ABB RED615 Technical Manual page 534

Section 4
Protection functions
528
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 277: 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
1YHT530004D05 D
615 series
Technical Manual