Starting Current Asymmetry - Siemens SIRIUS 3RW30 Manual

Product description
3.2 Functional principle of the SIRIUS 3RW30 and 3RW40 soft starters
3.2.2

Starting current asymmetry

With two-phase control the starting current is asymmetrical for physical reasons, because
the current in the uncontrolled phase is the sum of the currents in the two controlled phases.
This asymmetry can be as much as 30 to 40% during starting (current ratio: maximum
deviation from mean value to mean value).
Even though this cannot be influenced, it is not critical in most applications. It could cause an
insufficiently rated fuse to trip in the uncontrolled phase, for instance. For recommended fuse
ratings, refer to the tables in chapter Technical data (Page 127).
Figure 3-6 Starting current asymmetry
Note
If wye-delta starters are exchanged for soft starters in an existing system, you should check
the fuse ratings in the feeder in order to avoid false tripping. This is particularly important in
connection with heavy-duty starting or if the fuse that is installed has already been operated
close to the thermal tripping limit with the wye-delta assembly.
All elements of the main circuit (such as fuses, motor starter protectors, and switching
devices) must be dimensioned for direct starting and according to the on-site short-circuit
conditions, and ordered separately.
For recommended fuse and motor starter protector ratings for the feeder with soft starter,
refer to chapter Technical data (Page 127).
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SIRIUS 3RW30 / 3RW40
Manual, 10/2018, NEB535199502000/RS-AB/005