7SR105 Rho and 7SR17 Rho Applications Guide
©2018 Siemens Protection Devices
Chapter 7 Page 5 of 31
Section 2: Protection Functions
This section provides guidance on the application and recommended settings of the 7SR105 and 7SR105
protection functions.
Motor faults can be divided into two categories – system faults affecting plant up to the motor terminals and faults
within the motor.
2.1 Thermal Protection
The motor heats due to power lost to the windings. The loss of heat is proportional to I
2
. The motor heating time
characteristic is determined by its heat storage capability and heat transfer properties. Operating temperatures
will determine the life of the motor insulation, each occurrence of increased temperature reduces the life of the
motor.
A number of conditions can cause the temperature of the motor to increase beyond normal working levels and
beyond the thermal limits of the motor:
Motor running overloaded
Stalling (over-torque, load jam)
Frequent starting or excessive start time
Unbalance/NPS current heating e.g. caused by unbalanced supply voltages or single phasing.
When a motor is started the rotor is initially stationary followed by a period of acceleration i.e. a reducing level of
slip. The motor initially behaves as a Transformer with the rotor acting as a shorted secondary winding. The Line
input currents are at a maximum and can typically be up to times 6x the normal full load (I
FLA
) running level if the
motor is started Direct On Line (D.O.L.). As heating is proportional to I
2
the motor is subject to increased heating.
Figure 2-1 is a time/current plot illustrating the motor start (D.O.L.) and running currents, the current withstand
levels of the motor during running and stalled conditions and the operating characteristic of the thermal overload
protection.
TIME
CURRENT
Running Motor
Thermal Withstand
Characteristic
Stalled Motor
Thermal Withstand
Characteristic
Thermal Overload
Protection Operating
Characteristic
Motor I
START
Motor
I
RUN
Motor starting
characteristic
Figure 2-1 Thermal Overload Protection
The thermal overload protection above provides protection for all motor operate modes and will not operate during
motor starting.
Summary of Contents for 7SR105 Rho
Page 1: ...Answers for energy 7SR105 Rho User Manual Motor Protection Relay Reyrolle Protection Devices ...
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