7SR105 Rho and 7SR17 Rho Applications Guide
©2018 Siemens Protection Devices
Chapter 7 Page 17 of 31
In this configuration Gn-14-1 element (select Gn14-1 Control to ‘None’) can be used to provide protection against
excessive start time. This timer runs for current above Gn14-n setting to provide excessive start time protection.
Notes for Motors Where PF Correction Capacitors Are Fitted
The power factor of a 3-phase induction motor is inductive (typically 0.8 to 0.9). To correct the inductive current
(IQ) of the motor, a capacitor producing capacitive current (IQC) is used i.e. a capacitor is connected in parallel
with the motor.
Where the capacitor is connected on the motor side of the relay measuring point the 7SR105 and 7SR17
measures the corrected motor current and thus the relay settings must be adjusted to take account of the degree
of correction. Where the capacitor is connected on the ‘system’ side of the relay measuring point the relay will
measure pure motor current. In these cases the correction does not affect the relay settings.
Charging of the PF correction capacitors may cause transient inrush currents during motor starting.
MOTOR
P.F.
Correction
Capacitors
(where fitted)
2.3 Current Protection: Loss of load – Undercurrent (37)
Undercurrent elements are used to indicate that current has ceased to flow or that a low load situation exists.
Definite Time Lag (DTL) elements are used. Loss of Load protection is applied to detect:
•
Loss of pump flow e.g. motor cooling pumps.
•
Loss of airflow for fans
•
A failure in a mechanical transmission (e.g. failed shear pin).
The undercurrent setting where enabled can be set to the required level above motor no load current. The applied
current setting or time delay setting must take into consideration low load conditions e.g. the various operate
cycles of a compressor installation.
Where required the function can be inhibited from a binary input.
The undercurrent guard can be set to avoid inadvertent operation when the motor is stopped.
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