Specifications

Reference Manual
MHM-97408, Rev. 2
June 2008
CSI 9420
C-6
In general, the best way to detect a developing fault related to temperature is
to look for an increase in the temperature, relative to ambient, over time. This
implies that, for reliable alerting, the thresholds should change as ambient
temperature changes. In practice, this can be difficult to do because it
requires the operator constantly to monitor the ambient temperature and
adjust the alert levels accordingly. It is customary, therefore, to pick an
"average" ambient temperature (that is generally seasonal for outdoor
installations) and choose fixed thresholds based on this average. Be advised,
however, that there are issues with this methodology (i.e., it does not work
well) in areas with large variations in ambient temperature.
Thresholds may also be selected based on some absolute temperature limit.
In practice, this is much easier to maintain but it is not as effective at detecting
early failures as relative monitoring.
"Relative" Temperature
Monitoring
The recommended generic guidelines for setting the thresholds based on the
relative change are:
•T
Advise
= 10 °C increase
•T
Maintenance
= 15 °C increase
•T
Failed
= 20 °C increase
For example, assume ambient temperature is 25 °C. When operating at
steady-state, you have determined that the normal temperature at this point
on your equipment is 55 °C. So, your "baseline" relative difference is 30 °C.
Using the above guidelines, you should choose the Advise, Maintenance, and
Failed levels for a difference of 40 °C, 45 °C, and 50 °C, respectfully.
Assuming the ambient temperature is constant at 25 °C, this means the
thresholds become 65 °C, 70 °C, and 75 °C for Advise, Maintenance, and
Failed, respectively. Then, as the ambient temperature changes, the
thresholds should be changed accordingly (e.g., a 5 °C increase in ambient
temperature raises the alert thresholds by 5 °C).
Absolute Temperature
Monitoring
For monitoring a driven component (e.g., a pump or a fan), there are no
generic rules for determining the default levels without some a priori
knowledge of the steady-state baseline (good) value. In general, the Advisory
level should be set about 10 °C to 20 °C above this baseline, with the
Maintenance level about 10 °C above Advisory and the Failed level about
10 °C above Maintenance.
There are equations that define the suggested generic thresholds for
monitoring motor (driver) temperature. These are based on characteristics of
the motor as well as knowledge of the ambient temperature.
The first step is to determine the estimated winding temperature, which is
dependent on the following variables:
• Insulation type
• Motor type
• Ambient temperature
• Altitude
The estimated winding temperature, T
W
, is the rise temperature, T
rise
, for the
appropriate type of motor adjusted for high ambient temperature (T
a
) effects.