WEG SSW-08 Manual Del Usuario página 44

Determine the minimum thermal class:
1) Initially start at the standard thermal class, sometimes, but without the motor overheating;
2) Determine the correct starting time and find an average of the current using a multimeter with
a current probe to measure it; A current average can be found for any type of starting control;
For example:
Starting an 80 A motor using a voltage ramp. The current starts at 100 A and goes to 300 A,
returning afterwards to the nominal value in 20 seconds.
(100 A + 300 A)/2 = 200 A
200 A/80 A = 2.5 x ln of the motor
Therefore: 2.5 x ln @ 20 seconds.
Initial Voltage P101
Figure 5.9: Typical current curve during a voltage ramp start
3) Use this time to find the minimum class necessary to start the motor in cold condition. In the
item 4.8 Overload Protection it is possible to check the thermal class curves of the motor in
cold condition.
Figure 5.10: Checking the minimum class of curves in cold condition
Therefore the minimum class necessary to start the cold motor is Class 10. Class 5 will trip
during starting.
NOTE!
If the motor must start in the hot condition, class 10 will trip during the second start.
In the case a higher thermal class must be set.
Programming Information and Suggestions
U(V)
100 A
0
Enable Accel Time
t(s)
20 s
0
2.5 x ln of the motor
Start
100 % Un
P102
20 s
Cold
F.S.=1
15
10
5
xln
300 A
Motor
Current
t(s)
SSW-08 | 39
5