Medium Voltage Application Guide

SOFT STARTERS
Enclosed soft starters
Enclosed soft starters have varying levels of electrical and mechanical protection from the outside world. Housings
are made from a combination of metals, alloys and plastics with many different finishes. This style is more common
amongst low voltage soft starter products. In some cases, the housing provides enough protection that the soft
starter can be wall mounted and does not have to be fitted inside an electrical enclosure.
Common functionality and features
Soft starters vary widely in functionality and choosing the correct product depends mainly on the performance and
features required. Serviceability and product support are also important factors. In fewer cases, the main
consideration is product cost.
The most common features of industrial soft starters can be grouped into functional categories:
SCR control
A soft starter can control one, two or three phases:
Single phase controllers - These devices reduce torque shock at start but do not reduce start current. Also
known as torque controllers, these devices must be used in conjunction with a direct on-line starter.
Two phase controllers - These devices eliminate torque transients and reduce motor start current. The
uncontrolled phase has slightly higher current than the two controlled phases during motor starting. They
are suitable for all but severe loads.
Three phase controllers - These devices control all three phases, providing the optimum in soft start
control. Three phase control should be used for severe starting situations.
Common control methods include:
Adaptive control
Open loop, voltage ramp control
Closed loop, current control
Soft stopping
Special control formats
AuCom has developed a special control format known as Adaptive Control.
Adaptive Control is a new intelligent motor control technique that controls current to the motor in order to start
or stop the motor within a specified time and using a selected profile.
For soft starting, selecting an adaptive profile that matches the inherent profile of the application can help smooth
out acceleration across the full start time. Selecting a dramatically different profile can somewhat neutralise the
inherent profile.
For soft stopping, adaptive control can be useful in extending the stopping time of low inertia loads.
Page 26 Medium Voltage Application Guide 710-12280-00A