Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 80)

conveyors and large fans, the start times can extend to minutes. In the case of pumps and fans
it should be noted that the pumped material (liquid, air) contributes to the effective inertial mass.
The above given approximate values apply for direct starting. The times are correspondingly
extended with starter methods with reduced starting current and torque.
With respect to the permissible starting time of the respective motor, the manufacturer’s
documentation is definitive.
For the selection of contactors for heavy-duty starting, see Section 2.3.5.2 .
3.2.2 Reversing starters
In a reversing starter the motor is switched via two contactors, one for each direction. If the
motor is started from rest, the contactor is selected according to utilization category AC-3. Often
however the motor direction is changed while it is running (plugging), which means a corre-
spondingly higher loading of the contactors and hence requires selection according to utilization
category AC-4. Direct reversing requires a reversing delay between the contactors – for
example by means of a short-term delay – of around 40 ms, to prevent short-circuits between
phases. In addition to electrical interlocking of contactors of reversing starters, mechanical
interlocking is recommended.
Corresponding precautions as for reversing starters are required for plugging with stopping at
standstill. In this case when the motor comes to rest, the braking contactor (for example
controlled by a speed sensor) is switched off and the motor is hence disconnected from the
supply.
I> I> I>
Fig. 3.2-2
Reversing starter with motor protection-circuit breakers and mechanical interlock: Diagram and layout
3.3 Star-delta (Y-
Δ, wye-delta) starting
Star-delta (in North America the designation “wye-delta” is commonly used instead) starting is
the simplest method for reducing the starting current of a motor. The technique can be used
with all squirrel-cage induction motors that are delta-connected for normal operation and whose
windings ends are individually connected to terminals. The reduction of the motor current
causes a reduction of the starting torque. Star-delta starting is therefore especially suitable for
drives that are not loaded until after starting. The starting time is longer than with direct starting,
which is especially apparent when driving larger inertial masses.
A distinction should be made between
- normal star-delta starting
- star-delta starting with closed transition
- amplified star-delta starting.
LVSAM-WP001A-EN-P - April 2009 3-4