Slip-Ring Motors - Danfoss VLT MCD 500 Pocket Manual

Slip-Ring Motors: Are soft starters suitable for use
with slip-ring motors?
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 = open circuit rotor voltage
R
I = full load rotor current
R
38 | Danfoss Drives · DKDD.PB.551.A1.02