ABB RELION 611 Series Technical Manual page 302

Section 4
Protection functions
296
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 139: 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.
1MRS757454 D
611 series
Technical Manual