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2 Functions
Although the heat distribution at the rotor cage bars can range widely during motor
startup, the different maximum temperatures in the rotor do not necessarily affect the
motor restart inhibit (see Figure 2-53). It is much more important to establish a thermal
profile, after a complete motor startup, that is appropriate for protection of the motor's
thermal state. The figure shows, as an example, the heating processes during repeat-
ed motor starts (three startups from cold operating condition), as well as the thermal
replica of the protection device.
Restart Threshold
If the rotor temperature has exceeded the restart threshold, the motor cannot be re-
started. Only when the rotor temperature goes below the restart threshold, i.e. just
when a startup becomes possible without exceeding the rotor overtemperature limit,
the blocking signal is retracted. Therefore for the restart threshold Θ
maximum admissible rotor overtemperature:
Restart Times
The motor manufacturer allows a number of cold (n
subsequent renewed startup is allowed. A corresponding time — the restart time —
must expire to allow the rotor to cool down. This thermal behaviour is met as follows:
Each time the motor is shutdown, a leveling timer is started (address 6604 T EQUAL).
This takes into account the different temperatures of the individual motor components
at the moment of shutdown. During the leveling time the thermal profile of the rotor is
not updated but maintained constant to replicate the leveling processes in the rotor.
Then the thermal model cools down with the corresponding time constant (rotor time
constant · extension factor). During the leveling time the motor cannot be restarted. As
soon as the restart threshold is undershot, a new restart may be attempted.
The total time that must expire before motor restart equals to the leveling time and the
time calculated using the thermal model required for the rotor temperature to decrease
below the restart threshold:
with
T
k
150
– rotor temperature leveling time, address 6604
Leveling
– extension factor for the time constant = Kτ at RUNNING address
τ
6609 or Kτ at STOP address 6608
n
2
3
cold
Θ
[%] 50 %
66.7 %
Re.Inh.
) and warm (n
cold
C53000-G1176-C127-3
, related to
Re.Inh.
4
75 %
) startups. No
warm
7UM61 Manual
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