Danfoss VLT, Pocket Manual, Page: 38 / 64

38 VLT ® Soft Starter Pocket Guide
Soft starters are suitable for use with slip-ring motors provided that the
motor can still deliver the torque required to accelerate the load. Soft
starters are not suitable if the load requires extremely high start torque,
or if the slip-ring motor is intended to provide speed control. When
considering a soft starter for slip-ring applications, a trial should be
conducted to verify the performance.
To develop starting torque, some resistance must remain in the rotor
circuit during motor starting. This resistance must be bridged out using
a contactor (AC2 rated for rotor current) once the motor is running
close to full speed.
Rotor resistance (R) can be sized using the following formula:
Where V
R
= open circuit rotor voltage
I
R
= full load rotor current
Slip-Ring Motors: Are soft starters suitable for use
with slip-ring motors?