Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 123)

Protection
relay
I
o
Fig. 4.1-14
In the Holmgreen circuit, the current I
o
is measured in the common return conductor of the current
transformers. Because of the inaccuracy of the c.t.’s, the sensitivity is low.
Higher sensitivities can be achieved with core balanced current transformers (principle of the
residual current protection devices,
Fig. 4.1-15 ).
M
3~
Protection
relay
Fig. 4.1-15
In core balanced current transformers the iron core encloses all conductors leading to the motor like in a
residual current protection switch. High ground fault sensitivities can thus be achieved.
The shutdown of ground faults should be by means of a circuit breaker, as ground currents
frequently exceed the switching capacity of contactors. When normally shutting down via the
contactor, exceeding of its switching capacity must be prevented (inhibit). In this case, the
upstream short-circuit protection device takes over the job of clearing the fault.
4.1.2.5 Display, warning and control functions
In addition to the protective functions electronic motor protection relays provide valuable
information for monitoring and optimum operation of drive installations. The below functions are
possible:
ƒ Display of motor current flowing
ƒ Display of overloads
ƒ Display of the temperature status of the motor
ƒ Prewarning before protective tripping
ƒ Display of time to protective tripping (at constant load)
ƒ Control of the drive load (for example stone crushers, calenders) for optimum motor tempera-
ture and hence maximum production
ƒ Display of the required cooling time before the next start-up
ƒ Switching of star to delta as soon as the starting current falls
ƒ Closing the bypass of pumps or fans immediately after starting
ƒ etc.
LVSAM-WP001A-EN-P - April 2009 4-15