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 section usually consists of silicon controlled rectifiers. The inverter section for small systems
consists of power transistors, while larger systems use thyristors or silicon controlled rectifiers.
The output is usually formed from a six or twelve pulse generating bridge, with the pulse widths
being controlled by the microprocessor to simulate a sinusoidal wave shape. The frequency is
adjusted by the control system based on the changing process conditions. Use of microprocessor
based controls allows very stable frequencies to be produced, as well as allowing very small
changes to occur as required by the process. Suitable output inductors, capacitors and filters are
used to smooth the wave shape as necessary for the output motor device to utilize. Harmonic
filtering may be required on the input side, so that higher order harmonics are not transmitted to
the rest of the electrical system. Variable speed drive systems of the above types are now
available in sizes ranging from fractional horsepower to over 10,000 horsepower, and in speed
ranges from as low as a few rpm to over 6,000 rpm. Of course, the entire speed range is not
available in any one drive system, as usually drive systems, especially the motors are designed
for a smaller speed range, e.g., a large drive might be designed for from 3,600 to 5,400 rpm at
10,000 horsepower, or a small drive might be designed for from 30 to 1200 rpm at 2 horsepower.
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