Technical information

8 – THEORY OF OPERATION
Acceleration Control
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TruTorque acceleration control can be very useful for a variety of applications. However it is best
used to start centrifugal pumps, fans, and other variable torque applications. TruTorque generally
should not be used in applications where the starting load varies greatly during the start such as with
a reciprocating compressor, where the starting load is very low, or where the starting load varies
greatly from one start to another. TruTorque control is not recommended for the starting of AC
synchronous motors.
Initial Torque This parameter sets the initial torque level that the motor produces at the beginning of the starting
ramp profile. A typical value is 10% to 20%. If the motor starts too quickly or the initial torque 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 the value is set too low a “No Current at Run” fault
may occur.
Maximum Torque This parameter sets the final or maximum torque level that the motor produces at the end of the
acceleration ramp time. For a loaded motor, the maximum torque value initially should be set to
100% or greater. If the maximum torque 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 TruTorque acceleration parameter values or another
ramp profile, the Maximum Torque 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 TruTorque percent (TT%) meter on the display. Record the value displayed. The
Maximum Torque level should then be set to the recorded full load value of TT% plus an additional
10%. Restart the motor with this value to verify correct operation.
Note: When setting the Maximum Torque value, the motor must be monitored to ensure that the
torque level is high enough to allow the motor to reach full speed under worst case load conditions.
Note: Depending on loading, the motor many achieve full speed at any time during the TruTorque
ramp. This means that the Maximum Torque level many never be achieved. Therefore, the
maximum torque level is the maximum TruTorque level that is permitted. However the motor torque
may not necessarily reach this value during all starts.
Ramp Time When in TruTorque acceleration mode, the ramp time setting is the time it takes for the torque to go
from the initial torque setting to the maximum torque setting. To make the motor accelerate faster,
decrease the ramp time. To make the motor accelerate slower, increase the ramp time.
If the ramp time expires before the motor reaches full speed, the starter maintains the Maximum
Torque level until either the motor reaches full speed, UTS timer expires, or the motor thermal
overload protection trips.
Note: Setting the ramp time to a specific value does not necessarily mean that the motor takes that
exact amount of time to accelerate to full speed. The motor and load may achieve full speed before
the ramp time expires if the load does not require the set ramp time or set torque level to reach full
speed. Alternately, the motor and load may take longer than the set ramp time to achieve full speed
depending on the parameter settings and load level.