Configuration
6.2 Overload relays in motor feeders
SIRIUS 3RU thermal overload relays / SIRIUS 3RB electronic overload relays
Manual, 09/2016, A5E03656507420A/RS-AB/003
63
Contactor assembly for star-delta (wye-delta) start
Overload relays in contactor assemblies for star-delta (wye-delta) start
When using thermal or electronic overload relays in conjunction with contactor assemblies
for star-delta (wye-delta) start, you need to bear in mind that only 1/√3 times the motor
current flows through the line contactor. An overload relay mounted onto a line contactor has
to be set to this 0.58-times motor current.
Note
Internal ground-fault detection on the 3RB21 / 3RB31
If you are using the
3RB21 / 3RB31 electronic overload relay in conjunction with contactor
-delta (wye-delta) start, internal ground-fault detection must not be
activated because third harmonics can arise in the motor during delta operation. The
overload relay interprets these harmonics as a ground fault and they produce unint
entional
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