Automation Direct Stellar SR55 Series, User Manual

Page: 174 / 202

Page B–2 Stellar
®
SR55 Series Soft Starter User Manual – 1st Ed, Rev F – 09/18/2019
Appendix B: Soft Starter Application Considerations
b.1 – m
oToR
s
uITAbIlITy ANd
A
ssocIATed
c
oNsIdeRATIoNs
The SR55 Soft Starter is based on a series of microprocessor-based optimizing soft starters
which have been used world-wide in more than 100,000 critical and non-critical systems.
Since 1983, these soft starters have successfully operated with almost every type of load and
environment from the Antarctic to the jungle. The design has proven to be both reliable and
adaptable, and provides a powerful mechanism with which to control fixed-speed induction
motors.
However, due to the intrinsic differences between electronic and electro-mechanical starting
systems, there are a number of simple rules and observations to follow when using the SR55
Soft Starter. This section introduces guidelines for the user and those incorporating the unit
as part of their system design.
b.1.1 – s
uiTabiliTy
In principle, any three-phase induction motor can be started by a soft starter. Normally, the
breakaway torque of the load should be less than the full-load torque of the motor, unless a
motor with a high locked-rotor torque characteristic is employed. As a quick assessment, any
load which has a low or no-load start with a moderate starting time, or which can be started
with a star-delta (wye-delta) starter, auto transformer or other forms of reduced-voltage
starting, can be considered a potential application for a soft starter.
b.1.2 – i
nduCTion
m
oTor
C
haraCTerisTiCs
Three-phase induction motors are required to provide sufficient torque to accelerate the
motor and its load from standstill to full speed, and to maintain full speed efficiently at
all torque levels up to the design full-load torque. Most modern three-phase induction
motors have characteristics that are wholly suitable for use with soft starters. However, the
characteristics vary considerably between different manufacturers and design types.
It is important that the motor is capable of providing sufficient torque to drive the load
at all speeds from standstill to rated speed, to enable the SR55 to function properly. It is
particularly important that the motor to be soft started does not have a low pull-up or saddle
torque, or the load may not be accelerated correctly.
The primary function of the soft starter is to act as a torque-regulating device. It cannot
apply a torque greater than that which the motor generates. For this reason, problematic
applications for which many different starting methods have been tried but failed may need
analysis of the motor or load performance before a soft starter can be successfully applied.
b.1.3 – r
aTing
For most applications, except high inertia loads, the starting demands and the inertia
of the rotating masses are small enough to be insignificant. This means that no special
consideration needs to be given to the rating of the soft starter, other than to ensure that it is
equal or marginally greater than the rated voltage and current of the controlled motor.
Alternatively, if the number of poles of the motor and the moments of inertia of the load
(J
load
) and motor rotor (J
motor
) are known, a soft starter will be suitable if the figures comply
with the criteria given in the bottom row of Table B.1.3.
Table B.1.3
Number of Poles 2 4 6 8
Synchronous Speed (rpm @ 60 Hz) 3600 1800 1200 900
(J
load
)/(J
motor
) less than 5 15 20 25
Section B.2.13 contains a table showing the more common applications.