A device that converts dc to ac is known as an inverter. Nowadays, most engineers think of solid-state inverters in the context of applications such as electric vehicles and solar panels. Yet even today, not all inverters are solid state.
One early form of power inverter was based on mechanical switching and was appropriately called a vibrator. It is basically a relay whose coil gets power through a set of normally closed contacts on the relay itself. When the relay energizes, the contacts open and coil power immediately is interrupted. This break in the connection allows the normally closed contacts to reclose and start the sequence again. The process happens rapidly enough to generate an audible buzz.
Typically, the rapidly pulsing contact applies the rising and falling dc voltage to a transformer which can step it up to a higher voltages, produced a 50- or 60-cycle square wave. Besides generating ac, vibrators were also used to generate a dc higher than the input by filtering the transformer output.
Mechanical vibrators were typically used in radio equipment to power vacuum tubes. They are no longer around, but a second non-solid-state conversion method is still with us. Rotary converters resemble an electric motor minus the output shaft. Internally, on a single armature, dc motor windings, connected to the input, and ac generator windings, connected to the output, provided a good sine wave. AC equipment could run on a dc supply as long as the load did not exceed the rating of the converter so that RPM remained constant.
Entities such as the N.Y.C. subway system employed rotary converters for many years. These devices had their own difficulties including high initial cost and intermittent loads. Interestingly, it is still possible to find new designs employing rotary conversion. One from researchers in Malaysia at University Tenaga Nasional is aimed at power conversion on solar and wind energy hybrid systems. The Malaysian design has a first stage made up of direct current motor with a conventional stator and rotor. The stator contains wave or lapped windings divided into main and “improving” windings. The number of main winding poles set the motor speed while the improving windings are devoted to provide a good starting and running performance.
The inner dc stator of the rotary converter first stage consists of permanent-magnet poles on the inside part of the middle section of the rotary converter. The rotor of the first stage consists of permanent-magnet poles placed on the inside part of the
middle section of the rotary converter. When a dc voltage is applied to the inner dc stator of the first stage, the resulting magnetic field will interact with the permanent magnet poles on the inside part of the middle section causing the entire middle section to rotate. The second stage of the rotary converter is exactly the same as any conventional synchronous generator. The rotor of the second stage contains excitation field windings placed on the outer part of the middle section of the rotary converter. The armature may be comprised of multiple phases made up of a block of laminations mounted in a cast-iron or die-cast aluminum alloy frame.
The excitation field windings of the second stage of the rotary converter are acted on by the movement generated from the first stage. This rotation causes the excitation field to induce voltages on a multiphase outer stator of the second stage to generate ac multi-phase voltages at the output terminals.
That brings us to the most prevalent non-solid-state version of an inverter, the rotary phase converter. A rotary phase converter rotates to transform single-phase 220-V utility power into three-phase electricity. The point of the device is to take single-phase ac and convert it to three-phase ac so you can operate three-phase equipment such as motors on CNC machines or air compressors.
You might think that rotary phase inverters would just consist of three-phase generators powered by single-phase motors. But this approach is inefficient, heavy, and costly. The problem is a motor/generator converts the entire input power flow into mechanical energy and then back into electrical energy. In contrast, rotary phase inverters instead route some of the electrical energy directly from input to output. This technique lets the rotary converter be much smaller and lighter than an equivalent motor-generator set.
A rotary converter is basically a specially designed three-phase induction motor equipped with a run capacitor and balance capacitors. The converter induction motor is generally called an idler. Two of the input terminals (the idler inputs) get power from the single-phase line. The rotating flux in the motor produces a voltage on the third terminal.
The voltage induced in the third terminal is phase-shifted from the voltage between the first two terminals. Specifically, two of the motor windings act as a motor, and the third winding acts as a generator. Because two of the outputs are the same as the single-phase input, their phase relationship is 180°. This leaves the synthesized phase to be ±90° from the input terminals.
One problem is that a three-phase motor powered by a single phase won’t start by itself. So rotary converters need a starting capacitor in series with one of their inputs to shift the waveform sufficiently for the converter to begin turning. Another point to note is that because the third, synthesized phase is driven differently from the other two, it may respond differently to load changes. Most typically, this third phase sags more under load. Thus rotary phase converters typically incorporate running capacitors on their outputs to ensure phase voltages are all equal under maximum load.