Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 119)

max. 1 % voltage asymmetry. With larger asymmetries the motor loading should be reduced
(
Fig. 4.1-9 ).
Fig. 4.1-9
Power reduction as a consequence of voltage asymmetry
f
R
Reduction factor for motor power
ΔU Voltage asymmetry [%]
4.1.2.4.3 Phase failure protection
Star-connected motors
Small to medium-sized (stator-critical) motors in star configuration are in generally not endan-
gered by phase-loss. In accordance with
Fig. 4.1-10 , the currents in the motor windings both in
normal operation and with loss of one phase are the same as the currents in the external
conductors. The protective device also measures the current flowing through the windings in the
event of the failure. In both windings through which the current flows, because of the increasing
current at constant mechanical motor load, an increased power dissipation results. With the
third – de-energized – winding a temperature compensation occurs, so that a current-measuring
protective device would release in time in event of an overcurrent.
Fig. 4.1-10
Motor in star configuration under normal supply conditions and with phase failure
Motors in delta configuration
In the delta configuration and in normal operation, the windings currents I
P
are smaller than the
currents I
e
in the external conductors by a factor 1/
√3 = 0.58. If a phase is lost (
Fig. 4.1-11 ) the
current increases in one winding by around 15 % – assuming the current in the external
conductors remains constant (I
e1
= I
e
). In the two other windings that are now connected in
series, the current falls to a 1/
√3 times smaller value. In comparison to symmetrical operation,
the protective relay measures a current that is too low for the winding that lies between the
healthy phases, which can cause a thermal overload in this winding without the protective
device responding. Heat exchange between this and the other windings that carry less current
reduces the danger of a thermal overload in small motors up to around 10 kW.
In a real application, the mechanical load of the motor remains constant with the outage of one
phase. As the electrical power is only provided from two pole conductors, the current in the two
LVSAM-WP001A-EN-P - April 2009 4-11