MIL-HDBK-1003/3
APPENDIX D (Continued)
a) Define the operating profile of the load to which
the VFD is to be applied. Include any or all of the torques
listed in par. D-1.07. Using a recording true rms ammeter to
record the motor's current draw under all operating conditions
will help in doing this. Obtain the highest "peak" current
readings under the worst conditions. Also, see if the motor has
been working in an overloaded condition by checking the motor
full-load amperes (FLA). An overloaded motor operating at
reduced speeds may not survive the increased temperatures as a
result of the reduced cooling effects of the motor at these lower
speeds.
b) Determine why the load operation needs to be
changed. Very often VFD's have been applied to applications
where all that was required was a "soft start" reduced voltage
controller. The need for the VFD should be based on the ability
to change the load's speed as required. In those applications
where only one speed change is required, a VFD may not be
necessary or practical.
c) Size the VFD to the motor based on the maximum
current requirements under peak torque demands. Do not size the
VFD based on horsepower ratings. Many applications have failed
because of this. Remember, the maximum demands placed on the
motor by the load must also be met by the VFD.
d) Evaluate the possibility of required oversizing of
the VFD. Be aware that motor performance (breakaway torque, for
example) is based upon the capability of the VFD used and the
amount of current it can produce. Depending on the type of load
and duty cycle expected, oversizing of the VFD may be required.
D-1.09
Key VFD Specification Parameters. The most important
information to be included in a VFD specification are continuous
current rating, overload current rating, and line voltage of
operation.
a) Continuous run current rating. This is the maximum
rms current the VFD can safely handle under all operating
conditions at a fixed ambient temperature (usually 40 degrees C).
Motor full load sine wave currents must be equal to or less than
this rating.
b) Overload current rating. This is an inverse
time/current rating that is the maximum current the VFD can
produce for a given time frame. Typical ratings are 110 percent
to 150 percent overcurrent for 1 minute, depending on the
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