Digistart D3: User Guide
49
Issue: G
7.2.1 Constant Current
Constant current is the traditional form of soft starting, which raises the current from zero to a specified level and
keeps the current stable at that level until the motor has accelerated.
Constant current starting is ideal for applications where the start current must be kept below a particular level.
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1
3
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1:
Initial Current
(Pr 2C)
2:
Current Limit
(Pr 2B)
3: Full voltage current
Rotor speed (% full speed)
7.2.2 Current Ramp
Current ramp soft starting raises the current from a specified starting level (1) to a maximum limit (3), over an
extended period of time (2).
Current ramp starting can be useful for applications where:
•
the load can vary between starts (for example a conveyor which may start loaded or unloaded). Set the
initial current (Pr 2C) to a level that will start the motor with a light load, and the current limit (Pr 2B)
to a
level that will start the motor with a heavy load.
•
the load breaks away easily, but starting time needs to be extended (for example a centrifugal pump where
pipeline pressure needs to build up slowly).
•
the electricity supply is limited (for example a generator set), and a slower application of load will allow
greater time for the supply to respond.
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cur
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700%
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1
4
3
2
1:
Initial Current
(Pr 2C)
2:
Start Ramp Time
(Pr 2D)
3:
Current Limit
(Pr 2B)
4: Full voltage current
Time
7.2.3 Adaptive Control for Starting
In an adaptive control soft start, the Digistart D3 adjusts the current in order to start the motor within a specified time
and using a selected acceleration profile.
CAUTION
Adaptive Control cannot start the motor faster than a direct on-line (DOL) start. If the start ramp time (Pr
2D) is shorter than the motor's DOL start time, starting current may reach DOL levels.
Every application has a particular starting profile, based on characteristics of the load and the motor. Adaptive Control
offers three different starting profiles, to suit the requirements of different applications. Selecting a profile that
matches the inherent profile of the application can help smooth out acceleration across the full start time. Selecting a
dramatically different Adaptive Control profile can somewhat neutralise the inherent profile.
The Digistart D3 monitors the motor's performance during each start, to improve control for future soft starts.
