AC Power Source - Output Voltage Variation

This is about what you'd expect if your device contains just a variable ratio autotransformer (Variac). Without knowing more about what's inside, can't say much more about it.

Hi,

If I understand your post correctly you are saying that the output of your UPS is 300V when it has an A.C. input but that it only has 288V when supplied by the battery?

If so then the probability is that your meter is not true RMS reading and that the inverted waveform is not sinusoidal.

A typical inverter technique is to produce a width modulated pulse output with a peak of +/- 300V when the batteries are at full charge (2.4V/cell typ assuming lead acid) with the width of the pulse being regulated to give 230Vrms. As the battery voltage falls to 2V/cell the peak voltage will drop by 20% also and the width of the pulses is increased to maintain the 230Vrms. The limit is reached when the pulse width leaves no dwell time at 0V thus producing a square wave. At this point the inverter will usually shut down to protect the batteries from over-discharge.

This system works well for I.T. applications as most of the loads are SMPS units anyway. As the first stage of the SMPS rectifies the input it doesn't usually care about the supply waveform.

Check with a TRMS meter and/ or an oscillascope. See sketch below.

Hope this helps

Chas

Hi chas,

Thanx for your information.

Could you give me the RMS factor by how much DC voltage varies with respect to the AC voltage.

This will be useful for me to do calculation ...

Hari

Hi Hari,

The basis of this is that the peak voltage of the pulse = Peak voltage of a 230Vrms sine (325V^) and the width is calculated from that. The minimum voltage will be 230V (equivalent to typically 20V for a 24V battery) which will be a square wave, at which point the pulse width is at max.

I believe that you will find that at the max peak you will get a nearer to true value than when the waveform is square.

The actual peak values depend on the manufacturer of the equipment and their transformer design. The variation from nominal of +/-20% approx will also change if the batteries are of a chemistry other than lead acid.

Kind regards

Chas