SAF OPAL SS6 - Manual (Page 11)

OPAL SS6 Reduced Voltage Starters
Page 4
1.2.3 MOTOR AND LOAD TYPES
1.2.3.1 Squirrel Cage Motors
Squirrel cage motors, with a single winding and of any NEMA design may be connected to
the OPAL starter.
1.2.3.2 Wound Rotor Motors
Wound Rotor Induction Motors (WRIM) and solid state starters make an excellent
combination for frequent starting, jogging, plug-stopping and reversing applications. These
WRIM motors use multiple steps of resistors to provide various speed torque characteristics.
This allows a high starting torque characteristic while providing low starting current when
compared to NEMA B designs.
One disadvantage has been the high maintenance costs of the resistor and shorting contactor
bank. The use of an OPAL allows an improvement to this arrangement by allowing the
current to be limited without use of the multiple steps. Usually the load characteristic is such
that this starting may be accomplished with only one or two starting steps. By eliminating
several contactors a major savings is realised that often pays for the installation cost.
The common method is to add only one step of resistance at 20% (Rotor volts divided by
Rotor Amps times 20%). This resistance value will provide relatively high starting torque
and the OPAL will be used to limit the current. As little as 200% of motor full load current
can provide 100% starting torque unlike squirrel cage NEMA B motors which would require
500% current. The resistor bank may be shorted with a contactor at full speed to reduce the
slip and eliminate the watt losses in the resistors.