MIL-HDBK-1003/3
APPENDIX D
VARIABLE SPEED DRIVES (VFD)
D-1.00
Introduction
D-1.01
Scope and Criteria. This appendix is intended to
provide basic information on the principles of operation of
VFD's, description of different types of VFD's, guidance on the
proper application of VFD's, and installation guidelines to
ensure successful operation of VFD's. Over the last few years,
the VFD has become one of the most effective motor controllers
available for varying the speed of squirrel-cage induction
motors. VFD's save energy, reduce electrical consumption,
enhance equipment performance, are highly reliable, and have
become affordable. Much of the material presented in this
Appendix was developed by Mr. Solomon S. Turkel, Senior
Instructor and Course Author for Advanced Technologies Marketing
and Service (ATMS) Inc., Baltimore, MD.
D-1.02
Terminology. VFD's drives are sometimes called
adjustable frequency controllers (AFC's). It is incorrect to
call them inverters or even adjustable speed drives (ASD's). The
term "ASD" refers to many types of adjustable speed drives,
including belt or gear drives, eddy-current clutches, variable-
pitch sheave drives, and DC systems, as well as VFD's. The VFD
is the only type of motor drive that controls the speed of an AC
appropriately. The VFD microprocessor-based motor controller
incorporates an electronic control section, an electromagnetic
and semiconductor power section, and typical components used with
standard motor controllers. Currently, available sizes range
from 1/3 horsepower (hp) to thousands of hp.
D-1.03
Operation. The principles of operation of VFD's are
closely related to basic motor theory. During operation, the
stator's rotating magnetic field, which is created by the AC line
power to the motor, induces a voltage in the rotor. This induced
voltage causes a current to flow in the rotor, which creates
magnetic fields with north and south poles. The synchronous
speed of an AC induction motor is dependent upon the number of
poles in the stator and the frequency of the line power applied.
The basic equation is as follows:
Speed = (120 times frequency) divided by number
of poles
From this equation , a four-pole motor operating at 60
Hz will have synchronous speed of 1800 rpm. When a VFD supplies
power to an AC motor, it has the capability to provide a voltage
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