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2.4.1 Soft Start with Coast to Rest
Figure 7 shows the relationship of voltage and speed with
respect to time when a soft start is used with coast to rest. The
controller potentiometers have been set as follows.
U The initial voltage is set at approximately 30%.
T1 The start time setting is greater than 0.
T2 The stop time is set at 0 which allows the motor to coast
to a stop.
On the voltage/time graph, the voltage starts at U when the run
coil is energized and increases to 100% within the T1 time set-
ting. The voltage immediately drops to zero when the run coil is
de-energized.
The "speed/time" graph shows the motor accelerating from 0,
when the run coil is energized, to operating speed. The time
required to accelerate may be more or less than the T1 setting
depending on the connected inertia. The motor speed coasts to
zero when the motor is de-energized.
Figure 7 - Voltage and Time Curves for Soft Start
with Coast to Stop
Siemens Energy & Automation, Inc.
2.4.2 Soft Start with Soft Stop
Figure 8, like figure 7 , shows the voltage and speed curves for a
soft start but with controlled deceleration. The potentiometers
have been set as follows.
U The initial voltage is set at approximately 30%.
T1 The start time setting is greater than 0.
T2 The stop time setting is greater than 0 which allows the
motor to soft stop.
The motor starting ramp is similar to the one shown in figure 7 .
But, when the run coil is de-energized, a motor stopping ramp
is formed where the motor voltage starts at 100% and decreas-
es to 80% of U (the initial start voltage) within the T2 time set-
ting. Then the voltage immediately drops to zero. The time
required to decelerate may be more or less than the T1 setting
depending on the connected inertia.
When the run coil is de-energized, the speed decreases
throughout the T2 time period and then coasts to zero.
Figure 8 - Voltage and Time Curves for Soft Start with Soft Stop
9
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