What happens if you energize multiple inputs of a transformer

Question

I'm trying to automate an exhaust fan by using this ESPHome-driven relay:

I've got the relay setup working and now I'm puzzling over the actual AC motor control unit. The unit is Sentera STR-1. The transformer inside is STC-1-10L10. The unit has a rotary switch (Gottak GTK 46RCTH), which basically shorts different paths to the transformer to get the required voltage (if I understand it correctly).

My plan was to simulate different states of the rotary switch by connecting one relay per rotary knob position to the transformer's terminals.

I think that the lower left terminal on this is connected to the coloured wires in the top row in different positions. Here in this gif I cycle through all knob positions from OFF to 1 to 5 and you can see the mechanics behind the switching – the terminals are connected to the copper plate when the knob is turned. The plan is to connect wires to the appropriate terminal/rotary knob terminals and let the ESPHome switching do the magic.

However, before I blow up my motor or the relay, I want to ask:

What happens if I energize multiple inputs (e.g. 80V and 110V) on the transformer at the same time?

I'm using the interlock feature provided by ESPHome switch component. The ESPHome documentation has an angry red box warning that the software interlock might fail. I'm not sure what would happen with the relay/motor then. Hence the question.

Here's an image of what I think the current scheme looks like on the top, and what I would like to do on the bottom: (Disclaimer: the schematics are probably wrong, but I hope they give an idea of what I'm trying to do) Sorry for butchering the schematic, but several online tools didn't support all the necessary features, so I did an "MS Paint" on it. I hope it's understandable.

Ideally, I would appreciate two answers:

1. Would the transformer blow up if the interlock feature fails (it might fail momentarily – for a few dozen milliseconds, perhaps).
2. What would be a proper, reasonably cheap solution instead of separate relays?

Answer 1

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1. Series connection of the available changeover contacts ensures that only one tap can be energised at any time.

Answer 2

The schematics you have drawn are wrong.

The transformer is an autotransformer which is basically a single winding with multiple taps:

^{simulate this circuit – Schematic created using CircuitLab}

An autotransformer is basically a step-down transformer but with no isolation. You apply voltage (presumably 230V) across its L-N terminals only and take output from each tap (e.g. 170V output will have 170V across this tap and N). So, normally, no voltage is applied to the taps externally.

The relay box gets its inputs from autotransformer taps and directs only one of them to the output via relays. The main application of this is to drive an AC motor with different speeds.

Now the potential problem is having two relays direct their inputs to the output at the same time. This is probably what happens if interlock fails. In this case some taps will be shorted and therefore an unexpected voltage will be applied to the load. For example, if 140V and 110V switch relays are activated at the same time then L3 part of the auto transformer will be shorted. Therefore the voltage ratios will change so the output voltage at the output will be different. You can calculate the ratios and estimate the output voltage for different failures (i.e. tap shorts).