Ice NP900 Series, Инструкция по применению

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Application Guide - NP900 Series 238 (504
)
A996A
From the figure can be noted when the motor is loaded with constant current both of the
replicas (single and dual time constant) follow the motor heating quite accurately.
Difference in the operation comes from the cooling part. With single cooling time constant
the replica does not follow the actual cooling of the motor and it can be said that the
match is very poor. With dynamically controlled cooling time constants the response to
measured is very accurate. If this motor would be used for cyclic loads with cooling times,
most probably one time constant model would be overprotecting already in the next duty
cycle, possibly causing unnecessary alarms or even trip of the motor which is still running
in safe temperatures.
When studying the motor heating up in possible overload and start-up situations motor
thermal curves are useful. These are usually available on request from manufacturers and
the relay operation can be set according to these.
If motor thermal limit curves are
available from them can be seen the
time constants for the overloading and
also for the safe stall times for hot and
cold situation. In addition to this figure
the cooling time constant has to be
checked from the motor datasheet or if
not available the alternative option is to
measure it.
The cooling time constant as presented
in the previous example is very crucial in
case of variable duty cycle motor
applications. If the motor is constantly
running with constant load then the
cooling time constant is not that
significant and it can be estimated to be
eg. 2-3 times longer than heating time
constant.
It can be estimated from the figure that
the safe stall time cold is about 80
seconds and hot about 67 seconds.
Figure 3.3.2.1-114 Example thermal limit curves of a motor.