Technical information

8 – THEORY OF OPERATION
Acceleration Control
118
8.3 Acceleration Control
8.3.1 Current Ramp Settings, Ramps and Times
General
The current ramp sets how the motor accelerates. The current ramp is a linear increase in current
from the initial setting to the maximum setting. The ramp time sets the speed of this linear current
increase. The following figure shows the relationships of these different ramp settings.
Figure 17 – Current Ramp
Current
Time
Kick Time Ramp Time
Up To Speed Timer
Motor FLA
Initial
Current
Kick
Current
Max
Current
Start command
Initial Current The initial current should be set to the level that allows the motor to begin rotating within a couple of
seconds of receiving a start command.
To adjust the initial current setting, give the starter a run command. Observe the motor to see how
long it takes before it begins rotating and then stop the unit. For every second that the motor doesn’t
rotate, increase the initial current by 20%. Typical loads require an initial current in the range of 50%
to 175%.
Maximum Current For most applications, the maximum current can be left at 600%. This ensures that enough current is
applied to the motor to accelerate it to full speed.
The maximum current can also be set to a lower current limit. This is usually done to limit the
voltage drop on the power system or to limit the torque the motor produces to help prevent damage to
the driven load.
NOTE: The motor may achieve full speed at any time during the current ramp. This means that the
maximum current setting may not be reached. Therefore, the maximum current setting is the most
current that could ever reach the motor, and not necessarily the maximum current that reaches the
motor.
NOTE: When setting a current limit, the motor must be monitored to ensure that the current is high
enough to allow the motor to reach full speed under worst case load conditions.
Ramp Time The ramp time is the time it takes for the current to go from the initial current to the maximum
current. To make the motor accelerate faster, decrease the ramp time. To make the motor accelerate
slower, increase the ramp time.