Woodward MRM4, Manual

Page: 386 / 460

Protective Elements
•
Some RTD channels must be configured to measure the winding temperatures; and
•
These RTD channels must be enabled for trip.
In addition, at least one of these winding temperatures must be valid.
Knowing the maximum steady stator temperature qS (°C), the thermal capacity used can be estimated by the
following formula.
TC
Used
%
=
0
S
240

I
ef
2
∗
50
I
LR
2
∗
T
LR
 when
I
ef

ITH
∗
FLA
Take for example, ILR = 6FLA, TLR = 15, and thermal trip level of 100%. The relationship between the effective
current threshold and the stator temperature can be seen in the Stator Temperature Effect on Current Threshold
Curve.
Stator Temperature Effect on Current Threshold Curve
From the graph, it is seen that the lower the stator temperature, the higher the effective current threshold.
Without stator temperature, given the current threshold of 1.0 * Ib (FLA) and 2.0 * Ib (FLA) of the stator phase
current, the thermal model will use the full thermal capacity in 139.54 seconds. However, if the stator
temperature is known as 100°C (212°F), the effective ultimate trip current threshold is raised to 2.55 * Ib (FLA)
and the thermal capacity used will reach a steady state of 77.5%. As a result, the thermal model will never trip
under this condition. From this example, it can be seen that the stator RTD could keep the motor running under
overload condition. In this case, the appropriate direct stator temperature trip function must be enabled.
Page 386 EN MRM4 01/11
Effective Current Threshold vs. Maximum Stator Temperature
0
0.5
1
1.5
2
2.5
3
3.5
0 50 100 150 200 250
Stator Temperature (C)
C
u
rr
e
n
t
T
h
re
s
h
o
ld
M
u
lt
ip
le
o
f
F
L
A
Effective Current Threshold vs. Maximum Stator Temperature
Stator Temperature (°C)
C
ur
re
nt
T
hr
es
h
ol
d
M
ul
tip
le
o
f
Ib
(
F
L
A
)