Siemens 7SR105 Rho User Manual page 182

Gn49 TauC Cooling Constant
After an overload trip or when the motor is switched off the rotor slows until it stops. During the run-down and
standstill states the motor will cool down but as the rotor does not produce forced cooling the thermal time
constant will be different from the running state. The 'thermal capacity used' value decreases exponentially to
mimic the cooling characteristic of the motor.
The cooling time constant is set to reflect the time taken for a stopped motor to reach steady state ambient
temperature from its running temperature. The cooling time constant of the motor is always longer than the
heating time constant applicable for running. The factor of TauC / TauH is not typically specified by the motor
manufacturer. Typical factors are 5 to 10 x, however for large motors which are totally enclosed and also ones
that normally rely heavily on forced cooling due to motion of the rotor, the factor can be as high as 60x.
Gn49 Hot/Cold Ratio Setting
Most motors are designed thermally to withstand onerous starting conditions rather than running conditions,
motors therefore tend to run at a much lower temperature than their insulation class allows when thermal
equilibrium is reached.
Normally, approximately half of the thermal capacity is used when a motor is running at full load. A hot/Cold Ratio
setting of 50% will take this into account. Specific settings are often determined from the motor thermal damage
curves or locked rotor time (LRT) hot and cold data curves.
Selecting the Hot/Cold ratio (H/C) to 100% (i.e. a high weighting factor) results in identical hot and cold operating
time curves i.e. Hot Safe Stall Time (HSST) = Cold Safe Stall Time (CSST). A Hot/Cold Ratio setting approaching
100% may allow overheating of the motor as the thermal history of the motor has not been sufficiently considered.
A motor requiring negligible hot spot consideration may have a low weighting factor of 5%. In this case, when
operating at full load, the relay would indicate little remaining thermal capacity available, e.g. 95% used. The hot
operating time curve is then much faster than the cold operating time curve.
Disabling the Hot/Cold Ratio feature (i.e. H/C = 0) is appropriate to a motor with no hot spots or for static plant
such as cables. Full consideration is made of prior loading and the 'hot' trip time is at a minimum relative to the
'cold' trip time.
H/C = 100%
t
TRIP
( )
H
t α I 2 − 1 − I 2
C
P
H 11
= = 100%
C 11
2
t α I
HSST = CSST = 11s
Completely de-rated motor
0% of prior load considered
Longest hot curve trip time
(thermal history ignored).
Figure 2.1-4 Effect of Hot/Cold Curve Ratio Setting on Thermal Overload Operate Time
©2018 Siemens Protection Devices
H/C = 64%
( )
H
2 2
t α I − 1 − I
C
P
H 7
= = 64%
C 11
t α I 2 − 0.36I 2
P
HSST = 7s
CSST = 11s
Hot time to trip reduces
Thermal capacity used (meter) increases
7SR105 Rho and 7SR17 Rho Applications Guide
H/C = 9%
HSST = 1s
( )
H
t α I 2 − 1 − I 2
C
P
H 1
= = 9%
C 11
2 2
t α I − 0.91 I
P
CSST = 11s
Zero hot spot (e.g. cable)
100% of prior load considered
Shortest hot curve trip time
H/C = 0% (Disabled)
HSST = 0s
( )
H
t α I 2 − 1 − I 2
C
P
H 0
= = 0% (Disabled)
C 11
2 2
t α I − I
P
CSST = 11s
Chapter 7 Page 9 of 31