ABB RELION REX640 Technical Manual page 1007

1MRS759142 C
REX640
Technical Manual
GUID-56ECE503-FB21-493F-849D-3300E84EF1DD V1 EN
Figure 542: Typical motor starting and capability curves
The start-up supervision of a motor is an important function because of the higher
thermal stress developed during starting. During the start-up, the current surge
imposes a thermal strain on the rotor. This is exaggerated as the air flow for cooling
is less because the fans do not rotate in their full speed. Moreover, the difference of
speed between the rotating magnetic field and the rotor during the start-up time
induces a high magnitude of slip current in the rotor at frequencies higher than when
the motor is at full speed. The skin effect is stronger at higher frequencies and all these
factors increase the losses and the generated heat. This is worse when the rotor is
locked.
The starting current for slip-ring motors is less than the full load current and therefore
it is advisable to use the circuit breaker in the closed position to indicate the starting
for such type of motors.
The starting times vary depending on motor design and load torque characteristics.
The time taken may vary from less than two seconds to more than 60 seconds. The
starting time is determined for each application.
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.
Section 4
Protection functions
1001