CHAPTER 7 - APPLICATIONS AND PROGRAMMING
138
2) In the chart, at hot in P640, we can find the maximum Thermal
Class that the motor will withstand due to the locked rotor time at hot:
For 6.6 x In @ 12s, we select the closest lower Class: Class 40.
Now it is known that Thermal Class 10 allows one start and Thermal
Class 40 is the upper limit. Thus you must select a Thermal Class
between these two Thermal Classes by considering the number of
start per hour and the time interval between motor On-Off procedures.
The closer to Class 10 you select, more protected will be your motor,
less starts per hour are allowed and longer time intervals between
motor On-Off procedures are required.
The closer to Class 40 you select, nearer the upper motor limit you
will be, thus more starts per hour are allowed and shorter time intervals
between motor On-Off procedures can be used.
To determine the activation times of the hot Thermal Classes, i. and.,
when the motor is running at rated load with current lower than or
equal to 100% of the Nominal Current, use the multiplier factor shown
in Table 6.34 at P640, as a percentage of the current that the motor is
absorbing when running continuously.
For example:
A motor is running with 80% In and then is switched Off.
It is switched On again immediately.
The starting current is 3xIn @ 25s.
The selected Thermal Class, in the table 6.22, is the Class 10 with
33,7s @ 3xIn.
As shown in table 6.33, the correction factor for 80% In is 0.48.
The final activation time will be: 0.48 x 33.7s = 16.2s, i. and., the time
is reduced at cold start from 33.7s to 16.2s at hot start. Thus, a new
motor is not allowed before the thermal motor image decreases, i.and.,
cools down.
When the Service factor (S.F.) is different from 1.00, but its use is
required, you can find in the chart, cold, the points for the S.F. = 1.15
and a table for S.F. = 1.15.
If you want to know the thermal protection activation time for other
Service Factor (S.F.), displace the line xIn proportionally to the left.
7.2.1.3 Time reduction when
changing from cold starting
to hot starting
7.2.1.4 Service Factor
Figure 7.26
– Using the S.F. to find new times
t(s)
0
xIn
of the Motor
15
20
25
Cold
F.S.=1.15
F.F.=1.25
2x
3x
4x
Phone: 800.894.0412 - Fax: 888.723.4773 - Web: www.clrwtr.com - Email: [email protected]
