WEG SSW-07 - Time Reduction from Cold to Hot Starting

CHAPTER 5 - PROGRAMMING INFORMATION AND SUGGESTIONS
41
English
5.2.2 Time Reduction
from Cold to
Hot Starting
Example of how setting the thermal class:
Motor data:
Power:50hp
Voltage:380V
NominalCurrent(ln):71A
ServiceFactor(S.F.):1.00
lp/ln:6.6
Stalltime:12sinhotcondition
Speed:1770rpm
Motor + load starting data:
StartingbyVoltageRamp,startingcurrentaverage:
3 x the nominal current of the motor during 25 s (3 x ln @ 25 s).
1)Inthegraph,gure4.8incoldcondition,onecanseetheminimum
ThermalClassthatwillallowthestartwithareducedvoltage:
 For3xlnofthemotor@25s,thenexthighestisadopted:Class10.
2) In the graph, figure 4.9 in hot condition, one can see the
maximum Thermal Class that the motor can stand due to the
stalltimeinhotcondition:
For 6.6 x ln of the motor @ 12 s, the next lowest is adopted. Class 30.
One now knows that Thermal Class 10 allows a start and Thermal
Class 30 is the maximum limit. Thus, a Thermal Class between the
two should be adopted, according to the quantity of starts per hour
and the interval of time between Off/On procedures the motor.
The closer to Class 10, the more protected the motor will be, the
fewer the starts per hour and the greater the interval of time must
be between Off/On procedures the motor.
The closer to Class 30, the closer it gets to the maximum limit of the
motor, thus there can be more starts per hour and lower intervals
of time between Off/On procedures the motor.
To determine the working times of the thermal classes in hot condition,
when the motor is running at full voltage with a current lower that
100%ofthelnofthemotor,usethemultiplicationfactorintable
4.4, according to the current percentage that the motor is operating.
Forexample:
Amotorisrunningwith80%lnandisturnedoff.
Immediately afterwards it is turned back on.
The starting current is 3 x ln of the motor @ 25 s.
The Thermal Class selected is Class 10 with 33.7 s @ 3 x ln of the
motor.
Inthetable,thesettingfactorto80%lnofthemotoris0.48.
Thenalworkingtimewillbe:0.48x33.7s=16.2s,inotherwords,
the time was reduced from 33.7 s during a start in cold condition
to 16.2 s for a start in hot condition. A new start will not be allowed
before the thermal image of the motor decreases, in other words,
cools down.