ABB RED615 Technical Manual page 535

1YHT530004D05 D
615 series
Technical Manual
must be used for detecting whether the motor begins to accelerate or not. However,
if the safe stall time is longer than the startup 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 startup 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.
GUID-6E9B7247-9009-4302-A79B-B326009ECC7A V2 EN
Figure 278: Typical motor-starting and capability curves
Section 4
Protection functions
529