Multifunction Protection and Switchgear Control unit REF542plus
Manual: Motor Protection with ATEX-Certification
K = 10
IMn = 80A (rated current of the motor)
In = 100A (rated current of the current transformer)
If an open-circuit condition is present, the currents in the other two conductors are of
equal size and have a phase displacement of 180°. Furthermore, it is assumed that
both of the currents are identical with the rated current. Under this condition, the un-
balanced load current I2 is:
I2 = 0.577 In
If this value is inserted into the equation above, the tripping time is 30.9s.
As the rated current of the current transformer is not identical to the rated motor cur-
rent — in this example the rated current of the current transformer is 100A and the
one of the motor is 80A — the setting of the temperature rise constant has to be cor-
rected based on the relationship between the rated motor current and the transformer
rated current, as shown in the equation below:
⎥
⎥
⎦
⎤
⎢
⎢
⎣
⎡
⎟
⎠
⎞
⎜
⎝
⎛
−
⎟
⎠
⎞
⎜
⎝
⎛
=
2
Mn
2
Mn
AUS
n
s
nn
2
n I
I
I
I
I
I
I
I
tK
(21.)
The constant which is to be set at the REF542plus field control and protection device
for the above example is:
38,61,0
100
80
577,0
100
80
9,30
22
=
⎥
⎥
⎦
⎤
⎢
⎢
⎣
⎡
⎟
⎠
⎞
⎜
⎝
⎛
−
⎟
⎠
⎞
⎜
⎝
⎛
=K
(22.)
4.5.2 Tripping characteristic
The following paragraphs deal with the tripping characteristic at an assumed tempera-
ture rise constant K of 5. At the same time, it is assumed that the response threshold
or the current starting value is 0.1 or 10%. The tripping time can then be derived by
applying equation (13).
2
2
2
1,0
5
−
=
I
t
(23.)
The following figure shows the tripping characteristic, where the unbalanced load cur-
rent I2 is referred to the nominal current of the motor Imn.
1VTA100114-Rev. 04 en PTMV, 19.01.05 Motorprotection 29 / 66
Valid from Version V4D02
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