If a motor stalls while running, or is unable to start due to excessive loading, it draws a current equivalent to the
lock
ed rotor current. The level of starting current is equal to the level of locked rotor current. Therefore, it is not
possible to distinguish between 3 phase stalling and healthy starting by monitoring current alone.
In the majority of cases the starting time of a normal induction motor is less than the maximum stall withstand
time. Under these conditions you can discriminate on a time basis between the two conditions to provide
protection against stalling.
However, where motors are used to drive high inertia loads, the stall withstand time may be less than the starting
time. This means you cannot discriminate between start and stall conditions by time alone.
The P24xM relay provides extensive start and stall protection so that all of the aforementioned conditions can be
accounted for. The methods used to achieve this are discussed in more detail below.
This feature requires a 52a circuit breaker auxiliary contact mapped to an opto input to get the information
CBclosed/CB open.
14.8.2 EXCESSIVE START TIME/LOCKED ROTOR PROTECTION - STALL TIME > START TIME
A motor can fail to accelerate for a number of reasons. For example, loss of one supply phase, mechanical failure
or insufficient supply v
oltage. Failure of a machine to accelerate results in excessive current being drawn by the
motor. This current generates extremely high temperatures within the machine. The motor does not have the
added cooling normally provided by rotation, therefore irreparable damage will result very quickly.
Where the stall withstand time is less than the starting time, it is possible to utilize a contact from a speed sensing
device wired into a specified opto input (Speed Input) in conjunction with measurement of the phase current to
detect a safe start.
Three methods are available for detecting a start and they are menu selectable.
Setting guidelines
The starting current threshold should be set greater than full load current, but less than motor starting current.
Where the stall withstand time is greater than the starting time, timer â Prol. Start Time - should be set 1 or 2
seconds above the motor starting time and less than the cold stall withstand time. The Prolonged Start needs to
be set to Enabled for this function to operate.
Setting example:
Utilizing the previously specified motor parameters;
Starting current = 3 x Ith = 882A
Prolonged start time = 12 seconds.
14.8.3 STALL PROTECTION (STALL WHILE RUNNING)
Principle
An induction motor can stall for a number of r
easons, such as overloading or undervoltage. When a machine stalls
it runs down while drawing a current equal to the locked rotor current.
Where the stall withstand time is greater than the starting time, a stall condition during running is simply detected
by the line current exceeding the programmed threshold. If the current fails to fall back below this threshold before
the programmed time delay has elapsed, a trip can be initiated.
Note:
This function is disabled when the relay detects a start condition.
P24xM Chapter 6 - Current Protection Functions
P24xM-TM-EN-2.1 155
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