MIL-HDBK-1003/3
APPENDIX D (Continued)
high currents as being well beyond its capabilities and would go
into a protective trip or fail altogether. A simple solution for
this condition is to interlock the VFD run permissive circuit
with the service disconnects via an auxiliary contact at the
service disconnect. When the disconnect is closed, a permissive
run signal restarts the VFD at low voltage and frequency.
g) Power factor correction capacitors being switched
or existing on the intended motor loads. Switching of power
factor capacitors usually generates power disturbances in the
distribution system. Many VFD's can and will be affected by
this. Isolation transformers or line reactors may be required
for these applications.
Power factor correction at VFD-powered motor loads
is not necessary as the VFD itself does this by using DC
internally and then inverting it into an AC output to the motor.
VFD manufacturers warn against installing capacitors at the VFD
output.
D-1.07
Sizing VFD's for the Load. To properly size a VFD for
an application, you must understand the requirements of the load.
The torque ratings are as important as the horsepower ratings.
Every load has distinct torque requirements that vary with the
load's operation; these torques must be supplied by the motor via
the VFD. You must have a clear understanding of these torques.
a) Breakaway torque: torque required to start a load
in motion (typically greater than the torque required to maintain
motion).
b) Accelerating torque: torque required to bring the
load to operating speed within a given time.
c) Running torque:
torque required to keep the load
moving at all speeds.
d) Peak torque: occasional peak torque required by
the load, such as a load being dropped on a conveyor.
e) Holding torque: torque required by the motor when
operating as a brake, such as down hill loads and high inertia
machines.
D-1.08
Guidelines for Matching VFD to Motor. The following
guidelines will help ensure a correct match of VFD and motor:
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