ABB RELION REX640 Technical Manual page 1101

1MRS759142 F
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.
Figure 619: Typical motor-starting and capability curves
Setting of
Cumulative time Lim is calculated by
∑
t
REX640
Technical Manual
Cumulative time Lim
= − × +
( n ) 1 t margin
si
Protection functions
(Equation 283)
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