Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 99)

semiconductors can prevent these transient effects and reduce the loading of power supply and
drive.
The following features and options are characteristic in the use of soft starters:
ƒ Extended setting range of the starting characteristic or selection of various starting character-
istics for an optimum adjustment to the requirements of the working machine
ƒ Infinite variable characteristic of current, voltage and torque. No transient current peaks
ƒ Motor connection with only three lines with control in the motor supply lines
ƒ Increased rated power of the soft starter (factor 1.73) with control in the windings circuit and
motor connection with six lines
ƒ By-passing of power semiconductors after motor start to reduce the permanent losses
ƒ Limited number of starts per hour depending on starting conditions and thermal specifica-
tions of the soft starter
ƒ Extended coasting to stop and braking of drives
ƒ Crawl speed for positioning
ƒ Diagnostic and early warning functions such as overload, underload, locked rotor etc.
ƒ Integration in a communication network
ƒ Integrated (motor) protection functions
ƒ Current harmonics during the starting time by phase control
ƒ Drives with soft starters require for maintenance work on the motor a series disconnecting
device (for example disconnector switches, circuit breakers with isolating function).
Soft starters are available in a variety of different designs, each with specific technical charac-
teristics. For the selection of a device for a specific application the technical literature of the
manufacturer and its technical support have to be observed. See also IEC 60947-4-2 [5] and
[17]. For specific aspects of high efficiency motors see
1.7.1.2.1 .
3.9.1 Voltage ramp versus current limitation
The basic mode of operation of soft starters is to control the voltage across the motor by phase
control. Usually the phase control is performed in 3 phases and in both current half-cycles by
means of triacs or antiparallel connected thyristors. Economical solutions use controlled
semiconductors in only two or even only one (1) phase. The resulting asymmetries create
disadvantages with respect to the available torque related to the current consumption and for
example an increasing loading of the motor bearings because of torque asymmetries. The 1-
phase controller corresponds to the stator-resistance starting circuit (see Section
3.5 ).
The voltage across the motor can for example be controlled by
ƒ a (selectable) voltage ramp or
ƒ a fixed (reduced) voltage (quasi current-limiting)
ƒ in relation to a feedback variable such as
o motor current (current limitation) or
o speed (start following a speed characteristic)
Depending on the method chosen, typical torque and speed characteristics for starting are
produced (
Fig. 3.9-1 ). When starting with a voltage ramp and especially when starting with
current limitation, large acceleration torques in the range of the breakdown torque are generated
towards the end of the starting period.
LVSAM-WP001A-EN-P - April 2009 3-23