7.3.4 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.
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.
z 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.
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7 - THEORY OF OPERATION
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
Figure 31: Power Ramp
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