| 摘要 |
A variable speed 3-phase a-c motor is connected to a 3-phase source of alternating voltage via five controllable bidirectional solid state switches which are so arranged and controlled that motor speed can be reduced by skipping selected cycles of the source voltage. For example, for half speed operation three of the switches are arranged respectively to connect phases A, B and C of the source to phases A, C and B of the motor, and over two consecutive periods of the source voltage three separate, uniformly spaced "firing windows" are selected during which the conducting states of these switches are initiated in a predetermined pattern that results in the phase A-to-phase B source voltage being applied to the motor during the first window, the phase C-to-phase A source voltage being applied during the next window, and the phase B-to-phase C source voltage being applied during the last window.
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