ABB PSTX30 Installation And Commissioning Manual page 69

7.1 Voltage ramp
When using the Voltage ramp, the voltage increases
linearly from Initial start level to full voltage during start,
and decreases linearly from the Step down level to Stop
end voltage level during stop,
Since the torque depends on both the voltage and the
current, the torque will not always follow the voltage
curve. This has the effect that the the torque will not
increase or decrease linearly.
7.1.1 Voltage start ramp
When the softstarter is given a start signal, the softstarter
quickly increases the voltage to the Initial voltage level to
set Start ramp initial level. The softstarter then controls
the output voltage in a start ramp.
When the output voltage reaches Top of ramp, the
softstarter closes the by-pass, see Figure 78.
To reach Top of ramp the following conditions are needed:
• Pre-set Start ramp time have passed, i.e. 100% output
voltage.
• The current is below 1.2 x nominal motor current.
The Start ramp time is the time it takes to go from Initial
voltage level to full voltage. The actual time to reach the
Top of ramp state also depends on the current, which
means that the time to reach Top of ramp can be longer
than the set Voltage ramp time.
If the motor is to start with a very heavy load, the start
ramp time can be longer than usual.
Exampel: If the Start ramp time is set to 2 seconds and the
motor is supposed to start a heavy load. This can cause the
output current to not go below set 120% of nominal motor
current when the pre-set start ramp time is reached.
Voltage start ramp is configured with the following
parameters:
Parameter
02.01 Start mode
02.03 Start ramp Initial level
02.04 Start ramp time
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see Figure 77.
Description
Set the Start mode to Voltage ramp.
Set the voltage level from where the start ramp initiates
Set the effective time it takes for the voltage to reach
100%.
77
U
Start ramp
Figure 77: Start and stop ramp
78
U
Nominal voltage 100 %
Start ramp initial level
Start ramp time
Figure 78: Voltage start ramp
Setting range
Voltage ramp,
Torque ramp,
Full voltage ramp
10 ... 99%
1 ... 120s
Stop ramp Time
Top of ramp
Time
Default value
Voltage ramp
30%
10s
7