Page D–4 Stellar
®
SR55 Series Soft Starter User Manual – 1st Ed, Rev F – 09/18/2019
Appendix D: Sizing an SR55 Soft Starter
sTANdARd oVeRloAd cuRReNT pRofIle ANd duTy cycle
The SR55 has been designed for a specific Overload Current Profile and Duty Cycle as shown in
the previous SR55 Index Ratings section of this chapter.
The Overload Current Profile is expressed by two symbols, X and Tx.
X denotes the overload current as a multiple of I
e
and represents the maximum value of
operating current due to starting, operating, or maneuvering under overload conditions.
•
For example, X = 3.5 means that the maximum overload start current allowed is 3.5 times FLC.
Tx denotes the duration of the controlled overload currents during starting, stopping,
operating, or maneuvering.
•
For example, Tx = 17 means that the maximum allowed overload current is permitted for up
to 17 seconds only.
The Duty Cycle is expressed by two symbols, F and S which describe the duty and also set the
time that must be allowed for cooling.
F is the ratio of the on-load period to the total period expressed as a percentage.
•
For example, F= 90 means that the soft starter is ON for 90% of the time and then OFF for
10% of the time between each start.
•
If there are not multiple starts per hour, then the Duty Cycle is continuous.
S is the number of starts or operating cycles per hour.
•
For example, S = 5 means that the soft starter is capable of 5 equally spaced starts per hour.
These characteristics are summarized below in Figure 1.
Figure 1: Standard Overload Current Profiles and Duty Cycles
Model
Rated
Current
(A)
Class 10 O/L
Multiple
(X)
Class 10 O/L
Time
(Tx)
Starts / Hour
(S)
Duty
(F)
SR55-017
017
3.5 17
5
90%
SR55-021
021
SR55-027
027
SR55-034
034
SR55-040
040
SR55-052
052
SR55-065
065
SR55-077
077
SR55-096
096
SR55-124
124
SR55-156
156
SR55-180
180
SR55-242
242
3
SR55-302
302
SR55-361
361
SR55-414
414
SR55-477
477
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