ABB RELION REX640 Technical Manual page 1100

Protection functions
or mechanical constraint, this starting method is not suitable. The full-voltage
starting produces the highest starting torque. A high starting torque is generally
required to start a high-inertia load to limit the acceleration time. In this method,
full voltage is applied to the motor when the switch is in the "On" position. This
method of starting results in a great initial current surge, which is typically four to
eight times that of the full-load current drawn by the motor. If a star-delta starter is
used, the value of the line current will only be about one-third of the direct-on-line
starting current.
Figure 618: 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.
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1MRS759142 F
REX640
Technical Manual