Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 122)

0
100
250
I / I e
t
Start-up
Trip threshold high
overload/stalling
Operation
Fig. 4.1-13
Stalling protection recognizes high overloads and enables rapid intervention or protective shutdown
4.1.2.4.5 Underload protection
Motors that are cooled by the conveyed medium itself (for example submersible pumps, fans),
can become overheated as a result of underloading when the volume of the medium is absent
or reduced (obstructed filters, closed slides). These machines are often installed in inaccessible
places. The consequences are long repair time and correspondingly high costs.
When the current intake falls below a certain level, also a mechanical fault in the system may be
indicated (torn conveyor belts, damaged fan rotors, defective clutches, broken shafts or worn
down tools). Such conditions do not endanger the motor, but result in production outages.
The underload protection monitors the current intake of the motor and by rapid recognition of an
underload, helps to keep any disruptions and damages as small as possible.
4.1.2.4.6 Automatic switching-over during start-up
Monitoring the current consumption of a motor for falling below a defined threshold can also be
used for automatic control of switching-over of star-delta starters or auto transformer-starters.
The relay recognizes by the magnitude of the current, when the first phase of the starting
process has ended and immediately initiates switching-over to the second phase. Thus start-
ups can be kept to an optimally short length – even under changing starting conditions.
4.1.2.4.7 Ground fault protection
Damage to the insulation of motors is frequently caused by high voltage spikes. The sources
may be switching transients from the supply network, capacitor discharging, power electronics
devices or lightning strikes. Other causes are ageing and continuous or cyclical overload as well
as mechanical vibrations and foreign particles. In most cases, insulation damage results in
shorting against the grounded parts of the machine. In grounded supply systems, the ground
currents can quickly reach very high values.
The prompt detection and protective shutdown of a ground fault limits the extent of the resulting
damage and helps to reduce outages and repair costs.
A relatively simple ground fault protection method measures the zero sequence current
component of the current transformer-secondary currents (“Holmgreen”-circuit,
Fig. 4.1-14 ).
Because of the tolerances of current transformers and of the influence the 3rd harmonic a
sensitivity of 10 % can be achieved at best, typically around 30 %. This method is thus also
limited to application in solid grounded networks.
LVSAM-WP001A-EN-P - April 2009 4-14