SAF OPAL SS6 - Manual (Page 10)

OPAL SS6 Reduced Voltage Starters
Page 3
1.2.1.2 Current Ramp Mode
Applications
* Conveyors
* Extruders
* Mixers
* Stamping
Machines
* Cranes
* Shredders
*Hammer Mills
1.2.2 CHOOSING STARTING MODE
With any induction motor the design is such that starting currents and torques are very high.
Across the line starting for a basic NEMA B design motor creates an in-rush current of
600% or more of the nameplate rating. Often these levels are so high that they cause
problems with mechanical or auxiliary electrical. An example of this is the high starting
torques which cause belt slip and high currents, creating large voltage dips. The basis of
choosing the correct starting mode is determining the problem to be solved.
Torque Reduction on heavy loads is accomplished by using the Current Ramp Mode.
The idea is to reduce starting current to just above the level which will produce enough
torque to get the motor to move. This is commonly referred to as "break-away torque". As
the current ramps up from this point, the torque also increases as time passes. If at the
initiation of start, the motor torque is less than the load, the motor will remain stalled until
the torque ramps to a sufficient level for acceleration.
Soft Starting for light loads or reducing starting current are accomplished by using the
Constant Current Mode.
On some applications the starting time is still too fast in the current ramp mode because the
load is too light. In this application it is best to select the constant current mode which will
allow the acceleration torque to be better controlled, actually kept constant.
If the available current is limited and causes significant voltage drops, the constant current
mode will allow flexibility in setting the current draw. Remember that the current level must
be great enough to provide acceleration torque and this level is determined by the load. For
all other applications, or if in doubt, select the Current Ramp Mode.