISSN 0976 – 6553(Online) Volume 3, Issue 2, July- September (2012), © IAEME
The protection concept of increased safety is one intended for use in Zone 1 and less hazardous areas. Increased safety concept is developed by Germany and it is indicated by the use of symbol ‘e’ .
Increased safety protection method is being used worldwide for motors to use in zone 1 and 2 hazardous area but in India it is applicable in zone
2 area only as per IS 5571 [1].
During normal operating conditions, the induction machines produces synchronously rotating useful air gap field. However, the current in the stator end winding, produces stray field components. These components together with leakage components due to stator core magnetic saturation induce circulating current to flow in any closed conducting circuits. The current causes arc and spark at the joint of a multi section motor enclosure [2, 3, 4]. The stray end winding field will be the strongest during locked rotor and starting condition.
In increased safety Ex e motor “additional measures” are applied, so as to give increased security against possibility of excessive temperatures and occurrence of arcs or sparks in apparatus which under normal or specified abnormal condition do not produce any arc or spark. The specific measures are applied to electrical apparatus to avoid ignition of a surrounding explosive atmospheres [5]. It is fundamental concept that the external atmosphere which may be hazardous area may enter into the enclosure. Ignition must be prevented by suitable equipment design because the enclosure is not intended to provide any protection against transmission of an internal explosion to the external hazardous atmosphere. Therefore, in all design of Ex e motor in which potentially incendive may be generated, the appropriate protective measures should be implemented. The increased safety provides a type of explosive protection where, even though the ingress of an explosive atmosphere into the enclosure is not prevented, the apparatus does not spark, arc or become excessively hot in normal operation and in addition, is of such quality that it is unlikely to become faulty in a way which would make it a source of ignition for explosive atmospheres.
This security from fault is further increased by enclosure protection from its environment, reducing the risk of environmental conditions adversely affecting its operation. Therefore part of the protection depends upon the length of time for which such a situation can exist prior to the protective devices operating.
The essential principle of electrical apparatus with increased safety enclosure and design parameters is: i) the internal and the external surface temperature of the components should not exceed the ignition temperature of the surrounding gases under any circumstance. This is so as the surrounding explosive atmospheres are in contact with internal surfaces also. ii) time ‘t
E
’: is the time taken for an AC rotor or stator windings, when carrying the initial starting current I
A
(highest root mean square value of current absorbed by an A.C. motor when at rest at rated voltage and rated frequency), to be heated upto the limiting temperature from the temperature rest in rated service at the maximum ambient temperature at rated operating conditions during starting. iii) I
A/
I
N ratio: is between initial starting current I
A and rated current I
N of motor. iv) Starting torque: Direct on line starting of the squirrel cage induction motor will cause high starting current which will increase the copper loss and temperature rise of the motor during starting. The starting current and copper loss during starting can be reduced by reducing starting torque with reduced voltage method of starting which also reduces starting torque, starting current, loss of energy during starting and time t
E
is increased.
The increased safety HT motor is widely used in the hazardous area because they are cheaper than flameproof
(Ex d) and pressurized (Ex p) motors. The main purpose to control the ignition temperature in the explosive atmospheres due to spark/ arc or hot surface generated from any part of increased safety motor. The surface temperature of Ex e HT motor should always be less than the ignition temperature of explosive atmosphere where motor is installed. The temperature rise of a motor can vary due to losses i.e core loss, copper loss during normal operation and specified overload condition of the motor. The high starting current, stalling current, loading conditions, frequency of starting can cause for occurrence of spark/arc inside the motor. The occurrence of spark/arc inside the motor due to failure of winding insulation which increase motor temperature. The arc/spark can be controlled by proper designing of the motor as per the loading condition and duty cycle. The temperature rise depends on the inter-relationship between heating and cooling. Under steady-state conditions, the final temperature rise is reached when the rates of production and dissipation of heat are equal. Increased safety electrical machines are designed for a limited temperature rise. In fact, the continuous rating of a machine is that rating for which the final temperature rise equal to or just below the permissible value of temperature rise for the insulating material used for protection of the conductors. When the machine is overloaded for a long time, its final temperature rise is higher than the allowable limit, and the motor is likely to be damaged causing for explosion in the explosive area. In worst cases, this will result in an immediate thermal breakdown of the insulating material which will cause a short circuit in the motor, thus putting a stop to its functioning.
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International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print),
ISSN 0976 – 6553(Online) Volume 3, Issue 2, July- September (2012), © IAEME
Prior to the design of the Ex e induction motor, a review of the potential sources of ignition in electrical machines was carried out. It was cleared from the review, that ignition tends to occur during starting condition [2, 3,
4 ,5]. It was observed that high circulating current induced in the machine frame components during starting [2].
Clark et al [4] concluded that the sparking and arcing phenomenon occurs in the air gap at the end pockets of the machines. Bianco et al [7] investigate the possible incendive effect due to partial discharges in the insulation system.
Dymond [6, 8] indicated that contamination will lead to surface discharge and tracking on the windings, which is another potential sources of sparking and arcing. Hamer et al [9] elaborated on the relationship between hot rotor surfaces and flammable vapor ignition. The rotor of motor should not be blocked for longer time. This blocking time should be less than specified time t
E
.
The paper describes the design aspect and performance characteristics of normal Ex e HT induction motors rated 810KW/6.6KV/16 pole/ 3Phase/ 50Hz, 970KW/6.6KV/18 pole/ 3Phase/ 50Hz and 2200KW/6.6KV/2 pole/
3Phase/ 50Hz (designated as normal motor in this paper). The performance of same Ex e HT induction motors have been studied after designing the motors with increasing numbers of turns of stator winding in the same frame size by keeping the surface area and copper mass of winding of the motor constant. The motors are designed for insulation class F but temperature rise is calculated at insulation class B-10°C as per end user requiremnet.
The starting torque can be reduced upto the certain limit governed by load torque requirement. Generally, the motor is designed for hazardous area must have some optimization limit as shown in Figure 1. The above mentioned normal motors have about 15-20% optimization limit of torque which is optimized by reducing the starting torque to know other impact on the motor’s critical safety parameters (like t
E
and I
A
/I
N
ratio) and performance of the motors.
It is assumed from Figure 1 that starting torque of motor at 80% rated voltage (RV) produces between 15-80% of full load torque (FLT). This starting torque can be optimized within the limit of requirement by increasing the number of stator winding turns to know the effect on t
E
and I
A
/I
N
ratio of motor.
Motor at 100% rated voltage
(RV)
80%
Motor at 80% rated voltage (RV)
Optimization value Driven equipment
15%
0
100%
Figure 1. General s peed – torque characteristics of induction motor for pump.
A normal increased safety HT motor is designed keeping in view that the time t
E should not be less than the specified time limit. The time ‘t
E
’ should not be less than 5 seconds and I
A/
I
N ratio should not be more than 10 as per
IS/IEC 60079-7 [10]. The time t
E
is calculated for both stator and rotor winding of motor after designing the increased safety motor. In most of the cases, the time t
E
of rotor is always less than time t
E
of stator. In few cases i.e. in double cage increased safety induction motor, the time t
E
of rotor may be greater than time t
E of stator. The time t
E of rotor winding of increased safety motor is dependent on the different parameters like mass of rotor winding, specific heat of winding material, heat dissipation factor and rotor copper loss or starting torque of motor. The time
225

International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print),
ISSN 0976 – 6553(Online) Volume 3, Issue 2, July- September (2012), © IAEME t
E
of increased safety motor can be increased either by reducing starting torque and or increasing winding mass of rotor. The objective of this paper is to find out the effect of reduced starting torque on the time t
E
and other performance parameters of motor by keeping the frame size and other ratings of the motor same.
As we know that ∆Ө / t
E
= b × I
2
R / m × s [11] and I
2
R = Starting torque × KW rating of motor, so heat balance equation can be written as
∆Ө / t
E
= b × Starting torque × KW rating of motor / m × s where, m = mass of cage winding, s = specific heat of copper, b = ventilation factor, I
2
(1)
R=copper loss in rotor winding, ∆Ө = temperature difference and t
E
= time.
If s, ∆Ө , m, b and KW rating of motor are constant for particular motor design in the above so it can be written as t
E
α 1 / Starting torque i.e. time t
E
is inversely proportional to starting torque of the motor. Hence, time t
E of motor may be increased by decreasing the starting torque of motor upto optimization limit.
The Ex e squirrel cage induction motors of rating 810KW/6.6KV/16 pole/ 3Phase/ 50Hz, 970KW/6.6KV/18 pole/
3Phase/ 50Hz and 2200KW/6.6KV/2 pole/ 3Phase/ 50Hz are designed. The same Ex e motors are further designed with reduced starting torque of motor keeping frame size, KW rating and surface area of the motor same. The designed parameters of normal motors and with reduced starting torque of same normal motors are shown in the
Table 1.
Motor rating
Parameters
810KW-
Normal motor
Rated output in KW
Frequency in Hz
Rated Voltage in Volt
Connection Type STAR/ DELTA
810
50
6600
Star
Stator Outer Diameter in mm
Inner Diameter in mm
1120
870
Total Core length with Ducts in mm 1036
Total Core length without Ducts in 900 mm
No. of slots
Total Slot Height in mm
Slot Width in mm
Slot Wedge Height in mm
Slot tip height in mm 4.5
Elementary Conductor width in mm 5.5
Elementary Conductor height in mm 2.2
Main insulation thickness in mm
Height of middle spacer/separator in mm
Coil pitch
No. of effective conductors
No. of Parallel paths
Load Torque p.u. at slip s = 1
Load Torque p.u. at slip s = 0.8
144
63.9
9.3
2.4
1.3
3
8
1296
2
0.3
0.27
Load Torque p.u. at slip s = 0.6
Load Torque p.u at slip s = 0.4
Load Torque p.u at slip s = 0.2
Load Torque p.u at slip s = 0
No. of poles
Air gap width in mm
Rotor Inner Diameter in mm
Diameter of axial holes in rotor in mm
No of slots in rotor
Rotor Total Slot Height in mm
Rotor Slot Width (outer) in mm
Rotor Slot Width (inner) in mm
Rotor Slot tip height in mm
Rotor Slot opening in mm
Conductivity of Rotor Bar at 75°C in mho
Conductivity of SC ring at 75°C in
0.25
0.25
0.25
0.25
16
1.80
600
0
116
42
9.5
9.5
1
3
46
46
8
1440
2
0.3
0.27
0.25
0.25
0.25
0.25
16
1.80
600
0
116
42
9.5
9.5
1
3
46
144
63.9
9.3
2.4
4.5
5.5
1.9
1.3
3
810KW-
Motor with reduced starting torque
810
50
6600
Star
1120
870
1036
900
46
970KW-
Normal motor
8
1296
2
0.12
0.04
0.06
0.1
0.12
0.15
18
2.5
700
0
200
40
6.5
6.5
1
2.5
46
162
80.9
9.6
2.4
4.5
5.9
3.5
1.3
3
970
50
6600
Star
1400
1085
920
800
46
8
1458
2
0.12
0.04
0.06
0.10
0.12
0.15
18
2.5
700
0
200
40
6.5
6.5
1
2.5
46
162
80.9
9.6
2.4
4.5
5.9
3.1
1.3
3
970KW-
Motor with reduced starting torque
970
50
6600
Star
1400
1085
920
800
46
2200KW-
Normal motor
20
540
2
0.05
0.02
0.06
0.16
0.31
0.58
2
5
330
0
64
60
6.5
13
4.5
4.5
15
54
111.9
18
2.4
4.5
6.9
4.3
1.3
3
2200
50
6600
Star
1120
600
862
750
46
18
560
2
0.05
0.02
0.06
0.16
0.31
0.58
2
5
330
0
64
60
6.5
13
4.5
4.5
15
54
111.9
18
2.4
4.5
6.9
4.3
1.3
3
220KW-
Motor with reduced starting torque
2200
50
6600
Star
1120
600
862
750
46
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International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print),
ISSN 0976 – 6553(Online) Volume 3, Issue 2, July- September (2012), © IAEME mho
SC ring cross sectional area in mm
2
800
Overload Factor
Saturation factor
0.85
1.15
30 Time for thermal withstand characteristics in sec
Stator Winding temperature in °C
Rotor winding Temperature in °C
Moment of inertia of load in kg-m
2
120
120
0.59
17*8 No of cooling duct * width of each duct in mm
Effective iron length in mm
Ideal iron length in mm
No. of slots of rotor
Slot pitch of rotor
Tooth width in mm
Weight of yoke iron in kg
868.5
949
144
18.98
9.58
1359
800
0.85
1.09
30
120
120
0.59
17*8
868.5
949
144
18.98
9.58
1359
Weight of tooth iron in kg
Weight of active iron in kg
Core loss in watt/Kg of punching material
Slot/pole/phase of stator and rotor
681
2041
5.3
681
2041
5.3
Conductor/slot of stator
No. of elementary conductor in stator slot width
No. of elementary conductor in stator slot height
No. of parallel circuits in stator winding
Cross section of elementary conductor in mm
2
Effective total cross section of elementary conductor in mm
2
Chording coil pitch
Winding factor of stator
3
18
1
1
2
11.8
11.8
3
20
1
1
2
10.19
10.19
Auxiliary value of current density for stator in A/mm
2
Conductivity of stator winding/rotor bar/SC ring at 75 °C in mho
Stator Elementary Conductor width and height in mm
Stator Insulated element conductor
8/9
0.945
1552
46
5.5 &
2.2
5.9 &
8/9
0.945
1552 Half of Mean length of turn in Stator and mean length of Bar (MLB) for rotor
Total length of elementary
Conductor in mm
Single sided Axial winding overhang in mm
4024
230
4469
230
Length ratio (Ratio of average length of winding overhang to Height of insulated)
Magnetic stress during short circuit
Per phase stator winding resistance at 75 °C in m Ω
Stator phase voltage in volt
9.9
0.082 0.071
Wt. of copper used in rotor bar in kg 422.5
Total resistance of stator and Rotor 1522.55
405.2
1958.04 winding at 20 °C in m Ω
Total resistance of stator and Rotor winding at 75°C in m Ω
1853.54 2383.71
617.85 794.57
3811
94.8
10.1
3811
92.7 Rotor phase current at reference temperature in Amp
Current loading in stator in Amp
Current density in stator winding/Rotor bar/SC ring in A/mm
2
45.7
4.08
49.5
4.61
186 228
46
5.5 & 1.9
5.9 &2.3
800
0.85
1.09
30
120
120
0.71
15*8
772
852.7
162
21.04
11.31
1893
1014
2907
5.3
3
16
1
1
2
20.13
20.13
8/9
0.945
1459
3781
235
8
0.089
677.6
838.42
1020.68 1296.85
340.23
3811
117.9
45.6
2.98
136
46
5.9 &
3.5
6.3 &
800
0.85
1.06
30
120
120
0.71
15*8
772
852.7
162
21.04
11.31
1893
1014
2907
5.3
3
18
1
1
2
17.83
17.83
8/9
0.945
1459
4255
236
7.9
0.079
675.4
1065.27
432.28
3811
113
49
3.21
158
46
5.9 & 3.1
6.3 & 3.5
5000
1.25
1.01
30
120
120
0.004
14*8
723.8
818
54
34.91
17.31
2487
778
3266
3.5
9
20
2
1
2
28.93
28.93
20/27
0.877
2180
4708
446
13.3
0.415
1212.2
181.65
221.14
73.71
3811
216.4
62.5
1.89
118
46
5000
1.25
1.01
30
120
120
0.004
14*8
723.8
818.3
54
34.91
17.31
2487
778
3266
3.5
9
22
2
1
2
26.24
52.48
18/27
0.827
2082
4946
420
12.2
0.378
1155
210.41
256.15
85.38
3811
220
69.9
2.11
148
46
6.9 & 4.3 6.9 & 3.9
7.3 & 4.7 7.3 & 4.3
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International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print),
ISSN 0976 – 6553(Online) Volume 3, Issue 2, July- September (2012), © IAEME width & height
Main insulation thickness in mm
Width & height of insulated coil side of stator in mm
Slot dimension allowance in width and height in mm
2.6
2.6
8.6
0.6
2.6
8.6
0.6
Stator Slot width outer
Wedge height of stator
Stator Slot tip width and height in mm
9.3
--
9.7 &
4.5
63.9
9.3
--
9.7 & 4.5
Total slot height of stator and rotor in mm
Airgap between stator and rotor in mm
Mass of rotor in kg
Stator tooth size in mm
Rotor tooth size in mm
1.8
460.7
Bottom=
12.5
Middle=
11.1
Top
=11.3
Bottom=
13.7
Middle=
12.8
Top
=11.9
0.723
63.9
1.8
460.7
Bottom=12.5
Middle=11.1
Top =9.6
Bottom=13.7
Middle=12.8
Top =11.9
Maximum air gap flux density in
Tesla
Magnetic field strength at stator tooth middle in At/m
Magnetic field strength at rotor tooth middle in AT/m
Height of stator yoke in mm
1.997
1.893
0.665
1.055
1.015
61.1 61.1
Height of rotor yoke in mm
Width of copper bar of rotor in mm
90.2
9
90.2
9
Height of copper bar of rotor in mm 41.4
Winding cross sectional area
41.4
24.2mm
2
20.9mm
2
3.9
2.6
9
0.6
9.6
---
9.6
---
10 & 4.5 10 & 4.5
80.9
2.5
369.7
Bottom=
14.6
Middle=
13
Top
=11.3
Bottom=
10.3
Middle=
9.8
Top =9.3
0.721
1.482
1.221
2.6
9
0.6
80.9
2.5
469.7
Bottom=14.6
Middle=13
Top =11.3
Bottom=10.3
Middle=9.8
Top =9.3
0.647
0.816
0.767
76.6
149
6
76.6
149
6
39.4 39.4
41.3mm
2
36.58mm
2
2.6
17.3
0.6
0.1
18
18.4 &
4.5
111.9
5
447.4
Bottom=2
9.9
Middle=2
3.4
Top =17.3
Bottom=2
1.7
Middle=1
6
Top =10.3
0.431
0.168
0.315
2.6
17.3
0.6
0.1
18
18.4 & 4.5
111.9
5
447.4
Bottom=29.9
Middle=23.4
Top =17.3
Bottom=21.7
Middle=16
Top =10.3
0.431
0.162
0.268
148.1
65.5
6
148.1
65.5
6
59.4 59.4
29.67mm
2
26.91mm
2
Table 1. Designed parameters of Normal Ex e induction motors and Ex e induction motors of same rating with reduced starting torque.
Table 2 shows the variation of design parameters of Ex e motor with reduced starting torque from normal Ex e motor of same rating.
Motor rating
Changed design
Parameters
810KW-
Normal motor
810KW-
Motor with reduced starting torque
970KW-
Normal motor
970KW-
Motor with reduced starting torque
2200KW-
Normal motor
220KW-
Motor with reduced starting torque
50
9.3/9.5
80.9
9.6/6.5
80.9
9.6/6.5
111.9
18/105
111.9
18/105
Rotor Total Slot Height in mm
Stator/Rotor Slot Width (outer) in mm
Stator/Rotor Slot Width (inner) in mm
Saturation factor
Magnetic stress during short circuit
42
9.3/9.5
9.7/4.5
1.15
0.082
Stator/Rotor phase current at reference temperature in Amp
94.8
Current loading in stator in Amp 45.7
4.08 Current density in stator winding/Rotor bar/SC ring in
A/mm2
Auxiliary value of current density for stator in A/mm2
186
9.3/4.5
1.09
0.093
92.7
49.5
4.61
228
10/4.5
1.09
0.089
117.9
45.6
2.98
136
10.1/4.5
1.06
0.094
113
49
3.21
158
18.4/4.5
1.01
0.415
216.4
62.5
1.89
118
18.4/4.5
1.01
0.434
220
69.9
2.11
148
228

International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print),
ISSN 0976 – 6553(Online) Volume 3, Issue 2, July- September (2012), © IAEME
Stator tooth size in mm
Rotor tooth size in mm
Bottom=1
2.5
Middle=1
1.1
Top =11.3
Bottom=1
3.7
Middle=1
2.8
Top =11.9
0.723
Bottom=12.
5
Middle=11.
1
Top =9.6
Bottom=13.
7
Middle=12.
8
Top =11.9
0.665
Bottom=14.
6
Middle=13
Top =11.3
Bottom=10.
3
Middle=9.8
Top =9.3
0.721
Bottom=14.
6
Middle=13
Top =11.3
Bottom=10.
3
Middle=9.8
Top =9.3
0.647
Bottom=29.9
Middle=23.4
Top =17.3
Bottom=21.7
Middle=16
Top =10.3
0.431
Bottom=29.9
Middle=23.4
Top =17.3
Bottom=21.7
Middle=16
Top =10.3
0.431 Maximum air gap flux density in
Tesla
Magnetic field strength at stator tooth middle in At/m
Magnetic field strength at rotor tooth middle in AT/m
Height of stator yoke in mm
Height of rotor yoke in mm
Width of copper bar of rotor in mm
Height of copper bar of rotor in mm
1.997
1.893
61.1
90.2
9
41.4
1.055
1.015
61.1
90.2
9
41.4
1.482
1.221
76.6
149
6
39.4
0.816
0.767
76.6
149
6
39.4
0.168
0.315
148.1
65.5
6
59.4
0.162
0.268
148.1
65.5
6
59.4
Table 2. Change in designed parameters of normal Ex e motor and Ex e motor with reduced starting torque.
E
The 970kw,6.6kv,16pole rating motor is chosen here for providing the details of calculation with respect to increased safety concept. On the basis of designed parameters as given in Table 1 and Table 2 the calculation of temperature rise, time tE of rotor, time tE of stator winding, I
A
/I
N
ratio etc. is calculated after doing relevant test on all the three normal motors. The detail of calculation methodology is given below:
The cold resistance of stator winding and motor temperature was measured and recorded. Then motor was run for several hours at full load until thermal stabilization was attained. The temperature of stator body at every half an hour interval was taken. When the temperatures reading of stator body between two consecutive readings were same then it was confirmed that the motor was thermally stabilized. The motor was then switched off and hot resistance of stator winding was measured within the specified time and accordingly temperature rise of stator winding is calculated as per Eq. (2) [11].
Temperatur e riseT
R
( R hot
R cold
) / R cold
( 235
t
1
)
( t
1
t a
) , (2)
Where, t
R cold
= Cold resistance of stator winding of motor in Ω = 0.278 Ω ,
R hot
= Hot resistance of stator winding of motor in Ω = 0.323 Ω ,
1
= Machine Temperature at the initial cold resistance measurement in ºC = 29 ºC , t a
= Ambient temperature at the end of the examination corresponding to hot resistance in ºC , = 27.6
ºC ,
Temperature rise (T
R
) = (0.323 – 0.278) / 0.278 × (235 + 29) + (29 − 27.6) = 44.13 ºC.
E
Rate of temperature rise: ∆Ө / t
E
= a × j
2
× b ,
Where,
(3) [11]
∆Ө = Temperature difference between limiting temperature and Total temperature determined by resistance method in ºC ,
Total temperature (T) = temperature rise determined by resistance method (T a = 0.0065 for copper ºC / (A/mm
2
)
2 sec. ,
R
) + Ambient temperature (T
A
) b = 0.85 reduction factor for heat dissipation , t
E
= time in second , j = Current density at starting in A/mm
2
,
Hence, t
E for stator is calculated as follows by using Eq. (3):
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International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print),
ISSN 0976 – 6553(Online) Volume 3, Issue 2, July- September (2012), © IAEME
Starting Current of motor = 545.7 A ,
Winding copper cross sectional area = 41.3 mm
2
,
Insulation class of winding = F,
Limiting temperature for stator with respect to class F insulation = 170ºC (at ambient temp. 40ºC limited to insulation class B-10 ºC) [10] ,
Total temperature (T) = T
R
+ T
A
= 44.13 ºC + 40ºC = 84.13 ºC ,
Current density at starting = 545.7 / 41.3 = 13.21 A/ mm
2
,
∆Ө = Limiting temperature – T ,
= 170 – 84.13 = 85.87ºC ,
Rate of temperature rise = ∆Ө / t
E
= a × j
2
× b = 0.0065 × (13.21)
2
× 0.85 = 0.96 ºC / sec ,
Time ‘ t
E
’ = 85.87 / 0.96 = 89.44 sec.
Hence the time ‘ t
E
’ for stator is 89.44 seconds.
E
The cage rotor temperature rise of Ex e HT induction motor can be calculated with the help of Joules effect heat balance equation as given below: m × s × ∆Ө = b × I
2
R × t
E
, (4) where, m = mass of cage winding = 469.7 Kg , s = specific heat of copper = 0.396 , b = ventilation factor = 0.85 ,
I
2
R = copper loss in rotor winding = Starting torque × KW of motor ,
= 1.07 × 970KW (starting torque is 107.6% of rated torque) ,
∆Ө = Maximum allowable temperature (T3 class) – Maximum rated operating temperature (for insulation Class B) ,
∆Ө = (200 – 120) ºC = 80 ºC ,
By putting the above values in the Eq. (4) , t
E
= { (469.7 × 0.396 × 80) / (0.85 × 1.07 × 970) } ,
= 16.77 sec.
The time t
E for rotor is 16.77 sec.
As the time t
E for rotor is 16.77 seconds which is less than the stator time t
E i.e. 89.44 seconds, hence the declared time t
E
for motor will be 16.77 seconds and protective devices should operate according to the declared time value to protect Ex e motor in abnormal condition .
E
Temperature rise = New effective loss / Existing effective loss × Existing surface area / New surface area ×T
R
(5)
(6) Effective loss of motor = 0.45 core loss + stator copper loss + 0.3 rotor copper loss ,
So effective losses of 970KW / 6.6KV / 18 pole normal motor based on Eq. (6) is
= 0.45 × 10.86 + 14.66 + 0.3 × 8.0 = 21.94 KW ,
Similarly the effective losses of motor when starting torque is reduced is
= 0.45 × 8.56 + 17.04 + 0.3 × 10.47 = 24.03 KW ,
As the surface area of the motor is not changed and it remains same in both the design of Ex e motors and the estimated temperature rise of the motor as per Eq. (5) with reduced starting torque is;
Temperature rise = New effective loss / Existing effective loss × T
R
,
= 24.03 / 21.94 × 44.13 = 48.33 ºC.
E
Rate of Temperature rise as per Eq. (3) : ∆Ө / t
E
= a × j
2
× b ,
Where,
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International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print),
ISSN 0976 – 6553(Online) Volume 3, Issue 2, July- September (2012), © IAEME
∆Ө = Temperature difference between limiting temperature and total temperature determined by resistance method in ºC , a = 0.0065 for copper ºC / (A/mm
2
)
2
.sec. , b = 0.85 reduction factor for heat dissipation , t
E
= time in second , j = Current density at starting in A/mm
2
,
Hence t
E is calculated as follows by using Eq. no. (3):
Starting Current of motor = 448.9 A ,
Winding copper cross section area = 36.58 mm
2
,
Insulation class of winding = F ,
Limiting temperature for stator with respect to class F insulation = 170ºC (at ambient temp. 40ºC limited to insulation class B-10 ºC) ,
Total temperature = Estimated temperature rise + Ambient temperature ,
= 48.3 ºC + 40ºC = 88.30 ºC ,
Current density at starting , j = 448.9 / 36.58 = 12.27 A/ mm
2
,
∆Ө = Limiting temperature – Total Temp determined by resistance method = 170 – 88.30 = 81.70 ºC ,
Rate of temperature rise = ∆Ө / t
E
= 0.0065 × (12.27)
2
× 0.85 = 0.83 ºC / sec. ,
Time ‘ t
E
’ = 81.70 / 0.83 = 98.43 seconds .
Hence, calculated time ‘ t
E
’ for stator = 98.43 seconds.
E
The cage rotor temperature rise of Ex e HT motor can be calculated with help of Joules effect heat balance equation as per Eq. (4) as given below: m × s × ∆Ө = b × I
2
R × t
E
, where, m = mass of cage winding = 469.70 Kg , s = specific heat of copper = 0.396 , b = ventilation factor = 0.85 ,
I
2
R = copper loss in rotor winding = starting torque x KW of motor ,
= 0.842 × 970KW (starting torque is 84.2 % of rated torque) ,
∆Ө = Maximum allowable temperature (T3 class) – Maximum rated operating temperature (for Class B) ,
∆Ө = (200 – 120) ºC = 80 ºC ,
By putting the above values in the Eq. (4) , t
E
= { ( 469.7 × 0.396 × 80) / (0.85 × 0.842 × 970) } = 21.43 seconds .
The time t
E
for rotor winding of Ex e motor with reduced starting torque is 21.43 seconds. As the time t
E for rotor is
21.43 seconds which is less than the t
E of stator i.e. 98.43 seconds, hence the declared time t
E
for motor will be
21.43 seconds. The time t
E
for motor with reduced starting torque is 21.43 seconds which is higher than the normal
Ex e HT motor.
Similarly the temperature rise, Time t
E for stator, Time t
E for rotor, I
A
/I
N ratio and other parameters of normal
Ex e induction motors of rating 810KW and 2200KW and same Ex e induction motor with reduced starting torque of same rating are also determined. All the designed and calculated parameters of Ex e induction motors are shown in the Table 3.
Motor rating
Performance
Parameters
Temperature rise at full load
810KW-
Normal motor
47.88ºC
810KW-
Motor with reduced starting torque
55.59ºC
970KW-
Normal motor
44.13ºC
Time t
E for stator
Time t
E for rotor
Time t
E for tripping device of Ex e motor
Current density
28.21 sec
18.67 sec
≤ 18 sec
29.76 sec
24.71 sec
≤ 24 sec
89.44 sec
16.77 sec
≤ 16 sec
22.98 A/mm
2
21.31A/mm
2
13.21A/mm
2
Efficiency at full load 95.38%
Pull out torque in p.u. 2.180
94.92%
1.725
95.86%
2.388
970KW-
Motor with reduced starting torque
48.33ºC
98.43 sec
21.43 sec
≤ 21 sec
2200KW-
Normal motor
64.99ºC
61.78 sec
12.78 sec
≤ 12 sec
12.27 A/mm
2
13.52A/mm
2
95.13%
1.888
96.03%
1.834
2200KW-
Motor with reduced starting torque
73.47ºC
65.01 sec
15.40 sec
≤ 15 sec
12.87 A/mm
2
95.85%
1.617
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International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print),
ISSN 0976 – 6553(Online) Volume 3, Issue 2, July- September (2012), © IAEME
Speed at Full load
Slip at Full load
Starting current (I
A
)
Rated current (I
A
)
556.2 A
140A
Ratio Starting current/rated current
3.97
(I
A
/I
N
)
Starting torque in p.u. 1.135
Core losses at no load
372.1 rpm
0.01015
Iron loss of rotor tooth =
Iron loss of stator tooth +
Iron loss of stator yoke
=4.58KW+3.
43KW=8.01
KW
Total Iron loss
Pulsation
= loss in rotor
+ Iron loss of rotor tooth
=8.01KW+0.
74KW
=8.75KW
Stator copper losses at full load
Rotor copper losses at full load
Total Iron and Cu losses
No. of turns
17.19KW
6.62KW
32.56KW
1296
375 rpm
0.00783
445.4 A
140A
3.18
0.855
Iron loss of rotor tooth =
Iron loss of stator tooth +
Iron loss of stator yoke
=3.70KW+2.
78KW=6.48
KW
Total Iron loss
Pulsation
= loss in rotor
+ Iron loss of rotor tooth
=6.48KW+0.
60KW
=7.08KW
21.07KW
8.57KW
36.73KW
330.7 rpm
0.00536
545.7 A
160A
3.41
1.076
Iron loss of rotor tooth =
Iron loss of stator tooth +
Iron loss of stator yoke
=6.16KW+3.9
3KW=10.09
KW
Total Iron loss
= Pulsation loss in rotor +
Iron loss of rotor tooth
=10.09KW+0.
77KW
=10.86KW
14.66KW
8.0KW
33.52KW
333.3 rpm
0.01035
448.9 A
160A
2.80
0.842
Iron loss of rotor tooth =
Iron loss of stator tooth +
Iron loss of stator yoke
=4.85KW+3.
11KW=7.96
KW
Total Iron loss
Pulsation
= loss in rotor
+ Iron loss of rotor tooth
=7.96KW+0.
66KW
=8.56KW
17.04 KW
10.47 KW
36.07KW
2969.9 rpm
0.01003
401.3 A
180A
2.22
0.593
Iron loss of rotor tooth =
Iron loss of stator tooth +
Iron loss of stator yoke
=0.86KW+5.0
8KW=5.93
KW
Total Iron loss
= Pulsation loss in rotor +
Iron loss of rotor tooth
=5.93KW+0.9
0KW
=6.83KW
10.53 KW
23.06 KW
40.42KW
3000rpm
0.01119
346.5A
180A
1.92
0.492
Iron loss of rotor tooth =
Iron loss of stator tooth +
Iron loss of stator yoke
=0.80KW+4.
72KW=5.51
KW
Total Iron loss
Pulsation
= loss in rotor
+ Iron loss of rotor tooth
=5.51KW+0.
84KW
=6.35KW
12.61 KW
25.74 KW
44.7KW
1440 1296 1458 540 560
Table 3. Performance parameters of Ex e normal induction motor and Ex e induction motor with reduced starting torque.
The performance analysis of each Ex e induction motor with increased numbers of stator turns in resulting reduced starting torque is done by comparing it with the normal Ex e induction motor of the same rating. It has been observed that current density, starting current, core losses and I
A
/I
N
ratio decreases but time t
E
of rotor, time t
E
of stator and temperature rise increases. The efficiency of motors at full load with reduced starting torque is less than
0.77% of efficiency of all normal Ex e motors. The percentage values of variation of different important parameters are shown in the Table 4.
Performance analysis parameters with increased rotor mass
Increase in no. of turns
Decrease in starting torque
Increase of temperature rise
Increase of time t
E of rotor
Increase of time t
E of stator
Decrease in current density
Decrease in efficiency
Decrease in starting current
Increase in speed
Decrease in core losses
Increase in losses (Core losses at no load+
Stator copper losses at full load+ Rotor copper losses at full load)
Decrease in the I
A
/I
N
ratio
810KW-Motor with reduced starting torque
11.11%
24.67%
16.10%
32.35%
5.50%
7.26%
0.48%
19.92%
0.77%
19.08%
12.80%
19.90%
970KW-Motor with reduced torque starting
12.50%
21.74%
9.51%
27.78%
10.00%
7.11%
0.76%
17.73%
0.78%
21.17%
7.60%
17.89%
Table 4.
Performance analysis of Ex e induction motor with reduced starting torque.
2200KW-Motor with reduced starting torque
3.70%
17.03%
13.04%
20.50%
5.22%
4.88%
0.18%
13.65%
1.01%
7.02%
10.58%
13.51%
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International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print),
ISSN 0976 – 6553(Online) Volume 3, Issue 2, July- September (2012), © IAEME
The table 5 shows the effect of increase in stator numbers of turns by which starting torque, I
A
/I
N
ratio and iron losses are decreased in the same manner time t
E of rotor and stator of Ex e induction motor is increased. The
I
A
/I
N
ratio and time t
E
are very important parameters for any increased safety motors which is directly related with the safety of Ex e motor in the hazardous area. The graphical presentation of all above mentioned Ex e motors for time t
E
and I
A
/I
N
ratio are shown in the Figure 2 and Figure 3. The values of these parameters are more than the required standard value. It is also seen from the data of the Table 6 that efficiency and power factor is continuously increasing as load increased for all three Ex e motors with reduced starting torque with respect to normal Ex e motors.
The stator air gap was kept at slightly higher than the upper limit of ordinary design value for Ex e motor and the air gap is maintained same in both the cases. The designed stator air gap value is more than the specified value in the standard IS/IEC 60079-7. The stator air gap value of all Ex e motors under discussion is given in the Table 7.
Motor rating with reduced starting torque
2200KW/6.6KV/2 pole/
3Phase/ 50Hz
810KW/6.6KV/16 pole/
3Phase/ 50Hz
970KW/6.6KV/18 pole/
3Phase/ 50Hz
No. of turns increased
3.7%
11.11%
12.50%
Starting torque reduced
17.03%
21.74%
24.67%
Iron loss decreased
7.02%
19.08%
21.17%
Starting current decreased
13.65%
19.92%
17.73% t
E rotor of increased
20.50%
32.25%
27.78% t
E stator of increased
5.22%
5.50%
10.00%
I
A
/I
N
ratio decreased
13.51%
19.90%
17.89
Table 5. Effect of reduced starting torque on critical parameters of Ex e motor.
Load
%
25
50
75
100
810KW-Normal motor
810KW-Motor with reduced
970KW-Normal motor
970KW- Motor with reduced
2200KW-
Normal motor
2200KW- Motor with reduced starting torque starting torque starting torque
Efficiency p.f. Efficiency p.f. Efficiency p.f. Efficiency p.f. Efficiency p.f. Efficiency p.f.
92.07 38.1 93.19 46.3 91.9 33.3 93.21 41.1 91.83 87 91.88 88.3
94.98
95.51
95.38
60.9
72.1
77.1
95.32
95.43
94.92
68.5
76.9
79.4
95
95.8
95.9
55.3
67.6
73.9
95.56
95.89
95.62
63.6
73.6
77.4
95.09 93 95.06
95.91 93.2 95.82
96.03 91.9 95.85
93
92.7
90.5
Table 6. Load, efficiency and power factor parameters of normal Ex e motor and Ex e motor with reduced starting torque.
Important factor for
Ex e motor
IS/IEC
60079-7:2007 requirement
810KW-
Normal motor
810KW-
Motor with reduced starting torque
970KW-
Normal motor
970KW-
Motor with reduced starting torque
2200KW-
Normal motor
2200KW-
Motor with reduced starting torque
I
A
/I
N
Motor t
E
Air gap
4
14
1.7 mm
3.97 3.18
18 sec 24 sec
1.80mm 1.80mm
3.41
16 sec
2.5mm
2.80
21 sec
2.5mm
2.22
12 sec
5mm
1.92
15 sec
5mm
Table 7. I
A
/I
N
, air gap and Motor t
E
of normal Ex e motor and Ex e motor with reduced starting torque with respect to standards.
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International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print),
ISSN 0976 – 6553(Online) Volume 3, Issue 2, July- September (2012), © IAEME
Figure 2. I
A
/I
N
value with respect to Motor t
E
value as per standards.
30
25
20
24
21
18
14
16
15 15
10
5
4 3.97 3.18 3.41
2.8
12
2.22 1.92
IA/IN
Motor tE
0
Figure 3. I
A
/I
N
value with respect to Motor t
E
value of all Ex e designed motor and standards value.
Increased safety motor is designed to avoid explosion hazard in the hazardous atmospheres. Temperature rise of motor due to overloading, arc/spark of motor windings, high starting current or blocked rotor condition is very important parameter for increased safety Ex e squirrel cage induction motor in hazardous area.
The design aspect of squirrel cage induction motor has been described for increased safety in this paper. The aim of this paper is to know the effect of increasing numbers of turns of stator winding on time t
E
and I
A
/I
N
ratio of motor. It is observed that starting torque of motor is reduced which also increases the time t
E
of stator and rotor and decrease I
A
/I
N
ratio by increasing the numbers of turns of stator winding. Increase of time t
E
of motor and reduced
I
A
/I
N
ratio enhance the safety aspect of motor operation in explosive atmosphere. The protective relay/device of the motor would be getting sufficient time for switching off of power supply of motor during any abnormal condition of the motor which will cause high temperature rise. As temperature rise of the motor would be limited and less then
234

International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print),
ISSN 0976 – 6553(Online) Volume 3, Issue 2, July- September (2012), © IAEME the ignition temperature of the surrounding explosive atmosphere because of switching off of power supply by the current-dependent protective relay/device before time t
E be more safe. has elapsed. The motor operation in hazardous area would
The time t
E
of motor has been increased by reducing the starting torque of motor keeping the frame size and surface area of the motor same. It has been observed that the time t
E
of the motor has been increased considerably and performance characteristics of the motor has also been improved. However, there will be no effect on cost of the motor as well as on performance of motor by reducing the optimization starting torque.
The authors are grateful to Dr. A. Sinha, Director, CSIR-CIMFR, Dhanbad for his kind permission to publish this technical research paper and also wish to record sincere thanks to the Dr. A. K. Singh, Head of Flame & Explosion
Lab., CIMFR and Mr. N. Khare, Sr. Manager, BHEL, Bhopal for providing necessary help in carrying out this research work.
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New Delhi, Bureau of Indian Standards (BIS), 2009.
[2] J Bredthauer, L. B. McClung, “Risk of ignition due to transition currents in medium voltage motors for classified locations”, Paper No. PCIC-90-07, IEEE PCIC, 1990.
[3] J Bredthauer, N. Struck, “Starting of Large medium voltage motor: Design, protection and safety aspects”,
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[4] P Clark, N. Glew, R. Regan, “Solutions for MV motors and generators in hazardous locations”, Paper No.
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[5] IS/IEC 60079-0, Electrical apparatus for explosive gas atmospheres-General requirements, New Delhi,
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[6] J. H. Dymond, K. Younsi, N. Starnges, “Stator winding Failures: Contamination, surface discharges and tracking”, Paper No. PCIC-99-32, IEEE PCIC, 1999.
[7] L. Bianco, F. Feudale, F. razza, M. Sica, A. Contin, G. Rabach, “High voltage induction motors for use in hazardous area : Evoloution of legal aspects and technical investigations related to safety problems”, IEEE
Conf. publication 390, 1994.
[8] J. H. Dymond, “Sparking, Electrical discharge and heating in synchronous and induction machines: Can it be controlled?”, IEEE Transactions on Industry Application, Vol. 34, No. 6, 1998.
[9] P. S. Hamer, “Flammable vapor ignition intiated by hot rotor surface within an induction motor-reality or not?”, Paper No. PCIC-97-04, IEEE PCIC, 1997.
[10] IS/IEC 60079-7, Electrical apparatus for explosive gas atmospheres- Increased safety, New Delhi, Bureau of
Indian Standards (BIS), 2007.
[11] B. Ahirwal, A. K. Singh, R. K. Vishwakarma, A. Sinha, “Increased Safety Protection and Energy
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235