Schweitzer Engineering Laboratories SEL-749M - Page 285

F.11
Date Code 20080918 Instruction Manual SEL-749M Relay
Motor Thermal Element
Thermal Element Trip-Time Equations
RTC ≠ Auto
Equation F.14
Curve Method
Equations
Equation F.15 and Equation F.16 apply to the starting element when
12 ≥Ι≥2.5 and SETMETH = Curve. To obtain the trip time (cold rotor),
substitute and in
Equation F.10.
Equation F.15
To obtain the trip time (hot rotor), substitute
and in Equation F.10.
Equation F.16
Equation F.17 applies to the running element when 2.5 > Ι > SF and
SETMETH = Curve. To obtain the trip time (hot stator), substitute
and RTC in Equation F.11.
Equation F.17
T
p
60 RTC•
I
2
0.9 SF•()
2
–
I
2
SF
2
–
-------------------------------------
ln•=
U
0
0,=TD1.0,=T
O
2.08 CURVE•(),=I
L
6.0=
T
p
90 CURVE•
I
2
--------------------------------=
U
0
R,=
T
D1.0,=
T
O
2.08 CURVE•(),=I
L
6.0=
T
p
75 CURVE•
I
2
--------------------------------=
I
0
0.9 SF•(),=
T
p
2.5 CURVE•
36 0.9 SF•()
2
–
36 SF
2
–
---------------------------------------
ln
--------------------------------------------------
I
2
0.9 SF•()
2
–
I
2
SF
2
–
-------------------------------------
ln•=
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