Design of a 50 Hz step-up mini transformer

Question

I am a first-year B.Tech student majoring in electrical and electronics engineering. Currently, I am working on designing a 500 W mini inverter.

I have addressed all aspects to reduce the inverter's size to that of a 500 ml water bottle. However, I am encountering a challenge with the size of the transformer. My specifications include a 22.2 V voltage requirement with a power output exceeding 500 W. I am struggling to determine how to minimize the transformer's size while maintaining optimal efficiency.

I have considered employing two step-up transformers, each with a power output of 250 W, in parallel to achieve a step-up from 22.2 V to 220 V. The inverter operates on a 50 Hz sine wave with an H-bridge topology.

I would appreciate clarification on how to design such a transformer. Additionally, any recommendations on the design process and the necessary calculations for non-ideal conditions, such as determining the number of turns for voltage step-up, the size of the conductor wire, and the optimum core diameter, would be invaluable.

My goal is to create a compact transformer with an operational time of approximately 5-6 hours. While we have covered ideal cases in our studies, I am unsure of how to apply this knowledge in practical design. Any guidance on transforming theoretical knowledge into practical design solutions would be greatly appreciated.

Answer 1

A quick look at the RS catalogue's chassis mount transformer selection gives the following information for 500 VA transformers.

P/N 504-228 (E/I type) specification:

• Rating: 500 VA.
• Dimensions: 120 × 143 × 130 mm = 2230 cm³ = 2.23 L.
• Weight: 7.5 kg.
• Price: €231 (2024).

From their range of torroidal transformers, P/N 123-3997:

• Rating: 500 VA.
• Dimension: 136 dia. × 60 mm = 871 cm³ = 0.87 L.
• Weight: 3.5 kg.
• Price: €143 (2024).

On top of this you are going to have to house your electronics and heatsinking. (You haven't said where the battery will be located.) I don't think you can do this with a standard transformer topology. It may be possible with some exotic magnetic material but I'd guess that's going to be very expensive.

Answer 2

I am encountering a challenge with the size of the transformer

Your circuit topology is probably wrong...

The trick here is to generate a high-voltage DC power rail. To do this you can get-away with a small isolating ferrite transformer (because the switching waveforms are circa 100 kHz and not 50/60 Hz).

Trying to make a transformer to handle 500 watts at 50/60 Hz needs a lot of iron and definitely won't fit in your envelope.

So, once you have converted your incoming supply to around 300 volts, you can use a H-bridge to reproduce a sinewave at 50/60 Hz using fairly standard PWM techniques but, importantly, you don't need a transformer to do this.

Answer 3

Because you are using a true sinewave, I see no reason to design a transformer. An off-the-shelf 240 V : 24 V power transformer will do what you want when connected "backwards". The 24 V side now is the pr1mary and the 240 V side is the secondary. If you have a 600 W input signal, you will get a 500 W output.

Transformers in parallel often do not share well. The outputs never are perfectly matched, and a 1 V difference means that some of the higher-voltage transformer's energy is going into the lower voltage transformer, to be dissipated as heat in the core.

If you want to split your design into two transformers to change the size and shape, better to drive the two primaries in parallel, but connect the two secondaries in series. In this case, the two transformers would be 120 V : 24 V. The two 24 V windings are connected in parallel as the primary, and the two 120 V windings are connected in series as the secondary.

To be clear, the primaries and secondaries are different. The primaries can be connected in either series or parallel with no "sharing" issues (as long as the voltage scaling is correct). It is the secondaries that must be connected such that minor differences in the output voltages do not cause circulating currents.