Machine Design

Power Switching

Power-control modules are circuits comprising devices such as silicon-controlled rectifiers, triacs, power transistors, and power rectifier diodes. These circuits can function as pulse-width modulators or switches to provide variable power to specified loads.

Power modulators electronically control the ratio of on-time to total device operating time (duty cycle or firing angle). They are more controllable, more compact, have longer life, and are more efficient than older methods. Ac power-control modules essentially consist of trigger circuits which control load current, usually with a triac or SCR. Two switching methods are often used to control ac power: phase control and zero-voltage switching.

Phase-control switching: In phase control, continuously variable power is obtained by controlling the conduction period of a thyristor supplying the load. SCRs can be used singly for half-wave power control, or in combination for full-wave control. For partial range control of loads that are insensitive to nonsymmetrical waveforms, simple circuits for SCR/diode combinations can be used. However, commercially available modules are designed to produce symmetrical, half-wave, or full-wave voltages. For full-wave operation, the circuits are usually designed for use with a triac.

When a triac is triggered into conduction in a phase-controlled circuit, voltage is applied instantaneously to the load, the value of which depends upon the firing angle. The sudden surge of load current generates RFI in the circuit which is both propagated through space and reflected back into the power line. The closer the trigger point is to the peak of the sine wave, the greater the level of RFI. To minimize this condition, phase-controlled modules can be supplied with RFI filter networks consisting of resistors, capacitors, and inductances. The level of power determines the bulk and cost of the filter network required.

Zero-voltage switching: Zero-voltage switching is a form of on-off control that delivers power to a load in a series of full-cycle bursts. Thyristor turn-on and turn-off occur at, or very near the zero-load current point of the ac cycle. Thus, filtering is not normally required for the small amount of RFI generated. However, zero-crossing trigger circuits are more complicated than comparable phase-controlled circuits.

Features include differential input, internal power supply for external-component biasing, and protection circuits for opened or shorted sensors. Others are available with extended input compatibility for sensor impedances, internal protection against half cycling, bridge sensing with adjustable-hysteresis set points, ac or dc operation, and proportional control for increased accuracy.

Proportional control is used when simple on-off application of power to the load would result in unwanted overshoots (from set point) of the controlled variable. For example, in simple on-off control of a space heater, full power is applied to the heating element until the ambient temperature reaches set-point temperature. However, the system inertia usually causes additional ambient-temperature increase until the heater element cools to the temperature of the environment.

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