How will a PWM controlled relay affect a 800VA transformer on the primary side compared to the secondary side?

Question

Application: A machine that seals boxes with plastic wrapping. The box is sealed by applying a heating element to the plastic for a few seconds.

I have a 800VA (220VAC to 54VAC) transformer connected directly to a heating element.

My requirement is to make the transformer work with the heating element so that I can adjust the temperature, and also not break the transformer.

Initially I wanted to switch the input voltage by controlling a zero-cross solid state relay (Celduc SO842074) with a PWM signal (Arduino) on the primary side. I then learned that this may cause a DC-component or voltage spikes which could break the transformer.

My current idea is to place the zero-cross solid state relay on the secondary side of the transformer (untested). I'm uncertain as to why switching the primary side would cause a DC component but doing the same on the secondary side would not?

Will this secondary switching be safe for the transformer or will there still be a DC component / voltage spikes?

What exactly happens to a transformer when switching rapidly on the primary side compared to the secondary side?

Bonus question: What would have been a better solution?

Thank you for the help in advance

Answer 1

The standard solution when the heater does not react very quickly is to use proportional time control.

Figure 1. With zero-cross switching the result is that the waveform consists of multiple complete half-cycles. Image source: Opto-triacs, solid-state relays (SSR), zero-cross and how they work.

Now you have the advantage of being kind to the transformer, lowest possible EMC because of the zero-cross switching and a simple control strategy. For most applications a duty cycle of a few seconds works well but this depends on the heater's time constant.

There's further information on the topic in (my) linked article.