Technical information

8 – THEORY OF OPERATION
Acceleration Control
121
8.3.4 Power Control Acceleration Settings and Times
General
Power control is a closed loop power based acceleration control. The primary purpose of Power
controlled acceleration is to control and limit the power (kW) drawn from the power system and to
reduce the power surge that may occur as an AC induction motor comes up to speed. This power
surge can be a problem in applications that are operated on generators or other limited or “soft”
power systems. Power control also reduces the torque surge that can also occur as an AC induction
motor comes up to speed.
Figure 19 – Power Ramp
Motor Input
Power
Time
Kick Time Ramp Time
Up To Speed Timer
Motor Running
Power Level
Initial Power
Optional Kick
Current
Max Power
Start command
Power control acceleration can be very useful for a variety of applications. Power control generally
should not be used in applications where the starting load varies greatly during the start such as with
a reciprocating compressor. Power control is not recommended for starting of AC synchronous
motors.
Initial Power This parameter sets the initial power level that the motor draws at the beginning of the starting ramp
profile. A typical value is usually 10% to 30%. If the motor starts too quickly or the initial power
level is too high, reduce this parameter. If the motor does not start rotating within a few seconds after
a start is commanded, increase this parameter. If this value is set too low a “No Current at Run” fault
may occur.
Maximum Power This parameter sets the final or maximum power level that the motor produces at the end of the
acceleration ramp. For a loaded motor, the maximum power level initially should be set to 100% or
greater. If the maximum power level value is set too low, the motor may not produce enough torque
to reach full speed and may stall. On lightly loaded motors, this parameter may be reduced below
100% to produce smoother starts.
If the motor can be started by using the default Power acceleration parameter values or the Current
control ramp, the Maximum Power level can be determined more precisely so that the motor comes
up to speed in approximately the preset ramp time. In this case, while the motor is running fully
loaded, display the Power percent (KW%) meter on the display. Record the value displayed. The
Maximum Power level should then be set to the recorded full load value of KW% plus an additional
5% to 10%. Restart the motor with this value to verify correct operation.
Note: When setting the Maximum Power level, the motor must be monitored to ensure that the
starting power is high enough to allow the motor to reach full speed under worst case load conditions.