MIL-HDBK-1003/3
APPENDIX D (Continued)
at a frequency from less than 1 Hz to about 120 Hz. This means
that the motor may run extremely slow or very fast, depending on
the frequency supplied; and the amplitude of the voltage supplied
must be proportional to the frequency to ensure the proper
volts/hertz ratio for the specific motor. The output torque for
a motor is determined by the ratio of voltage applied to the
motor at a given frequency. Failure to maintain the proper
volts-per-hertz ratio will affect motor torque, temperature,
speed, noise, and current draw. Thus, for a motor to produce its
rated torque at variable speeds, it is also necessary to control
the voltage as well as the frequency supplied to the motor. For
example, a 460 volt motor operating at 60 Hz will have a
volts/hertz ratio of 7.67 to 1. For a VFD to operate this motor
at 30 Hz (half speed), the motor voltage must be reduced to 230
volts to maintain the same torque characteristics for the motor.
Motors are designed with specific torque characteristics and are
classified accordingly. While two motors may have the same
horsepower rating, their actual torque capability in the areas of
breakaway torque, pull-up torque, peak torque, and full-load
torque may be different, depending on their NEMA classifications.
Typical designs are Type A, B, C, and D. Refer to National
Electrical Manufacturers Association (NEMA) MG-1, Motors and
Generators, for a detailed explanation of these NEMA designs and
of motor torque capabilities. Although the motor is sized by
horsepower, the required torque at all speeds is the key to
successful, efficient operation of the VFD and its motor.
D-1.04
Types of VFD's. Although VFD's control the speed of an
AC induction motor by varying the motor's supplied voltage and
frequency of power, they do not all use the same designs in doing
so. The major VFD designs commonly used at the time of
publication of this handbook are:
a)
Pulse Width Modulated (PWM)
b)
Current Source Inverter (CSI)
c)
Voltage Source Inverter (VSI)
d)
Flux Vector Drive
An understanding of these different designs, along with their
advantages and disadvantages will be beneficial to correctly
match the VFD with the motor in a specific application.
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