Compatibility with power supplies

The electrical waveform provided by the mains power supply

The term « electrical waveform » refers to the shape of the wave supplied by a power source, such as the mains or a UPS. Electricity suppliers distribute a current with a sinus waveform, in other words, in the form of a sine wave.

The Sine Wave

This type of waveform is mandatory for supplying devices comprising motors or transformers: lamps, ATMs, radiators, laser printers, etc. Fifteen years ago, UPSs delivered an exclusively sinus output signal, as early computers contained numerous motors. In order to re-create a sine wave from a direct current signal, complex and costly technology is required. All MGE UPS SYSTEMS On-line UPSs implement this technology to produce a sinus output waveform.

The Square Wave

With the appearance of PCs and what is known as switch-mode power supplies, the earlier need almost disappeared. The power supplies on PCs and their peripherals use only electrical components which can cope with a lower-quality electrical waveform. This in turn led to the appearance, about ten years ago, of UPSs delivering a squarewave, rather than a sine wave. As this approach was easier to implement, significant savings were made possible.

However, the square wave has its share of hazards for computer systems: the vertical front of the signal tends to « stress » components in the power supply, which shortens the latters working life. In addition, the pulse width tends to increase towards the end of battery backup time, delivering too much power for the load. This may also damage components. These two reasons explain why Merlin Gerin does not use this technology, even on entry-level products.

The square wave may be adequate if the PCs are not supplied by it for too long. This may be the case for Off-line UPSs, which only convert the power in the event of an outage, or in other words for a few seconds or minutes, from time to time. On the other hand, this type of waveform should not under any circumstances be used on On-line UPSs which convert power continuously or on UPSs with long battery backup times (several hours).

Pulse Width Modulation (PWM)

On its Pulsar ES range entry-level products, Merlin Gerin proposes an intermediate electrical waveform, using pulse width modulation.
This signal combines the advantages of the other two waveforms:

battery runtime is extended by at least 15% versus sine wave due to better inverter efficiency,

it does not stress power supplies, on account of its oblique front,

it delivers continuous power, during the whole of the battery backup time,

it is much less expensive than a real sinus waveform.

For the above reasons, this waveform is often referred to as quasi-sine wave, which is not improper. The only restrictions on its use are that it is reserved for computer loads and not recommended for long battery backup times (in excess of two hours).

Output Waveform and Technology