ABB 615 series Technical Manual page 763

1MAC059074-MB A
615 series ANSI
Technical Manual
current decreases as the motor speed increases and the value of current remains close to
the rotor-locked value for most of the acceleration period.
The full-voltage starting or the direct-on-line starting method is used out of the many
methods used for starting the induction motor. If there is either an electrical 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 large 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.
GUID-F4C17D13-48CA-480A-BBE5-DFD7D6316DB8 V1 EN
Figure 380: 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.
Section 4
Protection functions
757