Appendix B: Soft Starter Application Considerations
Page B–3Stellar
®
SR55 Series Soft Starter User Manual – 1st Ed, Rev F – 09/18/2019
b.1.4 – maximum moTor Cable lengTh
The length of the cable between the output terminals of the starter and the motor should not
normally be greater than 100 meters [328 ft].
b.1.5 – poWer faCTor CorreCTion CapaCiTors
Power factor correction capacitors applied to a single motor must ALWAYS be connected by a
separate contactor placed on the SUPPLY side of the SR55 Soft Starter. Capacitors should be
switched into the circuit after top-of-ramp (full line voltage) is reached, and switched out of
the circuit before a stop is initiated.
It is important that any total system PFC scheme that automatically corrects for a range of
inductive loads is not operated in such a way as to leave it heavily over compensated since this
might introduce oscillations leading to damaging over-voltages.
b.1.6 – lighTly loaded small moTors
Lightly loaded small-sized (less than 2kW [2.7 hp]), star connected motors can produce
high voltages at the motor terminals when shut down by simply opening the line contactor.
As these voltages can damage the soft starter, it is safer to control the opening of the line
contactor with the soft starter run relay contacts.
b.1.7 – moTors insTalled WiTh inTegral brakes
Motors that include an integral, electrically operated brake internally connected to the motor
input terminals can only be soft started when the brake is re-connected to the supply through
its own contactor. (Do NOT soft start the brake.)
b.1.8 – older moTors
The action of the fully-controlled soft starter introduces harmonic currents and voltages to the
motor. Therefore, it is important to ensure that the motor employs techniques such as rotor
skewing in its construction to suppress the effects of harmonic fluxes and avoid rough starting.
This is rarely a problem with modern motors, because nearly all motors designed in the last 20
years employ these techniques.
b.1.9 – Wound-roTor or slip-ring moTors
Slip-ring induction motors ALWAYS need some resistance in the rotor circuit to ensure that
sufficient rotational torque is generated to overcome any alignment torque, which is present
at start-up. The resistance can be safely shorted out in the normal fashion with a contactor
controlled by the programmable relay set as ‘top-of-ramp’ contacts.
b.1.10 – enClosures
Thyristors are not perfect conductors, and the passage of current through them causes heat
dissipation in the body of the soft starter, which in turn causes the heatsink temperature to
increase. As a guide, the heat generated is 1 watt/amp/phase, which equates to a dissipation
of 30 watts from the heatsink for a line current of 10 amps. Therefore, all cabinets or
enclosures that house soft starters should have adequate ventilation. (For more detailed
information, refer to Chapter 1: Mechanical Installation.)
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