ABB RELION 620 Series Technical Manual page 770

Section 4
Protection functions
764
When the permissible stall time is less than the starting time of the motor, the stalling
protection is used and the value of the time delay setting should be set slightly less than
the permissible stall time. The speed switch on the motor shaft must be used for
detecting whether the motor begins to accelerate or not. However, if the safe stall time
is longer than the start-up time of the motor, the speed switch is not required.
The failure of a motor to accelerate or to reach its full nominal speed in an acceptable
time when the stator is energized is caused by several types of abnormal conditions,
including a mechanical failure of the motor or load bearings, low supply voltage, open
circuit in one phase of a three-phase voltage supply or too high starting voltage. All
these abnormal conditions result in overheating.
Repeated starts increase the temperature to a high value in the stator or rotor windings,
or both, unless enough time is allowed for the heat to dissipate. To ensure a safe
operation it is necessary to provide a fixed-time interval between starts or limit the
number of starts within a period of time. This is why the motor manufacturers have
restrictions on how many starts are allowed in a defined time interval. This function
does not allow starting of the motor if the number of starts exceeds the set level in the
register that calculates them. This insures that the thermal effects on the motor for
consecutive starts stay within permissible levels.
For example, the motor manufacturer may state that three starts at the maximum are
allowed within 4 hours and the start-up situation time is 60 seconds. By initiating three
successive starts we reach the situation as illustrated. As a result, the value of the
register adds up to a total of 180 seconds. Right after the third start has been initiated,
the output lock of start of motor is activated and the fourth start will not be allowed,
provided the time limit has been set to 121 seconds.
Furthermore, a maximum of three starts in 4 hours means that the value of the register
should reach the set start time counter limit within 4 hours to allow a new start.
Accordingly, the start time counter reduction should be 60 seconds in 4 hours and
should thus be set to 60 s / 4 h = 15 s / h.
1MRS757644 F
620 series
Technical Manual