WEG SSW-08 User Manual - page 44
5
Programming Information and Suggestions
SSW-08 | 39
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.
Motor
Current
P102
Accel Time
Initial Voltage P101
U(V)
Start
100 % Un
300 A
t(s)
20 s
0
100 A
Enable
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.
2.5 x ln of the motor
t(s)
20 s
0
xln
15
10
5
Cold
F.S.=1
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.