ABB REM 610, Technical Reference Manual

Page: 136 / 158

Generally, a set pickup 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 pickup value will be required.
6.1.6. Unbalance and phase reversal protection
The pickup value of the unbalance element, 46, is the NPS current which the motor
can continuously withstand without being damaged. The time constant, 46 TDIAL,
equals the motor constant, I
2
2
x t, i.e. it determines the rotor
’
s ability to withstand
heating caused by the NPS current.
The unbalance and the phase reversal protection element 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.
6.1.6.1. Selecting the pickup value for element 46
The pickup value of element 46 is selected as stated by the motor manufacturer. If
the 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 FLA of the motor.
For instance, if the start-up current of the motor is 6 x FLA and the maximum
allowed NPS voltage four per cent, the estimated NPS current will be 6 x 4% =
24%. Thus, 46 PICKUP will be 0.24 x FLA.
6.1.6.2. Selecting the time constant, 46 TDIAL
The time constant 46 TDIAL can be estimated as follows:
46 TDIAL
=
175
2
( ) I
start
(13)
I
start
= start-up current of the motor x FLA
For instance, if the start-up current of the motor is 5 x FLA, the estimated time
constant will be 175/5
2
= 7.
The operate time of the unbalance element should be set to be shorter than the safe
stall time stated by the motor manufacturer in case a phase is lost.
6.1.6.3. Connection with two phase current transformers
If two-phase connection is used, it is recommended that a current corresponding to
the sum of these two phases is connected to the input circuit of the missing phase;
see Fig. 6.1.6.3.-1. This has two advantages: the unbalance element does not have to
be set out of operation and the current measurement is more accurate compared to
two-phase measurement.
136
REM 610 REM 610 Motor Protection Relay
Technical Reference Manual - ANSI Version
1MRS755537