Inverter and “true” sine wave

Reading time 3 minutes

Aktualisiert – Mai 3, 2024

Inverter (english. Inverter), especially the "cheaper" ones, are often advertised with a "real" sine wave. A real sine wave is particularly important for switching power supplies. A more or less rounded square wave causes problems for the components of switching power supplies and can even destroy them in the long run.

Ein Wechselrichter sollte in seiner Leistung bei 12 V DC etwa maximal 20 %, bei 24 V DC 10 % der Batteriekapazität ausmachen. Beispiel: 1.000 W Wechselrichter-Leistung * 0,2 = 200 Ah Batteriekapazität.

Sine – mains voltage

The mains voltage is usually 230 V at a frequency of 50 Hz. The mains frequency means that the voltage changes between its minimum and maximum around the zero line 50 times per second.

The following diagram shows the sine curve of the mains voltage.

It is noticeable that the unit of measurement is set to 200 V (bottom left in the picture). Accordingly, two boxes represent 400 V. The peak of the curve represents the Effective value the nominal voltage of 230 V and is visually just over one and a half boxes, corresponding to around 300 V, or exactly 325V.

Sinus – APC UPS Back Pro 1500

APC is a renowned manufacturer of uninterruptible power supplies (UPS, english. UPS). It is assumed that these generate a decent sine wave, especially since they are usually used to protect against power failures, especially in computers.
They have switching times of around 18 ms, which is enough to protect the computers from an uncontrolled shutdown. Both laptops and desktop computers work with switching power supplies.
Since they are mainly powered by mains power, the operating times with UPS bridging are of little importance, which is why an approximate sine wave is tolerable, even if – actually – not desired. So – unfortunately – no real sine wave is delivered.

Sine – Victron Multiplus II

The inverter Victron Multiplus II and its relatives are designed for the permanent power supply of consumers, including computers. Like the previously mentioned APC UPS, they have a short switching time, which is why they can also function as UPS.

Since, depending on the battery capacity, this may involve a truly permanent supply of switching power supplies, the focus here is primarily on an actual - real - sine wave.

Measurement

If you want to measure the sine wave of your inverter yourself and have an oscilloscope, you can check the advertising claims live.

! – Danger - !
Working with voltages greater than 50 volts can be life-threatening if general safety regulations are not observed.

method

calibration

Each measuring input must be adapted and calibrated to the probe used.

The oscilloscope provides a fixed frequency output for this purpose. After connecting the BNC plug from the probe to the desired measurement input, the probe tip is connected to the eyelet of the fixed frequency output and the crocodile clip to the ground connection.

There is a small hole on the side of the probe plug through which a plastic adjustment screwdriver, which is included with the probe tip, can be inserted and a clean square signal can be set by turning it slightly in one direction or the other.

Adjust the divider ratio on the probe

The oscilloscope probe is freed from its pluggable ground connection (usually provided with a crocodile clip at the end). Only the probe tip itself is required.

The probe must switch from 1x on 10x This represents a division ratio. Since 230 V has an effective voltage of 650 V in total (based on the minimum and maximum values), but oscilloscope signal inputs are usually only designed for a maximum of 400 Vp (volt peak), the division ratio is set to 10:1 by switching to 10x. Accordingly, the measuring input does not have 650 V, but 65 V, which is within the permissible voltage values.

Oscilloscope settings

The oscilloscope input voltage range is 200V , signal coupling on AC and the ratio to 10x set.

Difference AC / DC coupling

When setting AC coupling the direct current component is subtracted and only the alternating current component is displayed.

When setting DC coupling the DC voltage component is used as an offset for the AC voltage component and the latter is displayed higher on the y-axis by this amount.

Measurement

First, the phase is determined using a voltage tester (screwdriver with glow lamp, also called a "lie detector"). The connected probe is then connected to this. The sine signal is displayed.

Some oscilloscopes also allow an overlay display of all measured values resulting from the applied signal, as shown in the following figure: