13
0
20
40
100
60
80
%
1.5 x I
θ
1.5 x I
θ
I = 0.8 x I
θ
I = 0.8 x I
θ
1.5 x I
θ
I = 0.8 x I
θ
Figure 8. The behaviour of the thermal level in a single time constant object with varying load
sequencies
The parameter p of
the thermal unit
One of the main settings of the thermal unit is
the thermal unit weighting factor p. This para-
meter is picturing the dualism of the thermal
properties of a motor.
A setting of p=100% produces a pure single time
constant thermal unit, which is useful for ap-
plications with cables etc. As can be seen from
Fig. 9., the hot curve with p=100% only allows
a stall time of about 10% of the cold safe stall
time. For a motor with a curve of e.g. t
6x
= 10 s,
the hot trip time would be only 1 s, whereas the
motor can withstand at least 5 or 6 seconds. To
allow full use of the motor a lower p-value
should be used.
As about one half of the thermal capacity is
narmally used when a motor is running at nomi-
nal load, this must also be handled by the pro-
tective unit. With the normal motor setting of
p=50 %, the relay will notify a 45...50% ther-
mal capacity usage at full load.
Generally a choice should be made between
50% for a standard motor started directly on
line, and 100% for a non-rotating object or a
soft-started motor. Only in special cases, where
a closer adjustment of the thermal characteris-
tic is needed and the thermal capacity of the
object is very well known, a value between 50
and 100% might be needed.
Note! For applications with e.g. three cold vs. two
hot starts a setting of p = 40% has sometimes
proved to be useful. Otherwise setting the p-value
much below 50% should be handled carefully be-
cause in this case there is a possibility to overload
the protected object as the thermal unit might al-
low too many hot starts or "forget" too much of the
thermal history background. In Fig.9. you can see
that the hot curve of p = 20% is quite close to the
cold curve. The cold curve is identical for all p-
values.
Figure 9. The influence of the p-value on the
hot trip time curve with t
6x
= 20 seconds.
100
500
200
50
10
20
5
1
2
3
4
30
40
300
400
12345106 8 I/I
θ
1.05
p
[%]
20
50
75
100
Cold
curve
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