P-Line Design Guide
10 Schrack Technik GmbH, Seybelgasse 13, 1230 Wien www.schrack.com LAKA/LAKS Design Guide V1.0
2.8 Frequently Asked Questions
•What is the minimum size of motor that can be
controlled using an LAKS closed loop soft
starter?
Motors connected to an LAKS soft starter must
have a full load current rating ≥50% of the LAKS
nameplate rating. All the motor protections are
based on this setting.
It is possible to operate an LAKS with a small
kW motor, for testing purposes. In this case, the
motor will effectively start direct on-line, and the
LAKS will not protect the motor. The starter will
not trip, because there is no undercurrent
protection on the LAKS.
•What is the minimum size of motor that can be
controlled using an LAKA open loop soft starter?
There is no minimum motor size when using an
LAKA open loop soft starter.
•What type of motor protection does the LAKS
have?
The LAKS has built-in motor overload protection
of the electronic thermal model type. The motor
current is continuously monitored and the
expected temperature is calculated based on
this monitored current.
The rate of rise of the calculated motor
temperature is determined by the Motor Trip
Class setting. The lower this setting, the faster
the rate of rise of calculated motor temperature.
A Motor Overload trip (2 x Ready LED flashes)
will occur when the calculated temperature
reaches 105%. This protection is similar to a
motor trip class setting on a standard thermal
overload relay.
An external motor protection device is not
required when using an LAKS soft starter. LAKS
is certified to conform to the IEC 60947-4-2
standard for electronic soft starters. The
reliability of the motor protection feature is part of
this standard.
•How do I select a LAKA/LAKS soft starter for
duty cycles different from those listed in the
standard ratings table?
The WinStart software package is available for
selecting soft starters for different duty cycles.
•What are the LAKA/LAKS operational ratings
before maintenance may be required?
The operational ratings for LAKA/LAKS are size-
dependent, and are due to the capability of the
internal bypass contactor:
18 ~ 200 A: 100,000 operations
•When would I use a line contactor?
A line contactor may be compulsory for a
specific installation.
•How do I size the fuses of the motor branch
circuit when using a LAKA/LAKS soft starter?
For current limit settings ≤350% and start times
≤15 seconds, the nominal rating of standard line
supply fuses should be 1.75 x Motor FLC. If
motor rated fuses are being used, their nominal
rating should be 1.5 x Motor FLC.
For current limit settings > 350% and start times
> 15 seconds, the nominal rating of standard line
supply fuses should be 2 x Motor FLC. If motor
rated fuses are being used, their nominal rating
should be 1.75 x Motor FLC.
•When would I use semiconductor fuses?
Either when specified for an installation, or when
Type 2 coordination is required.
The LAKA/LAKS is internally bypassed, so the
SCRs are in use only during starting and soft
stopping.
•What is the current consumption of the
LAKA/LAKS control supply?
The steady state consumption of the control
supply is 100 mA maximum.
However, the short time inrush current at control
supply switch-on can be as high as 10 A.
•How can the LAKS programmable output relay
be used?
The programmable output relay provides a
normally open contact which can be used for a
"Trip" or "Run" output.
Trip output:
The relay operates when the LAKS trips on any
fault. This can be used to operate a shunt-trip
mechanism of an upstream circuit breaker to
isolate the motor branch circuit. It could also be
used to signal LAKS "Trip" status to an
automation system.
Run output:
The relay operates on completion of start ramp.
