Medium Voltage Application Guide

09677.B
Current
Torque (% motor full load torque)
13474.A
SOFT STARTERS
Closed loop soft start control
Closed loop soft starters have one or more feedback loops, which monitor characteristics at the motor. The starter
adjusts the voltage to the motor, in order to control the monitored parameters.
Closed loop controller
Current transformer feedback
Common closed loop systems are:
Constant Current or Current Limit
Timed Current Ramp
Constant Acceleration
Constant current soft start
Constant current starters monitor the starting current. Increasing or decreasing the output voltage increases or
decreases the current supplied to the motor. As the motor accelerates, the stator impedance rises and in order to
maintain a constant current the voltage also rises. The exact relationship between voltage and speed depends on
the motor design.
With a constant current starter, full torque is available as the motor reaches full speed. It is important that the starting
current is high enough to accelerate the motor to full speed under all conditions. If the torque is insufficient for
acceleration at any time during the start, the motor will continue to run at the reduced speed. This will overheat the
motor unless there is excess start time protection.
Timed current ramp soft start
Timed current ramp soft starters increase the current from a selected start level to the maximum start current, at a
controlled rate. This caters for variation in starting torque requirements, or can deliver reduced starting torque
without limiting the maximum starting torque. Typical applications are conveyors which start under varying load
conditions, and pumps which require very low torque at low speed.
This method also suits motors running on generator supplies, as the starting load is gradually applied to the generator
set. This provides stable voltage and frequency control of the generator set during motor starting.
Constant acceleration soft start
Constant acceleration or linear acceleration starters monitor the motor speed, by means of a tacho generator
attached to the motor shaft. The voltage applied to the motor is controlled to deliver a constant rate of acceleration,
over a selected acceleration time. A current limiting circuit can also be used to limit the maximum starting current,
particularly in applications where a potential exists for jammed loads.
700
600
500
400
300
200
100
1
3
4
2
1
5
6
200
100
Full voltage start current
Current limit
Full voltage start torque
Torque output at current limit
Acceleration torque
Load torque curve
10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
Speed (%full speed)
710-12280-00A Medium Voltage Application Guide Page 21