AC-AC converters
With the firing angle at 90°, a phase controller delivers half the power to a heater, but its current carries 54 % third harmonic and the power factor drops to 0.71 even though the load is a resistor.
01AC voltage controllers
An AC voltage controller regulates the RMS voltage across a load without changing the frequency. It is built with two thyristors in antiparallel or, at low power, with a triac. There are two ways to drive it:
| Method | How it works | Use |
|---|---|---|
| Phase control | In each half-cycle it is triggered with a delay α; the load receives only the end of each half-wave | Light dimmers, universal motors, soft starters |
| Integral-cycle control | n complete cycles out of every N are let through, switching at the zero crossing | Heaters and furnaces with high thermal inertia |
With a resistive load and phase control, the RMS voltage across the load is:
A 1 kW heater at 230 V and 50 Hz with phase control. Top, the voltage across the load over one cycle, with the same shape as the current; bottom, its odd harmonics as a percentage of the fundamental.
With an inductive load, the current keeps flowing after the voltage zero crossing; if α is smaller than the load angle, control is lost. The soft starters for induction motors raise α gradually to limit the starting current and are then bypassed with a contactor.
02Changing the frequency
The cycloconverter builds a low-frequency voltage from segments of the line, using two thyristor bridges in antiparallel per phase; it was used for large low-speed drives, such as mills, with output frequencies of up to one third of the line frequency. The matrix converter connects each output phase to any input phase with bidirectional switches, with no intermediate storage elements. But the solution that dominates industry is indirect: a rectifier that charges a DC bus and an inverter that generates AC voltage at the desired frequency, the subject of the following pages.
03In the laboratory
With a triac, a diac and an RC network, build a dimmer for an incandescent lamp and measure the RMS voltage with a true-RMS multimeter and with an average-responding one.
With a power quality analyzer, measure the current spectrum of a dimmer at several angles and compare it with the lab.
Control a heater with a solid-state relay with zero-crossing detection and observe the current.
04Common mistakes
- Measuring the chopped voltage with an average-responding multimeter. Only a true-RMS one gives the correct result.
- Believing that a resistive load always has a power factor of 1. Not with phase control.
- Controlling a lamp with integral cycles. It visibly flickers.
- Triggering an inductive load with short pulses. The current does not reach the latching current and the thyristor does not stay on.
05Self-assessment
What fraction of the power does a resistive load receive with α = 90°?
Half: \( V_{ef}^2/V^2 = 1 - 1/2 = 0.5 \).
What is its power factor then?
\( \sqrt{0.5} \approx 0.71 \).
Why does integral-cycle control not generate harmonics on the line?
Because it switches at the zero crossings and lets complete sinusoidal cycles through; instead, it generates components of a frequency lower than the line frequency.
How is the frequency of an industrial motor changed today?
With a rectifier, a DC bus and an inverter.
06Further reading
- Muhammad H. Rashid. Power Electronics: Devices, Circuits, and Applications. 4th ed., Pearson. (Spanish edition: “Electrónica de potencia”.) AC voltage controllers and cycloconverters.
- Ned Mohan, Tore M. Undeland and William P. Robbins. Power Electronics: Converters, Applications, and Design. 3rd ed., Wiley, 2003. AC-AC converters within the whole subject.
- Bimal K. Bose. Modern Power Electronics and AC Drives. Prentice Hall, 2002. Cycloconverters and matrix converters in drives.