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1st Ed, Rev B 07/31/2019
SR44 Series Soft Starter User Manual
Chapter 3: Applications
3.1.4 – 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].
3.1.5 – 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 SR44 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.
3.1.6 – 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.
3.1.7 – 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.)
3.1.8 – 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.
3.1.9 – 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.
3.1.10 – 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. (Refer to the Mechanical Installation
procedures, sections 1.6 to 1.8 for more detailed information.)
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