If you already have a pretty good idea about the available methods for converting ac power to dc power, an inverter covers the other direction: changing dc into ac. Normally, when we speak of an inverter, we mean a device that generates line frequency power from a dc source, to power conventional ac-powered loads such as a personal computer, or industrial controls and instrumentation. A UPS integrates an inverter and battery charger in a single system, providing backup ac power for critical loads. Many modern UPS also include the battery and charging controls in the enclosure.

An inverter is different from an alternator, or ac generator, in that it’s static (no moving parts), being based entirely on semiconductor circuitry. In a traditional inverter, a transistor (or SCR) circuit switches the current (from a dc source) in the primary winding of a linear transformer to generate an ac voltage in the secondary. The transformer is designed to provide the required ac voltage; it also provides the necessary safety isolation from the battery or other dc source. Using this method, the transformer operates at line frequency, which means that it’s large and heavy, albeit reliable.

Inverters using line frequency transformers may have square wave or sine wave outputs. Square wave inverters are lower in cost, but are suitable only for simple linear loads, such as incandescent lighting. The square waveform has high harmonic distortion and may cause an unacceptable increase in temperature in transformer-operated equipment, such as another power supply or fluorescent lighting. They are also not very good at handling low power factor loads.

Line frequency inverters with sine wave outputs usually use ferroresonant transformers or brute force filtering and are generally more expensive than square wave inverters. An inverter described as “quasi sine wave” (or similar weasel-wording) is a square wave inverter with a limited pulse width, usually 120 to 126 degrees . This output is a little easier to filter, but a little harder on the inverter transistors.

Pulse width modulation (PWM) is a technique that can be used to drive the primary of a standard transformer with a “chopped” square wave that results in a pretty good synthesized sine wave. This is the preferred technology in most high-power inverters and UPS. The advantage is that the output can be easily filtered to produce a good low-distortion sine wave. The disadvantage is higher complexity in the inverter circuit. Despite the complexity, the cost is a little lower than a ferro inverter. PWM inverters have been used in economical line-interactive UPS and in “on-line” or double-conversion UPS. Modern inverter circuits also can use multiple power conversion stages, similar in operation to the SMPS, to generate a low-distortion ac output. The difference is that the last power conversion stage switches the isolated dc power to generate a line frequency ac, usually as a synthesized low-distortion sine wave.