135
6.1.4. Cumulative start-up time counter
The cumulative start-up time counter functions as backup to the thermal overload
protection and prevents too frequent motor start ups, i.e. ensures that the
recommendations from the manufacturer are followed.
There are two values to set: the restart disable value in seconds, 66, and the
countdown rate of the start-up time counter, 66 COOL/Δt.
The restart disable value is calculated as follows:
66 1=−×+()ntmargin
(11)
n = allowed number of motor start ups
t = start-up time of the motor (in seconds)
margin = safety margin (~10... 20%)
The countdown rate is calculated as follows:
66COOL
t
t
reset
=
(12)
t = start-up time of the motor (in seconds)
t
reset
= time during which the maximum number of
motor start ups stated by the manufacturer can be
made (in hours)
If the motor manufacturer has recommended a maximum of three start ups in four
hours and a start-up time of 60 seconds/start up, the restart disable will be activated
when the third motor start up has been initiated, thus preventing a fourth motor start
up. Consequently, the restart disable value is to be set to 130 seconds.
A maximum of three motor start ups in four hours means that the register’s value
should reach the set restart disable value four hours later to allow a new motor start
up. Consequently, the register’s value should decrease by 60 seconds in four hours,
i.e. 66 Cool / Δ = 60 s/4h = 15 s/h.
6.1.5. Short-circuit protection
It is recommended that the set pickup value of element 50P is set to be automatically
doubled during motor start up. Consequently, a pickup value lower than the start-up
current of the motor can be selected.
A pickup value of 70%...90% x the start-up current of the motor is normally
selected. This low a set pickup value together with a suitable set operate time will
enable tripping of the high-set overcurrent element if overcurrent due to a locked
rotor, for instance, is detected while the motor is running.
Motor Protection Relay
Technical Reference Manual - ANSI Version
REM 610REM 610
1MRS755537
To Next Page
Loading...
