SAF OPAL Pro MS6 - Manual (Page 9)

MS6 REDUCED VOLTAGE STARTER
Page 8
1.1.2 OPTIONAL FEATURES
• By-Pass Contactor
Once the motor has reached operational speed, the by-pass contactor is closed. This
shorts the SCR’s which eliminates the voltage drop across the SCR’s, in turn being more
efficient and ridding excess heat.
• Solid State Full Speed Reversing
Through the addition of 4 SCR’s and a reversing card, smooth motor reversing is
achieved without having to stop the motor.
• DC Injection Braking
Through the addition of SCR’s and a DC injection card, adjustable electric braking of the
AC motor is performed.
• Various communication options, which will allow connection to standard industrial
networks
• NEMA 4 or NEMA 12 enclosures
1.2 APPLICATION NOTES
1.2.1 OPERATION MODES
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.
1.2.2 CHOOSING STARTING MODE
Constant Current Mode
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.
Current Ramp Mode
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.