Soft Start Methods; Constant Current; Constant Current With Current Ramp - Danfoss VLT MCD 600 Operating Manual

VLT® Soft Starter MCD 600
Operating Guide
Application
Pump - submersible
Saw - bandsaw
Saw - circular
Shredder

9.8 Soft Start Methods

9.8.1 Constant Current

Constant current is the traditional form of soft starting, which raises the current from 0 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.
%
3
700
600
500
400
300
1
200
100
10 20 30
Rotor speed (% full speed)
Illustration 29: Example of Constant Current
1
Initial current (set in parameter 2-3 Initial Current)
2
Current limit (set in parameter 2-4 Current Limit)

9.8.2 Constant Current with Current Ramp

Current ramp soft starting raises the current from a specified starting level (1) to a maximum limit (3) over an extended period (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 parameter 2-3 Initial Current
to a level that will start the motor with a light load. Then, set parameter 2-4 Current Limit to a level that will start the motor with a
heavy load.
• The load breaks away easily, but starting time has to be extended (for example a centrifugal pump where pipeline pressure has
to build up slowly).
• The electricity supply is limited (for example a generator set), and a slower application of load allows greater time for the supply
to respond.
Danfoss A/S © 2020.12
Start mode Start
ramp
time [s]
Adaptive control (ear- 5
ly acceleration)
Constant current 1
Constant current 1
Constant current 1
2
40 50 60 70 80 90
Initial cur- Current
rent limit
(%FLC) (%FLC)
n/a 500
200 450
200 350
200 450
%
100
3 Full voltage current
Operation
Stop mode Stop time
[s]
Adaptive control 5
(late deceleration)
Coast to stop n/a
Coast to stop n/a
Coast to stop n/a
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