Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 98)

Ratings of the starter (start-up mode see Tab. 3.8-2 )
The stator contactor K1M (feeding contactor) is selected, corresponding to the rated operational
current I
e
of the motor under utilization category AC-2. A distinction is made in rotor contactors
between step contactors (K3M, K4M) and the final stage contactor (K2M). The rotor contactors
only have to connect and conduct the current briefly. Their poles are usually delta-connected.
The final stage contactor (K2M) must be designed for continuous duty AC-1; the loading is
0.58 · I
e rotor
. The step contactors (K3M, K4M) operate in starter circuits in short-time duty mode.
They can therefore be rated for this short-term loading or according to their making capacity.
Rating for starting with half-load full-load heavy duty
I
av rotor/
I
e rotor
0.7 1.4 2.0
Stator contactor K1M I
e AC-2
I
e (stator)
I
e (stator)
I
e (stator)
Rotor contactors (poles connected in delta)
Final stage contactor K2M I
e AC-1
0.58 · I
e rotor
0.58 · I
e rotor
0.58 · I
e rotor
2 steps K3M 0.20 · I
e rotor
0.35 · I
e rotor
0.50 · I
e rotor
3 steps K3M, K4M 0.18 · I
e rotor
0.30 · I
e rotor
0.43 · I
e rotor
Step contactors
4 steps K3M … K5M 0.15 · I
e rotor
0.25 · I
e rotor
0.35 · I
e rotor
Overload relay F1 I
e (stator)
I
e (stator)
I
e (stator)
Max. starting time per step 15 s 12 s 12 s
Max. frequency of operation (starts per hour) 120/h 30/h 12/h
I
e (stator)
Rated operational current of the motor (stator)
I
e rotor
Rated operational current of the rotor
I
av rotor
Mean rotor current during starting
Tab. 3.8-2
Factors for the rated operational currents of the motor for contactor selection according to the AC-2
catalog values
Permissible rated voltage for the rotor contactors
As the rotor contactors are only under voltage briefly during starting, in accordance with
IEC 60947-4-1, 5.3.1.1.2
the rated operational voltage of the rotor U
er
(rotor standstill voltage)
may exceed the rated isolation voltage U
i
of the contactor by 100 %. Contactors for 690 V may
therefore be used up to a rotor standstill voltage of 1380 V.
3.9 Electronic soft starters
Soft starters serve for a continuous adjustment of the starting characteristic of three-phase
asynchronous motors to the requirements of the load by controlling the voltage across the motor
and enable for an optimum integration of the drives in process control by means of various
complementary functions.
While when star-delta starters are used, the starting torque and starting current can be fix
reduced to around a third, with electronic soft starters the reduction can be set within a wide
range. It should be noted that the motor torque of a soft starter falls with the square of the
voltage and current reduction. With the same starting current as with a star-delta starter in star
connection (= 1/3 I
A Δ
), with a soft starter the motor torque falls to 1/9 T
A Δ
in comparison to 1/3
T
A Δ
for star-delta. See also Section 1.7.1.3 .
With the conventional starting procedures such as direct on line starters, starting transformers
or star-delta starters, the motor, supply and the entire drive chain is loaded by switching
transients. Each switching procedure also means a rapid current change (transient current
peaks) and hence generates high torque peaks in the motor. Electronic equipment with power
LVSAM-WP001A-EN-P - April 2009 3-22