Siemens SIRIUS 3RT Series System Manual page 765

Generalized phase control for soft starters
With SIRIUS soft starters, the rms value of the motor voltage is increased (from a settable
starting voltage) to the rated motor voltage within a definable start time by means of the
phase control of two anti-parallel thyristor pairs.
The motor current changes in proportion to the voltage applied to the motor. As a result, the
starting current is reduced by the factor of this voltage.
There is a quadratic relationship between the torque and the voltage applied to the motor. As
a result, the starting torque is reduced quadratically in relation to this voltage.
Heavy-duty starting
Heavy starting exists if a motor requires more than 10 to 15 seconds from being switched on
to reaching its rated speed on account of its special load conditions. When heavy starting
exists, the load torque of the machine to be driven is greater during startup than in rated
operation. It takes a long time for the rated speed to be reached because large centrifugal
masses need to be accelerated (e.g. on rolling mills, centrifuges). Special overload relays
(heavy-duty starting relays, solid-state overload relays) or thermistor motor protection
devices must be used to protect heavy-starting motors.
Heavy-duty starting for soft starters
Assuming that certain conditions and constraints apply, the SIRIUS soft starter size must be
at least one performance class higher than the motor rating for heavy-duty starting
(CLASS 20 startup). The tables in the relevant product manual provide sample set values
and device dimensions.
Holding power
The power input of the magnet coils of a contactor, which depends on the continuously
consumed current, and which is required to hold the magnet system in the ON state.
Infeed system for 3RA6
The infeed system for 3RA6 enables several compact starters to be fed using an infeed
system designed to modular principles with retained wiring.
Instantaneous short-circuit release
Release for a motor starter protector/circuit breaker, which provides short-circuit protection
for the downstream load or the cable. The instantaneous short-circuit release must disable
all poles of the motor starter protector/circuit breaker (acc. to UL) instantaneously or with a
short-time delay in the event of a short circuit.
SIRIUS Innovations
System Manual, 01/2011, A8E56203870002-03
Glossary
765