ABB RELION REX640 Technical Manual page 429

1MRS759142 F
R
SI
R
N
R
L
The rotor positive and negative sequence resistance vary depending on the motor
slip. The slip estimate values are used to calculate the rotor positive and negative
sequence resistance factors:
R
r+
R
r-
R
L
R
N
S
These resistance factors are then used in the thermal calculations to depict the
thermal behavior that the skin effect causes.
The skin effect describes the situation where the rotor bars are induced with high
amplitude and high frequency current which make the current flow in the rotor bars
to be distributed close to the rotor bar surface, increasing the resistance of the
rotor bar.
The rotor resistances depend on the relation of the rotating magnetic fluxes and the
rotor speed, i.e. slip. The faster the fluxes cut the rotor bars the higher amplitude
and higher frequency currents are induced to the rotor bars.
The rotor positive sequence resistance factor R
(S=1), and smallest when the slip is at nominal value (S≈0). The rotor negative
sequence resistance factor R
the intended direction since the magnetic flux that the unbalance creates rotates in
the opposite direction compared to the rotor, making the skin effect stronger.
STALL_IND output is provided for monitoring purposes or external logic. A TRUE
value indicates that the rotor is stalling. The output is functional only when the
motor is energized.
The rotational speed of the rotor is provided in monitored data at SPEED_RPM. The
monitored data is also only functional when the motor is energized.
4.1.11.6 Application
Rotor thermal overload protection function is intended to prevent the rotor from
overheating especially during motor starting and abnormal operating conditions
REX640
Technical Manual
is the initial stator resistance calculated from line voltages and
phase currents,
is the rotor resistance at nominal speed and
is the rotor resistance at locked rotor.
( )
=
( ) ∙(
)
=
is the rotor positive sequence resistance factor,
is the rotor negative sequence resistance factor,
is the rotor resistance at locked rotor,
is the rotor resistance at nominal speed and
is the motor slip.
is at its highest at motor standstill
r+
, however, grows larger as the rotor starts to revolve in
r-
Protection functions
(Equation 51)
(Equation 52)
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