CHAPTER 7 - APPLICATIONS AND PROGRAMMING
136
1) Initially start from the standard thermal class, sometimes, but without
heating the motor excessively.
2) Determine the correct starting time. Find an average of the current
through the P002 during the starting time. One can find a current
average for any kind of starting control.
For example:
When an 80A motor is started by voltage ramp, the current at P002
starts at 100A and increases to 300A and after 20s drops to the
rated current.
(100A+300A)/2 = 200A
200A/80A = 2.5 x In of the motor
then: 2.5 x In @ 20s.
7.2
PORTECTIONS AND
PROGRAMMING
7.2.1
Thermal Classes
7.2.1.1 Suggestions about thermal
class setting
Figure 7.23
– Typical current curve when started by voltage ramp
3) Use this time to find the minimum class necessary to start a cold
motor according to the descriptions of the P640 in chapter 6;
Figure 7.24
– Determining the minimum thermal class with cold motor
Thus the minimum required thermal class for starting the motor is the
Class 10. The Class 5 requires a shorter time for this current.
This Thermal Class allows motor cold start.
U(V)
Start
100%Un
20s
0
Enable
t(s)
P101
Voltage Ramp
P102
Current
in P002
100A
300A
t(s)
0
20s
2.5 x In of the Motor
5
10
xln
15
Cold
F.S.=1
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