When describing the output from an inverter, people refer to sine, quasi-sine and square waveforms. It is misleading to claim that any one type is better than another; it is all a question of compromise of cost, reliability and efficiency. Sine wave output is closest to replicating the AC mains supply, and at one time would have been costly and the least efficient. However, developments in power conversion technology mean that this is no longer the case & sine wave is now the choice for professional applications.

Quasi-sine units are now restricted to very low cost, low end applications and certain very high inrush applications, such as refrigeration. The descriptions below are more for historic interest, but remain true.

Square wave inverters, once common, were in their day the cheapest & probably the most efficient, but were also the most likely to cause operational problems due to their waveform. The quasi-sine wave is an intermediate approach which can replicate the key characteristics of the AC mains supply if properly controlled. Given proper control, and not all quasi-sine wave inverters do, the vast majority of loads can be successfully operated. At one time, quasi-sine inverters were the best cost-performance compromise, with true sine-wave restricted to waveform critical applications due to their high cost.

To understand how quasi-sine is designed to be a substitute for the mains, it is helpful to understand what the key characteristics of the mains are. For this discussion we shall refer to European system which is 50Hz and 230VAC, but the same principles apply to other voltages and frequencies.

The mains has four important characteristics:

Peak voltage: this is the peak voltage at the crest of a true sine wave. It reaches 325 volts on a 230 volt supply. This peak voltage needs to be maintained in order to successfully operate some electronic equipment, particularly microwave cookers.

Average voltage: this is maximum value reached by the integral against time of the voltage waveform, or in other words, the average voltage. A 230 volt sine wave supply has an average voltage 207volts. This characteristic is important for the successful operation of magnetic devices such as transformers and motors.

RMS voltage: (230V) this is a mathematically derived measure of the heating effect of a waveform when applied to a resistive load, and has been traditionally used to specify the magnitude of an AC voltage. This characteristic, which for a 230 volt mains supply is 230 volts, is important to ensure that heating and tungsten lighting equipment works to specification. If this is inaccurate or fluctuates then lighting may vary in intensity.

Stable frequency: (50Hz) this is necessary to ensure that timing devices using mains frequency operate accurately and that AC motors run at the correct speed. The following notes give an overview of how the three types of inverter mimics the mains.

The quasi-sine waveform cannot precisely replicate all these key voltage characteristics at the same time, so a compromise has to be arrived at. The compromise employed is one of the factors that differentiates different quasi-sine inverters.

Sine wave inverters set out to mimic the mains most closely. At one time. there were three common types; constant voltage transformer (CVT), pulse width modulated switch mode units (PWM) and so called linear units. The CVT's are the traditional units using a large transformer in which an oscillation is established. Simple and fairly rugged, they tended to be inefficient, large and heavy, and sometimes noisy too. The waveform can become quite severely distorted on some types of load. In contrast, the modern state of the art approach uses switchmode technology to reconstruct a sine wave; sometimes called PWM units. Relatively small and light, these are complex units, and used to be expensive, but have now become affordable. However, they can cause electrical interference and may have difficulty driving loads with a very high inrush current.

Quasi-sine inverters are simple, efficient and robust. Although they do not give a true sine wave, they can mimic the key voltage parameters fairly well, allowing the majority of equipment to run successfully. Not all quasi sine wave units are the same; many problems with quasi sine inverters stem from poor design, & not from the lack of a pure sinewave. Older type quasi-sine units can be designed to drive very high inrush loads; an area where they are still used today.

Square wave inverters are very simple but are really only suited to resistive loads such as heaters and incandescent lights. Due to their limitations, they are now quite rare.

Rotary inverters; for completeness it is worth being aware of this type of inverter. It uses a DC motor to drive an AC alternator, which gives a sine wave output. Although rugged and generally reliable, they suffer the disadvantage of needing brush maintenance (all static inverters are maintenance free), they are relatively inefficient and can be noisy.

Antares used to offer quasi-sine units, but with a difference. The control arrangement in our units maintained the key output voltage parameters over a wide range of operating conditions. Unlike many competitor units, there were no limitations on capacitive loads, such as presented by electronic equipment or appliances incorporating power factor correction or motor start capacitors.

However, Antares now offer exclusively sine wave units as they are now reliable & cost effective. Our quasi-sine units are no longer in general production, & are only available to special order when demanded by the application.