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Integral cycle control of load power

Pulse-width modulation and phase control are not the only way control of power can be achieved via solid-state switching devices. Thyristors can efficiently control load power by the integral-cycle technique, also known as burst modulation. In this scheme, the thyristor is switched to its fully on state (ISO-degree angle of conduc­tion), then turned off again for a number of integral cycles of the applied ac line voltage. Either, or both on and off periods can be manually adjusted or automatically varied by a sensing and feedback circuit. Heaters, because of their large thermal inertia, are particularly suitable for this control technique. Motors, too, can be satisfactorily controlled with a little extra care in design parameters. In such systems, the motor is not subjected to nonsinusoidal waveforms of the applied voltage. This reduces both electrical and mechanical losses and results in a quieter-running motor with less stress in its bearings.

Integral-cycle control requires triggering at zero line voltage and the process is, appropriately enough, often referred to as zero-voltage switching. It is possible to use discrete components in circuits for triggering thyristors at zero line voltage, but it is generally more convenient to use one of a number of available ICs designed for this purpose. The scheme, in any event, is best suited for use with triacs because of their full-wave operation. The salient features of integral-cycle control of load power are as follows:

1. There is less stress on the thyristor because of greatly reduced dv/dt, and for some loads, di/dt as well.

2. There is significantly less RFI and EMI. Often the RFI filter can be omitted. Altogether, interference from shock excitation and from harmonic energy are virtually eliminated.

3. False triggering from commutation dv/dt is eliminated.

4. Snubber requirements are relaxed; often snubber circuits can be omitted.

5. Certain loads operate under less abusive conditions.

6. Temperature effects on triggering are reduced, facilitating design and operation.

7. Measurements can be made with ordinary instruments.

8. Unity power-factor operation with resistive load is possible.

The more nearly resistive the load, the easier is the implementation of this switching technique. Motors often require extra considerations, but they, too, becomes essentially resistive loads when loaded.

It is only natural that a circuit technique possessing such features as integral-cycle control, or zero-voltage switching, must have some drawbacks, as well. Indeed, one disadvantage of this power-control method is its behavior with inductive loads. The worst time to introduce ac power to an inductive load is during the zero-cross interval of the applied voltage. Such timing produces the greatest surge current. The transient so produced in essentially a dc component and a time-decaying asymmetry of the ac current. Motors usually manifest themselves as equivalent L/R circuits; it turns out that control via zero-voltage switching can be practical if the L/R time constant is not too large. This implies that a partially loaded motor can cause less trouble with until current surges than an unloaded one. (This reasoning is more valid for small than for large motors because starting current for large motors can be very high even without zero-voltage switching.)

Another disadvantage of integral-cycle control is that benefits otherwise associated with the frequency of the chopped carrier are not forthcoming, but are limited by the average rate of interruption of the wave trains. Thus, a regulated power supply providing power made up of interrupted 20 kHz wave trains would be much harder to filter than conventional 20 kHz supplies; if the average interruption rate were in the vicinity of 1 kHz, the electrical and physical parameters of filter components would be dictated by 1 kHz ripple. Also, such a supply would show only the dynamic response of a 1 kHz switching rate.

A somewhat controversial topic concerns the matter of representing a stable feedback network for regulation purposes. The on and off nature of integral-cycle control can indeed introduce difficulties not encountered in systems operating at fairly steady levels. However, much depends upon the technical and experimental skills of the designer. The easiest integral-cycle systems to handle are probably those associated with heaters; these systems involve resistive loads, low frequencies, and low interruption rates. Furthermore, they are generally not high-precision systems, requiring only moderate feedback percentages. (4200)

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Polyphase versus single-phase power

Polyphase power systems have a number of advantages over more simple single-phase systems. Indeed, it is not always true that simplicity is necessarily associated with the single-phase approach. The advantages are well known because of the long exploitation of the three-phase format in utility and industrial power systems. Because of shortcomings in power-handling capability, reliability, and cost, solid-state power techniques were slow to make their appearance in polyphase systems. That era is over. Bipolar power transistors, power MOSFETs, and thyristors have evolved to the point where respectable power levels can be implemented. At the same time, logic devices and techniques make waveform synthesis feasible with surprisingly few 1C modules. All in all, polyphase power systems can be said to possess the following advantages over the single-phase format:

- Three-phase induction motors are superior to single-phase ac motors in nearly all respects.

- Two-phase motors are excellent for use in servo systems. They are cheaper and more maintenance free than are dc motors. And, unlike single-phase induction motors, they are self-starting.

- For airborne and space applications, three-phase transformers provide weight reduction over single-phase units working at the same power level.

- Rectified polyphase voltage is easier to filter than the rectified voltage from a single-phase source.

- A three-phase inverter or power supply maintains power balance in a three-phase source. By contrast, single-phase inverters or power supplies contrib­ute to the imbalance of power in a three-phase power source.

- Three-phase transformer windings can be connected so that much of the third-harmonic energy in a square or quasi-square wave is attenuated. Such electrical or electronic filtering reduces the need to rely upon bulky physical filters. This is of considerable importance in the operation of motors; harmonics can cause temperature rise and rough operation. The cancellation of the third harmonic also removes higher-order odd harmonics that often cause electrical interference with other systems.

- The manner of connecting the windings of three-phase transformers also provides the designer and user with additional flexibility in the selection of output voltage and available current.

- Logic circuitry for generating the polyphase format can be readily manipulated to provide various performance features. These include dead-time intervals, stepped shapes to simulate sine waves, changing phase sequence for reversing induction motors, harmonic manipulation, frequency variation for speed control of induction and synchronous motors, and output-voltage control. Such logic circuitry also facilitates feedback applications for maintaining constant speed, torque, or horsepower. (2520)