1MRS751802-MUM
REX 521
237
Protection Relay
Technical Reference Manual, Standard Configurations
If it is assumed that the motor is recommended to be started at most three times
within eight hours, and that the start-up time is 20 seconds each, the situation
described in the diagram (see Fig. 11.7.3.3.-1) occurs if all the allowed three starts
in a row are made:.
startup.tif
Fig. 11.7.3.3.-1 Operation of start-up time counter
The three 20 second starts add up to a total of 60 seconds. Right after the third start
has been initiated, the inhibit should be activated, and the fourth start attempt is
therefore not allowed. This means that the setting of the
Time limit
in this case
is set to 41 seconds. Note that the start sequence still proceeds even though the
inhibit is activated. The inhibit is only interrupting the close path to the circuit-
breaker, and thereby preventing further start-ups.
Furthermore, the recommendation of at most three starts within eight hours means
that the count-down should reach the inhibit level after 8 hours in order for it to allow
a single new start. In this example it means that it should be counted down 20
seconds in 8 hours, that is, the
Countdown rate
setting is 20 s/8 h=2.5 s/h.
The equation for Countdown rate setting is:
where
t
=starting time (in seconds)
t
reset
=time (in hours) to reset one start-up stress
For readability reasons the time scaling in the diagram is not the
same for start-up versus cooling-down sequences.
t
t
reset
-----------
=
Summary of Contents for REX 521
Page 1: ...Protection Relay REX 521 Technical Reference Manual Standard Configurations ...
Page 2: ......
Page 12: ...12 ...
Page 22: ...22 ...
Page 62: ...62 ...
Page 72: ...72 ...
Page 128: ...128 ...
Page 132: ...132 ...
Page 172: ...172 ...
Page 198: ...198 ...
Page 249: ......
