Circuit for switching between two AC voltage sources

Question

I have a circuit in which I have two different AC voltage sources (both 230 V) and a common load.

The aim of the circuit is that voltage source 1 is connected to the load; if voltage source 2 is also connected, voltage source 1 should be disconnected from the circuit.

I would only switch the life conductors with the semiconductors (optocouplers).

Briefly about me I have a BSc in automotive engineering, so I have some basic knowledge but not enough to realise the project by myself.

I have now made a schematic and would be happy if you could give me some feedback before I order the components. Especially whether it works at all and whether I still need capacitors or similar as a filter because 230 V AC voltage is at 50 Hz.

I am also open to try completely different approaches.

Please find the technical datasheets (TDS) and the schematic

TDS SSR

TDS Optocoupler

TDS Inverter

Answer 1

That's not going to work. Redraw your circuit using only V1, V2, X1 and replace the SSRs with switches.

Now figure out what happens when SSR1 is closed, SSR2 is closed and both are closed.

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1. Equivalent circuit for analysis.

Now, what is the optoisolator doing in your circuit?

^{simulate this circuit}

Figure 2. A simplified circuit that doesn't switch neutrals and includes protection devices.

• There is no need to switch neutrals as in your two diagrams. It complicates things, leads to errors and makes troubleshooting more difficult.
• I'm not recommending this circuit because it doesn't address synchronisation, interlocking or switchover delay.
• I highly recommend installation of fuses or MCBs on both supplies.
• SSRs are probably more likely to fail short-circuit than a relay contact will. (I don't have any numbers to back this up but have used both for > 40 years in industry.)

Answer 2

Are the two AC sources in perfect phase alignment? If not, two things.

a) the load might not like an instantaneous voltage change of up to 750 V.

b) the two AC sources might not like being connected directly together for a few milliseconds.

Many (most?) solid state relays have a TRIAC as the switching device. This is a solid state device that can be turned on at any point in the AC cycle. However (the bad part), when you remove the gate signal in the middle of a cycle, the TRIAC stays on until the next zero-crossing of the AC waveform. If the two AC sources are not in perfect phase alignment, this can cause a huge cross-conduction current between the two sources as one TRIAC comes on before the other one turns off.

One solution for this is to create a dead band, a short time delay between when one AC source is turned off and the other is turned on.

UPDATE:

Based on new information, a single component that will do what you want is a DPDT relay powered by AC source #2. This will disconnect AC #1 from the load and connect AC2 only when AC2 voltage value is above the actuation voltage of the relay. Note that this must be a double-pole relay because the two neutrals are not phase-aligned.

Answer 3

A relay or a contactor should suffice.