Transformer winding tap switch without relay
You may use magnetic latch relays which require only a short pulse to set or reset.

Here's a schematic, using a diode matrix, 3 push button switches and 3 magnetic latch relays. 



Freewheeling diodes for the relay coils and LED indicators are not shown.

'NO' contacts of the relays K1, K2 &amp; K3 are to be used to select the taps.

How it works:


The 'S' (set) coil of a relay switches the coil one direction and it remains there by magnetic action.
The 'R' (reset) coil of a relay switches the coil in the opposite direction and it remains there by magnetic action.
Pressing S1 momentarily 'sets' K1 and 'resets' K2 and K3.
Pressing S2 momentarily 'sets' K2 and 'resets' K1 and K3.
Pressing S3 momentarily 'sets' K3 and 'resets' K1 and K2.
The diodes prevent backfeed from one switch into the circuit of either of the others.