ABB 615 Series ANSI, Technical Manual

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1MAC050144-MB C
Section 6
Supervision functions
615 series ANSI 467
Technical Manual
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 249: Typical motor starting and capability curves
The startup supervision of a motor is an important function because of the higher thermal
stress developed during starting. During the startup, 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 startup 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 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.