Short-Circuit Protection; Unbalance And Phase Reversal Protection; Selecting The Start Value For Stage I; Selecting The Time Constant, K - ABB REM 610 Technical Reference Manual

1MRS 752263-MUM
5.1.5.
5.1.6.
5.1.6.1.
5.1.6.2.
Motor protection Relay
Technical Reference Manual
A maximum of three motor start ups in four hours means that the register's value
should reach the set restart inhibit 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. ∆Σt
/∆t = 60 s/4 h = 15 s/h.
s

Short-circuit protection

It is recommended that the set start value of stage I>> is set to be automatically
doubled during motor start up. Consequently, a start value lower than the start-up
current of the motor can be selected.
A start value of 70%...90% x the start-up current of the motor is normally selected.
This low a set start value together with a suitable set operate time will enable
tripping of the high-set overcurrent stage if overcurrent due to a locked rotor, for
instance, is detected while the motor is running.
Generally, a set start value as low as 75% of the start-up current of the motor has
proved useful, but if the inrush current causes tripping during motor start up, a
higher set start value will be required.

Unbalance and phase reversal protection

The start value of the unbalance stage, I
continuously withstand without being damaged. The time constant, K
2
motor constant, I x t, i.e. it determines the rotor's ability to withstand heating
2
caused by the NPS current.
The unbalance and the phase reversal protection stage can be separately selected or
set out of operation. The phase reversal protection should be set out of operation in
applications where the motor rotates in the opposite direction.

Selecting the start value for stage I

The start value of stage I
maximum allowed NPS voltage and not the current is stated, the NPS current will
be approximately the same as the product of the NPS voltage and the ratio of the
start-up current and the FLC of the motor.
For instance, if the start-up current of the motor is 6 x FLC and the maximum
allowed NPS voltage four per cent, the estimated NPS current will be 6 x 4% = 24%.
Thus, I > will be 0.24 x I
2

Selecting the time constant, K

The time constant can be estimated as follows:
175
------------------
K =
2
2
( )
I
start
where
I = start-up current of the motor x FLC
start
For instance, if the start-up current of the motor is 5 x FLC, the estimated time
2
constant will be 175/5
>, is the NPS current which the motor can
2
>
2
> is selected as stated by the motor manufacturer. If the
2
.
n
2
= 7.
REM 610
, equals the
2
121