Electric motor control applications: Pulse-width modulation in a package

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.One of the important methods of controlling the performance of motors is by inverters that are pulse-width modulated. Pulse-width modulation can be brought about by circuitry made up of discrete devices, with appropriate use of general-purpose ICs, or with a dedicated IC designed expressly for the function. The latter approach is probably the best and affords a neat and economical way to achieve a vital circuit function with a single IC module. Motorola, Silicon General, Texas Instruments, Presley, and National Semiconductor are among those marketing similar IC modules for achieving pulse-width modulation. No matter how large the motor being con trolled, such an IC pulse-width modulator can be used as the logic for the system. Some of the features and advantages of using these specialized ICs are as follows:

• Worthwhile savings in cost, production time, and troubleshooting. Ease of duplication.

• Maximum duty cycle is about 45 percent, therefore eliminating the possibility of destructive simultaneous conduction in push-pull or bridge inverters.

• Dual-output driver stages enable the use of either push-pull or single-ended inverters.

• Current-limit amplifier provides external component protection.

• On-chip thermal-limit protection against excessive junction temperature and output current.

• External RC network enables frequency adjustment generally to well over 100 kHz.

• Five-volt, 50-mA linear regulator output is available for auxiliary purposes.

• Symmetry control by an injected dc voltage (on some models).

• Oscillator synchronization by injected frequency (on some models).

• Dead-time adjust (on some models).

• Inhibit terminal (electronic shutdown).

If for no other reason, you can expect superior performance from these pulse-width modulator ICs because of the elimination of numerous connecting leads which necessarily attend the use of discrete devices, separate operational amplifiers, and gates. Of ten well-founded fears of unstable and erratic circuit operation need no longer be entertained. With the advent of these single-chip modules, pulse-width modulation is nearly as easily realized as straight amplification. Moreover, in applications where wide- range speed control of induction motors is required, it’s feasible to simultaneously obtain both frequency and amplitude control of the voltage applied to the motor (induction motors require higher applied voltage as the frequency is increased).

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