2.25 Restart Inhibit for Motors (ANSI 66, 49Rotor)
149
7UM61 Manual
C53000-G1176-C127-3
2.25 Restart Inhibit for Motors (ANSI 66, 49Rotor)
The rotor temperature of a motor generally remains well below its maximum admissi-
ble temperature during normal operation and also under increased load conditions.
However, with startups and resulting high startup currents caused by small thermal
time constants it may suffer more thermal damage than the rotor. To avoid multiple
startup attempts causing tripping, a repeated startup of the motor must be prevented,
if it may be assumed that admissible rotor heating would be otherwise be exceeded.
Therefore the 7UM61 device provides a motor restart blocking feature. An inhibit
signal is issued until a new motor startup is admissible (restarting threshold). This
blocking signal must be allocated to a binary output of the device whose contact is in-
serted in the motor starting circuit.
2.25.1 Functional Description
Determining Rotor
Overtemperature
Because rotor current cannot be measured directly, stator currents must be used. The
rms values of the currents are used for this. Rotor overtemperature
Θ
R
is calculated
using the highest of the three phase currents. For this it is assumed that the thermal
limits for the rotor winding are based on manufacturer’s data regarding nominal startup
current, maximum admissible starting time, and the number of starts permitted from
cold (n
cold
) and warm (n
warm
) state. From this data, the device calculates values for the
thermal rotor profile and issues a blocking signal until this profile decreases below the
restarting threshold allowing restart.
Figure 2-53
Temperature Curve at the Rotor and the Thermal Profile during Repeated Start-
Up Attempts
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