Technical information

8 – THEORY OF OPERATION
Acceleration Control
123
8.3.5 Open Loop Voltage Ramps and Times
General
The open loop voltage ramp provides soft starting of a motor by increasing the voltage applied to
motor from the Initial Voltage setting to full (100%) line voltage. The ramp time sets the speed at
which the voltage is increased. Because this is an open loop control profile, the motor current during
starting tends to be reduced; however, the current is not limited to any particular level. This starting
mode it is not commonly used except in special circumstances. In most applications, the use of one
of the other closed loop starting profiles is recommended.
Figure 20 – Voltage Ramp
Voltage
Time
Kick Time Ramp Time
Initial Voltage
Optional Kick
Current
Full Voltage
Start
command
Initial Voltage This parameter sets the initial voltage level that is applied to the motor. To adjust the starting voltage
level, give the starter a run command and observe the motor operation. If the motor starts too quickly
reduce the initial voltage level. If the motor does not start rotating immediately or starts too slowly
then increase the initial voltage level until the motor just starts to rotate when a start command is
given. If the initial voltage level is set too low, a Fault 39 – No Current at Run may occur. In this
case increase the initial voltage level to permit more current to initially flow to the motor.
Ramp Time The ramp time setting is the time that it takes for the applied voltage to go from the initial voltage
level to the full voltage (100%) level. To make the motor accelerate faster, decrease the ramp time.
To make the motor accelerate slower, increase the ramp time.
Note: Setting the ramp time to a specific value does not necessarily mean that the motor takes this
time to accelerate to full speed. The motor and load may achieve full speed before the ramp time
expires if the application does not require the set ramp time and/or voltage to reach full speed.
Alternatively, the motor and load may take longer than the set ramp time to achieve full speed.