7SR105 Rho and 7SR17 Rho Applications Guide
©2018 Siemens Protection Devices
Chapter 7 Page 10 of 31
Gn49 Capacity Alarm Setting
This setting is expressed as a percentage of the thermal state. It is settable from 50 to 100% of the set capacity
and gives an alarm output if its setting is exceeded. Typically this would be set to 95% to indicate that a potential
trip condition exists and that the system should be scrutinised for abnormalities.
Gn 49 Load Alarm
An alarm is provided to indicate load currents above a user set value. High levels of load current may be
indicative of bearing wear e.g. due to excessive vibration or loss of lubrication. The supply to the motor can be
removed before further damage occurs.
The alarm level is set as a multiple of the thermal overload setting. The range is 0.5 to 1.0 x Itheta Thermal
Overload and a measured current above the set value will initiate the alarm if set.
Thermal Restart Inhibit
This feature is used to prevent motor starting if there is insufficient thermal capacity to satisfactorily complete the
start operation. This can be initially set to 50% in the absence of further information.
During commissioning, before starting the protected motor, check the thermal state at the motor by accessing the
“Thermal Capacity Used” meter and note the value. The motor should then be started with its normal load and,
when up to speed the “Thermal Capacity Used” value can be re-checked. The difference between these two
values indicates the magnitude of thermal capacity used to start. This amount must always be available before a
restart is permitted. For safety, this figure should be multiplied by 1.25. For example, if 20% of capacity is used
during starting, then 20% x 1.25 = 25%, and the Thermal Restart Inhibit setting should be 75%.
Alternatively auto-setting can be selected.
2.1.2 Stall Protection (14)
TIME
CURRENT
Running Motor
Thermal Withstand
Stalled Motor
Thermal Withstand
Thermal Overload
Protection
Motor I
START
Motor
I
RUN
Motor starting
characteristic
Figure 2-5 Thermal Overload Protection
The motor will stall when the load torque exceeds the breakdown torque. The motor will draw a stall current equal
to or approaching locked rotor current. The thermal overload protection may provide protection against stalling
during running and starting, however, where this is not possible then the additional dedicated stall protection
functions can be used. Quick motor shut down can reduce thermal damage as well as damage to gears, bearings
and other drive-train components.
Stalling may occur during running or may be a failure to accelerate during starting e.g.
Excess shaft load prior to motor start-up (e.g. failure to open the pump’s discharge gate)
Sudden change of increased shaft load torque during normal operation (e.g. bearing failures)
Summary of Contents for 7SR105 Rho
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