Electricity: motive power systems – Induction motor systems – Primary circuit control
Patent
1990-08-07
1992-01-14
Shoop, Jr., William M.
Electricity: motive power systems
Induction motor systems
Primary circuit control
388811, 3889072, 318599, H02P 500
Patent
active
050814095
ABSTRACT:
A pulse-width modulated (PWM) circuit applies a voltage across a load in accordance with a PWM command signal. The command signal is converted into first and second trains of pulses, by electronically comparing the command signal with a triangular dither signal and also with the dither signal whcih has been shifted by half its period. These two pulse trains, and two more pulse trains obtained by taking the complements of the original pulse trains, are applied to a pair of bridge circuits. The load is connected to both bridge circuits. Both bridge circuits include electronic switching devices actuated by the pulses. The bridge circuits are configured such that when a pulse is present, from either the first or the second pulse train, no net voltage appears across the load. At all other times, substantially the entire power supply voltage is applied across the load. In another embodiment, one or more additional pairs of bridge circuits are connected across the load. The pulse trains controlling the switches in the bridge circuits are generated using dither signals which are shifted in time. In general, there can be n pairs of bridge circuits, the pulse trains of one pair being shifted from the next pair by t/2n, where t is the period of the dither signal. The greater the number of pairs of bridge circuits, the less the effective ripple current in the load. The invention is especially suitable for use in controlling the gradient coils in a magnetic resonance imaging system.
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Reprint: Motion, Jul./Aug. 1987, "Basic PWM Servo-Amplifier Theory".
Eilberg William H.
Martin David
Performance Controls, Inc.
Shoop Jr. William M.
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