Abstract: An SCR firing circuit allows the firing point to be changed. The firing circuit includes a ROM which has an output for each gate of each SCR. The ROM is programmed to fire the SCRs in a desired sequence. The ROM is driven by a counter, which counts pulses from an oscillator, the frequency of which is variable. The difference between the zero cross of the waveform and the ROM output is compared and averaged. The comparator output is summed with a variable DC demand voltage. The sum is applied to an integrator, which provides an integrated output to the oscillator to change the frequency of the oscillator if the firing point is changed.

Abstract: A variable speed drive for an AC motor has features to reduce the speed of the motor if excess temperature is encountered in the variable speed drive system. The system has a rectifier that converts AC supply voltage into DC voltage on the positive and negative rails. Frequency switches switch the DC voltage to power conductors in a sequence and at a rate to determine the selected frequency. The frequency switches are controlled by a DC demand voltage. A temperature sensor provides an electrical response corresponding to the temperature in the system. The temperature response is compared to a reference voltage, and if it exceeds the reference voltage, the comparator provides an output that draws down the demand voltage by a value corresponding to the difference. Lowering the demand voltage lowers the speed of the motor to allow the system to cool under reduced load conditions.

Abstract: A speed control for an AC motor varies the frequency and amplitude. The speed control includes a rectifier which converts the AC power supplied to negative and positive DC. A switch connects each power conductor leading to the motor with one of the DC voltages. The switches are switched on and off to provide alternately positive and negative voltage. Controls for the switches include an oscillator which provides pulses of frequency that can be varied. A binary counter counts the pulses from the oscillator and provides binary outputs. A memory unit provides a programmed output to the switches for each binary output received. In one embodiment, the amplitude is varied by comparing a triangular wave from the oscillator with the DC input level. Enabling pulses equal to the excess of the triangular wave over the DC input enable the memory unit. In another embodiment, the enabling pulses are provided to an amplitude switch which is cycled to vary the DC rail voltage.

Abstract: A variable speed drive for an AC motor has features to improve the power factor. The system uses a rectifier for converting AC supply voltage into DC voltage on a positive rail and a negative rail. Frequency switches are controlled to alternately connect the rails to the power conductors at a variable rate to define a selected voltage frequency. An amplitude switch is located on one of the rails and controlled to vary the potential between the rails in proportion to the frequency. A current sensor senses the current waveform in one of the conductors and applies it to a phase detector which detects the difference in phase between the current and the voltage. Phase difference pulses are produced which are averaged into an average DC value. The DC value is applied to a demand voltage that controls the amplitude switch to reduce the amplitude to improve power factor.