SAF OPAL Pro MS6 - Manual (Page 10)

MS6 REDUCED VOLTAGE STARTER
Page 9
1.2.3 MOTOR AND LOAD TYPES
Squirrel Cage Motors
Squirrel cage motors, with a single winding and of any NEMA design may be connected to the
Opal Pro starter.
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 Pro 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 realized 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 Pro 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.
Other Types of AC Motors
Opal Pro starters may also be used to start 2-speed motors with multiple windings or
connections. Proper contactor selection and sequencing is required for the specific motor.
Resistive and Inductive Loads
The Opal Pro MS6 is well suited for use as a phase controller or current regulator on resistive
or inductive loads such as heaters. When applied with a Temperature PI controller, the Opal
Pro MS6 will maintain smooth control to avoid the overheating associated with on/off type
controllers. Improved temperature accuracy and longer heater element life can be expected
over standard control methods.
1.2.4 STARTING AC MOTORS IN PARALLEL
Opal Pro MS6 starters can be used to start motors in parallel but they must have individual
overload devices added.
When the paralleled motors are also connected to the same load, mechanically or through the
material, then load sharing must be considered.
When the motors are mechanically tied, the load sharing is determined by how closely the
motor characteristics are to each other. It is important to keep the motors exactly the same.
Even changing the manufacturers of the motors can cause some variations in load sharing. The
Opal Pro size is determined by the total of the motor current ratings.