Allen-Bradley SMC-50, User Manual

Page: 15 / 292

Rockwell Automation Publication 1560F-UM001A-EN-P - June 2019 15
Product Overview Chapter 1
Starting Modes
Soft Start
This method covers the most general applications. The motor is given an initial
torque setting, which is user adjustable. From the initial torque level, the output
voltage to the motor is steplessly increased (ramped) during the acceleration
ramp time, which is user-adjustable. A user-adjustable current limit value is also
available. This limits the current throughout the soft start.
Figure 1 - Soft Start Timing Diagram
Starting Mode Page
Soft Start 15
Sensorless Linear Speed Acceleration 16
Torque Control Start 17
Current Limit Start 17
Selectable Kickstart 17
Pump Control Mode 18
Dual Ramp Start 20
Emergency Bypass Run (Full Voltage Start) 20
TIP For any of the following starting modes, the motor only accelerates in the
programmed time if that time and the current limit setting is appropriate for
the connected load. If the set time is too short, or the load is too great, the
motor will take longer to reach full speed.
TIP
A motor’s torque curve is not a linear function and depends on both applied
voltage and current. As such, if the soft starter ramped voltage applied to
the motor is sufficient for it to develop torque high enough to overcome the
inertia of the load, the motor could quickly accelerate to full speed in less
than the configured ramp time when using the Soft Start mode.
Current Limit
Time (seconds)
Ramp Time
Start Run
%
Vo
ltag
e
Initial
Torque
100%