I know well this circuit because I use it in a battery charger project.
The circuit is very simple and works well.
It must however comply with certain precautions.
1)For safety, the potentiometer must have a plastic shaft and be mounted on an epoxy insulating plate.
2) the circuit generates a pulse train, which is correct for an inductive load.
But pulses are very short (2 to 3 microseconds) and are insufficient to ensure that the current reaches the holding/latching current with highly inductive load.
For proper and safe operation, add an incandescent lamp 100W 220V in parallel with the transformer.
3) I replaced the MOV by a bridge rectifier and 4 zener diodes in serie of 15V 1,3W.
Choose 3 quadrants snuberless triac (with snubber added for more reliability).
To test the circuit, use only the 100W lamp as load, without the transformer.
Nb: for auxiliary triac, be sure to use the Z0103, not the Z0107, nor Z0109, nor Z0110 because it will not generate pulse train with lower sensitivity auxiliary triacs. (higher gate current)
With transformer, you must have a symetrical triggering of the triac to avoid create a DC voltage on the primary of the transformer.
If this occur, transformer will saturate and a very high current will blow your mains fuse and perhaps also your triac.
With a 100W incandescent lamp in parallel with the primary of the transformer (resistive load low enough to reach the latching current), you are sure that no false triggering can occur and it will works with reliability.