OPERATION
9.3
Soft Start Methods
Soft starters offer a variety of methods to control motor starting. Each soft start method uses a different primary control parameter.
Soft Start Method
Parameter Controlled
Performance Parameters Influenced
Timed Voltage Ramp
Voltage
Start current, start torque, acceleration
Constant Current
Current
Start torque, acceleration
Torque Control
Torque
Start current, acceleration
Adaptive Control
Acceleration
Start current, start torque
Best results are obtained by selecting the soft start method that directly controls the parameter of most importance for the
application. Typically soft starters are used to limit motor start current or control load acceleration and/or deceleration. The EMX3
can be set to either Constant Current or Adaptive Control.
To Control
Use
Motor Start Current
Constant Current
Motor/Load Acceleration
Adaptive Control
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.
Cu
rr
ent
(%
m
ot
or
ful
l l
oa
d c
ur
re
nt
)
700%
600%
500%
300%
100%
400%
200%
10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
1
3
2
1:
Initial Current
(parameter 2C)
2:
Current Limit
(parameter 2D)
3: Full voltage current
Rotor speed (% full speed)
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
(parameter 2C) to a level that will start the motor with a light load, and the current limit (parameter 2D) 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.
Cu
rr
ent
(%
m
ot
or
ful
l l
oa
d c
ur
re
nt
)
700%
600%
500%
300%
100%
400%
200%
1
4
3
2
1:
Initial Current
(parameter 2C)
2:
Start Ramp Time
(parameter 2B)
3:
Current Limit
(parameter 2D)
4: Full voltage current
Time
32
| User Manual (710-04840-00M)
