14.2 LOCKED ROTOR DURING STARTING – (STALL TIME < START TIME)
For applications such as motors driving high inertia loads, the stall withstand time can be safely exceeded during
star
ting. This can be done without an over temperature condition within the motor. The stall withstand time is less
than the start time, therefore time alone cannot be used to distinguish between a start and a stall condition.
The P24xM relay overcomes this problem by using a contact from a speed sensing device wired into a specified
opto input (Speed Input). Change of state of this contact indicates successful acceleration of the motor. If the line
current exceeds the value set in the Starting current and the speed of the motor is equal to zero, the relay trips
following the programmed Stall Time (Stall Rotor-Strt).
Note:
The breaker status must be mapped for this function and the breaker must be closed (52A input high) for the relay to operate
if a locked rotor is detected.
The figure below shows the principle of this feature:
90 ms
0
Starting
Current
Threshold
Current
T0
T0 + Tstart
0
1
T0 T0 + Tstart
O/O CB
interlock
0
1
T0 T0 + Tstart
«Speedswitch»
Information
Prolonged Start Time
0
1
T0 T0 + Tstart
Locked rotor during start
Information due to speed
switch not being asserted
before stall time delay has
elapsed
STALL TIME
Time
Time
Time
Time
P0652 ENb
Figure 73: Locked rotor detection
The S
tall Rot-Strt setting must be enabled. After the detection of the start, both timers Prol.Start Time and Stall
Time are running. A typical setting could be 5 s for the prolonged start timer and 1 s for the stall timer.
If the current falls below the starting current threshold before the end of the Stall time and if the speed switch
doesn't detect the rotation of the motor, this feature will NOT generate a tripping order.
P24xM Chapter 6 - Current Protection Functions
P24xM-TM-EN-2.1 149
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