Rockwell Automation Low-Voltage Switchgear and Controlgear User Manual
Page 86
Switching from star to delta produces the phasor diagram
Fig. 3.3-11
Phasor diagram for connections of the motor phases according to Fig. 3.3-10. This produces a high
transient current surge because of the adverse vector position.
Counterclockwise sense of rotation
To run the motor in the counterclockwise direction, it is not enough to swap around two phases
at any point. This would produce the same relationships as described above. In order to keep
the transient current surge from star to delta connection as small as possible the wiring must be
arranged as in
Fig. 3.3-12
Correct connection of the motor phases for counterclockwise rotation of motor
3.3.3
Influence of the third harmonic on motor protection relays
In motors, in which there is relatively little core iron (e.g. refrigeration motors, submersible pump
motors etc.), with delta connection the third harmonic and its harmonics are excited in the
windings as a consequence of iron saturation. Because of the triple frequency the currents of
the third harmonic have the same vector position in all three windings. This harmonic current
basically flows in a circle through the windings and is not noticeable in the feeding lines. With
star connection, no third harmonics can form as the motor star point is not connected to the
mains.
Experience shows that the harmonic current values can be up to 30 % and more of the basic
current. With measuring instruments that show the true r.m.s. value, the r.m.s. value of the
entire windings current can be measured; on the other hand, the harmonic component cannot
be correctly measured by instruments that only show the mean value.
The third harmonic contributes to heating of the motor windings. This is taken into account by
the motor manufacturer so that the rated load is not compromised. Therefore motor protective
devices for direct starting in delta mode should always be set to the motor rated operational
current (= current in feeding lines).
In delta mode in a star-delta starter the motor protective device is connected in series to the
motor windings (
). If it is normally set to 0.58 · I
e
it may release prematurely due to the
additional harmonics. In these cases the actual r.m.s value of the windings current should be
measured and the setting of the protective device should be increased by the percentage of the
LVSAM-WP001A-EN-P - April 2009
3-10